1,25-(OH2)D3 Alters the Transforming Growth Factor β Signaling Pathway in Renal Tissue

Abstract

Background. 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) plays an important role in regulating immune responses, in addition to its effects on bone metabolism. The cytokine transforming growth factor β (TGF-β) regulates diverse biological processes, including cellular proliferation and differentiation, immune modulation, and modulation of extracellular matrix deposition. 1,25-(OH)2D3 interacts in vitro with Smad proteins, important regulators of TGF-β signal transduction. We hypothesized that exogenous 1,25-(OH)2D3 would alter levels of TGF-β1 and TGF-β1 signaling proteins in renal tissue.Methods. C57BL6 mice and Lewis rats were placed on diets with or without 1,25-(OH)2D3 for 14 days. Renal lysates were examined for TGF-β1, vitamin D receptor (VDR), and Smad3 protein levels using a cell proliferation assay and Western blot analysis. Coimmunoprecipitation was used to determine if any interaction between VDR and Smad3 proteins occurs in vivo. Reverse transcription–polymerase chain reaction (RT-PCR) was used to assess messenger RNA (mRNA) levels for all of these molecules.Results. Vitamin D supplementation decreased VDR and Smad3 protein levels. Coimmunoprecipitation of VDR and Smad3 revealed a Smad3–VDR interaction in vivo. Vitamin D–treated rats had a significant (P = 0.001) reduction in bioactive renal TGF-β1. RT-PCR demonstrated no difference in mRNA expression for either VDR or TGF-β1.Conclusion. Our results suggest that vitamin D has a significant effect in regulating levels of bioactive TGF-β1 and appears to affect aspects of the TGF-β1 signaling system. These effects, in combination with the immunomodulatory actions of vitamin D, may alter the evolution of chronic rejection in renal transplants.