Steroid receptor coactivators: from basic research to translational opportunities

Although SRCs can function as transcriptional repressors in specific contexts ( 113 , 126 ), their primary role is to activate gene expression. Deregulation of SRC activity is commonly linked to cancer, and they are traditionally classified as oncogenes ( 64 , 127 ). While their oncogenic activity is most well established in hormone-dependent cancers—reflecting their primary role as coregulators of hormone-responsive NRs—their oncogenic functions extend beyond hormone-related malignancies. In the following section, we focus on the roles of SRCs in hormone-related cancers, while directing readers to other articles for a discussion of their functions in non-hormone-related cancers ( 64 , 128 , 129 ). In ERα + breast cancer (BC), SRC-1 acts as an oncogene through regulation of ERα-induced levels of stromal cell–derived factor 1 alpha (SDF-1α; also known as CXCL12), a cytokine that controls BC cell proliferation and invasion ( 130 ). In hormone receptor negative BC, SRC-1 expression positively correlates with HER2 expression and poor prognosis ( 131 ). In the mouse mammary tumor virus (MMTV)–polyoma middle T antigen (PyMT) BC model, loss of SRC-1 does not impair primary tumor initiation or growth but significantly reduces lung metastases ( 132 ). Mechanistically, SRC-1 enhances metastatic potential by promoting cell migration and invasion via activation of Twist expression, achieved through its interaction with the TF polyoma enhancer activator 3 (PEA3) ( 133 ), highlighting the role of SRC-1 in driving metastasis also in non-hormone-dependent BC. Beyond its primary role in supporting metastases development, SRC-1 drives therapy resistance in BC by promoting signaling pathways that bypass hormone dependency ( 134 ). For instance, SRC-1 promotes resistance to aromatase inhibitors (AIs) by coactivating Ets2-driven transcription of c-myc and MMP-9, further supporting tumor progression independent of hormonal signals ( 135 ). SRC proteins are often considered master transcriptional regulators, in part due to their ability to integrate the transcriptional activity of numerous transcription factors, extending well beyond just NRs ( 136 ). It was reported that members of the STAT family of TFs, including STAT1, which has been linked to endocrine treatment resistance ( 137 ), were coactivated by SRC-1 ( 138-141 ). Another pathway by which SRC-1 drives ERα-independent oncogenicity in BC is through regulation of ADAM22, a member of the ADAM protein family, which is well established as a promoter of cancer progression ( 142-144 ). In the endocrine-resistant LY2 derivative of the MCF-7 BC cell line, ADAM22 expression was positively correlated with the expression of SRC-1. Moreover, KD of ADAM22 significantly reduced the migratory capacity of both endocrine-resistant and ERα-negative cell lines. These findings highlight the SRC-1–ADAM22 axis as an alternative pathway that drives the shift from steroid-responsive to steroid-resistant BC, enhancing migratory potential and metastatic progression ( 145 ). Consistent with this, SRC-1 expression positively correlates with resistance to endocrine therapy and AIs ( 131 , 135 , 146 , 147 ). The canonical mechanism by which SRC proteins co-activate NRs involves ligand binding to the NR, which induces conformational changes that enable interaction with coactivators ( 148 ). Ligand-independent activation of NRs, on the other hand, can also occur, and it is frequently associated with uncontrolled cell proliferation and cancer ( 149 , 150 ). It has been demonstrated that cyclin D1, which is frequently amplified in BC, can mediate ligand-independent interaction between ERα and SRC-1 ( 151 ). The formation of a transcriptionally active ternary complex (D1–SRC-1–ERα) in the absence of ligand suggests that SRC-1 participates in unregulated activation of ERα in BC. Aberrant DNA methylation, frequently involving hypermethylation of tumor suppressor genes, is commonly observed in cancer cells ( 152 ). Although the majority of SRCs’ activity is associated with transcriptional activation, it has been demonstrated that SRC-1 can also promote oncogenicity by functioning as a global transcriptional repressor ( 153 ). Specifically, in endocrine-treatment-resistant models, it has been shown that SRC-1 acts as an integrator in the formation of an epigenetic remodeling complex. SRC-1 mediated recruitment of methyl-CpG-binding domain (MBD) proteins, such as MECP2 and MBD2, which bind methylated DNA and mediate transcriptional repression, along with HDAC2, a known MBD binding partner ( 154 ). This coordinated activity leads to the silencing of a set of differentiation-related genes and results in an increased proliferative capacity specifically in endocrine-resistant BC models. Steroid receptor coactivator 2 has also been linked to BC tumorigenesis; however, compared with SRC-1 and SRC-3, its role in BC is relatively minor and remains poorly characterized. Like SRC-3, SRC-2 can compensate for SRC-1 deficiency and restore the expression of an ERα target gene, SDF-1α, in the BC cell line ( 130 ). Two other studies demonstrated that in MCF-7 cells the expression of pS2, another ERα-responsive gene, as well as DNA synthesis and full-scale ERα transcriptional activity, also require SRC-2 ( 155 , 156 ). These observations imply the supportive role of SRC-2 in an ERα-driven proliferative signaling in cancer cells. Treatment with tamoxifen in ERα-positive BC patients induces the expression of all 3 SRCs ( 157 ), suggesting that like SRC-1 and SRC-3, SRC-2 may also contribute to the development of hormone-therapy resistance. Two recent studies demonstrated a more direct oncogenic role for SRC-2 in BC cells. One study reported that the NCOA2 gene is amplified in 5% to 14% of BC cases and is essential for BC cell growth, including a triple-negative subtype, suggesting functions beyond ERα signaling ( 158 ). Indeed, the authors of this study showed that NCOA2 perturbation in the TNBC cells MDA-MB-231 led to reduced MAPK/ERK signaling. In a separate study, SRC-2 was implicated in epithelial–mesenchymal transition (EMT) regulation through its control of Lyn kinase, a known EMT marker ( 159 ). Depletion of SRC-2 altered the transcriptional program of MCF-7 cells toward a more differentiated luminal gene expression signature, accompanied by reduced expression of genes associated with invasion, suggesting that SRC-2 may contribute to maintaining invasive potential even within luminal BC cells ( 159 ). Steroid receptor coactivator 3 is also referred to as amplified in BC 1, which underscores its overexpression and strong association with ER-positive BCs ( 160 ). Amplification and overexpression of SRC-3 have been observed in ∼5% to 10% and 30% to 60% of BC biopsies, respectively ( 46 , 161 ). Both alterations are associated with adverse clinical outcomes, including tamoxifen resistance, aggressive disease, and poor prognosis ( 162 , 163 ). Mutations in the ligand-binding domain (LBD) of the ESR1 gene generate ERα variants with ligand-independent activity, conferring resistance to AIs and reducing sensitivity to some selective ER degraders (SERDs), such as fulvestrant ( 164 ). Although the newer SERDs elacestrant and imlunestrant significantly prolong progression-free survival in ESR1-mutant BC patients, durable long-term remission remains a clinical challenge ( 165 , 166 ). Unlike the WT ER, whose interactions with SRCs in normal physiological conditions are for the most part estrogen dependent, mutant ER variants can recruit SRC-3 even in the absence of a ligand ( 12 , 167 ). These ligand-independent interactions drive persistent oncogenic transcription, highlighting the critical role of SRC-3 as an oncoprotein in ER-positive BCs. Interestingly, SRC-3 overexpression in BC has been associated with the absence of estrogen and PRs, suggesting that SRC-3 may also drive tumorigenesis independently of its canonical NR partners, or potentially without engaging NRs at all ( 168 , 169 ). Cyclin D1 is a key regulator of the cell cycle, enabling the transition from the G1 to the S phase. In ER-positive BCs, overexpression of Cyclin D1 drives mitogenic signaling and is associated with poor prognosis ( 170 ). ER and Cyclin D1 interact in a positive feedback loop, enhancing each other's mitogenic functions ( 171-174 ). Evidence from ER-positive BC cells shows that estradiol (E2) induces Cyclin D1 overexpression ( 173 , 175 ). Importantly, SRC-3 contributes to BC progression by coactivating ER, a function that is essential for ER-mediated induction of Cyclin D1 expression ( 176 ). Several mouse models have further established SRC-3 as an oncogene by demonstrating its critical impact on BC development in vivo . In MMTV–v-ras transgenic mice, loss of SRC-3 markedly reduced tumor incidence and completely prevented tumor formation in ovariectomized animals ( 177 ). Similarly, in the MMTV–Erbb2 mice model, ERBB2-driven mammary tumor development was entirely suppressed in the absence of SRC-3, highlighting its requirement for ERBB2 oncogenic signaling ( 178 ). Furthermore, SRC-3 overexpression, modeled by MMTV–SRC3 transgenic mice, was sufficient to drive spontaneous malignant mammary tumor formation ( 179 ). Consistent with SRC-3’s dominant role in the mammary gland, in a chemically induced carcinogenesis model, SRC-3 deficiency protected the mammary tissue, but not the skin, from tumorigenesis ( 180 ). Using the MMTV–PyMT animal model SRC-3 has also been shown to promote BC metastasis by mediating the enhanced expression of MMP-2 and MMP-9 through coactivation of the TF PEA3 ( 181 ). Interestingly, a study using human BC cells showed that SRC-3 regulates MMP-7 and MMP-10 expression through activation of the TF AP-1 ( 182 ), underscoring its role in promoting metastasis by enhancing cancer cell invasion through upregulation of MMPs. Collectively, this body of evidence establishes all 3 SRCs as key oncogenes in BC tumorigenesis through their roles in tumor initiation, progression, and metastasis. Steroid receptor coactivator 1 functions as an important coactivator of androgen receptor (AR). It has been shown that male mice lacking SRC-1 exhibit reduced responsiveness to androgen stimulation and that SRC-1 deficiency results in smaller prostate glands compared with WT controls ( 65 ). Additionally, SRC-1 coactivation of AR supports AR-driven cell proliferation involved in prostate development and tissue regeneration ( 183 ). In prostate cancer, increased expression of SRC-1 is correlated with aggressiveness of the disease as evidenced by higher levels of SRC-1 in metastatic lesions compared with primary tumors ( 184 ). In vitro studies have also demonstrated that SRC-1 drives the invasive behavior of prostate cancer cells, independently of AR expression ( 185 ). Additionally, in samples from prostate cancer patients, SRC-1 expression levels were higher in tumor tissues compared with normal prostate tissues ( 185 ). Interestingly, in a spontaneous in vivo prostate cancer mouse model, SRC-1-deficient mice exhibited tumor development similar to that of WT mice. However, in both SRC-1 KO and WT mice, prostate cancer development was accompanied by SRC-3 overexpression, suggesting a potential redundancy in driving the disease in WT animals and a compensatory role for SRC-3 in SRC-1-deficient mice ( 186 ). Nonetheless, despite the apparent nonessential role of SRC-1 in tumorigenesis in the spontaneous prostate cancer transgenic model, findings in human samples suggest that SRC-1 plays a critical role in promoting the development and aggressiveness of the disease. Steroid receptor coactivator 2 serves as the primary coactivator of AR and is the most strongly linked SRC family member to prostate cancer progression and metastasis ( 187 , 188 ). Genomic amplification of SRC-2 occurs in ∼8% of primary and 37% of metastatic prostate cancers ( 189 ). Beyond its oncogenic role, SRC-2 is a key regulator of energy homeostasis ( 95 , 190 ) and has been shown to promote metastasis by reprogramming prostate cancer cell metabolism. Activated by glutamine-driven mTORC1 signaling, SRC-2 coactivates the TF sterol regulatory element-binding protein 1 to enhance de novo lipogenesis. Consistent with this metabolic role, disruption of SRC-2 function has been found to impair prostate cancer growth and metastases ( 191 ). Like the other 2 SRC family members, SRC-3 is also strongly involved in AR-related development and progression of prostate cancer ( 192 , 193 ). Consistent with its strong association with coactivation of AR, elevated SRC-3 levels in prostate cancer patients positively correlated with higher tumor grade and increased risk of recurrence ( 188 , 194 ). Analysis of 58 RNA samples from prostate cancer patients revealed that overexpression of SRC-3 is positively correlated with invasion and metastasis ( 195 ). Supporting these findings, in vitro assays have shown that SRC-3 plays an essential role in prostate cancer cell motility and invasiveness through the direct induction of key MMPs, MMP-2, and