Insulin-like growth factor-1 causes a switch-like reduction of endogenous growth hormone mRNA in rat MtT/S somatotroph cells

Reduction of mRNA expression from the endogenous GH gene by insulin-like gr owth factor 1 (IGF-1) in somatotroph-like rat MtT/S cells was measured, GH mRNA levels were reduced by 65 nM IGF-1 treatment in a time-dependent manne r over 5 d of culture with a calculated GH mRNA half-life of 50 h, in line with previous values from primary cultures. Inhibition of inositol 3-phosph ate kinase by wortmannin or LY-294,002 treatment was ineffective in blockin g IGF-1 decreases in GH mRNA, as was inhibition of MAP kinase activity by P D 098059, The inhibition by IGF-1 also did not regulate Pit-1 (GHF-1) mRNA levels, which were constant during 65 nM IGF-1 treatment. MtT/S cells were shown to have both IGF-1 and insulin receptors as detected by Western blott ing. There was also shown to be the suggestion of "hybrid" receptors contai ning different beta chains from each of these related heterotetrameric rece ptors, Analysis of the effects of IGF-1 and insulin on MtT/S cells showed t hat each reduced CH mRNA in a dose-dependent manner gave a calculated EC50 of 15.5 nM for IGF-1 and 0.6 nM for insulin, suggesting that the respective receptors for each hormone were activated. However, GH mRNA response to IG F-1 treatment was "ultrasensitive," exhibiting a switch-like effect; below 10 nM IGF-1, there was no decline in GH mRNA, but then maximal reduction oc curred at IGF-1 concentrations above 20 nM, The degree of this ultrasensiti ve effect was calculated from the Hill equation for cooperativity, with a H ill coefficient of -4.1, greater than the classic cooperativity exhibited b y hemoglobin binding to oxygen, The ultrasensitive response was specific fo r IGF-1, as insulin did not display this effect. These results suggest that the response evoked by the IGF-1 receptor could act as a binary molecular switch controlling GH mRNA expression in somatotrophs.