Independent and Interactive Effects of Tetradecanoyl Phorbol Acetate On Growth and Differentiated Functions of Frtl5 Cells

In studies of regulation of the growth and differentiated function of the thyroid follicular cell, we have employed the FRTL5 cell line to evaluate both the effects of agents that activate protein kinase-C (PKC) and their interaction with other agents that influence the growth and/or function of the FRTL5 cell. The PKC activator tetradecanoyl-phorbol acetate (TPA) alone induced a time- and concentration-dependent stimulation of the incorporation of [3H]thymidine into the DNA of quiescent FRTL5 cells, an effect anteceded by an increase in the levels of the mRNAs of the proto-oncogene c-myc and associated with a stimulation of cell replication. TPA also produced a dose-dependent inhibition of the low levels of radioiodine uptake in quiescent FRTL5 cells. These effects of TPA were unaccompanied by any change in the cellular cAMP concentration. TPA also modified a variety of responses to TSH, attenuating the TSH-induced stimulation of [3H]thymidine incorporation into DNA, cell replication, cAMP generation, and iodine uptake. Inhibition of TSH-stimulated growth and iodine uptake by TPA could not be ascribed solely to a decrease in cAMP generation, since TPA also inhibited the increase in [3H]thymidine incorporation and iodide uptake induced by the cAMP analog (Bu)2cAMP. In contrast, the independent stimulatory effects of TPA and insulin-like growth factor I (IGF-I) on [3H]thymidine incorporation and cell replication were at least additive when the two stimulators were added together. We have previously reported that both TSH and (Bu)2cAMP amplify the enhancement of DNA synthesis and cell replication in FRTL5 cells induced by IGF-I, and that the response of DNA synthesis to IGF-I is also enhanced if cells are preincubated with either TSH or (Bu)2cAMP. Both the former amplification of mitogenesis and the latter priming effect were decreased by exposing cells to TPA concomitant with their exposure to TSH or (Bu)2cAMP. The effects of TPA were mimicked by other activators of PKC, but not by a phorbol ester that fails to activate this enzyme. In general, we conclude that in the FRTL5 cell, regulation of cell growth is extremely complex; there are at least three mitogenic pathways that are separate from but interact with one another. The first is the well known cAMP-dependent pathway, which is activated by TSH. The second is activated by IGF-I and is cAMP independent. These two pathways interact to produce a marked amplification of their individual mitogenic effects. The third pathway is that stimulated by TPA and involves activation of PKC.