MMP-13, by partnering with the transcription factors AP-1 and PEA3 ( 195 ) . At the molecular level, SRC-3 has been shown to cooperate with the transcription factor AP-1 to upregulate gene expression in the insulin-like growth factor/AKT signaling pathway, thereby promoting cancer cell proliferation and survival, including in prostate cancer cell lines ( 193 , 196 ). In the spontaneous transgenic adenocarcinoma of the mouse prostate (TRAMP) model, elevated SRC-3 expression was observed in prostate tumor cells as they advanced to more malignant stages. Loss of SRC-3 suppressed tumor growth and progression and significantly prolonged the survival of TRAMP mice ( 197 ). These findings indicate that SRC-3 promotes tumor aggressiveness and plays a critical role in the progression of prostate tumorigenesis toward the metastatic stage. In normal endometrial tissue, SRC-1 mRNA levels positively correlate with ERα expression ( 198 ). In endometrial carcinoma, SRC-1 expression is reduced compared with normal proliferative-phase endometrial tissue, and this decrease in SRC-1 levels is associated with loss of ER and PR expression ( 199 ). Conversely, another study has reported that all 3 SRCs have upregulated mRNA levels in endometrial carcinoma relative to normal endometrium. However, no significant correlation was observed between the overexpression of SRCs and key pathological parameters, such as disease stage or invasiveness ( 198 ). Steroid receptor coactivator 2 is the least studied among the 3 SRC family members in female genital cancers. However, it was reported that both SRC-2 and SRC-3 are elevated in the endometrium of women with PCOS ( 74 , 76 ), a condition associated with increased risk of developing estrogen-driven endometrial hyperplasia and cancer ( 200 ). Steroid receptor coactivator 3 has been associated with the pathogenesis of both endometrial and ovarian cancers. In a mouse model with MMTV-driven SRC-3 overexpression, elevated SRC-3 levels were associated with increased endometrial tumor formation ( 179 ). In human endometrial tumors, high SRC-3 expression correlates with advanced disease stage, increased myometrial invasion, and unfavorable clinical outcomes ( 80 , 201 ). Similarly, in ovarian cancer, SRC-3 is often amplified, and its overexpression was found in >60% of high-grade tumors ( 46 , 202 ). Experimental evidence further indicated that loss of SRC-3 impairs ovarian cancer cell migration and motility ( 203 ). Among the 3 SRC family members, SRC-1 and SRC-3 are most strongly linked to endocrine therapy resistance. Tamoxifen, a selective ER modulator, is a standard antiestrogen therapy for premenopausal patients with ERα-positive BC. Treatment with tamoxifen is associated with increased expression of both SRC-1 and SRC-3 ( 147 , 157 ). In the T47D BC cell line, overexpression of SRC-1 has been shown to enhance ERα transcriptional activity in response to E2, effectively converting tamoxifen into a transcriptional activator in these cells ( 204 ). Furthermore, in MCF-7 cells that acquired tamoxifen resistance after continuous exposure, SRC-1 levels decreased once the tamoxifen treatment was lifted, suggesting that SRC-1 expression is specifically associated with conferring resistance in ERα-positive BC cells during treatment ( 205 ). Immunohistochemical analyses of tissue microarray revealed that the co-association of SRC-1 and SRC-3 with ERα is linked to reduced disease-free survival (DFS) and endocrine therapy resistance in patients ( 134 ). SRC-1 may also contribute to therapy resistance by its role in regulating ADAM22, a nonprotease member of the ADAM disintegrin family, that was identified as an independent marker of poor DFS ( 145 ). In tamoxifen-resistant cell lines, ADAM22 has been associated with cell migration and differentiation, and its mRNA expression is increased in tamoxifen-resistant tumors in xenograft mouse models ( 145 ). Examination of patient samples from ERα-positive BCs revealed that tumors expressing PAX2 but lacking SRC-3 showed the most favorable DFS outcomes compared with other PAX2/SRC-3 expression profiles. Using human BC cell lines, the authors of this study have demonstrated that tamoxifen–ER complexes repress HER2 transcription, whereas SRC-3 competes with the tamoxifen-recruited transcriptional repressor PAX2, leading to HER2 upregulation and promoting HER2-related tumor aggressiveness ( 145 ). These findings suggest a possible mechanism in which SRC-3 outcompetes the tamoxifen-recruited transcriptional repressor PAX2, thereby promoting HER2-driven drug resistance in ERα-positive BC. Aromatase inhibitors suppress estrogen biosynthesis and are widely used to block ERα signaling in postmenopausal women with hormone-dependent cancer. Examination of tumors from AI-resistant patients and the AI-resistant cell-line model revealed a significant association of SRC-1 expression with disease recurrence and drug resistance. SRC-1 was essential for maintaining the enhanced metastatic potential of AI-resistant cells, while its KD restored cellular differentiation and reduced migratory capacity. Interestingly, ERα KD had no significant impact on the migratory capacity of AI-resistant cells, suggesting that SRC-1 mediates resistance and the aggressive phenotype of the disease through steroid-independent mechanisms. In endocrine-resistant BC cells, SRC-1 interacts with other transcription factors, including Ets2—a member of the ETS family and a downstream effector of HER2 and MAPK pathways ( 131 , 135 , 206 ). This suggests an alternative mechanism by which SRC-1 promotes an aggressive disease phenotype while bypassing canonical ERα signaling. Estrogen receptor α mutations, which typically occur within the LBD, often lead to ligand-independent oncogenic transcriptional activity following AI treatment ( 207 ). It has been demonstrated that all SRC family members show enhanced recruitment to estrogen response elements when bound to mutant forms of ERα LBD ( 12 , 208 ). Moreover, the constitutive transcriptional activity of these mutants is associated with their enhanced ability to recruit coregulators such as SRC-3 ( 12 , 209 ), suggesting an additional mechanism through which SRCs may contribute to endocrine therapy resistance. Beyond their primary roles in tumor initiation, progression, and metastasis in hormone-dependent cancers, SRCs also play a dominant role in the development of resistance to endocrine therapy. This adds an additional layer to their oncogenic potential, highlighting the promise of SRC-targeted therapeutic strategies.

