A novel role for Gq alpha in alpha-thrombin-mediated mitogenic signalling pathways

Gαi/o-dependent Ca(2+) mobilization and Gαq-dependent PKCα regulation of Ca(2+)-sensing receptor-mediated responses in N18TG2 neuroblastoma cells

A functional Ca(2+)-sensing receptor (CaS) is expressed endogenously in mouse N18TG2 neuroblastoma cells, and sequence analysis of the cDNA indicates high homology with both rat and human parathyroid CaS cDNAs. The CaS in N18TG2 cells appears as a single immunoreactive protein band at about 150 kDa on Western blots, consistent with native CaS from dorsal root ganglia. Both wild type (WT) and Gαq antisense knock-down (KD) cells responded to Ca(2+) and calindol, a positive allosteric modulator of the CaS, with a transient increase in intracellular Ca(2+) concentration ([Ca(2+)]i), which was larger in the Gαq KD cells. Stimulation with 1 mM extracellular Ca(2+) (Ca(2+)e) increased [Ca(2+)]i in N18TG2 Gαq KD compared to WT cells. Ca(2+) mobilization was dependent on pertussis toxin-sensitive Gαi/o proteins and reduced by 30 μM 2-amino-ethyldiphenyl borate and 50 μM nifedipine to the same plateau levels in both cell types. Membrane-associated PKCα and p-PKCα increased with increasing [Ca(2+)]e in WT cells, but decreased in Gαq KD cells. Treatment of cells with 1 μM Gӧ 6976, a Ca(2+)-specific PKC inhibitor reduced Ca(2+) mobilization and membrane-associated PKCα and p-PKCα in both cell types. The results indicate that the CaS-mediated increase in [Ca(2+)]i in N18TG2 cells is dependent on Gαi/o proteins via inositol-1,4,5-triphosphate (IP3) channels and store-operated Ca(2+) entry channels, whereas modulation of CaS responses involving PKCα phosphorylation and translocation to the plasma membrane occurs via a Gαq mechanism.

The rapid activation of N-Ras by alpha-thrombin in fibroblasts is mediated by the specific G-protein Galphai2-Gbeta1-Ggamma5 and occurs in lipid rafts

alpha-thrombin is a potent mitogen for fibroblasts and initiates a rapid signal transduction pathway leading to the activation of Ras and the stimulation of cell cycle progression. While the signaling events downstream of Ras have been studied in significant detail and appear well conserved across many species and cell types, the precise molecular events beginning with thrombin receptor activation and leading to the activation of Ras are not as well understood. In this study, we examined the immediate events in the rapid response to alpha-thrombin, in a single cell type, and found that an unexpected degree of specificity exists in the pathway linking alpha-thrombin to Ras activation. Specifically, although IIC9 cells express all three Ras isoforms, only N-Ras is rapidly activated by alpha-thrombin. Further, although several Galpha subunits associate with PAR1 and are released following stimulation, only Galpha(i2) couples to the rapid activation of Ras. Similarly, although IIC9 cells express many Gbeta and Ggamma subunits, only a subset associates with Galpha(i2), and of those, only a single Gbetagamma dimer, Gbeta(1)gamma(5), participates in the rapid activation of N-Ras. We then hypothesized that co-localization into membrane microdomains called lipid rafts, or caveolae, is at least partially responsible for this degree of specificity. Accordingly, we found that all components localize to lipid rafts and that disruption of caveolae abolishes the rapid activation of N-Ras by alpha-thrombin. We thus report the molecular elucidation of an extremely specific and rapid signal transduction pathway linking alpha-thrombin stimulation to the activation of Ras.

The extreme COOH terminus of the retinoblastoma tumor suppressor protein pRb is required for phosphorylation on Thr-373 and activation of E2F

The retinoblastoma protein pRb plays a pivotal role in G(1)- to S-phase cell cycle progression and is among the most frequently mutated gene products in human cancer. Although much focus has been placed on understanding how the A/B pocket and COOH-terminal domain of pRb cooperate to relieve transcriptional repression of E2F-responsive genes, comparatively little emphasis has been placed on the function of the NH(2)-terminal region of pRb and the interaction of the multiple domains of pRb in the full-length context. Using "reverse mutational analysis" of Rb(DeltaCDK) (a dominantly active repressive allele of Rb), we have previously shown that restoration of Thr-373 is sufficient to render Rb(DeltaCDK) sensitive to inactivation via cyclin-CDK phosphorylation. This suggests that the NH(2)-terminal region plays a more critical role in pRb regulation than previously thought. In the present study, we have expanded this analysis to include additional residues in the NH(2)-terminal region of pRb and further establish that the mechanism of pRb inactivation by Thr-373 phosphorylation is through the dissociation of E2F. Most surprisingly, we further have found that removal of the COOH-terminal domain of either RbDeltaCDK(+T373) or wild-type pRb yields a functional allele that cannot be inactivated by phosphorylation and is repressive of E2F activation and S-phase entry. Our data demonstrate a novel function for the NH(2)-terminal domain of pRb and the necessity for cooperation of multiple domains for proper pRb regulation.

