# Involvement of the Orexinergic System in Cancer: Antitumor Strategies and Future Perspectives
## 3 Orexins and Cancer
### Colon Cancer  
All colon tumor samples studied expressed OXR1 but not OXR2 and, importantly, this was independent of the genetic alterations, grade state and location; OXR1 was also observed in dysplastic polyps which are precancer lesions leading adenocarcinomas [16,60]. OXR1 mRNA has been detected in many colon cancer cell lines (SW48, SW480, SW620, Colo205, LS174T, T84, LoVo, Caco-2, HT29-D4, HT29-FU, HT-29) but not in HCT-116 cells; in addition, orexins A and B promoted apoptosis in LS174T, T84, Colo205, LoVo, Caco-2, SW480, SW620 and SW48 cell lines [82]. Moreover, orexins promoted apoptosis and blocked tumor growth in experimental animals [23]. OXR1 mediated mitochondrial apoptosis in colon cancer Caco-2, LoVo and HT-29 cells, a mechanism that was blocked using NSC-8787 (a SHP-2 inhibitor) or PD-169,316 (a p38 inhibitor). Orexins A and B favored apoptosis but did not alter cell proliferation in human HT29-D4 colon cancer cells, in which OXR1 but not OXR2 was expressed [92]. In HT29-D4 colon cancer cells, OXR1 mediated orexin-induced calcium transients and apoptotic mechanisms have been detected since caspases 3 and 7 were activated, cytochrome c was released into the cytoplasm, chromatin condensation/DNA fragmentation were observed, and cell shape was changed [92]. However, these mechanisms were not observed in normal colonic epithelial cells and orexins promoted a pro-apoptotic effect in LoVo, SW480 and Caco-2 colon tumor cells expressing OXR1, but this was not observed in normal colonic epithelial cells [92]. All the primary colorectal tumors and hepatic metastasis studied showed OXR1 expression, whereas adjacent normal colonocytes/hepatocytes were devoid of such expression [60]. Moreover, human colon cancer cell lines established from metastases or primary tumors also expressed OXR1 mRNA and suffered apoptotic mechanisms (through the activation of caspase 3) after treatment with orexin A [60]. Orexin A also showed an antitumor effect in xenografted human colon cancer cells by decreasing tumor size [60]. Normal colonic epithelial cells do not express OXR1, but this receptor is aberrantly expressed in colorectal tumors and after local (lymph node) or distant (lung, liver) metastasis [93]. Orexins promoted apoptosis not only in colorectal tumor cells but also in those cells resistant to the treatment with the chemotherapy drug 5-fluorouracil. The administration of orexin A decreased tumor volume in in vivo experiments, slowed the tumor grow and reverted the development of well-established tumors (xenografted LoVo or HT-29 colon cancer cells) by promoting apoptotic mechanisms [60; 93]. Importantly, endogenous orexin A did not exert an intrinsic effect on tumor development; thus, the orexin A circulating level was very low and neither orexin A nor orexin B were observed in the tumor environment. Previous findings suggest that the orexinergic system (OXR1, orexins) is involved in metastasis and chemoresistance and hence OXR1 is a promising antitumor target and orexins could be used as antitumor agents against tumor cells showing chemoresistance. OXR1, via G(q) protein but not via G\(\alpha\)(q) activation of PLC, promoted apoptosis; thus, the \(\beta\gamma\) dimer of G(q) stimulated src-tyrosine kinase leading to the phosphorylation of the immunoreceptor tyrosine-based inhibitory motif in OXR1 and subsequent recruitment by this receptor of the phosphotyrosine phosphatase SHP-2, which is then activated, followed by the activation of p38 mitogen-stress protein kinase and the entry of Bax protein into the mitochondria [17; 93; 94; 95]. The immunoreceptor tyrosine-based switch motif presented in OXR1 is required for OXR1-medited apoptosis [17; 95]. An OXR1 structural model has been developed and it has been shown that the spatial localization of phosphotyrosines in the immunoreceptor tyrosine-based switch/immunoreceptor tyrosine-based inhibitory motifs in OXR1 was compatible with their interaction with the two SH\({}_{2}\) domains of SHP-2 [95].