The most fundamental determinant of eukaryotic transcription is the interaction of RNA polymerase II (Pol II) with the genome. Pol II is a 12-subunit protein complex that functions in concert with general transcription factors (GTFs) to assemble into an even larger preinitiation complex, comprised of several dozen subunits ( 30 , 31 ). This degree of complexity provides the transcription process with a high level of regulatory precision. Beyond the core preinitiation machinery, sequence-specific TFs further complicate the process. These factors interpret upstream signals and chromatin context to exert gene-specific control, ultimately dictating the fate of transcription at the level of individual genes ( 32 ). Furthermore, as was first proposed in the early 1970s, the regulatory landscape of gene expression involves an additional class of proteins, coregulators, that modulate the activity of TFs ( Fig. 2 ). Originally, the hypothesis of the existence of helper proteins, that partner with NRs, was introduced in the context of the activity of the progesterone receptor (PR)—as essential facilitators of its hormone-dependent transcriptional activity ( 14 , 15 ). These proteins were first termed “acceptor proteins” or “acceptor molecules” to reflect their role in determining the nature of functional hormone receptor binding to its tissue-specific chromatin “acceptor cites” ( 13 , 33 , 34 ). Ultimate proof for the existence of NR coregulators came 2 decades later with 2 publications that identified the first corepressor and the first coactivator of NRs. The proof of principle that NRs are interacting with partner proteins that regulate their activity was first exemplified by the repressor protein SSN6; by binding to the TAF1 domain of the ER, SSN6 suppresses the hormone-dependent transcriptional activity of ER and PR. Mutations in SSN6, that disrupted its protein–protein interaction capability, led to increased hormone-dependent activity of both ER and PR ( 17 ). These findings suggested a dual role for the hormone: it not only promotes DNA binding but also induces a conformational change of the NR (ie, ER/PR) that antagonizes the activity of a corepressor protein (eg, SSN6). Shortly after this foundational demonstration of NR coregulation, another study reported that transcriptional activity of rat glucocorticoid receptor (GR) in yeast is enhanced upon its interaction with SWI proteins. Coprecipitation of the GR complex with SWI proteins was detected in wild-type (WT) yeast extracts but was absent in extracts from SWI–mutant strains, which also exhibited impaired GR-mediated transcriptional activation ( 18 ). These 2 seminal studies established the concept that NRs rely on “helper proteins” for the regulation of their transcriptional activity. This group of proteins, known as coregulators, includes coactivators and corepressors, which regulate NRs by subjecting them to either positive or negative regulation of their transcriptional activity ( 19 ). More than 30 years after the discovery of the first coregulators, several hundred have now been reported in the literature, with more likely to be discovered in the future ( 35 , 36 ). Within the NR coregulator family, SRCs comprise one of the most important groups, acting as integrators of transcriptional activity not only of NRs but also of other TFs as well. SRC-1 was among the first coregulators to be cloned ( 20 ) after the discovery of the general coactivator/integrator CBP ( 37 ). Initially identified as an ER-associated protein, it was found to bind ER in a specific and ligand-dependent manner, implicating it as a central mediator of ER-dependent transcriptional activity ( 38 ). Subsequent cloning and characterization revealed SRC-1 to be a common coactivator that interacts with multiple NRs, including PR, GR, ER, TR, and Retinoid X Receptor (RXR) ( 20 ). Later studies further expanded its functional scope by demonstrating its ability to coactivate non-NR transcription factors, such as nuclear factor-kappa B (NF-κB), thereby positioning SRC-1 as a broadly acting coactivator ( 39-41 ). The discovery and characterization of SRC-1 was followed shortly by the cloning and identification of TIF2/SRC-2 ( 42 ) and RAC3/ACTR/SRC-3 ( 43-46 ), establishing the SRCs as the first characterized coactivator family and revealing significant structural similarity among the 3 members ( 45 , 47 ). Of note, while SRCs are part of the broader nuclear coactivator (NCOA) family—comprising 7 human coregulators (NCOA1-7) ( 48 )—they represent a distinct subgroup of 3 closely related members. All SRCs have a molecular weight of ∼160 kDa and share several conserved structural domains ( Fig. 2B and 2C ); the most conserved domain is the basic helix-loop-helix (bHLH)/Per/Arnt/Sim (bHLH-PAS) domain, which is located at the N-terminus and shares ∼60% amino acid identity between the SRC family members ( 46 ). bHLH-PAS is required for the protein–protein interaction capability of the SRCs ( 49-51 )—including interactions with specific TFs ( 52 , 53 )—and it also contains a bipartite nuclear localization signal that directs the SRCs into the nucleus. The central region of the SRCs contains 3 LXXLL α-helical motifs (X represents any amino acid) with flanking sequences around them that together form the NR interaction domain, which is required for the activating or repressor interactions of the SRCs with their target NRs ( 54 ). The α-helical amphiphilic LXXLL motifs are essential for the SRCs to interact with NRs ( 55 ), while the flanking sequences dictate the strength and specificity of binding ( 56-58 ). The 2 activation domains, AD1 and AD2, are located at the C-terminus of SRCs and serve as key mediators of SRC interactions with essential transcriptional cofactors; the AD1 domain primarily recruits histone acetyltransferases such as p300 and CBP, while AD2 engages histone methyltransferases (HMTs) such as CARM1 and PRMT1 ( 21 , 58 , 59 ). Accordingly, AD1 is commonly referred to as the CBP-interacting domain, and AD2 is known as an HMT domain. Functional domains of the SRCs: (A) Eukaryotic transcription as a multilayered, hierarchical process, with