Leucine-rich repeat-containing G-protein coupled receptor 5/GPR49 activates G12/13-Rho GTPase pathway

Leucine-rich repeat-containing G-protein coupled receptor 5 (LGR5/GPR49) is highly expressed in adult stem cells of various tissues, such as intestine, hair follicles, and stomach. LGR5 is also overexpressed in some colon and ovarian tumors. Recent reports show that R-spondin (RSPO) family ligands bind to and activate LGR5, enhancing canonical Wnt signaling via the interaction with LRP5/6 and Frizzled. The identity of heterotrimeric G-proteins coupled to LGR5, however, remains unclear. Here, we show that Rho GTPase is a downstream target of LGR5. Overexpression of LGR5 induced SRF-RE luciferase activity, a reporter of Rho signaling. RSPOs, ligands for LGR4, LGR5, and LGR6, however, did not induce SRF-RE reporter activity in the presence of LGR5. Consistently, LGR5-induced activity of the SRF-RE reporter was inhibited by Rho inhibitor C3 transferase and RhoA N19 mutant, and knockdown of Gα12/13 genes blocked the reporter activity induced by LGR5. In addition, focal adhesion kinase, NF-κB and c-fos, targets of Rho GTPase, were shown to be regulated by LGR5. Here, we have demonstrated, for the first time, that LGR5 is coupled to the Rho pathway through G12/13 signaling.

Targeting NFĸB mediated breast cancer chemoresistance through selective inhibition of sphingosine kinase-2

Resistance to chemotherapy remains a significant obstacle in the treatment of hormone- independent breast cancer. Recent evidence suggests that altered sphingolipid signaling through increased sphingosine kinase activity may be an important mediator of breast cancer drug resistance. Sphingosine kinase-1 (Sphk1) is a proposed key regulator of breast cancer tumorigenesis, proliferation and resistance. There is, however, conflicting data on the role of sphingosine kinase-2 (Sphk2) in cancer biology and resistance, with some suggesting that Sphk2 has an opposing role to that of Sphk1. Here, we studied the effects of the novel selective Sphk2 inhibitor, ABC294640 (3-(4-chlorophenyl)-adamantane-1-carboxylic acid (pyridin-4-ylmethyl) amide), on human breast cancer. ABC294640 blocked both viability and survival at low micromolar IC(50) concentrations in the endocrine therapy-resistant MDA-MB-231 and chemoresistant MCF-7TN-R cell systems. Treatment with the inhibitor significantly reduced proliferation, as seen in immunofluorescence staining of Ki-67 in vitro. Interestingly, pharmacological inhibition of Sphk2 induced apoptosis through the intrinsic programmed cell death pathway. Furthermore, ABC294640 also diminished NF-ĸB survival signaling, through decreased activation of the Ser536 phosphorylation site on the p65 subunit. Xenografts of MCF-7TN-R cells growing in immunocompromised mice were utilized to validate the therapeutic efficacy of the sphingosine kinase-2 inhibitor. Treatment with 50 mg of ABC294640/kg completely blocked tumor volume in this model. These results indicate that pharmacological inhibition of Sphk2 with the orally bioavailable selective inhibitor, ABC294640, has therapeutic potential in the treatment of chemo- and endocrine therapy- resistant breast cancer.