Targeting bone marrow adipose tissue and the FABP family increases efficacy of dexamethasone in MM

Abstract

It is well known that multiple myeloma (MM) cells have a dependency on the bone marrow (BM) microenvironment for survival and proliferation. Targeting stromal cells of the BM, rather than just the tumor cells themselves, can be useful for reducing tumor burden in mice and humans. Our lab has recently explored targeting the bone marrow adipocyte (BMA) due to correlations between obesity to increased risk of MM and increased BM adiposity. To test the effects of BMAs on MM1S in vivo, 5-week old SCID-beige female mice were treated with saline or an anti-sclerostin antibody (Scl-Ab, 100 mg/kg once per week IP) for 5 weeks (a treatment shown to increase bone volume and decrease BMAs). Then, 5x10ˆ6 human GFP+luc+MM1S cells were injected i.v. followed by 5 additional weeks of Scl-Ab or saline treatments. Mice were treated with or without dexamethasone (dex) (9mg/kg IP) during weeks 4-6 after MM1S injection. Mice were assessed with biweekly whole body bioluminescent imaging (BLI) and survival. Dex treatments led to a significantly lower total flux BLI signal (p<0.05 vs MM1S alone), and Scl-Ab+dex combination treatments had the least total flux BLI signal (p<0.01 vs MM1S alone) at day 33. Interestingly, Scl-Ab+dex combination treatments increased mouse survival significantly versus dex alone, Scl-Ab treatment alone, or no treatment. Thus, our in vivo data suggested that decreasing bone marrow adiposity enhanced anti-myeloma effects of dex. Next, to specifically examine the effects of BMAs on myeloma cells, we generated human BMAs from BM mesenchymal stem cells with adipogenic media. MM1S cells were cultured in conditioned media from BMAs (BMA-CM); BMA-CM from many, but not all donors, supported MM1S proliferation, migration, or drug resistance. Specifically, some BMAs protected MM1S cells against dex-induced apoptosis, assessed by Annexin V/DAPI flow cytometry and bioluminescence cell counting. Gene expression and protein data from MM1S cells treated with BMA-CM revealed a significant increase in fatty acid binding protein 4 (FABP4), a fatty acid shuttling molecule with broad signaling effects (pcr: p<.01, ELISA: p<0.0001). While unknown in MM, the FABP family has demonstrated pro-tumorigenic effects in other cancers. We used inhibitors of FABP4 (iFABP4), and its compensatory FABP5 family member, to study their effects on basal and dex-induced apoptosis, cell number, cell cycle and proliferation in MM1S cells. After 72 hours, FABP4i and FABP5i alone, and in combination, significantly decreased cell number and proliferation. In combination, FABP4i/FABP5i improved efficacy of dex. Decreases in MM1S cell number and proliferation, (assessed by BLI, Ki67 and cell cycle flow cytometry analyses) from FABP4i, FABP5i, or the FABP4i/FABP5i combination were attributed to increased sub G1/G0 population. Overall, targeting BMAT and FABPs may be novel approaches to inhibiting progression and decreasing the drug resistance properties of MM cells in the BM.