Differential effects of phorbol ester on growth and protein kinase C isoenzyme regulation in human hepatoma Hep3B cells

PMA has both mitogenic and antiproliferative effects on human hepatoma Hep3B cells. In response to low PMA concentration (10 nM), Hep3B cells displayed an increasing proliferation potentiation. At high PMA concentration (1 µM) Hep3B cells exhibited modest cytostatic effects. Determinations of protein kinase C (PKC) activity in PMA-treated cells revealed that alterations in PKC activity are associated with proliferative capacity. The decrease in PKC activity mediated by a high dose of PMA was accompanied by cell growth inhibition. Increases in PKC activity mediated by a low dose of PMA were consistent with proliferation stimulation. Immunoblot analysis showed that there are at least six PKC isoenzymes: α, δ, ε, µ, ζ and ι/λ, constitutively expressed in Hep3B cells. Cellular fractionation and immunocytochemical staining results demonstrated that both 10 nM and 1 µM PMA treatments induced a marked translocation of PKC-α from cytosol to membrane or nuclear fraction within 5–30 min. At the same time PKC-δ and ε were translocated from the membrane to nuclear fraction. In addition, prolonged treatment with 1 µM PMA, but not with 10 nM PMA, selectively mediated the down-regulation of these three PKC isoenzymes. The distinct effects of different concentrations of PMA on cell proliferation and PKC-α, δ and ε isoenzyme modulation support the involvement of these three PKC isotypes in the mechanism of action of Hep3B cells in cell growth events.