Labile proteins are necessary for T3 induction of growth hormone mRNA in normal rat pituitary and rat pituitary tumor cells.

Abstract

Previous studies from our laboratory have shown that ongoing protein synthesis is required for L-tri-iodothyronine (T3) regulation of rat hepatic genes. In this report we have examined the role of ongoing protein synthesis in T3 regulation of the growth hormone gene (GH) in rat pituitary and GC cells. T3 (200 micrograms/100 g body weight injected intraperitoneally or 10(-8) M added to the media) induced a 3-fold rise of GH mRNA concentration after 8 h in rat pituitary (from 5.9 +/- 1% to 17.8 +/- 1.4% of an internal standard (IS), p less than 0.01) and a 5-fold rise in GC cells after 6 h (from 0.1 +/- 0.01% to 0.54 +/- 0.03% of IS, p less than 0.01). Cycloheximide (1 mg/100 g body weight intraperitoneally or 25 microM added to the media) completely blocked the increase in GH mRNA when given before the hormone in rat pituitary (7.5 +/- 0.5% of IS after 8 h) and GC cells (0.095 +/- 0.01% of IS after 6 h). Similar results were observed when emetine and puromycin were used to block protein synthesis. GH transcription rate in pituitaries from hypothyroid animals was 20 +/- 8 ppm and increased to a value of 295 +/- 85 parts per million 2 h after the injection of T3, and a similar 8-fold increase was observed in GC cells after 2 h. However, when cycloheximide was given before the hormone, the T3-induced increase in transcription was completely blocked. We also demonstrate here that the differences observed in GH transcription rate between hypo- and euthyroid rat pituitaries fully account for the differences observed in mRNA concentration. We conclude that short-lived proteins are involved in T3 regulation of the GH gene in rat pituitary and GC cells.