Requirement for Rho-mediated Myosin Light Chain Phosphorylation in Thrombin-stimulated Cell Rounding and Its Dissociation from Mitogenesis

Abstract

Thrombin treatment causes a dose-dependent rounding of 1321N1 astrocytoma cells. This cytoskeletal response is rapid, peaking 2 h after thrombin stimulation, and reverses by 50% after 24 h. The thrombin receptor peptide SFLLRNP also induces cell rounding, whereas other G protein-linked receptor agonists such as carbachol, lysophosphatidic acid, or bradykinin fail to do so. Results of studies using pharmacological inhibitors do not support a requirement for phosphatidylinositol 3-kinase, mitogen-activated protein kinase, or Ca 2؉ mobilization in this response. Inhibition of protein kinase C or tyrosine kinase produces minimal blockade. Pertussis toxin treatment is also without effect. However, thrombin-induced rounding is fully blocked by the C3 toxin from Clostridium botulinum, which specifically ADP-ribosylates and inactivates the small G protein Rho. Thrombin also leads to a rapid, 2.4-fold increase in 32 P incorporation into myosin light chain while carbachol does not. Myosin phosphorylation, like cell rounding is inhibited by inactivation of Rho with C3 exoenzyme, suggesting that myosin phosphorylation is necessary for this cytoskeletal response. This is supported by the observation that thrombin-induced rounding is also blocked by the myosin light chain kinase inhibitor KT5926. However, treatment with KT5926 fails to inhibit mitogenesis. Thus, cell rounding is not prerequisite to thrombin-induced DNA synthesis. We conclude that stimulation of the heterotrimeric G protein-coupled thrombin receptor in 1321N1 cells activates Rhodependent pathways for both DNA synthesis and cell rounding, the cytoskeletal response being mediated in part through increases in myosin phosphorylation.Thrombin is a protease known to act through a specific seven-transmembrane spanning G protein-coupled receptor. Thrombin activates its receptor through a proteolytic cleavage mechanism which unmasks a new amino terminus that functions as a tethered ligand to activate receptor signaling (1-4). A synthetic peptide analogous to the first five or six NH 2 -terminal amino acids of the receptor elicits responses similar to thrombin in many cell types (4 -8). The best characterized physiological responses to thrombin are its effects on platelet function and DNA synthesis (8 -12). Thrombin can also elicit cytoskeletal responses, and has been shown to induce cell rounding in neurons and astrocytes (5,13,14). Previous work investigating the mechanism by which thrombin changes cell morphology has shown the process to be activated by the thrombin receptor peptide and thus to be mediated through a cell surface thrombin receptor rather than by other non-receptor-mediated proteolytic mechanisms (5, 14, 15).In 1321N1 astrocytoma cells, thrombin receptor activation stimulates phospholipase C through G q , increasing intracellular Ca 2ϩ , and activating protein kinase C (PKC) 1 (10, 16 -18). The muscarinic receptor in 1321N1 cells is also coupled to activation of phospholipase C through Gq, and carbachol elicits changes in phospholipid metabolism, Ca 2ϩ...

A Rho Exchange Factor Mediates Thrombin and Gα12-induced Cytoskeletal Responses

“…Microinjection of C3 exoenzyme also inhibited thrombin-induced rounding (Fig. 4), confirming the inhibitory effect of extracellularly applied C3 exoenzyme on the response to thrombin in these cells (9). Cell rounding induced by activated Rho was also inhibited by C3 exoenzyme injection (Fig.…”

“…Microinjection of constitutively activated G␣ 12 or G␣ 13 expression plasmids induced a loss of stellation and produced a round morphology in 40 -50% of the injected cells. The morphology induced by microinjection of activated G␣ 12 QL or G␣ 13 QL was identical to that seen with thrombin stimulation of astrocytoma cells (9). Microinjection of the same concentration of constitutively activated G␣ q cDNA resulted in a significantly lower level of cell rounding (ϳ20% of injected cells) than that seen with activated G␣ 12 or G␣ 13 .…”

“…Notably, GPCR agonists that activate phospholipase C through G q are not equally efficacious for inducing cytoskeletal changes. For example, thrombin but not carbachol causes cell rounding in astrocytoma cells (9), and LPA but not bradykinin causes process retraction in PC-12 cells (6). Several lines of evidence support the conclusion that thrombin-induced cell rounding is independent of phospholipase C activation, Ca 2ϩ mobilization, or pertussis toxin-sensitive G protein activation (9,10).…”

“…Our laboratory has also demonstrated a requirement for Rho in thrombin-stimulated rounding and retraction of processes in 1321N1 astrocytoma cells (9). Similar cytoskeletal rearrangements are believed to be crucial in growth cone guidance and cell migration and may be involved in development and plasticity of neuronal and glial cells.…”

“…For example, thrombin but not carbachol causes cell rounding in astrocytoma cells (9), and LPA but not bradykinin causes process retraction in PC-12 cells (6). Several lines of evidence support the conclusion that thrombin-induced cell rounding is independent of phospholipase C activation, Ca 2ϩ mobilization, or pertussis toxin-sensitive G protein activation (9,10). Thus, heterotrimeric G proteins other than those of the G␣ q or G␣ i/o family are likely to be responsible for linking activation of selected GPCRs to Rho-mediated actin rearrangements.…”