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

Abstract

Fumonisin B(1) (FB(1)) is a worldwide corn contaminant and has been epidemiologically linked to the high incidence of human esophageal cancer in South Africa and China. FB(1) is hepatocarcinogenic in rats by an unknown mechanism. Inhibition of ceramide synthase and disruption of membrane phospholipids have been shown to be mechanisms of toxicity. Here we show overexpression of cyclin D1 protein in both preneoplastic and neoplastic liver specimens obtained from a long-term feeding study of FB(1) in rats. In rats fed FB(1) short-term, 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 FB(1)-treated samples showed a distinct pattern of migration (presence of less negatively charged form of the protein) that differed from controls. Recently, it has been shown that phosphorylation of cyclin D1 by glycogen synthase kinase 3beta (GSK-3beta) on a single threonine residue (Thr-286) positively regulates proteosomal degradation of cyclin D1. In FB(1)-treated samples we detected GSK-3beta phosphorylated on serine 9; activated protein kinase B (Akt) appears to be responsible for this activity-inhibiting phosphorylation. These findings suggest that overexpression of cyclin D1 results from stabilization due to a lack of phosphorylation mediated by GSK-3beta. We also observed an increase in cyclin dependent kinase 4 (Cdk4) complexes with cyclin D1 in FB(1)-treated samples; additionally, elevated Cdk4 activity was shown by increased phosphorylation of the retinoblastoma protein. In summary, the activation of Akt leads to increased survival, inhibition of GSK-3beta activity and post-translational stabilization of cyclin D1, all events responsible for disruption of the cell cycle G(1)/S restriction point in hepatocytes. This is the first report suggesting the mechanism by which FB(1) acts as a carcinogen.

Inhibition of Hepatocarcinogenesis by ArtinM via Anti-proliferative and Pro-apoptotic Mechanisms

“…ArtinM treatment increased nuclear p21 and p27 staining which could restore their tumor-suppressing function. Moreover, previous studies reported the importance of cyclin D-CDK complex for cell cycle transition and the intrinsic relationship between p21 levels and cyclin D1 activity (29,30). In accordance, animals exposed to DEN, showed a significant increase of cyclin D levels, which were restored to same levels as in the control group by ArtinM treatment.…”

Post-transcriptional regulation of cyclin D1 expression during G2 phase

“…For examples, growth suppressive treatments decreased cyclin D1 protein synthesis with minimal changes in message levels (Brewer and Diehl, 2000;Mettouchi et al, 2001;Muise-Helmericks et al, 1998;Palakurthi et al, 2000;Pervin et al, 2001). Fumonisin B1, a carcinogenic mycotoxin, stabilizes cyclin D1 protein resulting in high levels of protein expression without a change in message levels (Ramljak et al, 2000). Growth suppressive chemicals can also destabilize cyclin D1 message and/or protein (Agami and Bernards, 2000;Dufourny et al, 2000, Hashemolhosseini et al, 1998Lin et al, 2000).…”

Inherent growth advantage of (pre)malignant hepatocytes associated with nuclear translocation of pro-transforming growth factor α

“…Since proTGFa translocates to the nucleus in the G1-phase of the cell cycle (Grasl-Kraupp et al, 2002), the elevated presence of nuclear proTGFa in (pre)malignant hepatocytes may be evidence for a G1-status of these cells. Several groups reported that premalignant liver cells show an increased expression of c-myc and cyclin D1, known inducers of the transition from G0 to G1 of the cell cylce (Deguchi and Pitot, 1995;Ramljak et al, 2000). On the other hand, the expression of the WAF1/CIP1 gene product, p21 and the c-myc antagonist mad were decreased in hepatocarcinogenesis (Martens et al, 1996).…”