It is well-recognized that de novo long chain fatty acid (FA) synthesis, driven by the key enzyme fatty acid synthase (FASN), is crucial for the growth and survival of many types of cancer cells. We and others have observed FASN protein expression in diffuse large B-cell lymphoma (DLBCL) tumors. Furthermore, we have shown that higher levels of FASN in DLBCL tumors strongly predicted inferior survival, which was independent from the international prognostic index. We also recently demonstrated that, in addition to FA synthesis, various cancer cells can acquire FA from circulating lipoproteins, using the secreted enzyme lipoprotein lipase (LPL), and that this promotes cell growth. DLBCL, however, has never been examined in this regard. In this study, we investigated the functional significance of both de novo FA synthesis via FASN and exogenous FA uptake via LPL in DLBCL.
Levels of FASN and LPL mRNAs in DLBCL cell lines (SUDHL4, SUDHL10, OCI-LY3, OCI-LY19) were studied using reverse transcriptase polymerase chain reaction. We determined FASN and LPL protein expression by flow cytometry using a novel anti-LPL antibody that we developed. DLBCL cell lines were cultured +/− Cerulenin (an inhibitor of FASN), Orlistat (an inhibitor of FASN and LPL), or in lipoprotein-depleted serum +/− supplementation with very low density lipoprotein (VLDL) particles. The MTT assay was used to assess cell proliferation.
DLBCL cell lines exhibited >10-fold variation in levels of FASN mRNA. Cerulenin and Orlistat each caused dose-dependent inhibition of proliferation of each cell line. The cells were partially rescued by the addition of palmitic acid, the FA product of FASN. Surprisingly, flow cytometry revealed that SUDHL4 and OCI-LY3 cells, which did not secrete LPL or show detectable LPL activity, displayed the enzyme on the cell surface. Moreover, in stark contrast to several other cancer cell lines, DLBCL cells were exquisitely sensitive to withdrawal of lipoproteins from the culture media. Indeed, 75–95% of the cells underwent apoptosis after only 24 hours in lipoprotein-depleted serum. In complete serum, the provision of VLDL particles did not rescue DLBCL cells from FA synthesis inhibition using Cerulenin, suggesting that the serum contains sufficient lipoproteins to saturate the FA uptake system. This prediction was validated in experiments utilizing lipoprotein-depleted serum, in which add-back of VLDL particles completely rescued the cells from Cerulenin-induced demise in a dose-related manner, with full restoration at approximately 100–200mcg/ml of VLDL.
Our data demonstrate that DLBCL cells employ both de novo FA synthesis via FASN and exogenous FA uptake using LPL to satisfy their strict requirement for FA. Interference with either pathway, using FASN inhibitors or lipoprotein-depleted serum, is cytotoxic indicating that neither alone is sufficient to support proliferation. Further, DLBCL cells show a striking dependency on exogenous FA of dietary origin compared with all other cancer cells we have examined. The observation that the cell lines can be rescued by provision of VLDL particles strongly supports the functional significance of the exogenous FA uptake pathway for DLBCL. Our data thus demonstrate that the extracellular lipase LPL is critical for the growth and survival of DLBCL cells. Surprisingly, the cells deploy LPL to their surface, and we speculate that this promotes efficient FA acquisition from circulating lipoproteins. Recognition that DLBCL relies on both synthesis and uptake of FA will provide guidance for drug development and dietary modifications to effectively target the metabolic requirements of this tumor.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.