Adrenal phenotype in TPIT insufficient animals cannot be restored by short-term treatment with physiological doses of ACTH and N-POMC

Tpit is a positive regulator for late POMC cell differentiation. Accordingly, inactivation of the Tpit gene results in loss of POMC expressing cells in the pituitary while heterozygous mice (Tpit+/–) display a normal adrenal phenotype. Adrenal growth and function is dependent upon hormonal stimulation from the pituitary. While ACTH is a potent stimulator of adrenal steroidogenesis, N-terminal POMC peptides have been implicated to act as potent adrenal mitogens and suppressors of adrenal steroidogenesis. To dissect specific effects of ACTH and N-POMC(1–28) on adrenal function, both peptides were injected over 7 days in Tpit+/– and Tpit–/– mice. In addition, as a model of acute secondary adrenal insufficiency, another group of Tpit+/– mice was pretreated with dexamethasone (dex) over 6 days prior ACTH and N-POMC substitution. Adrenal weight, morphology, and steroid production were assessed. Sham treated Tpit–/– mice had lower adrenal weights (0.9±0.3mg) in comparison to dexa treated Tpit+/– (2.6±0.1mg, p=0.0006) and sham treated Tpit+/– animals (3.6±0.1mg, p<0.0001) with smaller cortical areas and a higher number of cells per HPF indicating cellular hypotrophy. In addition, morphological and immunohistochemical evaluation in Tpit–/– animals revealed a prominent outer zone composed of cells negative for steroidogenic enzymes and SF-1 which was not evident in dexa treated or sham treated Tpit+/– mice. Neither ACTH (at high physiological doses) nor N-POMC substitution had a significant effect on adrenal weight and morphology compared to sham treated animals from each genotype. Although RT-PCR and immunohistochemistry demonstrated expression of ACTH receptor and steroidogenic enzymes (SCC, 3βHSD, StAR) in Tpit–/– adrenals, ACTH treatment did not stimulate corticosterone secretion. Taken together, these results suggest that physiological levels of ACTH cannot restore adrenocortical morphology and function in Tpit–/– mice and treatment with N-POMC does not significantly affect adrenal growth or steroidogenesis in this particular experimental setting.