Direct promoter induction of p19Arf by Pit-1 explains the dependence receptor RET/Pit-1/p53-induced apoptosis in the pituitary somatotroph cells.

Somatotrophs produce growth hormone (GH) and are the most abundant secretory cells of the pituitary. Somatotrophs express the transcription factor Pit-1 and the dependence receptor RET, its co-receptor GFRa1 and ligand GDNF. Pit-1 is a transcription factor essential for somatotroph proliferation and differentiation and for GH expression. GDNF represses excess Pit-1 expression preventing excess GH. In the absence of GDNF, RET behaves as a dependence receptor, becomes intracellularly processed and induces strong Pit-1 expression leading to p53 accumulation and apoptosis. How accumulation of Pit-1 leads to p53 expression is unknown. We have unveiled the relationship of Pit-1 with the p19Arf gene. There is a parallel correlation of RET processing, Pit-1 increase and ARF protein and mRNA expression. Interfering the pathway with RET, Pit-1 or p19Arf siRNA blocked apoptosis. We have found a Pit-1 DNA-binding element within the ARF promoter. Pit-1 directly regulates the CDKN2A locus and binds to the p19Arft promoter inducing p19Arf gene expression. The Pit-1-binding element is conserved in rodents and humans. RET/Pit-1 induces p19Arf/p53 and apoptosis not only in a somatotroph cell line but also in primary cultures of pituitary somatotrophs, where ARF siRNA interference also blocks p53 and apoptosis. Analyses of the somatotrophs in whole pituitaries supported the above findings. Thus Pit-1, a differentiation factor, activates the oncogene-induced apoptosis (OIA) pathway as oncogenes exerting a tight control in somatotrophs to prevent the disease due to excess of GH (insulin-resistance, metabolic disease, acromegaly).