DIFFERENTIAL REGULATION OF THE EXPRESSION OF FAST-TYPE SARCOPLASMIC-RETICULUM CA2-ATPASE BY THYROID-HORMONE AND INSULIN-LIKE GROWTH-FACTOR-I IN THE L6 MUSCLE-CELL LINE()

The aim of this study was to investigate the mechanism(s) underlying t he thyroid-hormone (L-tri-iodothyronine, T-3)-induced elevation of fas t-type sarcoplasmic-reticulum Ca2+- ATPase (SERCA1) levels in L6 myotu bes and the potentiating effect of insulin-like growth factor-I (IGF-I ) [Muller, van Hardeveld, Simonides and van Rijn (1991) Biochem. J. 27 5, 35-40]. T-3 increased the SERCA1 protein level (per mu g of DNA) by 160%. The concomitant increase in the SERCA1 mRNA level was somewhat higher (240%). IGF-I also increased SERCA1 protein (110%) and mRNA lev els (50%), whereas IGF-I+T-3 increased SERCA1 protein and mRNA levels by 410% and 380% respectively. These SERCA1 mRNA analyses show that th e more-than-additive action of T-3 and IGF-I on SERCA1 expression is, at least in part, pre-translational in nature. Further studies showed that the half-life of SERCA1 protein in L6 cells (17.5 h) was not alte red by T-3. In contrast, IGF-I prolonged the half-life of SERCA1 prote in 1.5-1.9-fold, which may contribute to the disproportional increase in SERCA1 protein content compared with mRNA by IGF-I. Measurements of SERCA1 mRNA. half-life (as determined by actinomycin D chase) showed no difference from the control values (15.5 h) in the presence of T-3 or IGF-I alone. When T-3 and IGF-I were both present, the SERCA1 mRNA half-life was prolonged 2-fold. No significant effects of T-3 and IGF- I were observed on the half-life of total protein (37.4 h) and total R NA (37.0 h). The absence of an effect of T-3 on SERCA1 protein and mRN A stability, when it was present alone, suggested transcriptional regu lation, which was confirmed by nuclear run-on experiments, showing a 3 -fold increase in transcription frequency of the SERCA1 gene by T-3. W e conclude that the synergistic stimulating effects of T-3 and IGF-I o n SERCA1 expression are the result of both transcriptional and post-tr anscriptional regulation. T-3 acts primarily at the transcriptional le vel by increasing the transcription frequency of the SERCA1 gene, wher eas IGF-I seems to act predominantly at post-transcriptional levels by enhancing SERCA1 protein and mRNA stability, the latter, however, onl y in the presence of T-3.