Abstract A18: Investigating the role of bone marrow adiposity on macrophage function and osteoclast differentiation

Prostate cancer (PCa) is the second leading cause of cancer-related deaths among men. Although PCa is slow to progress, patients eventually develop metastases at distant sites, predominantly bone. Once established in the bone marrow, PCa becomes a devastating and incurable disease. The emerging evidence suggests that overweight and obesity are associated with shorter time to disease recurrence and poor patient survival. One major consequence of obesity is the accumulation of adipocytes within the bone marrow, a process which we have previously shown to accelerate tumor progression in the skeletal sites. We demonstrated that prostate tumor cells exposed to bone marrow adipocyte-derived factors (Adipo CM) exhibit lipid accumulation, increased invasiveness, and changes in cell morphology that promote an invasive phenotype. We have also shown that increased marrow adiposity results in augmented levels of tumor-promoting inflammatory factors in bone. Therefore, in the present study, we investigated the effects of adiposity on bone marrow macrophage (BMM) function, phenotype, and differentiation to mature osteoclasts. Utilizing a co-culture system of BMMs and PCa cells, we demonstrate that BMMs exposed to Adipo CM have increased expression and secretion of osteopontin (OPN), a key regulator of macrophage function that has been implicated in prostate tumor progression. We also observed that BMMs are more invasive toward PCa cells exposed to Adipo CM. Our studies show that IL-10 expression, a chemokine regulated by OPN, is suppressed in BMMs co-cultured with Adipo CM and PCa cells. Conversely, arginase-1 and CD163, markers of M2 macrophage phenotype, are highly induced by macrophage-tumor cell interactions. Our results suggest a direct role for OPN in regulation of BMM phenotype in the bone-tumor microenvironment. Furthermore, our investigations of adiposity-driven bone remodeling in tumor progression in bone reveal that adipocyte-derived factors accelerate BMM differentiation into osteoclasts as evidenced by TRAcP staining, overexpression of osteoclastogenesis-associated genes, and increased proteolytic cleavage of DQ-collagen I. Utilizing functional fluorescence-based assays and western blotting approaches we also show that expression and proteolytic activity of cathepsin K, a bone-degrading cysteine protease, is upregulated in osteoclasts differentiating in the presence of Adipo CM. This further supports the involvement of adipocyte-derived factors in tumor-induced bone disease. In summary, our work highlights novel roles of adipocyte-derived factors in macrophage function, behavior, and osteoclastogenesis in the bone marrow niche. Studies are currently underway to determine how lipid uptake affects their function and tumor-promoting activities in bone. Citation Format: Aimalie Lynnette Hardaway, Mackenzie K. Herroon, Erandi N. Rajagurubandara, Izabela Podgorski. Investigating the role of bone marrow adiposity on macrophage function and osteoclast differentiation. [abstract]. In: Abstracts: AACR Special Conference on Cellular Heterogeneity in the Tumor Microenvironment; 2014 Feb 26-Mar 1; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2015;75(1 Suppl):Abstract nr A18. doi:10.1158/1538-7445.CHTME14-A18