Background. Serum leptin levels correlate with fat cell mass and are elevat
ed in patients with massive obesity and type 2 diabetes mellitus, which are
strong risk factors for the development of glomerulosclerosis. We have pre
viously shown in cultured glomerular endothelial cells that leptin stimulat
es cellular proliferation and expression of the prosclerotic cytokine trans
forming growth factor-beta1 (TGF-beta1). Although the effect of leptin on t
he hypothalamus to regulate energy homeostasis is well known, the effect of
leptin on the kidney, and specifically on the glomerular mesangial cell, i
Methods. The obese, diabetic db/db mouse, which lacks the functional full-l
ength Ob-Rb leptin receptor, is a suitable model to assess the effects of h
yperleptinemia on peripheral tissues that express other receptor isoforms.
The effects of leptin on glucose uptake, the TGF-beta system, and type I co
llagen production were evaluated in db/db mouse mesangial cells in culture.
A phosphatidylinositol-3 kinase (PI-3K) inhibitor was used to assess the r
ole of PI-3K in mediating the effects of leptin.
Results. A short form of the leptin receptor (Ob-Ra), but not Ob-Rb, was pr
esent 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 depe
ndently and stimulated gene expression of TGF-P type II receptor (T beta RI
I) and alpha1(I) collagen, but not TGF-beta1. 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 supp
ressed by a PI-SK inhibitor, LY294002. Mesangial cells pretreated with lept
in exhibited increased responsiveness to exogenous TGF-beta1, as evidenced
by a greater production of type I collagen protein in leptin-pretreated cel
ls exposed to low-dose TGF-beta1 (0.5 ng/mL). The addition of both TGF-beta
1 (2 ng/mL) and leptin (100 ng/mL) increased type I collagen production mor
e than addition of either TGF-beta1 or leptin alone.
Conclusions. Leptin increases glucose uptake and type I collagen in db/db m
esangial cells through a PI-3K-dependent pathway. We postulate that increas
ed leptin levels may transmit a signal through the short-form leptin recept
or to up-regulate T beta RII and activate the intraglomerular TGF-beta syst
em, which may contribute to the glomerulosclerosis of obesity or type 2 dia