1,25-dihydroxyvitamin D3 stimulates transforming growth factor-beta1 synthesis by mouse renal proximal tubular cells

1,25-dihydroxyvitamin D3 [1,25-(OH)2 D3] is a secosteroid hormone with effects on cell growth, differentiation and immunoregulatory functions in a number of tissues not primarily involved in mineral metabolism. We recently demonstrated growth-regulating effects of 1, 25-(OH)2 D3 on human mesangial cells and proximal tubular cells. To investigate whether 1,25-(OH)2 D3 might also affect the synthesis of cytokines and growth factors in proximal tubular cells, we assessed in the present study the expression and secretion of transforming growth factor-beta1 (TGF-beta1) in a mouse proximal tubular cell line (MCT) in vitro. TGF-beta1 synthesis was measured by a monospecific ELISA in culture supernatant. The secreted TGF-beta1 was proven to be biologically active by means of a bioassay system (CCL-64 mink lung epithelial cell proliferation assay). TGF-beta1 gene expression was assessed by RT-PCR. To analyze whether TGF-beta1 expression mediates the 1,25-(OH)2 D3-induced antiproliferative actions in MCT, proliferation studies in the absence or presence of a blocking monoclonal anti TGF-beta1-3 antibody were performed. 1, 25-(OH)2 D3 (10(-11) to 10(-7) M) specifically increased the TGF-beta1 protein secretion in MCT with a maximum at 10(-8) M. No detectable effect was found with 25 D3 at 10 times higher concentrations. A synthetic 20-epi analogue, MC 1288, increased TGF-beta1 secretion up to similar amounts at equimolar concentrations as the natural hormone 1,25-(OH)2 D3. Steady-state TGF-beta1 mRNA concentration in MCT was transiently increased by 1, 25-(OH)2 D3 between 12 and 24 h, returning to control values at 48 h. Blocking TGF-beta1 did not reduce or abrogate the antiproliferative effect of 1,25-(OH)2 D3. In conclusion, 1,25-(OH)2 D3 stimulates TGF-beta1 expression in renal proximal tubular cells, a growth factor with anti-inflammatory and profibrotic actions which plays an important role in the development and progression of nephrosclerosis.