A potential mechanism for fumonisin B-1-mediated hepatocarcinogenesis: cyclin D1 stabilization associated with activation of Akt and inhibition of GSK-3 beta activity

Fumonisin B-1 (FB1) is a worldwide corn contaminant and has been epidemiolo gically linked to the high incidence of human esophageal cancer in South Af rica and China. FB1 is hepatocarcinogenic in rats by an unknown mechanism. Inhibition of ceramide synthase and disruption of membrane phospholipids ha ve been shown to be mechanisms of toxicity. Here we show overexpression of cyclin D1 protein in both preneoplastic and neoplastic liver specimens obta ined from a long-term feeding study of FB1 in rats. In rats fed FB1 short-t erm, cyclin D1 protein levels in liver were increased up to five-fold in a dose-responsive manner. Northern blot analysis demonstrated no increase in mRNA levels of cyclin D1, 2D electrophoresis of cyclin D1 protein in FB1-tr eated samples showed a distinct pattern of migration (presence of less nega tively charged form of the protein) that differed from controls. Recently, it has been shown that phosphorylation of cyclin D1 by glycogen synthase ki nase 3 beta (GSK-3 beta) on a single threonine residue (Thr-286) positively regulates proteosomal degradation of cyclin D1, In FB1-treated samples we detected GSK-3 beta phosphorylated on serine 9; activated protein kinase B (Akt) appears to be responsible for this activity-inhibiting phosphorylatio n, These findings suggest that overexpression of cyclin D1 results from sta bilization due to a lack of phosphorylation mediated by GSK-3 beta, We also observed an increase in cyclin dependent kinase 4 (Cdk4) complexes with cy clin D1 in FB1-treated samples; additionally, elevated Cdk4 activity was sh own by increased phosphorylation of the retinoblastoma protein. In summary, the activation of Akt leads to increased survival, inhibition of GSK-3 bet a activity and post-translational stabilization of cyclin D1, all events re sponsible for disruption of the cell cycle G(1)/S restriction point in hepa tocytes, This is the first report suggesting the mechanism by which FB1 act s as a carcinogen.