FASN, a protein playing important roles in the endogenous fatty acid production, has been recognized to contribute to oncogenesis in solid tumors. It has been well documented that FASN is over-expressed in many types of epithelial malignancies, such as those derived from the prostate and breast. However, the relevance of FASN in hematologic malignancies has not been fully examined. The expression status and functional significance of FASN has never been examined in MCL. In the present study, we report that FASN is consistently expressed at a high level in MCL cell lines and tumors. These findings are in contrast with the undetectable FASN level in benign lymphoid tissues, including normal lymphocytes present in the mantle zone. The lack of FASN expression in normal peripheral blood mononuclear cells also has been previously reported 
. Of note, the high expression level of FASN is not a universal phenomenon in hematologic malignancies. Based on our literature search, we identified reports describing a high level of FASN expression in myelomas 
and in diffuse large B-cell lymphomas 
. In contrast, the expression of FASN was not detected in cDNA microarray studies of chronic lymphocyte leukemia, another CD5-positive B-cell lymphoproliferative disorder 
We found that the high level of FASN expression in MCL is biologically important. Specifically, our results suggest that FASN confers anti-apoptotic effects in these cells. In support of this concept, the use of FASN pharmacologic inhibitors (including C75 and Orlistat) was found to induce apoptosis in a dose-dependent fashion in all MCL cell lines examined as well as 3 samples of primary MCL leukemic cells. FASN inhibition using siRNA was also found to induce apoptosis, associated with a downregulation of cyclin D1 and β-catenin. We noted that the siRNA-induced inhibition of FASN did not induce apoptosis as efficient as the two pharmacological drugs did. This observation can be explained by the fact that FASN siRNA did not completely abrogate the expression of FASN; approximately 20 to 30% of FASN protein remained to be detectable in MCL cell lines after the siRNA treatment. Alternatively, it is likely that the pharmacologic agents had off-target effects, as this concept is supported by our observation that pre-treatment of palmitic acid did not completely abrogate the cell-growth inhibitory effects of Orlistat ().
The mechanisms by which FASN is upregulated in cancer cells have been described in a number of epithelial malignancies. For instance, the Akt signaling pathway and HER-2 have been reported to play a role in upregulating FASN expression in breast cancer 
as well as in thyroid papillary carcinoma 
. While the Akt pathway may contribute to the high expression level of FASN in MCL cells, we believe that it is highly unlikely that it is the sole mechanism, as constitutive activation of the Akt pathway is largely restricted to blastic MCL cases in one study 
. To further examine the mechanism by which FASN expression is driven in MCL cells, we analyzed the promoter region of the FASN
gene, in an attempt to identify specific transcriptional factors that may be responsible for the high level of FASN expression in MCL cells. Our analysis led us to identify the consensus binding sequence for β-catenin (unpublished finding). In keeping with the importance of β-catenin in inducing FASN in MCL, we found that inhibition of β-catenin in MCL cells using siRNA led to a substantial decrease in the protein level of FASN ( and figure S1
). Rather surprisingly, the level of FASN
mRNA was not significantly downregulated. These observations suggest that β-catenin does not exert transcriptional control over FASN
expression in MCL cells; instead, it is likely that it regulates the FASN protein level by modulating its stability/protein degradation. In this regard, two previous reports have observed that the protein expression of FASN in prostate and breast cancer is regulated at the post-transcriptional level 
. One of these two reports highlights the role of USP2a (ubiquitin-specific-protase 2a), an isopeptidase 
. In light of this information, we asked if β-catenin may contribute to the overexpression of FASN protein by decreasing USP2a. As shown in , it turns out to be the case. Interestingly, our analysis of the human promoter of USP2a
has revealed the presence of TCF/LEF binding site (unpublished finding). Taken together, it appears that β-catenin increases the protein expression of FASN in MCL cells by promoting it stabilization via modulation of USP2a. We also asked if the mTor pathway is involved. No modulation of mTor activation was observed after siRNA β-catenin.
We also have demonstrated, for the first time, the existence of a positive feedback regulatory loop involving β-catenin and FASN. Specifically, we have shown that inhibition of FASN by using siRNA leads to a significant downregulation of β-catenin expression in MCL cells. In view of the biological significance of β-catenin in MCL 
, our findings suggest that one mechanism by which FASN exerts its oncogenic function in MCL cells is by upregulating β-catenin. These new information suggests that inhibiting FASN and β-catenin in combination may be a useful therapeutic approach.
Despite the fact that FASN overexpression has been observed in different forms of cancer, there is still no clear mechanism explaining how FASN mediates its oncogenic effects. It has been postulated that FASN, by virtue of its normal functions, provide a source of fatty acids for membrane production as well as energy supply 
. Moreover, FASN has been associated with post-translational modifications (such as palmitoylation) of proteins 
, a phenomenon that has been described to promote the activation of the Src-family tyrosine kinases 
. As mentioned above, FASN may mediate its oncogenic effects by upregulating β-catenin, a protein known to carry oncogenic functions.
To conclude, the present study describes the high level of FASN expression is a consistent finding in MCL cell lines and tumors. Our data has supported the concept that FASN confers anti-apoptotic effects in MCL cells. Our results also uncovered a positive feedback loop involving FASN and β-catenin, a signaling protein previously reported to be important in the pathobiology of MCL. Thus, inhibition of FASN, possibly in combination with the blockade of β-catenin, may be a useful approach to treat MCL.