Lysophosphatidic acid-mediated signal-transduction pathways involved in the induction of the early-response genes prostaglandin G/H synthase-2 and Egr-1: a critical role for the mitogen-activated protein kinase p38 and for Rho proteins

Abstract

During inflammatory processes of the kidney, lesions of the glomerulus lead to aggregation of thrombocytes and infiltration of macrophages, which can release bioactive mediators. One of these important signalling molecules is lysophosphatidic acid (LPA). Incubation of rat mesangial cells with LPA induced mRNA and protein expression of the early-response genes pghs-2 (for prostaglandin G/H synthase-2/cyclo-oxygenase-2) and egr-1. As shown by antisense experiments, induction of egr-1 was related to the strong mitogenic effect of LPA. LPA-mediated gene expression was inhibited by pertussis toxin, indicating coupling to G-proteins of the Gi family. Specific inhibition of proteins of the small G-protein subfamily Rho with toxin B from Clostridium difficile led to changes in mesangial cell morphology without induction of apoptosis. LPA-mediated expression of pghs-2 and egr-1 was reduced to base-line levels by toxin B, indicating a role for Rho proteins in LPA-mediated gene induction. Of the two mitogen-activated protein kinase (MAPK) pathways investigated, the MAPK kinase-extracellular signal-regulated kinase pathway was involved in the induction of both pghs-2 and egr-1 mRNA expression, as shown by the inhibitory effect of PD98059. Activation of the MAPK p38, however, was only related to pghs-2 expression, whereas egr-1 expression was not affected by treatment of mesangial cells with the specific inhibitor SB203580. Taken together our data provide evidence that LPA-mediated activation of MAPK kinase and Rho proteins leads to the induction of the functionally distinct early-response genes pghs-2 and egr-1, whereas activation of MAPK p38 revealed considerable differences between the regulation of these two genes.

The platelet-derived-growth-factor receptor, not the epidermal-growth-factor receptor, is used by lysophosphatidic acid to activate p42/44 mitogen-activated protein kinase and to induce prostaglandin G/H synthase-2 in mesangial cells

“…We have shown previously that LPA-mediated PGHS-2 induction is PTX-sensitive, indicating involvement of a G-protein of the G i family [5]. In line with these results, activation of p42\44 MAP kinases by LPA was abrogated by pretreatment with PTX, whereas activation by PDGF or 5-HT was not affected ( Figure 7A).…”

“…Induction of PGHS-2 by LPA was inhibited by the MAP kinase kinase inhibitor PD98059 [5]. The concentration-dependence was comparable with that observed when the cells were stimulated with PDGF ( Figure 1A).…”

“…Further downstream, common signalling modules, such as activation of p42\44 mitogen-activated protein (MAP) kinase, were part of both pathways [5][6][7]. Induction of PGHS-2 in mesangial cells was not restricted to G-protein-coupled signalling.…”

“…Previous studies showed activation of p38 MAP kinase to be part of the LPA-mediated signal-transduction pathway leading to PGHS-2 expression [5]. Preincubation of mesangial cells with SB203580, a rather specific inhibitor of p38 MAP kinase, also decreased 5-HT-mediated PGSH-2 induction, indicating an involvement of p38 MAP kinase in 5-HT signalling ( Figure 5A ; 58.9p2.7 % inhibition by 5 µM SB203580, meanpS.D., n l 3).…”

“…In mesangial cells, we and others have previously characterized induction of PGHS-2 by activation of heptahelical receptors coupled to heterotrimeric G-proteins. Thromboxane and 5-hydroxytryptamine (5-HT ; ' serotonin ') [3], endothelin [4] or lysophosphatidic acid (LPA) [5] rapidly induced a transient up-regulation of PGHS-2 mRNA and protein, whereas ATP, another activator of G-protein-coupled receptors, was not able to induce PGHS-2 expression [3]. Induction by LPA and 5-HT was mediated by different types of G-proteins, pertussis-toxin (PTX)-sensitive and -insensitive respectively.…”