coregulator proteins at its highest regulatory level. (B) The molecular structure of SRC family members contains several conserved structural domains: a bHLH/period aryl hydrocarbon/simple-minded (bHLH-PAS) domain, a serine/threonine-rich (S/T) domain, a nuclear receptor interaction domain (NRID), a histone acetyltransferase (HAT) domain, and an HMT domain. (C) SRCs act as scaffold proteins to integrate the interaction of NRs and chromatin remodelers with the genome and promote assembly of the Pol II transcriptional machinery. Due to their modular multidomain architecture, SRCs are ideal bridging molecules capable of simultaneously interacting with NRs, other TFs, and essential coregulatory elements such as GTFs and histone–chromatin remodelers ( 60 ). This enables SRCs to function as an epicenter of Pol II transcriptional complexes and to facilitate histone modifications critical for the regulation of gene expression. Moreover, it has been shown that the SRC-1 and SRC-3 have an intrinsic acetyltransferase activity ( 45 , 61 ). This finding highlights an additional layer of functional flexibility in SRCs. Further versatility arises from posttranslational modifications (PTMs), which enhance the interaction specificity of SRCs and broaden the amplitude of their coregulatory activity. Among these, phosphorylation is a key PTM observed across all SRC family members. Generally, phosphorylation enhances SRC interactions with NRs, resulting in increased transcriptional activity, which in some cases has been associated with a potential risk of tumorigenesis ( 62 , 63 ). Other PTMs such as ubiquitination, SUMOylation, and acetylation have also been observed. These modifications regulate SRC stability, cellular concentration, and overall activity, adding further complexity to their regulatory network ( 64 ). Altogether, the ubiquitous expression of SRCs, their ability to recruit a broad spectrum of regulatory proteins, and the added diversity of their activity conferred by PTMs position them as central players in gene expression regulation, ultimately reflecting their broad significance in human physiology and disease.

Steroid receptor coactivators mediate the transcriptional regulation of virtually all NRs and many other TFs. They perform diverse functions in human biology and disease, including key roles as oncogenic drivers, metabolic regulators, central controllers of the reproduction system, and modulators of immune responses. Since the establishment of the coregulation concept and discovery of the first SRC family member, the vast scope of SRC-mediated regulatory functions has rapidly been extensively characterized, positioning them as compelling therapeutic targets, especially in oncology. Initially, due to their “undruggable” nature, early drug discovery efforts focused on identifying ligands that could disrupt the interface between SRCs and their main target proteins, namely NRs. These efforts generated large public libraries of compounds that interfered with NR–coactivator interactions, though it was unclear whether the compound targeted the NR or the coactivator. Nevertheless, this laid the groundwork for more focused screening efforts aimed at identifying ligands that directly target SRCs, ultimately leading to the discovery of several naturally occurring SMIs, followed by the development of synthetic molecules with improved pharmacological properties. In addition to inhibitors, SMSs of SRCs have also been identified. While initially raising concerns, due to SRCs’ well-established oncogenic functions, it was later shown that the SRC stimulator MCB-613 exerts paradoxical anticancer effects. By hyperactivating SRCs—which, along with other growth factors, are already overexpressed and operating at maximal capacity in many cancers—MCB-613 overwhelms cancer cells—inducing lethal levels of cellular stress and ultimately triggering cell death. In addition to their roles as oncogenes and in canonical hormone-governed physiological processes such as reproduction and metabolism, SRCs are increasingly recognized for their significant functions in immunology—a field where their biology is actively investigated but not yet fully characterized. Nonetheless, their involvement in specific immune cells and immune-related processes is well established, particularly in regulating macrophage activity and the proliferation, differentiation, and function of lymphoid cell subsets, including a key role in governing the Treg–Th17 transition axis. The significance of SRCs in Th17-driven autoimmunity is demonstrated by the resistance to EAE observed in SRC-1- and SRC-3-deficient mice ( 100 , 101 ). SRC-1 and SRC-2 play contrasting roles in directing CD4⁺ T-cell differentiation between the Treg and Th17 lineages. SRC-1 favors a transition toward an inflammatory phenotype by promoting Th17 cell development over Treg formation, while SRC-2 facilitates the commitment of naive CD4⁺ T cells to the regulatory phenotype. In macrophages, SRCs act both directly and indirectly to promote anti-inflammatory phenotypes, primarily by activating the expression of various anti-inflammatory factors. The identification of drug-like molecules capable of directly targeting SRCs marks a major advance in the therapeutic potential of SRC modulation. However, due to the high structural similarity among SRC family members, achieving greater specificity remains a key challenge. The nuanced and selective roles of SRCs in regulating immune system functions underscore the need for the development of highly specific modulators. Such modulators may enable the extension of SRC targeting beyond cancer cell inhibition to broader immunomodulatory applications. Nonetheless, the existing arsenal of SRC small-molecule modulators has provided a foundation for progress in this direction. For example, the observation that an SRC inhibitor, SI-2, can both suppress tumor growth directly and enhance antitumor immune response ( 224 ) provided the impetus for developing a potential cell therapy using SRC-3 KO Tregs ( 227 ). This approach has shown highly promising preclinical results in achieving long-term remission in several solid