Fumonisin B1 induces protein kinase C translocation via direct interaction with diacylglycerol binding site

Abstract

Fumonisins are carcinogenic to rats and are suspected human carcinogens. However, the mechanism(s) of carcinogenesis of fumonisin B1 (FB1) is poorly understood. Multiple signal transduction pathways such as protein kinase C (PKC) have been shown to play an important role in carcinogenesis. This study was undertaken to evaluate whether FBI affects PKC activation. Similar to tumor-promoting phorbol ester, phorbol 12-myristate-13-acetate (PMA), PKC is also catalytically activated by FB1. Protein kinase C activity and its redistribution in response to FB1 were determined in rat cerebrocortical slices. Cytosolic and membranous PKC activities were determined by histone phosphorylation in the presence of [γ32P]ATP, phosphatidyl-l-serine, PMA, and Ca 2+. Distribution of γPKC isozyme in the presence of FB1 was also assessed by immunoblotting using affinity purified anti-peptide antibodies. Similar to PMA, FB1 added in vitro to rat cerebrocortical slices facilitated PKC translocation from cytosol to membrane in a concentration-dependant manner. This FB1-induced PKC translocation was inhibited by incubation with the inactive 4α-phorbol 12,13-didecanoate. The effects of FB1 and PMA were neither additive nor synergistic. In addition, PMA and FB1-induced PKC enzyme redistribution were inhibited by pretreating tissues with sphingosine. A concentration-related FB1 attenuation of specific phorbol dibutyrate, [3H]PDBu, binding was also observed when cortical membranes were incubated with either PMA or sphingosine. This is the first report of FB1-induced PKC translocation via a direct action on the diacylglycerol site that also binds phorbol esters. Because phorbol esters are well known tumor promoters, we provide a plausible cellular mechanism to explain the carcinogenicity of FB1.