Transforming growth factor-betas 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-beta1 (TGFbeta1) significantly reduced both basal and cAMP-induced somatostatin messenger RNA (mRNA) levels and somatostatin secretion. This inhibitory effect was dose- and time-dependent and not mediated by glial cells, as it was also observed in glial-free hypothalamic cell cultures treated with cytosine arabinonucleoside. TGFbeta2 and -beta3 mimicked the actions of TGFbeta1, which indicated that the three isoforms of the TGFbeta family expressed in the central nervous system displayed similar effects on the somatostatinergic neurons. The blockade of synthesis of proteins with either cycloheximide or puromycin for 24 h prevented the inhibitory effect of TGFbeta1 on somatostatin mRNA. This implied that the reduction of this mRNA by TGFbeta1 required de novo protein synthesis. We next studied whether TGFbeta1 acted at the transcriptional or posttranscriptional level by altering the stability of somatostatin mRNA. Examination of the rate of disappearance of somatostatin mRNA by Northern blot, after inhibition of mRNA transcription with either actinomycin D (AcD) or 5,6-dichloro-1beta-ribofuranosyl benzimidazole revealed that TGFbeta1 did reduce the stability of somatostatin mRNA. This effect was observed when we pretreated the cultures with TGFbeta1 4 h before the addition of AcD, but not when we administered TGFbeta1 simultaneously with AcD or 5,6-dichloro-1beta-ribofuranosyl benzimidazole. Altogether these results demonstrated that the treatment of hypothalamic cells in culture with TGFbeta1, TGFbeta2, or TGFbeta3 resulted in a decrease in somatostatin mRNA levels and somatostatin secretion. TGFbeta1 reduced the steady state levels of somatostatin mRNA by inducing the synthesis of a protein (s), that appears to accelerate the degradation of the mRNA of somatostatin. Whether TGFbeta1 has additional effects on the transcription of the somatostatin gene will require further study.