Hyperphosphorylation of retinoblastoma protein and p53 by okadaic acid, a tumor promoter

A potent tumor promoter, okadaic acid, induced hyperphosphorylation of tumor suppressor proteins, retinoblastoma protein and p53, by in vitro incubation with nuclei isolated from rat regenerating liver as well as by incubation with primary human fibroblasts. Most of the retinoblastoma protein migrated to a hyperphosphorylated position in electrophoresis. The phosphorylation of p53 was increased at a rate 8 times that in non-treated primary human fibroblasts. Hyperphosphorylation of tumor suppressor proteins, mediated through inhibition of protein phosphatases 1 and 2A, is involved in tumor promotion by okadaic acid. The significance of hyperphosphorylation of the retinoblastoma protein and p53 is discussed in relation to the regulation of the cell cycle.