Fatty Acid Synthase and Diffuse Large B Cell Lymphoma

Abstract

Diffuse Large B-cell Lymphoma (DLBCL) is considered to be the most common type of lymphoma in adults, accounting for 30–40 percent of cases of non-Hodgkin's lymphoma. The incidence of DLBCL is about 40–60% in Saudi Arabian population. The reason of high rate for DLBCL in Saudi Arabian population is not known, however recent studies suggests that differences in molecular signature as compared to western population accounts for this incidence. Although patients with DLBCL are potentially curable with combination chemotherapy, the disease proves fatal in approximately 50% of patients. The cause of most DLBCLs remains unknown; however several studies suggest that dysregulated survival/apoptosis or defective repair pathways have been implicated. Many human cancers, particularly those with a poor prognosis express high levels of fatty acid synthase (FASN), a key metabolic enzyme linked to the synthesis of membrane phospholipids in cancer cells. Over-expression of FASN can be largely attributed to activation of phosphatidylinositol-3′-kinase (PI3K)/AKT pathway. However, the role of FASN in the pathogenesis of DLBCL has not been elucidated. Therefore, in this study, we investigated the role of FASN in a large series (301) of DLBCL patient samples and a panel of DLBCL cell lines. Using immunohistochemistry, FASN was detected in appreciable number of DLBCL tumors and was strongly associated with the expression of p-AKT protein. We next examined the effect of C-75, a synthetic slow binding inhibitor of FASN activity on DLBCL cell lines (SUDHL4, SUDHL5, SUDHL8 and OCI-LY19) in vitro and found that C-75 treatment inhibits growth and induces apoptosis in all 4 DLBCL cell linesused in the study. We show based on in vitro studies employing a variety of experimental tools using different FASN inhibitors, FASN siRNA and AKT siRNA that FASN exert its oncogenic action in DLBCL cells via activated AKT. Our data show that inhibition of FASN leads to de-phosphorylation of p-AKT and it's down stream effectors, FOXO1 and GSK-3. This, in turn leads to activation of the intrinsic apoptotic pathway by, initially causing conformational changes of the Bax protein leading to changes in the mitochondrial membrane potential and release of cytochrome c into cytosole. This causes activation of caspases-9 and -3 and cleavage of PARP. zVAD-fmk, a universal inhibitor of caspases prevents caspase-9 and -3 activation and abrogates apoptosis induced by C-75 treatment. Finally, C-75 treament of DLBCL cell lines causes down-regulation of the inhibitor of apoptosis proteins, XIAP, cIAP1 and Survivin. In summary, data presented here demonstrate a significant correlation between the expression of FASN and active AKT in DLBCL and indicate that inhibition of PI3K/AKT signaling synergize the FAS inhibitors to induce apoptosis in DLBCL cell lines with constitutively active AKT. This may have significant clinical implications. Therefore, FASN has become a promising target for anti cancer drug development.