New perspectives of the molecular mechanisms to potentiate IGF signals

在许多细胞类型中，igf - 1已被证明具有多种生物活性。尽管IGFs的这些作用丰富多样，但IGFs的体外生物学效应相对较弱，除非有其他激素或生长因子存在，否则通常无法证明。这些发现表明igf作为允许因子来增强信号或其他因子。我们已经证明，在甲状腺滤泡细胞系FRTL-5中，TSH和IGF-I协同刺激细胞生长，cAMP预处理对于增强IGF-I依赖性DNA合成至关重要。在该细胞系中，cAMP预处理导致酪氨酸激酶活性增加和细胞内蛋白（如125-kDa蛋白（p125））酪氨酸磷酸化，这与cAMP启动效应对IGF-I诱导的DNA合成增强密切相关。我们最近发现磷酸酪氨酸p125结合到PI-3激酶p85调控亚基上，LY294002 （PI-3激酶的一种）阻断了camp的启动作用。结合酪氨酸激酶和PI-3激酶活性是CAMP依赖性Gl细胞周期蛋白增加所必需的数据，我们的研究结果表明，CAMP刺激通过CAMP诱导的酪氨酸磷酸化变化将静止细胞招募到细胞周期中。另一方面，我们证明了cAMP预处理增强了IGF-I诱导的IRS-2和Shc酪氨酸磷酸化，尽管cAMP预处理不影响IGF-I受体的自磷酸化。cAMP预处理增加了Grb2与IRS-2和Shc的结合，并且cAMP刺激也增强了IGF-I诱导的MAP激酶活化。此外，cAMP预处理增加了IRS-2与PI-3激酶p85亚基的关联，并且cAMP预处理增强了igf -i诱导的与IRS-2结合的PI-3激酶活性。最后，在IGF-I治疗期间，PD98059（一种MEK抑制剂）或LY294002的存在消除了IGF-I依赖性DNA合成的camp依赖性增强。这些结果表明，cAMP刺激通过cAMP依赖性增强IGF-I受体底物的酪氨酸磷酸化来放大IGF-I信号。总之，热带激素依赖途径和IGF-I依赖途径之间的相互作用通过camp依赖过程引发细胞对IGF-I的反应和IGF-I有丝分裂活性的扩增，从而导致细胞增殖的增强。少

In many cell types IGF-I has been shown to possess a variety of bioactivities. Despite the profuseness and diversity of these effects of IGFs, the in vitro biological effects of IGFs are relatively weak and often are not demonstrable except in the presence of other hormones or growth factors. These findings suggest that IGFs act as permissive factors to augment the signals or other factors. We have shown that in FRTL-5, a thyroid follicular cell line, TSH and IGF-I stimulate cell growth synergistically and cAMP pretreatment is essential for the potentiation of IGF-I-dependent DNA synthesis. In this cell line, cAMP pretreatment caused an increase in tyrosine kinase activity and tyrosine phosphorylation of intracellular proteins such as a 125-kDa protein (p125), which was well correlated with a cAMP-priming effect on potentiation of DNA synthesis induced by IGF-I.We recently found that the phosphotyrosyl p125 bound to a PI-3 kinase p85 regulatory subunit, and LY294002 (a PI-3 kinase inhi … More bitor) blocked the cAMP-priming effect. Taken together with the data that tyrosine kinase and PI-3 kinase activities are necessary for cAMP-dependent increases in Gl cyclins, our results suggest that CAMP stimulus recruit the quiescent cells into the cell cycle through cAMP-induced changes of tyrosine phosphorylation. On the other hand, we demonstrated that cAMP pretreatment potentiated IRS-2 and Shc tyrosine phosphorylation induced by IGF-I, although pretreatment with cAMP did not affect autophosphorylation of the IGF-I receptor. cAMP pretreatment increased Grb2 binding to IRS-2 and Shc, and MAP kinase activation induced by IGF-I was also enhanced by cAMP stimulus. Furthermore, cAMP pretreatment increased IRS-2 association with the PI-3 kinase p85 subunit, and IGF-I-induced PI-3 kinase activity bound to IRS-2 was enhanced by cAMP pretreatment. Finally, the presence of PD98059 (a MEK inhibitor) or LY294002 during IGF-I treatment abolished the cAMP-dependent augmentation of IGF-I dependent DNA synthesis. These results suggest that cAMP stimulus amplifies the IGF-I signals through cAMP-dependent potentiation of tyrosine phosphorylation of IGF-I receptor substrates. In conclusion, interaction between tropic hormone-dependent and IGF-I-dependent pathways leads to an augmentation of cell proliferation through the cAMP-dependent process of priming the cells to respond to IGF-I and amplification of IGF-I mitogenic activities. Less