Leptin stimulates type I collagen production in db/db mesangial cells: Glucose uptake and TGF-β type II receptor expression

Leptin stimulates type I collagen production in db/db mesangial cells: Glucose uptake and TGF-β type II receptor expression. Serum leptin levels correlate with fat cell mass and are elevated in patients with massive obesity and type 2 diabetes mellitus, which are strong risk factors for the development of glomerulosclerosis. We have previously shown in cultured glomerular endothelial cells that leptin stimulates cellular proliferation and expression of the prosclerotic cytokine transforming growth factor-β1 (TGF-β1). Although the effect of leptin on the hypothalamus to regulate energy homeostasis is well known, the effect of leptin on the kidney, and specifically on the glomerular mesangial cell, is unclear. The obese, diabetic db/db mouse, which lacks the functional full-length Ob-Rb leptin receptor, is a suitable model to assess the effects of hyperleptinemia on peripheral tissues that express other receptor isoforms. The effects of leptin on glucose uptake, the TGF-β system, and type I collagen production were evaluated in db/db mouse mesangial cells in culture. A phosphatidylinositol-3 kinase (PI-3K) inhibitor was used to assess the role of PI-3K in mediating the effects of leptin. A short form of the leptin receptor (Ob-Ra), but not Ob-Rb, was present by reverse transcription-polymerase chain reaction in the kidney and mesangial cells of both nondiabetic db/m and diabetic db/db mice. In db/db mesangial cells, leptin increased 2-deoxy-D-glucose (2DOG) uptake dose dependently and stimulated gene expression of TGF-β type II receptor (TβRII) and α1(I) collagen, but not TGF-β1. Protein production of type I collagen (enzyme-linked immunosorbent assay) was also increased by leptin. Both leptin-stimulated 2DOG uptake and type I collagen production were suppressed by a PI-3K inhibitor, LY294002. Mesangial cells pretreated with leptin exhibited increased responsiveness to exogenous TGF-β1, as evidenced by a greater production of type I collagen protein in leptin-pretreated cells exposed to low-dose TGF-β1 (0.5 ng/mL). The addition of both TGF-β1 (2 ng/mL) and leptin (100 ng/mL) increased type I collagen production more than addition of either TGF-β1 or leptin alone. Leptin increases glucose uptake and type I collagen in db/db mesangial cells through a PI-3K–dependent pathway. We postulate that increased leptin levels may transmit a signal through the short-form leptin receptor to up-regulate TβRII and activate the intraglomerular TGF-β system, which may contrib