We have previously shown that obesity is an independent predictor of leukemia relapse in children. We have also shown that obese mice transplanted with syngeneic leukemia cells have poorer survival after chemotherapy, even when they are dosed proportional to body weight. Since interactions between leukemia cells and cells of the bone marrow niche are considered important for chemotherapy resistance and relapse, and adipocytes can comprise ~50% of the bone marrow niche, we developed in vivo and in vitro models to investigate the role of adipocytes in the leukemia microenvironment.
Obese C57Bl/6J mice were transplanted with GFP+ murine preB cell ALL (“8093”) cells and then treated with vincristine (0.5 mg/kg/week × 3 weeks). At the time of relapse, we found that GFP+ leukemia cells persisted in the fat pads of the mice. We then developed an in vitro co-culture system in which human or murine leukemia cells were cultured together with adipocytes (differentiated 3T3-L1s). Undifferentiated 3T3-L1 cells, which are fibroblastic in nature, were used as a control. In this model, adipocytes severely diminished the anti-leukemic effect of all chemotherapeutics tested against murine 8093 cells, including vincristine, dexamethasone, nilotinib, daunorubicin, and L-asparaginase. Adipocytes also protected murine T-cell ALL and human SD-1, RCH-ACV, and BV173 cells from vincristine and daunorubicin. Adipocyte protection of leukemia cells occurred independent of cell contact.
Further experiments demonstrated that media conditioned by adipocytes was able to protect 8093 cells from a 3-day exposure to 25 nM dexamethasone (viable cells were at 40±12% of their plated value in regular media, 66±17% in fibroblast-conditioned media, and 109±24% in adipocyte-conditioned media, p<0.05). Surprisingly, adipocyte-conditioned media did not protect leukemia cells from daunorubicin. However, media conditioned by the presence of both adipocytes and leukemia cells simultaneously conferred a high degree of resistance to the leukemia cells (n=3, p<0.05 compared to all other media types).
In summary, adipose tissue is a reservoir for relapsed leukemia cells in vivo. Adipocytes engender protection from multiple chemotherapies in murine and human leukemia cell lines. Adipocytes secrete factor(s) that confer dexamethasone and daunorubicin resistance to leukemia cells, though for the latter drug it appears that a two-way communication between leukemia and adipocytes may be necessary for this protection.
Disclosures: No relevant conflicts of interest to declare.