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The role exerted by protein kinase C (PKC) on estrogen-induced DNA synthesis has been investigated in hepatic and mammary gland cells, HepG2 and MCF7. 17-beta-estradiol stimulated DNA synthesis in HepG2 and MCF7 cells, maximal effect occurring at 10 nM. DNA synthesis stimulation was prevented by anti-estrogen ICI 182,780 and by inhibitor of PKC, Ro 31-8220. The rapid estradiol effects in MCF7 cells were determined by following the inositol trisphosphate (IP(3)) production and PKC-alpha membrane translocation. After estradiol treatment the increase of IP(3) production, prevented by anti-estrogen or by phospholipase C (PLC) inhibitor (neomycin), was present in MCF7 cells. In MDA cells, devoid of estrogen receptor, no effect was observed. The PKC-alpha presence on the membranes appeared unchanged in MCF7 cells. The PLC inhibitors, neomycin and U73,122, and PKC-alpha down regulator, phorbol 12-myristate 13-acetate (PMA), were able to prevent estradiol-induced DNA synthesis in hepatoma cells, but ineffective in mammary cells; wortmannin, an inhibitor of phosphoinositide 3-kinases (PI3-K), blocked DNA synthesis in both cell lines. These data show that beta-estradiol, via an estrogen receptor-mediated mechanism, activates more signal transduction pathways, and consequently different PKC isoforms in two responsive cell lines. In both cell lines PI3-K/PKC pathway is functional to the estrogen regulation of DNA synthesis, whereas in HepG2 cells the parallel involvement of the PLC/PKC-alpha pathway is present. The reported results indicate that the DNA synthesis stimulation by beta-estradiol requires the estrogen receptor and utilises one or more activated pathways in dependence on the cell equipment.

The intracellular concentration of cholesterol is regulated by the balance between endogenous synthesis and exogenous uptake. Oestrogens have been reported to be involved in the physiological regulation of cellular cholesterol content. Relevant reports have focused on long‐term responses and there is a lack of information about the relationship between the timing of the oestrogen effect and the regulation of cholesterol homeostasis. The aim of this work has been to set up a systematic picture of the short‐term effects induced by oestrogen on hepatic lipid metabolism in vivo and the involvement of some relevant signal transduction pathways. At intervals after oestrogen administration (30 min to 6 h), oestrogen receptor expression and changes in liver cAMP, IP3 and protein kinase C‐α (PKC‐α) were followed. Changes in the expression of the low density lipoprotein receptor at mRNA and protein levels, and of hydroxy‐methyl‐glutaryl‐CoA reductase activity have been verified. At the same time, the content of hepatic cholesterol, ubiquinone and dolichol and of plasma cholesterol have been determined. Changes of rab 5 and rab 8, small GTP‐binding prenylated proteins involved in the transfer of neosynthesised proteins through the cell, have been also checked. In vivo treatment with oestradiol produced no change in cyclic AMP but a rapid increase in IP3, increased PKC‐α localisation on the membranes and enhanced expression of the low density lipoprotein receptor in the liver occurred. PKC inhibition completely prevented any increase in low density lipoprotein receptor mRNA in isolated and perfused rat liver. Early changes of ubiquinone and dolichol content and a later reduction in hepatic hydroxy‐methyl‐glutaryl‐CoA reductase activity and plasma cholesterol content were also detectable. A functional role of the IP3 ‐protein kinase C‐α pathway in the induction of the low density lipoprotein receptor is suggested.

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