Transforming growth factor-beta induces cellular injury in experimental diabetic neuropathy

The mechanism/s leading to diabetic neuropathy are complex. Transforming growth factor-beta1 (TGF-beta1) has been associated with diabetic nephropathy and retinopathy but not neuropathy. In this study, changes in TGF-beta isoforms were examined in vivo and in vitro. Two groups of animals, streptozotocin diabetic with neuropathy and non-diabetic controls were examined at 4 weeks (n=10/group) and 12 weeks (n=8/group). In diabetic DRG using quantitative real-time PCR (QRT-PCR), TGF-beta1 and TGF-beta2 mRNA, but not TGF-beta3, was increased at 4 and 12 weeks. In sciatic nerve TGF-beta3 mRNA was primarily increased. Immunohistochemistry (DRG) and immunoblotting (sciatic nerve) showed similar differential protein expression. In sciatic nerve TGF-beta formed homo- and hetero-dimers, of which beta(2)/beta(3), beta(1)/beta(1), and beta(1)/beta(3) were significantly increased, while that of the TGF-beta(2)/beta(2) homodimer was decreased, in diabetic compared to non-diabetic rats. In vitro, pretreatment of embryonic DRG with TGF-beta neutralizing antibody prevents the increase in total TGF-beta protein observed with high glucose using immunoblotting. In high glucose conditions, combination with TGF-beta2>beta1 increases the percent of cleaved caspase-3 compared to high glucose alone and TGF-beta neutralizing antibody inhibits this increase. Furthermore, consistent with the findings in diabetic DRG and nerve, TGF-beta isoforms applied directly in vitro reduce neurite outgrowth, and this effect is partially reversed by TGF-beta neutralizing antibody. These findings implicate upregulation of TGF-beta in experimental diabetic peripheral neuropathy and indicate a novel mechanism of cellular injury related to elevated glucose levels. In combination, these findings indicate a potential new target for treatment of diabetic peripheral neuropathy.