Effect of tumour promoting agents on protein phosphorylation in human placenta

1. The effects of okadaic acid (OA) and phorbol-12-myristate-13-acetate (PMA) on protein phosphorylation were studied in human term placentas. 2. When samples treated with tumour promoters were compared with untreated samples, the phosphorylation of a 135 kDa protein was significantly decreased; OA also produced a decrease in phosphorylation of a 24 kDa protein. 3. Both substances produced an alteration in the proportions of bands of masses 170, 65 and 24 kDa, relative to total phosphorylation; PMA treatment also affected the band of mass 135 kDa. 4. Placental cell extracts were also subjected to Western blotting with a protein kinase C (PKC) antibody, reportedly specific for the alpha- and beta-isoforms. 5. Two immunoreactive proteins were detected; an 80 kDa band, presumably corresponding to the alpha- or beta-PKC, and a 64 kDa protein, which could be a degradation production of the 80 kDa protein or it could correspond to another form of the enzyme. The expression of PKC did not change on treatment with PMA.