Interaction between growth factors and retinoic acid in the induction of kidney tubulogenesis in tissue culture

Abstract

Kidney tubulogenesis is the initial step in renal organogenesis. The precise molecular determinants of this pattern formation are presently unknown, although soluble factors, such as growth factors, and insoluble factors, such as extracellular matrix molecules, most likely play fundamental roles in this process. To define the molecular determinants of renal proximal tubule morphogenesis, primary cultures of rabbit renal proximal tubule cells in hormonally defined, serumfree media were treated with transforming growth factor-β₁ (TGF-β₁), epidermal growth factor (EGF), and the retinoid, all trans-retinoic acid (RA), singly or in combination. Utilizing phase contrast and light and transmission electron microscopy, the simultaneous administration of TGF-β₁ (10 ng/ml), EGF (1 nM), and RA (0.1 nM) transformed a confluent monolayer of renal proximal tubule cells within 5 to 6 days into three-dimensional cell aggregates containing lumens within the interior of the cell clusters. The lumens were bordered by tubule cells possessing a polarized epithelial cell phenotype with extensive microvilli formation and tight junctional complexes along the luminal border. All three factors were necessary and sufficient to induce this phenotypic transformation. Further studies demonstrated that RA promoted the deposition of the A and B₁ chains of laminin, a cell attachment protein of the basement membrane, in a small subset of proximal tubule cells in culture, as deduced by indirect immunofluorescent microscopy. Additional studies demonstrated that soluble purified laminin fully substituted for RA in this system to promote renal tubulogenesis when combined with TGF-β₁ and EGF. These results demonstrate that the growth factors, TGF-β₁ and EGF, and the retinoid, RA, promote tubulogenesis in adult renal proximal tubule cells in tissue culture in a manner reminiscent of inductive embryonic kidney morphogenesis. These observations define a coordinated interplay between growth factors and retinoids to induce pattern formation and morphogenesis. Furthermore, the demonstration of RA-induced laminin deposition as a critical event in this morphogenic process identifies laminin as a possible target protein for RA to act as a morphogen.