Pbx-1 is mandatory for proper adrenocortical growth and steroidogenesis and interacts with the SF-1 pathway

Pbx-1 deficiency leads to adrenal agenesis in mice, and thus, similar to the established transcription factors WT-1, SF-1 and Dax-1, this homeodomain DNA binding protein has been defined as an important factor necessary for proper adrenal development. Accordingly, adult Pbx-1 haploinsufficient mice (Pbx-1+/-) have a significant lower adrenal weight (4.9±0.2mg vs. wild type 7.7±0.4mg, p=0.0002), and the X-zone is significantly smaller compared to the wild type (wt) animals. Furthermore, PCNA RNA and protein levels are remarkably lower (45±9% vs. 100±20%), and unilateral adrenalectomy leads to impaired contralateral compensatory adrenal growth, both indicating less proliferation in the context of Pbx-1 haploinsufficiency. The IGF system plays a key role in the adrenal cortex. Real time PCR demonstrates significant lower expression levels of IGF1 receptor (46±6% vs. 100±10 p=0.007), and an upregulation of IGFBP2 by 10 fold in these mice. On a functional level, while baseline corticosterone levels are similar in Pbx-1+/- and wt animals, corticosterone levels are lower in Pbx-1+/- mice after ACTH stimulation (184.4±51.5ng/ml vs. wild type 316.7±32.7ng/ml, p=0.06) and restrained stress experiments (375.0±36.3 vs. wild type 495.9±51.2ng/ml, p=0.14), while ACTH levels are significantly higher in Pbx-1+/- mice after restrained stress (1177.5±57.8pg/ml vs. wild type 797.3±112.8pg/ml, p=0.02). Taken together, these data are in line with a compensated primary adrenal insufficiency in Pbx-1 haploinsufficient mice. Thus, the overall phenotype in Pbx-1+/- mice is reminiscent to that of SF-1+/- animals, indicating that Pbx-1 and SF-1 might interact on a molecular level. Interestingly, SF-1 RNA and protein levels in Pbx-1+/- adrenals are not significantly different from those of wild type controls, and Pbx-1-transfection of Y1 cells does not alter SF-1 expression levels. In contrast, SF-1 overexpression in Y1 cells leads to Pbx-1 transcription. Hence, SF-1 directly affects Pbx-1 promoter activity. Ongoing experiments aim at the other possible intersections between Pbx-1 and SF-1.