ELEVATED GLUCOSE STIMULATES TGF-BETA GENE-EXPRESSION AND BIOACTIVITY IN PROXIMAL TUBULE

Our previous studies have demonstrated that raising ambient glucose from 100 to 450 mg/dl significantly inhibited the proliferation of mouse renal proximal tubule cells in culture. This effect was demonstrated after a latent period of 24 to 48 hours. Because transforming growth factor-beta (TGF-beta) inhibits cell proliferation in most epithelial cell lines, we hypothesized that the inhibitory effect of high glucose levels on cell proliferation may be mediated by TGF-beta. The present studies were performed to test the hypothesis that TGF-beta is an autocrine cytokine whose activity can be modulated by ambient glucose. Exogenous TGF-beta inhibited [H-3]-thymidine incorporation in a dose-dependent fashion and with high affinity (picomolar range), but with slightly lower potency in high versus normal glucose media. Northern analysis of mRNA demonstrated that proximal tubule cells constitutively express TGF-beta-1 transcripts, and that the steady state level of TGF-beta-1 mRNA was, on average, 63% higher in the cells grown for 48 hours in high versus normal glucose media. Furthermore, the conditioned media of cells exposed to 450 mg/dl glucose exhibited endogenous TGF-beta bioactivity as measured by inhibition of cell proliferation. The addition of a rabbit antiporcine TGF-beta neutralizing antibody significantly increased basal thymidine incorporation in high glucose media to levels approaching those of cells grown in normal glucose media. In contrast, the anti-TGF-beta antibody did not have a significant effect on the growth of cells in the normal glucose media. Control rabbit IgG did not stimulate growth. We conclude that TGF-beta functions as an autocrine cytokine in proximal tubule cells and that expression of TGF-beta is stimulated by high glucose concentration. The growth inhibitory effect of elevated glucose levels may be largely mediated by increased endogenous TGF-beta production and/or activity.