tumors ( 227 ). Moreover, previous observations indicating that SRCs contribute to reparative processes ( 230 ) and regulate anti-inflammatory programs in key regenerative cells—particularly macrophages—have supported the idea of extending SRC-targeted therapies beyond oncology. The therapeutic potential of the SMSs MCB-613 and MCB-10-1 in regenerative medicine has been demonstrated in preclinical models of tissue repair following MI and ischemic brain injury. These pro-reparative effects are likely driven by the SMS-related stimulation of SRCs in macrophages, where SRCs are predominantly associated with promoting an anti-inflammatory phenotype. Overall, current data highlight the strong and diverse therapeutic potential of targeting SRCs. Continued efforts to develop more selective SRC inhibitors will likely build on early successes and broaden their applications. For example, given the well-established roles of SRCs in metabolic regulation and the first clinical trial investigating SRC-1 deficiency as an indicator of metabolic disorders, SRC modulation may offer novel therapeutic strategies for metabolic diseases such as diabetes. Moreover, based on encouraging data showing SMS-mediated pro-reparative shaping of microglia and the central role of these cells in neurodegeneration, targeting SRCs in microglia holds promise for treating neurodegenerative diseases like Alzheimer's disease. Further exploration of SRC modulation in additional immune subsets where they exert dominant biological functions, such as Th17 cells, may open new therapeutic opportunities, including the treatment of autoimmune disorders. Promising preclinical results demonstrating the therapeutic potential of SRC-3 KO Tregs in solid tumors highlight an opportunity to leverage advances in cell-based and gene-editing technologies for the clinical translation of SRC-3-targeted cellular therapies. Together, these insights underscore the vast and still-unfolding therapeutic potential of SRC modulation, fueling strong optimism for future breakthroughs in oncology, immune-related disorders, and beyond. In summary, the early reasoning that NRs require assistant proteins to achieve the full scale of their biological activity led to the establishment of NR coactivation as a core branch of molecular biology, ultimately culminating in the discovery of one of the most important gene regulatory proteins—the SRCs. Decades of fundamental research have now been translated into challenging yet exciting opportunities for the development of new therapeutic approaches for unmet medical needs.

As multifaceted oncoproteins, SRCs represent compelling therapeutic targets. Given their pivotal role in modulating hormone receptor activities, targeting SRCs—rather than the NRs themselves—offers a promising alternative, particularly as traditional hormone-based therapies often lose effectiveness due to resistance ( 127 ). However, SRCs have long been considered challenging to target pharmacologically due to their structural flexibility and absence of a well-defined LBD. As a result, initial strategies focused on disrupting SRC–NR interactions instead of directly targeting the SRC proteins themselves ( 23-25 ). High-throughput screening combined with expression reporter assays led to our discovery of several naturally occurring inhibitors that directly target SRCs ( 26 , 210 , 211 ). This platform ultimately enabled the identification of the first-generation synthetic compounds that directly target the SRCs and bring about either inhibition (SMIs) ( 27 ) or stimulation (SMSs) ( 28 ) of their activity ( Fig. 5A ). For example, the SMI SI-2 has been shown to physically interact with SRC-3, leading to a reduction in SRC-3 protein levels in cancer cells. By depleting this oncoprotein, SI-2 selectively induced cytotoxicity in cancer cell lines in vitro and suppressed tumor growth in vivo ( 27 ). The successful development of synthetic small molecules that directly target SRCs paved the way for second-generation SMIs with improved drug-like properties ( 212 , 213 ). Targeting SRCs: emerging therapeutic strategies: (A) Inhibition or stimulation of SRCs with either SMI or SMS suppresses cancer cell growth: Inhibition of SRCs with SMIs induces their degradation, resulting in reduced proliferation and enhanced apoptosis. In contrast, stimulation of SRCs with SMSs causes hyperactivation of SRC transcriptional activity, triggering cancer cell–specific cytotoxic stress. (B) Immunomodulatory effects of SRC small molecule moderators: SMI SI-2 triggers an antitumor immune response characterized by the secretion of pro-inflammatory cytokines and recruitment of cytotoxic lymphocytes into the TME (left). Stimulation of SRC-3 with SMSs leads to an increased presence of anti-inflammatory macrophages, promoting the establishment and maintenance of a pro-reparative environment that facilitates regeneration of damaged tissue (right). (C) SRC-3 regulates the immunosuppressive function of Tregs: SRC-3 KO in Tregs results in a remodeled, antitumor TME, characterized by increased levels of pro-inflammatory cytokines and enhanced recruitment of cytotoxic lymphocytes, which ultimately drives eradication of solid tumors in mouse models. Further advancements in SRC SMI-based cancer therapy have come from exploring combination strategies with other molecular targets. One study demonstrated that combining SRC SMIs with CDK4/6 inhibitors not only synergistically enhanced cancer cell killing but also reduced the expression of immunosuppressive markers in surviving cells, potentially reducing immune evasion ( 214 ). In another study, a genome-wide screen in ER⁺ MCF-7 cells identified a combinational treatment that enhanced sensitivity to the derivative of SI-2 known as SI-12 across multiple types of cancer cell lines, suggesting pan-anticancer potential that can be achieved by SRC inhibition. Notably, the transcriptional signature of SI-12 differed from that of the ERa inhibitor ICI, indicating distinct mechanisms of action and supporting SRC SMIs as alternatives to traditional hormone therapies ( 215 ). Initially, there were concerns that the SRC stimulator molecules