Intracellular pathways mediating Na+/H+ exchange activation by platelet-derived growth factor in rat hepatic stellate cells

Background & Aims: The Na+/H+ exchanger is the main intracellular pH regula tor in hepatic stellate cells (HSCs), and its activity is increased by plat elet-derived growth factor (PDGF), Amiloride, an Na+/H+ exchange inhibitor, reduces PDGF-induced HSC proliferation, suggesting that the Na+/H+ exchang er plays a role in regulating HSC proliferative response. The aim of this s tudy was to characterize the intracellular pathways mediating activation of the Na+/H+ exchanger by PDGF in HSCs, Methods: The activity of the Na+/Hexchanger acid HSC proliferation rate were evaluated under control conditio n and after incubation with PDGF in the absence or presence of specific inh ibitors of the main intracellular pathways of signal transduction. Na+/H+ e xchange protein expression was evaluated by means of Western blot, Results: PDGF induced a significant increase in the activity of the Na+/H+ exchange r without modifying protein expression. Inhibition of the calcium/calmoduli n- and protein kinase C-dependent pathways resulted in a significant inhibi tion of both Na+/H+ exchange activity and of PDGF-induced HSC proliferation . The involvement of the two pathways was confirmed by showing that incubat ion of HSCs with both phorbol-12-myristate-13-acetate, a potent protein kin ase C activator, and thapsigargin, which increases intracellular calcium le vels, significantly increased both the Na+/H+ exchanger activity and HSC pr oliferation rate. Inhibition of the protein kinase A pathway did not modify either PDGF-induced Na+/H+ exchange activation or PDGF-induced HSC prolife ration. On the contrary, inhibition of the mitogen-activated protein kinase - and of phosphatidylinositol 3-kinase-dependent pathways significantly red uced PDGF-induced HSC proliferation without affecting the activity of the N a+/H+ exchanger. Conclusions: Activation of the Na+/H+ exchanger by PDGF in HSCs is mediated by calcium/calmodulin- and protein kinase C-dependent pat hways. PDGF-induced HSC proliferation is mediated by Na+/H+ exchange-depend ent and -independent pathways.