Resistance to apoptosis is a recurrent theme in colon cancer. We have shown previously that the 7-transmembrane spanning receptor OX1R for orexins promotes robust apoptosis in the human colon cancer cell line HT29 through an entirely novel mechanism involving phosphorylation of tyrosine-based motifs in OX1R. Here, we investigated the status of OX1R in a large series of human colorectal tumors and hepatic metastases. All primary colorectal tumors regardless of their localization and Duke's stages and all hepatic metastases tested expressed OX1R mRNA and/or protein. In sharp contrast, adjacent normal colonocytes or hepatocytes as well as control normal tissues were negative. Next, we showed that nine human colon cancer cell lines established from primary tumors or metastases expressed OX1R mRNA and underwent important apoptosis on orexin-A challenge. Most interestingly, orexin-A also promoted robust apoptosis in cells that are resistant to the most commonly used drug in colon cancer chemotherapy, 5-fluorouracil. When human colon cancer cells were xenografted in nude mice, orexin-A administered at day 0 strongly slowed the tumor growth and even reversed the development of established tumors when administered 7 days after cell inoculation. Orexin-A also acts by promoting tumor apoptosis in vivo because caspase-3 is activated in tumors on orexin treatment of nude mice. These findings support that OX1R is an Achilles heel of colon cancers, even after metastasis or chemoresistance. They suggest that OX1R agonists might be novel candidates for colon cancer therapy. Cancer Res; 71(9);

Orexins acting at the G protein-coupled receptor (GPCR) OX1R have recently been shown to promote dramatic apoptosis in cancer cells. We report here that orexin-induced apoptosis is driven by an immunoreceptor tyrosine-based inhibitory motif (ITIM) (IIY(358)NFL) present in the OX1R. This effect is mediated by SHP-2 phosphatase recruitment via a mechanism that requires Gq protein but is independent of phospholipase C activation. This is based on the following observations: 1) mutation of Y(358) into F abolished orexin-induced tyrosine phosphorylation in ITIM, orexin-induced apoptosis, and uncoupled OX1R from Gq protein in transfected Chinese hamster ovary (CHO) cells; 2) orexin-induced apoptosis in CHO cells expressing recombinant OX1R and in colon cancer cells expressing the native receptor was abolished by treatment with the tyrosine phosphatase inhibitor PAO and by transfection with a dominant-negative mutant of SHP-2; 3) orexins were unable to promote apoptosis in fibroblast cells invalidated for the G alpha q subunit and transfected with OX1R cDNA, whereas they promoted apoptosis in cells equipped with G alpha q and OX1R; and 4) the phospholipase C inhibitor U-73122 blocked orexin-stimulated inositol phosphate formation, whereas it had no effect on orexin-induced apoptosis in CHO cells expressing OX1R. These data unravel a novel mechanism, whereby ITIM-expressing GPCRs may trigger apoptosis.