Activation of protease-activated receptor 4 (PAR-4) in tubular epithelial cells contributes to diabetic nephropathy

Abstract

BACKGROUND: Protease-Activated Receptors (PARs) have been implicated in the pathogenesis of many inflammation-associated disorders, however their roles in diabetic nephropathy (DN) have not yet been explored. Further to our findings on the up-regulation of PAR-4 expression in human renal proximal tubular epithelial cells (PTEC) in response to high glucose stimulation, herein we investigated the pro-inflammatory potential of this receptor in tubular cells and examined its expression in renal tissues of DN. METHODS: Renal biopsies from patients with proven DN were immunohistochemically examined for the expression of PAR-4. To elucidate the role of PAR-4, human PTEC were cultured with 1) selective PAR-4 activating peptide or 2) high glucose medium in the presence of PAR-4 antagonist, and the effects on the expression of pro-inflammatory genes were studied. RESULTS: High PAR-4 expression was detected in tubular cells of all DN biopsies whereas very low expression was found in normal control subjects. In vitro, PAR-4 activating peptide induced chemokine (C-C motif) ligand 2 (CCL-2) expression and to a lesser extent, interleukin 6 (IL-6) expression in PTEC in a time- and dose-dependent manner. Inhibition of PAR-4 by a specific antagonist partially suppressed high glucose-induced mitogen-activated protein kinase (MAPK) p42/p44 signaling and the subsequent CCL-2 and IL-6 production. CONCLUSIONS: Our data demonstrated an increased expression of tubular PAR-4 in human DN biopsies and an enhanced secretion of PAR-4 stimulated inflammatory cytokines from PTEC. These findings suggest a novel role for PAR-4 in mediating diabetic tubular injury.