Thyrotropin and insulin-like growth factor I regulation of tyrosine phosphorylation in FRTL-5 cells : interaction between cAMP-dependent and growth factor-dependent signal transduction

Pretreatment of rat FRTL-5 thyroid cells with thyrotropin (TSH) markedly potentiated the mitogenic response to insulin-like growth factor I (IGF-I) (Tramontano, D., Moses, A. C., Veneziani, B. M., and Ingbar, S. H. (1988) Endocrinology 122, 127-132; Takahashi, S.-I., Conti, M., and Van Wyk, J. J. (1990) Endocrinology 126, 736-745). The present study was undertaken to determine whether this synergism between TSH and IGF-I in FRTL-5 cells was correlated with changes in tyrosine phosphorylation of intracellular proteins. Tyrosine phosphorylation in intact cells was determined by gel electrophoresis and immunoblotting using monospecific anti-phosphotyrosine antibodies. Cells were preincubated for up to 24 h with TSH, dibutyryl cAMP, forskolin, or cholera toxin and then incubated for an additional 1 min in the absence or presence of IGF-I. As reported by others, IGF-I rapidly increased tyrosine phosphorylation of a 175-kDa protein as well as a less intense band of 90-100 kDa. Pretreatment for 6-12 h with either TSH or other agents that elevate intracellular cAMP potentiated the IGF-I-dependent tyrosine phosphorylation of the 175-kDa substrate by 3-5-fold. Since TSH did not increase IGF receptor number of kinase activity, the effect of TSH is assumed to be exerted at a step distal to IGF receptor tyrosine kinase. Surprisingly, IGF-I-independent tyrosine phosphorylation was also increased by pretreatment with TSH. When intact cells were analyzed TSH produced a time- and concentration-dependent increase in tyrosine phosphorylation of a prominent 120-125-kDa substrate and less prominent 100- and 80-kDa substrates. Assays using Triton X-100-soluble extracts incubated with MgCl2, ATP, and orthovanadate demonstrated that TSH pretreatment increased tyrosine phosphorylation over that observed in untreated cells. In this cell-free assay, TSH pretreatment enhanced the phosphorylation of multiple substrates. These studies suggest that a cAMP stimulus that initiates a trophic effect can be propagated indirectly through multiple pathways including enhancement of tyrosine phosphorylation