Despite recent advances in treatment modalities for advanced prostate cancer (PCa) and castration-resistant prostate cancer (CRPC), PCa continues to be a major cause of morbidity and mortality in American men. An improved understanding of the tumor microenvironment and immune surveillance has opened new avenues for targeted therapeutic strategies for the treatment of PCa. The primary cause of treatment failure is the development of cellular resistance to the cytotoxic effects of cancer therapeutics. Both cell intrinsic and extrinsic mechanisms can underlie this acquired therapeutic resistance. More recently, the tumor microenvironment, and more importantly tumor-related inflammation, have emerged as a crucial players in promoting cancer initiation and progression, invasion and metastasis, and resistance to both conventional and targeted therapies. In particular, there have been fundamental leaps in our understanding of the complex roles of tumor-infiltrating myeloid cells (TIMs). TIMs are a heterogeneous population of monocytic and granulocytic myeloid derived suppressor cells (MDSCs) and tumor-associated macrophages (TAMs) that have a profound impact on tumor development. TIMs have been reported to promote tumor growth. In PCa, first-line radiotherapy (RT) and blockade of androgen signaling have potent anti-cancer effects, but PCa patients inevitably develop resistance mechanisms to evade therapy. Recently, elevated inflammation has been demonstrated after RT and androgen inhibition, yet the impact of this aberrant inflammation and the potential therapeutic options for preventing resistance in PCa have been largely unexplored. Therefore, we sought to further understand the impact of inflammation in acquired resistance to these two important therapeutic strategies, and whether combinatorial therapies may prevent resistance and improve therapeutic effectiveness. Our laboratory and others have shown a critical role for macrophage colony-stimulating factor-1 (M-CSF-1, CSF-1) signaling through its receptor, CSF1R, in the recruitment of TIMs to tumors, which promote cancer progression. Using murine models of prostate cancer, we found that RT resulted in pronounced increases in CSF-1 expression in PCa cells, which was transcriptionally regulated by the non-receptor tyrosine kinase, ABL1 (c-Abl), highly correlating with increased TIM recruitment. Importantly, utilizing a CSF1R inhibitor, PLX3397, in combination with RT resulted in significant depletion of TIMs and delayed RT tumor recurrence. In parallel, the potential impact of TIMs in therapeutic resistance to androgen blockade therapy (ABT) was also interrogated. Similarly to RT, we found that inhibiting androgen signaling using the novel androgen receptor inhibitor MDV3100 (Enzalutamide) resulted in increased expression anti-inflammatory cytokine IL-10 and Th2 cytokine IL-13 in PCa cells, as well as CSF-1 expression in PCa cells and in mice sera. Moreover, cellular interactions between macrophages and PCa cells treated with MDV3100 skewed macrophages to a protumorigenic "alternative" activation state. In addition, tumors grown in surgically castrated mice showed increased CSF-1 expression in serum and in whole tumors compared to sham surgery mice, resulting in a considerable increase in TAM infiltration. More importantly, TAM blockade using PLX3397 in combination with castration resulted in a significant delay in the onset of CRPC. Lastly, we sought to further investigate potential mechanisms underlying TIM-mediated resistance to androgen depletion. Cytotoxic effects of androgen depletion are primarily due to induction of apoptosis after inhibition of androgen receptor (AR) signaling. Insulin-like growth factor 1 receptor (IGF1R) signaling is a well-established mode of CRPC development due to its anti-apoptotic and mitogenic functions. Furthermore, the Th2 cytokine IL-13, is known to induce IGF-1 expression in macrophages. Interestingly, we found that the increased IL-13 expression from MDV3100 treated Myc-CaP cells, correlated with increased macrophage IGF-1 expression. Subsequently, macrophage-derived IGF-1 levels correlated with increased p-IGF1R staining in tumors from castrated mice versus sham surgery controls. Additionally, tumors grown in castrated mice showed a significant increase in IGF-1 expression, and a subsequent reduction of IGF-1 levels upon macrophage depletion with PLX3397. Macrophage depletion also correlated with decreases in Ki67 proliferation index, as well as delayed onset of CPRC. Collectively, these data suggest that a strong interplay between TIMs and tumor cells generate a pro-tumor microenvironment that favors therapeutic resistance in PCa.
Overall, these studies increase our understanding of the complex role of TIMs in the prostatic microenvironment, and argue in favor of their contribution to PCa progression and therapeutic resistance to RT and therapies targeting androgen signaling. The rational targeted therapies pursued here and other combinatorial approaches hold promise to improve the long-term efficacy of new therapeutics and ultimately improve patient lives.