TRANSFORMING GROWTH FACTOR-BETA-S INHIBIT SOMATOSTATIN MESSENGER-RIBONUCLEIC-ACID LEVELS AND SOMATOSTATIN SECRETION IN HYPOTHALAMIC CELLS IN CULTURE

Treatment of hypothalamic cells in monolayer culture with transforming growth factor-beta 1 (TGF beta 1) significantly reduced both basal an d cAMP-induced somatostatin messenger RNA (mRNA) levels and somatostat in secretion. This inhibitory effect was dose-and time-dependent and n ot mediated by glial cells, as it was also observed in glial-free hypo thalamic cell cultures treated with cytosine arabino-nucleoside. TGF b eta 2 and -beta 3 mimicked the actions of TGF beta 1, which indicated that the three isoforms of the TGF beta family expressed in the centra l nervous system displayed similar effects on the somatostatinergic ne urons. The blockade of synthesis of proteins with either cycloheximide or puromycin for 24 h prevented the inhibitory effect of TGF beta 1 o n somatostatin mRNA. This implied that the reduction of this mRNA by T GF beta 1 required de novo protein synthesis. We next studied whether TGF beta 1 acted at the transcriptional or posttranscriptional level b y altering the stability of somatostatin mRNA. Examination of the rate of disappearance of somatostatin mRNA by Northern blot, after inhibit ion of mRNA transcription with either actinomycin D (AcD) or 5,6-dichl oro-1 beta-ribofuranosyl benzimidazole revealed that TGF beta 1 did re duce the stability of somatostatin mRNA. This effect was observed when we pretreated the cultures with TGF beta 1 4 h before the addition of AcD, but not when we administered TGF beta 1 simultaneously with AcD or 5,6-dichloro-1 beta-ribofuranosyl benzimidazole. Altogether these r esults demonstrated that the treatment of hypothalamic cells in cultur e with TGF beta 1, TGF beta 2, or TGF beta 3 resulted in a decrease in somatostatin mRNA levels and somatostatin secretion. TGF beta 1 reduc ed the steady state levels of somatostatin mRNA by inducing the synthe sis of a protein (s), that appears to accelerate the degradation of th e mRNA of somatostatin. Whether TGF beta 1 has additional effects on t he transcription of the somatostatin gene will require further study.