Lipid-Rich Macrophages at the Tumor-Adipose Interface: A New Target for Locally-Invasive Prostate Cancer Therapy

SUMMARY Macrophages are important contributors to the biology of the tumor microenvironment. As tumors penetrate the prostatic capsule they invade a lipid-rich depot known as the peri-prostatic fat (PPF). An ability to utilize this energy source is important for tumor progression. PPF is a white adipose tissue (WAT) that encases the prostate

and is a depot of adipocytes and resident macrophages termed adipose tissue macrophages (ATMs). PPF and activated ATMs are relatively understudied components of the tumor microenvironment (TME). Adipose tissues represent a nutrient reservoir from which tumor cells can gain access to adipokines. “Browning” of WAT results

in a more vascular and multilocular fat phenotype and it is not clear whether this represents an adaptive metabolic response in PPF to suppress or accelerate tumor growth. There is heterogeneity in lipid availability in the prostate TME: ATMs reside in a lipid-rich storage environment composed primarily of adipocytes whereas tumor

associated macrophages (TAMs) inhabit stroma where there is a paucity of adipocytes. Metabolic reprogramming in tumor cells is well documented; however, limited recent studies have reported that ATMs in visceral fat or breast TME can acquire a metabolically activated phenotype (MMe) distinct from the M1/M2

phenotypes. Our preliminary studies identified a potentially critical immuno-metabolic signaling axis in macrophages involving pro-inflammatory receptor interacting protein 140 (RIP140), lipid-regulating fatty acid binding proteins 4/5 (FABP4/5) and Diacylglycerol O-acyltransferase 1 (DGAT1) that synthesizes neutral lipids

and is required for fat absorption and storage. RIP140, a co-activator of NF-kB, is important for promoting pro- inflammatory cytokines characteristic of M1 activation, in macrophage M1/M2 polarization, and in browning of WAT. Both FABPs and DGAT1 can be regulated in vivo and in vitro using small molecule inhibitors some of

which are in development and being tested as potential therapies. The proposed study will investigate a central hypothesis: PPF is a highly dynamic depot, and increased lipid stores in the TME can promote tumor progression by triggering lipid-rich ATMs to secrete pro-tumorigenic cytokines/chemokines and lipid mediators, thus,

switching ATMs to a harmful pro-inflammatory MMe phenotype. We will use in vivo and in vitro models to test whether altering DGAT1 or FABP4/5 activity is tumor suppressive. Our aims will define mechanisms regulating the immuno-metabolic axis in ATMs and TAMs in prostate cancer (PCa), and determine whether the MMe

phenotype subpopulation is increased in locally invasive PCa. To our knowledge, this is the first study to investigate the role and functional significance of an understudied phenotype, MMe, that merges both inflammatory and lipid signaling pathways in the prostate TME. Targeted therapies will be tested in multicellular

organoid cultures and relevant murine xenograft models. Successful completion of these studies will set the stage for more comprehensive pre-clinical investigations and potential translation to the clinic in the medium term.