Folic acid: Regulation of growth of colon and gastric cancer cell lines

Abstract

Results from several laboratories, including our own, suggest that folic acid may be chemopreventive against colon cancer. However, little is known about the regulation of this process. We hypothesize that folic acid exerts its chemopreventive effect on the colon by inhibiting proliferation and enhancing, differentiation and/or apoptosis. To test this hypothesis, we examined the changes in proliferation and protein levels of bax (whose expression is positively related to apoptosis) and connexcin (C-43; involved in cell-cell communication; an indicator of differentiation)in two colon cancer cell lines (HCT-116 and CaCo-2). These parameters were also measured in a gastric cancer line (Kato-III) to determine whether folic acid may also be chemopreventive against gastric cancer. In each of these cell lines, folic acid inhibited proliferation in a dose-dependent manner. However, the 15o dose of folic acid was lower in colon cancer cell lines compared to the gastric cancer cells (125 ng/ml vs 626 ng/ml). In these cell lines, bax and Cx-34 levels, as assessed by Western-immunoblot, was increased significantly by 30-50% after 48 h of exposure to folic acid (625 ng/ml). We further hypothesize that EGF-receptor (EGFR) may play a role in regulating folic acid-induced changes in proliferation, differentiation and apoptotic processes. Indeed, we found folic acid (625 ng/ml) to significantly inhibit (40-60%) EGFR tyrosine kinase activity and tyrosine phosphorylation of EGFR as well as the relative concentration of the 14 kDa precursor form of TGF-a (one of the ligands of EGFR) in membranes of Kato-III as well as HCT-116 and CaCo-2 cells. Our data suggest a chemopreventive role for folic acid in colon and gastric cancers, as evidenced by the inhibition of proliferation and stimulation of differentiation and apoptotic processes in colon and gastric cancer cell lines. We also suggest that diminished activation of EGFR tyrosine kinase resulting from decreased membrane accumulation of TGF-a may partly be involved in regulating these processes. Supported by grants from the NIH/NIA