Lysophosphatidic Acid‐Induced Neurite Retraction in PC12 Cells: Neurite‐Protective Effects of Cyclic AMP Signaling

Abstract

Effects of the cyclic AMP second messenger system were studied on the retraction of neurites elicited by the phospholipid mediator lysophosphatidic acid (LPA) in PC12 cells. LPA stimulation inhibited adenylyl cyclase, indicating that the LPA receptor couples to the heterotrimeric G, proteins. However, pertussis toxin or expression of dominant negative Ras did not prevent neurite retraction. In contrast, cholera toxin, forskolin, and application of dibutyryl-cyclic AMP prevented neurite retraction. The neurite-protective effect of forskolin was blocked by Rp-adenosine 3',5'-phosphorothioate. Forskolin and dibutyryl-cyclic AMP both failed to protect neurites in Al 26-1 B2 and 123.7 cells, which lack cyclic AMP-activated protein kinase. Data indicate that elevation of cyclic AMP levels triggers a cyclic AMP-activated protein kinase-dependent mechanism that opposes the functioning of the morphoregulatory signaling activated by LPA. ADP-ribosylation of Rho by the Clostridium botulinum C-3 toxin in 123.7 cells caused neuronal differentiation, indicated by neurite extension, and blocked LPAinduced neurite retraction. LPA activates G 0-and G~linked signaling in parallel; therefore, a morphoregulatory signaling network hypothesis is proposed versus the simplistic approach of a signaling pathway. The signaling network integrates the receptor-activated individual, sequential, and parallel signaling events into an interactive network whose individual components may fulfill required and permissive functions encoding the cellular response. Key Words: Lipid mediator-Signal transduction -Adenylyl cyclase-Forskol in -Receptor-operated Ca2+ influx-Rp -Adenosine 3 ',S' -phosphorothioate-Cyclic AM P-dependent protein kinase-C-3 ADP-ribosyltransferase-Rho.

Dual Regulation of Actin Rearrangement through Lysophosphatidic Acid Receptor in Neuroblast Cell Lines: Actin Depolymerization by Ca2+-α-Actinin and Polymerization by Rho

“…LPA-induced actin polymerization results in process retraction and cell rounding in neuronal cells such as N1E-115, or stress fiber formation in fibroblast cells (Ridley and Hall, 1992;Jalink et al, 1993). These cellular responses are mediated by actomyosin interactions through activation of the Rho pathway, produc- ing contractile forces that induce cell shape changes (Jalink et al, 1994;Tigyi et al, 1996;Kozma et al, 1997;Hirose et al, 1998;Kranenburg et al, 1999;Fukushima et al, 2000;Weiner et al, 2001). Herein, we have also shown that Rho is activated by LPA in TR cells and LPA-induced process retraction is blocked by a ROCK inhibitor or CD, which indicates the possible involvement of Rho and actomyosin for process retraction in TR cells (Figure 10).…”

Mitogenic signaling by lysophosphatidic acid (LPA) involves Gα12

“…LPA stimulation of Ga 12 V Radhika et al stimulate Ga 13 or any other a-subunits in a different cellular or physiological context. Results from different laboratories, including ours, indicate that the stimulation of a specific Ga-subunit by LPA is dependent on the cell-type or as well as the physiological context, presumably determined by the type of LPAR expressed (Tigyi et al, 1996). While LPA readily activates Ga 12 in NIH3T3 cells, it has little or no effect on Ga 13 .…”

“…A role for pertussis toxin (PTX)-sensitive Ga i has been recently proposed in LPA-stimulated cell proliferation (Tigyi et al, 1996). However, the pretreatment of Ga 12 WT-NIH3T3 or pcDNA3-NIH3T3 cells with 15 U of PTX failed to inhibit LPA-stimulated proliferation of these cells (number of cells in Figure 2 LPA stimulates Ga 12 in NIH3T3 cells.…”

“…However, the identity of the asubunit that transduces mitogenic signals from LPARs has remained unresolved. While LPA-mediated proliferation has been shown to be associated with G i -family of G proteins in neuronal cells (Tigyi et al, 1996), the weak mitogenic-activity of Ga i (Radhika and Dhanasekaran, 2001) casts uncertainties on the role of Ga i as the mediator of LPA-mediated mitogenic responses. In this context, the strong mitogenic activity and the ability to couple to LPARs points to Ga 12 as a potential candidate for transducing cell proliferation signals from LPAR to intracellular effectors.…”

“…Previous studies have shown that LPA 1 -receptor specifically couples to PTX-dependent signaling pathway (Tigyi et al, 1996). Since PTX-treatment failed to have any effect on LPA-stimulated proliferation of Ga 12 WT-NIH3T3 cells (number of cells in Ga 12 WT þ LPA ¼ 16.070.5 Â 10 5 compared to 13.671.1 Â 10 5 in Ga 12 WT þ LPA þ PTX group), it is more likely that the LPAR coupled to Ga 12 in stimulating mitogenic response is LPA 2 -receptor.…”