Fatty Acid Metabolism in Diffuse Large B-Cell Lymphoma (DLBCL): Interaction with Oncogenic Cell Signaling Pathways and the Identification of a Novel Treatment Paradigm.

Fatty acid synthase (FASN) is a key enzyme of fatty acid synthesis and is upregulated in many cancers. Increased FASN in cancer is associated with poor prognosis, while inhibition of FASN results in cancer cell death. The MEK/ERK signal transduction is one of the primary pathways that activate tumor-related FASN. Lipoprotein lipase (LPL) is also involved in fatty acid metablishm as it releases free fatty acid (FFA) from circulating lipoproteins, making them available for cellular uptake. Notably, these concepts have emerged primarily from solid tumor studies; there is a comparative paucity of data in lymphoma. We examined the functional roles of FASN and LPL in DLBCL cells and their interaction with oncogenic signal transduction pathways including MEK/ERK and an upstream target, hypoxia inducible factor-1 alpha (HIF-1a). We also investigated potential therapeutic implications of targeting fatty acid metabolism for the treatment of DLBCL.

We used the DLBCL cell lines OCI-LY3, OCI-LY19, SUDHL4, and SUDHL10 in normoxic or hypoxic (0.2% O₂) conditions. Cerulenin (FASN inhibitor) and Orlistat (FASN and LPL inhibitor) were utilized to examine the effect of fatty acid enzyme inhibition on cell signaling and cell death. We assessed cell viability with the MTT assay and apoptosis by flow cytometric analysis of Annexin-V/propidium iodide (PI). FASN and LPL mRNAs were quantified in DLBCL cell lines by RT-PCR as well as through gene expression profiling (GEP) analysis (by cell of origin) using the CaBIG dataset. Further, FASN and associated signaling pathways (MEK, ERK, and HIF-1a) were analyzed by Western blot. Finally, for investigation of potential interactions between FASN and HIF-1a, or MAPK signaling, we utilized short hairpin RNA interference (shRNA) to knock down (KD) pathways of interest.

FASN protein expression was readily detectable in all DLBCL cell lines in normoxia, while the expression of LPL was barely detectable in most cells, except in SUDHL10 and only in hypoxic conditions. RT-PCR showed that all DLBCL cell lines tested expressed high levels of FASN mRNA, while minimal levels of LPL could be detected; GEP showed that FASN was expressed more prominently in germinal center (GC) DLBCL (p=0.0006 vs GC control and p=0.0001 vs non-GC DLBCL), whereas LPL was preferentially expressed in non-GC DLBCL (p<0.0001 vs non-GC control and GC DLBCL). We next examined FASN expression following KD of MEK, ERK, or HIF-1a using shRNA in OCI-LY3 and SUDHL10 cells. HIF-1a KD significantly decreased FASN expression; this result was most prominent in OCI-LY3 cells, although it was also evident in SUDHL10. Interestingly, MEK and ERK KDs had minimal effect on FASN or LPL. Pharmacologic treatment with cerulenin, however, resulted in inhibition of MEK and ERK phosphorylation in OCI-LY3 cells. Additionally, treatment with Cerulenin or Orlistat (0.25–4 μg/mL for 48 hours) resulted in dose-dependent cytotoxicity across several DLBCL cell lines (OCI-LY3, SUDHL4, and SUDHL10) with an approximate IC₅₀ of 1μg/mL in all lines. Furthermore, treatment with Cerulenin resulted in induction of apoptosis, which was mediated by caspase cleavage (caspases 3, 8 and 9) in SUDHL4 and OCI-LY3 cells.

We demonstrated that FASN is constitutively activated in DLBCL with expression in part dependent on cell of origin, while LPL protein or message were mostly down-regulated. HIF-1a is a constitutively activated oncogenic pathway in DLBCL (Evens AM, et al. Br J Haematol 2008) and it appeared here to directly regulate FASN expression. In addition, we showed that targeting fatty acid metabolism may be harnessed as a potential therapeutic strategy. Further investigations are required to delineate the mechanisms through which MAPK and HIF-1a regulate FASN expression and to determine the in vivo implications of FASN inhibition on DLBCL tumor growth.

No relevant conflicts of interest to declare.