We investigated the effects of glucose concentration in serum-free media on the proliferative growth response of a cultured aiurine mesangial cell line. Raising the ambient D-glucose concentration from 100 mg/dl to 450 mg/dl stimulated cell proliferation after 24 to 48 hours but had a growth inhibitory effect after 72 to 96 hours of incubation. This biphasic proliferative response to high glucose concentration was not mediated by the elevated osmolarity of the medium and did not occur when L-glucose was used. The early phase of glucose-induced proliferation was associated with increased expression of the immediate early genes c-myc and egr-1 as well as with induction of the S-phase related proliferating nuclear cell antigen (PCNA). Several lines of evidence indicated that the late phase of glucose-induced growth inhibition was mediated by the bioactivation of endogenous transforming growth factor beta (TGF-β). Neutralizing antibody against TGF-β prevented the late inhibitory effects of glucose on proliferation. On the other hand, exogenous TGF-β (1 ng/ml) significantly inhibited basal proliferation in mesangial cells. Furthermore, Northern blot analysis revealed that TGF-β1 mRNA was induced by 450 mg/dl glucose in the medium after 48 to 72 hours, but not after 24 hours. Cell cycle analysis demonstrated that mesangial cells incubated in high glucose for 24 hours have a higher percentage of cells in the S-G2 phase of the cell cycle compared with cells grown in normal glucose concentration. After 48 hours of culture in elevated glucose concentration, the percentage of cells in S-G2 phase was decreased, and became comparable to that of cells in normal glucose concentration. However, the addition of neutralizing anti-TGF-β antibody stimulated the progression of cells towards S-G2 in high glucose medium after 48 hours. The findings of this study demonstrate a biphasic growth response of mesangial cells when they were cultured in high glucose concentration; initially there was a transient stimulation of replication for 24 to 48 hours followed by a sustained inhibition after longer incubation periods. This inhibition may be mediated by the glucose-induced synthesis and/or bioactivation of TGF-β which can inhibit proliferation of mesangial cells in an autocrine fashion.
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