might activate dormant cancer cells and promote tumor growth ( 16 ). Surprisingly, the SRC SMS MCB-613 showed inhibitory effects on cancer cells both in vitro and in vivo . This anticancer effect likely results from the fact that cancer cells already overexpress SRCs and exhibit elevated activation of other growth-promoting genes; further stimulation by SMS overwhelms these cells by inducing stress responses and ultimately suppresses their growth ( 28 ). Consistent with anti-inflammatory roles of SRCs in macrophages, it has been demonstrated that a pan-SRC stimulator, MCB-613, promotes a pro-reparative environment in injured cardiac tissue, highlighting the potential therapeutic applications of SRC targeting beyond cancer ( Fig. 5B ) ( 216 ). Specifically, SRC pharmacological activation induces an anti-inflammatory macrophage phenotype, marked by increased infiltration of CCR2⁻ macrophages expressing the anti-inflammatory and cardioprotective protein TSC22D3 (also known as GILZ) ( 217 , 218 ). This shift in macrophage composition suggests that the beneficial effects of SRC stimulation are mediated, at least in part, through modulation of macrophage polarization toward a reparative, anti-inflammatory state ( 216 ). Furthermore, a derivative of the MCB-613 SRC stimulator with improved drug-like characteristics, named MCB-10-1, was tested for its ability to protect the brain in the setting of ischemic injury, aiming to extend the reparative benefits observed in cardiac tissue to the brain ( 219 ). Indeed, pharmacological stimulation of SRCs promoted microglial polarization toward a pro-reparative M2-like phenotype, marked by expression of ARG1—an enzyme recognized for its inflammation-resolving role in macrophages ( 220 , 221 ). M2-polarized microglia supports neuronal recovery and attenuates neurodegenerative processes ( 222 ). Therefore, by promoting M2-microglial phenotype, MCB-10-1 facilitates a neuroprotective response following ischemic injury. These findings suggest that pharmacological activation of SRCs promotes anti-inflammatory macrophages, suppresses pro-inflammatory and apoptotic pathways such as IL-1β signaling and oxidative stress, and induces antioxidant transcriptional programs, highlighting the potential value of SRC SMSs in regenerative medicine ( 216 , 219 , 223 ) ( Fig. 5B ). Intriguingly, the antitumor activity of the SMI SI-2 observed in immunocompetent mice was lost in immunocompromised models, suggesting that its therapeutic effect involves not only inhibition of SRC-3's oncogenic activity in tumor cells but also modulation of the tumor immunity ( 224 ). Indeed, low-dose treatment with SI-2 induced a moderate cytokine response, bringing about increased infiltration of cytotoxic immune cells and decreased presence of Tregs within the TME, establishing a direct link between SRC-3 inhibition and the activation of antitumor immunity, characterized by increased levels of cytokines with known anticancer functions—such as IFNγ and CXCL9 ( 225 , 226 ). These observations led to the hypothesis that selectively targeting SRC-3 in Tregs could enhance antitumor immune responses. Indeed, using a syngeneic BC mouse model, durable tumor eradication was achieved following cKO of SRC-3 in Tregs induced by tamoxifen-activated Cre recombinase ( 227 ). Moreover, a similar outcome was observed in mouse models of prostate, glioblastoma, lung, colon, and pancreatic cancers, suggesting that SRC-3-deficient Tregs may exert a TCR-independent, pan-cancer therapeutic effect ( 16 , 227 ). SRC-3 KO in Tregs led to increased infiltration of cytotoxic immune cells into the tumor, along with elevated levels of IFNγ and CXCL9 within the TME. Based on these findings, the proposed mechanism of the antitumor immunity promoted by SRC-3 KO Tregs relies on tumor-specific accumulation of the edited Tregs, which secrete IFNγ within the TME. This increased IFNγ then stimulates tumor and stromal cells to produce CXCL9 ( 228 ) to bring about the recruitment of CXCR3-expressing cytotoxic immune cells—such as CD8⁺ T cells and NK cells—into the tumor, thereby promoting tumor clearance ( Fig. 5C ). Remarkably, when cancer cells were reintroduced into mice previously that had achieved long-term remission from BC after SRC-3 KO Tregs treatment, new tumors failed to form, indicating a durable immune memory response. Notably, SRC-3-deficient Tregs isolated from tamoxifen-treated SRC-3f/f:FOXP3Cre-ERT2/+ transgenic mice can be adoptively transferred into tumor-bearing animals, resulting in an antitumor effect similar to genetically engineered mice. This underscores the translational potential of developing SRC-3 KO Treg-based cell therapy. To support clinical translation, a protocol was developed for generating SRC-3-deficient human Tregs using CRISPR-Cas9-mediated gene editing. This was achieved by introducing Cas9-sgRNA ribonucleoprotein complexes into Tregs via a standard nucleofection procedure, enabling efficient and precise KO of SRC-3 ( 229 ). Importantly, the anticancer effect of SRC-3 KO Tregs was not accompanied by any detectable adverse side effects. The absence of autoimmune complications can be attributed to the preferential accumulation of the SRC-3 KO Tregs within the TME, where they secrete IFNγ and trigger the IFNγ-CXCL9 signaling cascade ( 227 ). However, the mechanisms underlying the biased accumulation of SRC-3-deficient Tregs in the TME remain incompletely understood and warrant further investigation. Overall, once considered “undruggable” targets, SRCs were made targetable through an extensive discovery approach that led to the development of synthetic molecules capable of either stimulating or inhibiting their activity. Initially designed for anticancer applications, these molecules have also proven applicable for immune modulation, with potential uses in regenerative medicine. Furthermore, immune-stimulatory effects observed in tumor-bearing mice after treatment with the SRC SMI SI-2, ultimately led to the discovery of antitumor immunity conferred by SRC-3-deficient Tregs in vivo , suggesting the potential for developing cell-based therapeutic strategies for solid tumors.

