Differential regulation of protooncogenes c-jun and jun D expressions by protein tyrosine kinase, protein kinase C, and cyclic-AMP mitogenic pathways in dog primary thyrocytes: TSH and cyclic-AMP induce proliferation but downregulate C-jun expression.

The expressions of the protooncogenes c-jun and jun D have been investigated in dog thyrocytes in a primary culture whose proliferation is stimulated by three distinct intracellular signaling pathways (1) the thyrotropin (TSH) or forskolin-cyclic-AMP-mediated cascade; (2) the protein kinase C pathway activated by diacylglycerol (DAG) and phorbol esters (TPA); (3) a protein tyrosine kinase system activated by epidermal growth factor (EGF). While the first cascade is compatible with the differentiated state of the cell, the two latter pathways induce dedifferentiation. Following the stimulation by TPA or EGF, the expression of c-jun was increased and the expression of jun D was faintly increased. Both expressions are superinduced in the presence of cycloheximide as in mitogenically stimulated fibroblasts but, in the presence of cycloheximide alone, the expressions of c-jun and jun D are clearly unstable with time. This indicates that cycloheximide controls should be included at all time points examined in such experiments. Increasing intracellular concentrations of cyclic-AMP by forskolin or TSH was followed by an inhibition of the expression of c-jun. This inhibition was independent of protein synthesis. Similarly, the TPA or EGF stimulation of c-jun expression was also inhibited by TSH or forskolin, as in fibroblasts in which cyclic-AMP inhibits proliferation. Our results show that the expression of c-jun is not universally correlated with the stimulation of cell proliferation. The stimulation of c-jun expression is not common between the three mitogenic pathways. It thus represents another of the very different responses elicited by the cyclic-AMP cascade as compared to the more studied tyrosine kinase and protein kinase C mitogenic pathways.