CLL and Fat Cells Utilize Similar Metabolic Pathways.

Unlike their normal counterparts which are resting B-cells, CLL cells do proliferate. What energy source CLL cells use and which metabolic pathway they recruit to support proliferation is unknown.

Because the gene expression profile of CLL cells is skewed towards that of adipocytes, and because they proliferate at similar rates we hypothesized that like adipocytes CLL cells utilize free fatty acid (FFA).

To study this hypothesis, we cultured CLL cells in closed medium and measured the concentration of dissolved oxygen (dO₂) prior to and after adding FFA, assuming that if the cells oxidize the acid, the dO₂ will fall. Indeed, 48 hours after adding FFA dO₂ levels were markedly reduced. In contrast_, levels of dO₂ did not change when normal B-cells were incubated under similar conditions or if CLL cells were incubated in the presence of ibrutinib. Because in adipocytes fat is stored in intracellular vacuoles we wondered whether CLL cells also use similar strategy. Oil Red O` staining confirmed that lipid deposits are present in the bone marrow of patients with CLL, and by electron microscopy we detected vacuoles in the cytoplasm of CLL cells, suggesting that just like adipocytes, CLL cells store lipids in intracytoplasmic lipid vacuoles.

In analogy to adipocytes CLL cells express lipoprotein lipase (LPL). Similar to adipocytes we found LPL in the cell membrane and in the cytoplasm of CLL cells. Furthermore, after knocking-down the expression of LPL, the cells could no longer utilize fat, suggesting that lipid metabolism in CLL is LPL-dependent.

Because STAT3 is constitutively active in CLL cells, and because the LPL gene harbor STAT3 binding sites, we hypothesized that STAT3 activates the LPL gene. Indeed, chromatin immunoprecipitation (ChIP) confirmed that STAT3 binds to the LPL promoter. Furthermore, transfection of CLL cells with STAT3-shRNA downregulated LPL, confirming that STAT3 activates the LPL gene.

Conclusions

CLL cells are undergoing metabolic reprograming and use strategies physiologically utilized by adipocytes. This process is driven by STAT3 and is inhibited by ibrutinib.