Background: Because accumulation of extracellular matrix is a prominent characteristic of the microangiopathy that complicates long-term diabetes, a pathogenetic role for transforming growth factor beta (TGF-beta) is being considered. Having observed that glucose levels mimicking diabetic hyperglycemia induce in vitro endothelial cell overexpression of extracellular matrix molecules, decreased replication, and increased levels of TGF-beta mRNA, we have examined whether the effects of high glucose are mediated by autocrine TGF-beta.
Experimental design: TGF-beta levels were measured by bioassay in the media conditioned by human umbilical vein endothelial cells cultured in the presence of high (30 mM) or normal (5 mM) glucose concentrations. The effect of high glucose was tested on the proliferation of two epithelial cell lines, one (Mv1Lu) exquisitely sensitive to TGF-beta and the other (DR mutants) insensitive to the cytokine. To examine whether high glucose and TGF-beta affect cellular programs in a similar manner, the effects of high glucose and exogenous TGF-beta were compared on proliferation and gene expression of endothelial cells.
Results: Media conditioned by endothelial cells cultured in high or normal glucose contained similar amounts of TGF-beta (4.9 +/- 3.5 and 3.7 +/- 2.5 ng/10(6) cells, respectively (mean +/- SD)), all in the latent form. The replication of parental Mv1Lu cells and their DR mutants was decreased by high glucose to the same extent. Whereas the inhibitory effect of high glucose on endothelial cell replication was reversible, that of TGF-beta was not. Both perturbations induced up-regulation of fibronectin expression, but the effects were additive. Only TGF-beta induced overexpression of Type IV collagenase.
Conclusions: These combined observations indicate that (a) endothelial cells exposed to high glucose do not secrete TGF-beta in excess of control cells, (b) there are growth-inhibitory effects of high glucose that are independent of TGF-beta, and (c) high glucose and TGF-beta exert their effects through distinct pathways and at different loci.