Hyperglycemia induces NF-κB activation and MCP-1 expression via downregulating GLP-1R expression in rat mesangial cells: inhibition by metformin

Hyperglycemia impairs glucagon-like peptide-1 receptor (GLP-1R) signaling in multiple cell types and thereby potentially attenuates the therapeutic effects of GLP-1R agonists. We hypothesized that the downregulation of GLP-1R by hyperglycemia might reduce the renal-protective effects of GLP-1R agonists in diabetic nephropathy (DN). In this study, we examined the effects of high glucose on the expression of GLP-1R and its signaling pathways in the HBZY-1 rat mesangial cell line. We found that high glucose reduced GLP-1R messenger RNA (mRNA) levels in HBZY-1 cells and in the renal cortex in db/db mice comparing with control groups. In consistence, GLP-1R agonist exendin-4 induced CREB phosphorylation was attenuated by high glucose but not low glucose treatment, which is paralleled with abrogated anti-inflammatory functions in HBZY-1 cells linked with nuclear factor-kappa B (NF-kappa B) activation. In consistence, GLP-1R inhibition aggravated the high glucose-induced activation of NF-kappa B and MCP-1 protein levels in cultured HBZY-1 cells while overexpression of GLP-1R opposite effects. We further proved that metformin restored high glucose-inhibited GLP-1R mRNA expression and decreased high glucose evoked inflammation in HBZY-1 cells. On the basis of these findings, we conclude that high glucose lowers GLP-1R expression and leads to inflammatory responses in mesangial cells, which can be reversed by metformin. These data support the rationale of combinative therapy of metformin with GLP-1R agonists in DN.