Dissociation between protein kinase C content and biological responsiveness to phorbol esters in tumor promoter-sensitive (MCF-7) and resistant (RPh-4) cells

A cell line (RPh-4) insensitive to the effects of phorbol esters has been isolated from MCF-7 human breast cancer cells. The growth pattern of RPh-4 cells in the presence of 50 ng/mL (80 nM) 12-O-tetradecanoylphorbol 13-acetate (TPA) is similar to that of parental MCF-7 cells in the absence of TPA. While phorbol esters inhibit MCF-7 cell proliferation and increase cell volume and protein content, no such effects are observed in RPh-4 cells. TPA affects MCF-7 but not RPh-4 cell cycle in two ways: a G1 block and a delayed passage through G2 phase. Profound alterations in protein kinase C content and activity are observed in RPh-4 versus MCF-7 cells, i.e. (i) a dramatic decline in the cellular enzyme content; (ii) a loss of the capacity to translocate upon acute TPA stimulation for the remainder enzyme; and (iii) a lack of stimulation by phorbol esters of the endogenous Mr 28,000 substrate. However, these striking changes are only transient and rapidly reverse when RPh-4 cells are subcultured in TPA-free medium, with a 60% and an almost total recovery, respectively, after 15 days and 3 months. By contrast, a much lower rate of reversion is observed in terms of cell growth responsiveness to TPA with a total insensitivity to phorbol ester after 80 days and a 50% inhibition of RPh-4 cell proliferation after 3.5 months. Our data clearly demonstrate an apparent dissociation between the cellular protein kinase C content and the biological responsiveness to phorbol ester in the variant RPh-4 cells. Moreover, they suggest that the Mr 28,000 protein phosphorylation event is not directly related to the cell growth arrest induced by phorbol esters in MCF-7 cells.