The transcriptional activity of NRs relies on and is regulated by coregulator proteins The SRC family comprises 3 paralogous proteins—SRC-1, SRC-2, and SRC-3—that represent the most extensively studied coactivators and participate in key physiological processes including reproduction, metabolism, immunity, and oncogenesis SRCs were long considered “undruggable” due to their large size and lack of a well-defined ligand-binding pocket, until the identification of the first SRC-targeting compound, gossypol, in 2011 The development of synthetic, drug-like small-molecule modulators—both stimulators and inhibitors—enabled exploration of SRCs as potential therapeutic targets in cancer The immunomodulatory role of SRCs is most consequentially demonstrated by the finding that inhibition of SRC-3 in Tregs reprograms the tumor immune environment, enhances local inflammation, promotes cytotoxic lymphocyte infiltration, and results in tumor eradication in mouse models of solid tumors Nuclear receptors (NRs) are a family of transcription factors (TFs) that regulate a broad range of physiological processes ( 1 ). For many years, it was believed that all members of the NR family require ligand (hormone) binding to become biologically active. However, the identification of structurally related proteins that lack known ligands ( 2 , 3 ), thus referred to as “orphan” receptors ( 3 , 4 ), changed this paradigm. It became clear that NRs actually comprise a much larger group of TFs sharing a high degree of homology and that not all members necessarily depend on hormone molecules—or indeed any ligand at all—for their activity ( 5 ). In fact, of the 48 NRs identified in humans, several—such as NR4A1 (Nur77), PNR (NR2E3), and COUP-TFII (NR2F2), which play essential physiological roles in processes such as development and immunity ( 6-8 )—remain classified as “true orphans,” distinguishing them from “adopted orphans” for which endogenous ligands have been identified ( 1 , 5 , 9 ). Importantly, mutations in certain ligand-dependent NRs, particularly estrogen receptor α (ERα), can confer ligand-independent activity, including the aberrant recruitment of cofactor proteins that supports unregulated transcription and is associated with cancer progression ( 10-12 ). However, under normal physiological conditions—whether ligand-dependent or orphan—NRs cannot drive transcription independently. The idea that for a full scale of their activity, NRs rely on a distinct family of proteins was introduced in the early 1970s ( Fig. 1 ) ( 13-15 ) but was not accepted without initial skepticism ( 16 ). Nearly 2 decades after this hypothesis was proposed, the first coregulators—a corepressor and coactivator—were cloned ( 17 , 18 ). These seminal discoveries opened the door to a plethora of publications, and in fact, it became clear that the entire scope of NR activity is regulated by “partner” proteins that can either activate or repress NR-mediated transcription. It did not take long after these pioneer discoveries for coregulators to be recognized as critical components of gene regulation ( 19 ). This ultimately led to the identification of the first—and arguably most dominant—coactivator family: the steroid receptor coactivators (SRCs), initiated by the discovery of the first family member, SRC-1 ( 20 ). Since the discovery and identification of their biological activity, the 3 members of the SRC family—SRC1/2/3—have been established as central pillars of gene regulation that coordinate the assembly of multiprotein transcription complexes and facilitate the recruitment of key chromatin-modifying enzymes such as CREB-binding protein (CREBBP/CBP), p300 (EP300), and coactivator-associated arginine methyltransferase 1 (CARM1) ( 21 ). By mediating these molecular interactions, the SRCs exert widespread control over gene expression, with broad implications for both physiological function and disease states ( 22 ). The central role of SRCs in diverse physiological processes—mediated through their interactions with numerous NRs and other TFs—has positioned them as attractive platforms for therapeutic intervention with broad clinical potential. However, SRCs were long regarded as “undruggable” targets due to their large size, intrinsic structural disorder, and absence of a conventional ligand-binding pocket ( 16 ). Initial efforts to overcome the perceived “undruggability” of SRCs focused on disrupting their protein–protein interactions with NRs, rather than by developing compounds that act on SRCs as primary molecular targets ( 23-25 ). The longstanding challenge in developing pharmacological strategies that act directly on SRCs was eventually met through the use of high-throughput screening technology combined with luciferase reporter assays, which were designed to detect changes in SRC-mediated transcriptional activity and protein abundance ( 21 , 26 ). This approach led to the discovery of the first-in-class synthetic SRC small-molecule inhibitor (SMI) ( 27 ), as well as the small-molecule stimulator (SMS) ( 28 ) for applications in cancer treatment. These breakthroughs paved the way for the development of second-generation SRC SMIs and SMSs and laid the foundation for additional applications beyond cancer treatment ( 29 ). Timeline highlighting key milestones in the discovery and characterization of SRCs, from the emergence of the coregulation concept to the development of SRC-targeted therapeutics. In this review, we first outline how NR coregulation has evolved into a central discipline within modern molecular-endocrine biology. We then focus on the SRC family—arguably the most dominant and functionally versatile family of coregulators—and describe their major physiological and oncogenic roles that underscore their significance as therapeutic targets. We further describe the discovery and development of small-molecule inhibitors and activators of SRCs, which were long considered undruggable oncogenes. Finally, we highlight emerging opportunities that arise from modulating SRC activity beyond tumor suppression, particularly in achieving therapeutically beneficial immune modulation.