No abstract

Protein kinase C is important in the transduction of signals generated at the plasma membrane. The physiological activators of protein kinase C are diacylglycerols, and the tumour-promoting phorbol esters, such as 12-O-tetradecanoyl-phorbol-14 acetate (TPA), constitute another group of specific activators. Many cellular substrates for phosphorylation by protein kinase C have been described, but proteins that directly control transcription in response to protein kinase C activation are yet to be identified. TPA treatment leads to induction of various proto-oncogenes, growth factor genes, and genes encoding secreted proteases. In addition. TPA increases the activity of viral enhancer elements. To identify trans-acting factors that mediate the transcriptional response to TPA we chose the simian virus 40 (SV40) enhancer as a model, because it is known to be composed of several discrete cis-acting elements which are recognized by multiple transacting factors. We report here that the SV40 enhancer contains at least four different TPA responsive elements whose activity is dependent on cell-type. The induction response is likely to involve at least two distinct post-translational steps which modulate the activity of the proteins that recognize these elements.

The exact physiological role of metallothionein (MT) is not clear. It has been suggested that these low-molecular-weight, highly inducible, heavy-metal-binding proteins serve in the regulation of intracellular Zn metabolism. Among the Zn-requiring systems are several enzymes involved in DNA replication and repair. Therefore, during periods of active DNA synthesis there is likely to be an increased demand for Zn, which could be met by elevated MT synthesis. For that reason, we examined whether stimulation of cellular proliferation leads to increased expression of MT. We report here that treatment of cultured mammalian cells with serum growth factors and activators of protein kinase C, all of which are known to have growth stimulatory activity, led to induction of MT mRNA. One of the required steps in the signal transduction pathways triggered by these agents, ending in MT induction, appears to be the activation of protein kinase C.Zinc (Zn) is an important component of many enzymes, including alkaline phosphatases, carboxypeptidases, dehydrogenases, RNA and DNA polymerases, and the protein synthesis machinery (see references 28 and 34 for reviews). Zn deficiency leads to severe growth retardation, developmental abnormalities, immunodeficiency, eczematoid dermatitis, and several other disorders (28). As an important ion, the homeostasis of Zn is finely maintained. Metallothioneins (MTs), which are low-molecular-weight heavy-metal-binding proteins (see reference 12 for a review), are likely to occupy a central role in the regulation of zinc metabolism (7,29). Several Zn-requiring apoenzymes can be reactivated by the transfer of Zn from MT to the apoenzyme (33). Therefore, MTs may constitute a regulatory system whose function is analogous to that of calmodulin in calcium metabolism. In addition, MTs probably serve as an intracellular reservoir of Zn. In support of these roles it has been reported that induction of MT synthesis is responsible for the increased cellular uptake of Zn after treatment with glucocorticoid hormones (13,15,16).As the major Zn-binding proteins in the cell (16), MTs can potentially contribute to many important biological processes that involve Zn-requiring enzymes, including replication, repair, transcription, protein synthesis and turnover, and energy metabolism. It is expected that during periods of active cell growth and proliferation there is an increased demand for Zn that could be met by elevated MT synthesis. In this report we describe the relationship between cellular proliferation and MT synthesis. More specifically, we examined the effect of serum and various growth factors as well as direct activators of protein kinase C, all of which are stimulators of cellular proliferation (25, 26), on the expression of MT mRNA in various mammalian cell cultures. Although MT synthesis is not directly coupled to the cell cycle, it is stimulated in response to treatment with serum, peptide growth factors, and two different activators of protein kinase C. MATERIALS AND METHODSChemicals. 12-0-Tetra...