Macrophages are conventionally classified into M1 and M2 subtypes according to their differentiation status and functional role in the immune system. However, accumulating evidence suggests that this binary classification system is insufficient to account for the remarkable plasticity of macrophages that gives rise to an immense diversity of subtypes. This diverse spectrum of macrophage subtypes play critical roles in various homeostatic and immune functions, but remain far from being fully characterised. In addition to their roles in normal physiological conditions, macrophages also play crucial roles in disease conditions such as cancer. In this review, we discuss the roles tumour-associated macrophages (TAMs) play in regulating different steps of tumour progression and metastasis, and the opportunities to target them in the quest for cancer prevention and treatment.

Metastatic castration-resistant prostate cancer (CRPC) is a fatal disease, primarily resulting from the transcriptional addiction driven by androgen receptor (AR). First-line CRPC treatments typically target AR signaling, but are rapidly bypassed, resulting in only a modest survival benefi t with antiandrogens. Therapeutic approaches that more effectively block the ARtranscriptional axis are urgently needed. Here, we investigated the molecular mechanism underlying the association between the transcriptional coactivator MED1 and AR as a vulnerability in AR-driven CRPC. MED1 undergoes CDK7-dependent phosphorylation at T1457 and physically engages AR at superenhancer sites, and is essential for AR-mediated transcription. In addition, a CDK7-specifi c inhibitor, THZ1, blunts AR-dependent neoplastic growth by blocking AR/MED1 corecruitment genome-wide, as well as reverses the hyperphosphorylated MED1-associated enzalutamide-resistant phenotype. In vivo , THZ1 induces tumor regression of AR-amplifi ed human CRPC in a xenograft mouse model. Together, we demonstrate that CDK7 inhibition selectively targets MED1-mediated, AR-dependent oncogenic transcriptional amplifi cation, thus representing a potential new approach for the treatment of CRPC. SIGNIFICANCE: Potent inhibition of AR signaling is critical to treat CRPC. This study uncovers a driver role for CDK7 in regulating AR-mediated transcription through phosphorylation of MED1, thus revealing a therapeutically targetable potential vulnerability in AR-addicted CRPC.

Mutations in SOX10 cause neurocristopathies which display varying degrees of hypopigmentation. Using a sensitized mutagenesis screen, we identified Smarca4 as a modifier gene that exacerbates the phenotypic severity of Sox10 haplo-insufficient mice. Conditional deletion of Smarca4 in SOX10 expressing cells resulted in reduced numbers of cranial and ventral trunk melanoblasts. To define the requirement for the Smarca4 -encoded BRG1 subunit of the SWI/SNF chromatin remodeling complex, we employed in vitro models of melanocyte differentiation in which induction of melanocyte-specific gene expression is closely linked to chromatin alterations. We found that BRG1 was required for expression of Dct, Tyrp1 and Tyr, genes that are regulated by SOX10 and MITF and for chromatin remodeling at distal and proximal regulatory sites. SOX10 was found to physically interact with BRG1 in differentiating melanocytes and binding of SOX10 to the Tyrp1 distal enhancer temporally coincided with recruitment of BRG1. Our data show that SOX10 cooperates with MITF to facilitate BRG1 binding to distal enhancers of melanocyte-specific genes. Thus, BRG1 is a SOX10 co-activator, required to establish the melanocyte lineage and promote expression of genes important for melanocyte function.

SUMMARY Microphthalmia-associated transcription factor (MITF) is a survival factor in melanocytes and melanoma cells. MITF regulates expression of anti-apoptotic genes and promotes lineage specific survival in response to ultraviolet (UV) radiation and to chemotherapeutics. SWI/SNF chromatin remodeling enzymes interact with MITF to regulate MITF target gene expression. We determined that the catalytic subunit, BRG1, of the SWI/SNF complex protects melanoma cells against UV-induced death. BRG1 prevents apoptosis in UV- irradiated melanoma cells by activating expression of the melanoma inhibitor of apoptosis (ML-IAP). Down-regulation of ML-IAP compromises BRG1 mediated survival of melanoma cells in response to UV-radiation. BRG1 regulates ML-IAP expression by cooperating with MITF to promote transcriptionally permissive chromatin structure on the ML-IAP promoter. The alternative catalytic subunit, BRM and the BRG1 associated factor, BAF180 were found to be dispensable for elevated expression of ML-IAP in melanoma cells. Thus, we illuminate a lineage-specific mechanism by which a specific SWI/SNF subunit, BRG1, modulates the cellular response to DNA damage by regulating an anti-apoptotic gene and implicate this subunit of the SWI/SNF complex in mediating the pro-survival function of MITF. SIGNIFICANCE SWI/SNF enzymes interact with the Microphthalmia-associated transcription factor (MITF) a lineage addiction oncogene, to promote MITF target gene expression in melanoma cells. In this study we determined that the SWI/SNF component, BRG1, promotes melanoma survival in response to UV-radiation, by activating expression of the melanoma inhibitor of apoptosis, ML-IAP gene. Our data show that BRG1 and MITF co-operate to establish permissive chromatin structure on the ML-IAP promoter and alter the association of other epigenetic regulators. Thus, we have elucidated a mechanism by which a component of the SWI/SNF complex promotes the pro-survival function of MITF. We further demonstrate that the BRG1 associated factor, BAF180, is not required for the activation of ML-IAP, suggesting that a specific configuration of the SWI/SNF complex mediates distinct activities. These results provide insight into how SWI/SNF function is deregulated in melanoma.

Accumulating evidence suggests that presence of macrophages in the tumor microenvironment add to the invasive and tumor-promoting hallmarks of cancer cells by secreting angiogenic and growth factors. RKIP is a known metastasis suppressor and interferes with several steps of metastasis. However, the mechanistic underpinnings of its function as a broad metastasis suppressor remain poorly understood. Here, we establish a novel pathway for RKIP regulation of metastasis inhibition through the negative regulation of RANTES/CCL5 thereby limiting tumor macrophage infiltration and inhibition of angiogenesis. Using a combination of loss- and gain-of-function approaches, we show that RKIP hinders breast cancer cell invasion by inhibiting expression of the CC chemokine CCL5 in vitro. We also show that the expression levels of RKIP and CCL5 are inversely correlated among clinical human breast cancer samples. Using a mouse allograft breast cancer transplantation model, we highlight that ectopic expression of RKIP significantly decreases tumor vasculature, macrophage infiltration and lung metastases. Mechanistically, we demonstrate that the inhibition of the CCL5 expression is the cause of the observed effects resulting from RKIP expression. Taken together, our results underscore the significance of RKIP as important negative regulator of tumor microenvironment.