Alpha-thrombin-mediated phosphatidylinositol 3-kinase activation through release of Gbetagamma dimers from Galphaq and Galphai2

Chinese hamster embryonic fibroblasts (IIC9 cells) express the Galpha subunits Galphas, Galphai2, Galphai3, Galphao, Galpha(q/11), and Galpha13. Consistent with reports in other cell types, alpha-thrombin stimulates a subset of the expressed G proteins in IIC9 cells, namely Gi2, G13, and Gq as measured by an in vitro membrane [35S]guanosine 5'-O-(3-thio)triphosphate binding assay. Using specific Galpha peptides, which block coupling of G-protein receptors to selective G proteins, as well as dominant negative xanthine nucleotide-binding Galpha mutants, we show that activation of the phosphatidylinositol 3-kinase/Akt pathway is dependent on Gq and Gi2. To examine the role of the two G proteins, we examined the events upstream of PI 3-kinase. The activation of the PI 3-kinase/Akt pathway by alpha-thrombin in IIC9 cells is blocked by the expression of dominant negative Ras and beta-arrestin1 (Phillips-Mason, P. J., Raben, D. M., and Baldassare, J. J. (2000) J. Biol. Chem. 275, 18046-18053, and Goel, R., Phillips-Mason, P. J., Raben, D. M., and Baldassare, J. J. (2002) J. Biol. Chem. 277, 18640-18648), indicating a role for Ras and beta-arrestin1. Interestingly, inhibition of Gi2 and Gq activation blocks Ras activation and beta-arrestin1 membrane translocation, respectively. Furthermore, expression of the Gbetagamma sequestrant, alpha-transducin, inhibits both Ras activation and membrane translocation of beta-arrestin1, suggesting that Gbetagamma dimers from Galphai2 and Galphaq activate different effectors to coordinately regulate the PI 3-kinase/Akt pathway.

Thrombin induces Sp1-mediated antiviral effects in cytomegalovirus-infected human retinal pigment epithelial cells

Human cytomegalovirus (HCMV) retinitis causing retinal detachment and destruction of the blood-retina barrier is closely related to retinal hemorrhage/coagulation. However, the effects of procoagulants on HCMV (re)activation in retinal cells have not been investigated yet. Therefore, we studied whether thrombin modulates the expression of HCMV immediate early (IE) and late (L) genes in cultured human retinal pigment epithelial cells (RPE). Thrombin specifically stimulated the protease-activated receptor-1 (PAR-1) on RPE and, surprisingly, inhibited basal and 12,0-tetradecanoylphorbol 13-acetate-stimulated HCMV IE gene expression in infected RPE. On the other hand, HCMV strongly induced Sp1 DNA binding activity, which was prevented by thrombin/PAR1-mediated Sp1 hyperphosphorylation. Our data suggest that thrombin/PAR-1 may inhibit Sp1-dependent HCMV replication, which might be an important regulatory mechanism for HCMV persistence and replication in RPE.

Thrombin induces nitric-oxide synthase via Galpha12/13-coupled protein kinase C-dependent I-kappaBalpha phosphorylation and JNK-mediated I-kappaBalpha degradation

An imbalance between thrombin and antithrombin III contributed to vascular hyporeactivity in sepsis, which can be attributed to excess NO production by inducible nitric-oxide synthase (iNOS). In view of the importance of the thrombin-activated coagulation pathway and excess NO as the culminating factors in vascular hyporeactivity, this study investigated the effects of thrombin on the induction of iNOS and NO production in macrophages. Thrombin induced iNOS protein in the Raw264.7 cells, which was inhibited by a thrombin inhibitor, LB30057. Thrombin increased NF-kappaB DNA binding, whose band was supershifted with anti-p65 and anti-p50 antibodies. Thrombin elicited the phosphorylation and degradation of I-kappaBalpha prior to the nuclear translocation of p65. The NF-kappaB-mediated iNOS induction was stimulated by the overexpression of activated mutants of Galpha(12/13) (Galpha(12/13)QL). Protein kinase C depletion inhibited I-kappaBalpha degradation, NF-kappaB activation, and iNOS induction by thrombin or the iNOS induction by Galpha(12/13)QL. JNK, p38 kinase, and ERK were all activated by thrombin. JNK inhibition by the stable transfection with a dominant negative mutant of JNK1 (JNK1(-)) completely suppressed the NF-kappaB-mediated iNOS induction by thrombin. Conversely, the inhibition of p38 kinase enhanced the expression of iNOS. In addition, JNK and p38 kinase oppositely controlled the NF-kappaB-mediated iNOS induction by Galpha(12/13)QL. Hence, iNOS induction by thrombin was regulated by the opposed functions of JNK and p38 kinase downstream of Galpha(12/13). In the JNK1(-) cells, thrombin did not increase either the NF-kappaB binding activity or I-kappaBalpha degradation despite I-kappaBalpha phosphorylation. These results demonstrated that thrombin induces iNOS in macrophages via Galpha(12) and Galpha(13), which leads to NF-kappaB activation involving the protein kinase C-dependent phosphorylation of I-kappaBalpha and the JNK-dependent degradation of phosphorylated I-kappaBalpha.