Arg188 drives RhoC membrane binding.

RhoA and RhoC GTPases are 92% identical but demonstrate unique regulation and function. Phosphorylation of Ser188 has widely been reported to inhibit RhoA activity. RhoC possesses Arg188 in place of Ser188 but retains a canonical upstream PKA recognition sequence. We report here that RhoC-R188S was a PKA substrate in vitro and exhibited less GTP loading compared to wild-type RhoC when expressed in cells. Transiently expressed RhoC was found to be significantly more membrane associated than RhoA. Membrane association of RhoC-R188S and RhoC-R188A were similar to each other and wild-type RhoA, suggesting that Arg188 directly promotes RhoC membrane binding. The positive influence of Arg188 on RhoC membrane association was evident in a constitutively active (Q63L) background. In accordance, RhoA-S188R was significantly more membrane associated than either RhoA or RhoA-S188A. Altogether, these data suggest that swapping residue 188 identity effectively flips the membrane binding profile of wild-type RhoA and RhoC through positive arginine contribution rather than negative phosphoserine regulation.

Lysophosphatidic Acid Stimulates Urokinase Receptor (uPAR/CD87) in Ovarian Epithelial Cancer Cells.

BACKGROUND/AIM: Lysophosphatidic acid (LPA) is a bioactive lipid positively linked with ovarian cancer progression. The multi-functional urokinase receptor (uPAR), a cell-surface glycoprotein, binds and facilitates activation of uPA and laterally regulates integrin and tyrosine kinase receptor activities in promotion of cell migration and invasion. We hypothesized that LPA stimulates uPAR expression and activity in ovarian epithelial cancer cells. MATERIALS AND METHODS: Ovarian epithelial cancer cell lines OVCA 429 and OVCA 433 were stimulated with LPA and examined for uPAR mRNA expression and protein localization. uPA binding to OVCA plasma membranes was measured through enzymatic analysis of affinity-isolated cell-surface proteins. RESULTS: LPA drove cell-surface uPAR aggregation and mRNA expression concomitant with increased cell-surface binding of uPA. Both control and LPA-stimulated uPAR expression and uPA cell-surface association involved phosphatidylinositol 3-kinase, but not p38 or p42 mitogen-activated protein kinase, signaling. CONCLUSION: These data provide mechanistic insight into ovarian epithelial cancer cell progression by demonstrating that LPA drives uPAR expression and uPA binding.

Divergence of Rho residue 43 impacts GEF activity.

RhoA, RhoB and RhoC GTPases are over 85% identical at the amino acid level, with RhoA and RhoC differing at only one residue (43) across the initial two-thirds of their sequences. A putative regulatory distinction between the molecules is their capacity to be uniquely activated by guanine nucleotide exchange factors (GEFs). We hypothesize that variation of amino acid residue 43 between RhoA/B (valine) and RhoC (isoleucine) impacts GEF activity. Direct participation of residue 43 in GEF-catalyzed exchange was confirmed by the observation that mutation of this position to a threonine reduced GEF-catalyzed nucleotide exchange activity in vitro (Vav2, XPLN, GEFT, Dbl and Dbs) and greatly depressed RhoA and RhoC GTP-loading profiles in cell lysates. Using a residue swap approach, substitution of RhoA Val 43 with an Ile was found to significantly promote basal nucleotide exchange activity and enhance GTP-loading in cells. Substitution of Val 43 with an Ile in RhoB negatively affected nucleotide exchange in vitro. Substitution of RhoC Ile 43 with a Val increased GEF-catalyzed exchange in vitro. In addition, RhoC-I43V was more efficacious at driving ovarian cancer cell invasion through matrigrel than wild-type RhoC, RhoC-I43T, wild-type RhoA, RhoA-V43I or RhoA-V43T GTPases. These findings suggest that a divergence between RhoA/B and RhoC at residue 43 impacts basal and GEF-stimulated nucleotide exchange activity.