Zeng, Li
(1996)
Enzymes of cyclic AMP metabolism in hepatocytes.
PhD thesis, University of Glasgow.
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Abstract
Incubation of hepatocytes or the SV40-DNA immortalised hepatocyte P9 cell line with cholera toxin led to a time-dependent activation of adenylyl cyclase activity which occurred after a defined lag period. When added together with cholera toxin, each of the hormones insulin and vasopressin was capable of attenuating the maximum stimulatory effect achieved by cholera toxin over a period of 60min through a process which could be blocked by the compounds staurosporine and chelerythrine. Attenuating effects upon cholera toxin- stimulated adenylyl cyclase activity could also be elicited using either tlie protein kinase C stimulating phorbol ester PMA (12-O-tetradecanoyl phorbol-13- acetate) or the protein phosphatase inhibitor okadaic acid. Alkaline phosphatase treatment of membranes reversed the inhibitory effect of PMA. Cholera toxin also stimulated the adenylyl cyclase activity of intact CHO and NIH-3T3 cells but this activity was insensitive to the addition of PMA. Overexpression of various protein kinase C isoforms in CHO cell lines did not confer sensitivity to inhibition by PMA upon cholera toxin stimulated adenylyl cyclase activity. It is suggested that the protein kinase C mediated phosphorylation of a membrane protein attenuates cholera toxin-stimulated adenylyl cyclase activity in hepatocytes and P9 cells. The cellular selectivity of such an action may be due to the target for this inhibitory action of protein kinase C being a particular isoform of adenylyl cyclase. This is believed to be type V adenylyl cyclase which provides the major activity in hepatocytes and P9 cells but is absent from both CHO and NIH-3T3 cells. Multiple families of phosphodiesterase are differential.
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