Parathyroid hormone-related protein induces hypertrophy in podocytes via TGF- 1 and p27Kip1: implications for diabetic nephropathy

Background. Hypertrophy of podocytes is characteristic in diabetic nephropathy (DN). Previously, we observed the upregulation of parathyroid hormone-related protein (PTHrP) and its receptor PTH1R, in experimental DN, associated with renal hypertrophy. Herein, we test the hypothesis that PTHrP participates in the mechanism of high glucose (HG)-induced podocyte hypertrophy. Methods. On mouse podocytes, hypertrophy was assessed by protein content/cell and [H3]leucine incorporation. Podocytes were stimulated with HG (25 mM), PTHrP(1–36) (100 nM), angiotensin II (AngII) (100 nM) or TGF-β1 (5 ng/mL) in the presence or absence of PTHrP-neutralizing antibodies (α-PTHrP), the PTH1R antagonist JB4250 (10 μM), PTHrP silencer RNA (siRNA) or TGF-β1 siRNA. Protein expression was analysed by western blot and immunohistochemistry. Results. HG-induced hypertrophy was abolished in the presence of either α-PTHrP or PTHrP siRNA. This effect was associated with an inhibition of the upregulation of TGF-β1 and p27Kip1. JB4250 also inhibited HG-induced p27Kip1 upregulation. Interestingly, whilst HG and AngII were unable to stimulate the expression of p27Kip1 on PTHrP siRNA-transfected podocytes, TGF-β1 was still able to upregulate p27Kip1 in these cells. Moreover, HG and PTHrP-induced hypertrophy as well as p27Kip1 upregulation were abolished on TGF-β1 siRNA-transfected podocytes. Furthermore, the glomeruli of transgenic PTHrP-overexpressing mice showed a constitutive overexpression of TGF-β1 and p27Kip1 to a degree similar to that of diabetic animals. Conclusions. PTHrP seems to participate in the hypertrophic signalling triggered by HG. In this condition, AngII induces the upregulation of PTHrP, which might induce the expression of TGF-β1 and p27Kip1. These findings provide new insights into the protective effects of AngII antagonists in DN, opening new paths for intervention.