Transforming growth factor-beta 1 regulates the expression of Pax-2, a developmental control gene, in renal tubule cells.
The pattern-forming event of kidney tubulogenesis is initiated by the inductive transition of mesenchymal cells to epithelial phenotype; a transition that is critically dependent on the regulated expression of the developmental control gene, Pax-2. Because of a defined role in in vitro renal tubulogenesis, the effects of epidermal growth factor (EGF), transforming growth factor (TGF)-beta 1 and retinoic acid on Pax-2 gene expression in proximal tubule cells (PTC) were evaluated. Rabbit PTC were isolated and grown in tissue culture. Under confluent quiescent conditions, the effect of various factors on Pax-2 gene expression was assessed by Northern blot analysis. To assess whether the effect of TGF-beta 1 to alter Pax-2 gene expression was due to transcriptional or posttranscriptional events, nuclear run-on assays were also undertaken. Under control, confluent growth conditions, PTC expressed high levels of Pax-2. A 24-hour exposure to EGF (10 nM), a potent mitogen of PTC, increased this level of expression. In contrast, Pax-2 gene expression was suppressed by treating PTC with retinoic acid (10 mM), a well-described differentiating factor, and with TGF-beta 1 (10 ng/ml), a recognized antiproliferative agent for these cells, which suggests that Pax-2 has a role in renal cell proliferation. The mechanism of the effect of TGF-beta 1 on Pax-2 mRNA levels was further detailed. TGF-beta 1 did not affect Pax-2 transcription rates, as assessed by nuclear run-on assays; however, in a dose-dependent manner, it diminished the stability of Pax-2 mRNA. At a concentration of 10 ng/ml, TGF-beta 1 reduced Pax-2 mRNA stability from a control half-life of 120 min to a half-life of less than 60 min. This study demonstrates that various soluble inductive factors affect Pax-2 gene expression in renal tubule cells. Also, TGF-beta 1 downregulates Pax-2 gene expression through a posttranscriptional process, an acknowledged mechanism for modulating important growth regulatory gene products.