OX1 orexin receptors activate extracellular signal-regulated kinase in Chinese hamster ovary cells via multiple mechanisms : the role of Ca2+ influx in OX1 receptor signaling

• Activation of OX1 orexin receptors heterologously expressedin Chinese hamster ovary (CHO) cells led to a rapid, strong,and long-lasting increase in ERK phosphorylation (activation).Dissection of the signal pathways to ERK using multiple inhibitorsand dominant-negative constructs indicated involvement of Ras,protein kinase C, phosphoinositide-3-kinase, and Src. Most interestingly,Ca2+ influx appeared central for the ERK response in CHO cells,and the same was indicated in recombinant neuro-2a cells andcultured rat striatal neurons. Detailed investigations in CHOcells showed that inhibition of the receptor- and store-operatedCa2+ influx pathways could fully attenuate the response, whereasinhibition of the store-operated Ca2+ influx pathway alone orthe Ca2+ release was ineffective. If the receptor-operated pathwaywas blocked, an exogenously activated store-operated pathwaycould take its place and restore the coupling of OX1 receptorsto ERK. Further experiments suggested that Ca2+ influx, as such,may not be required for ERK phosphorylation, but that Ca2+,elevated via influx, acts as a switch enabling OX1 receptorsto couple to cascades leading to ERK phosphorylation, cAMP elevation,and phospholipase C activation. In conclusion, the data suggestthat the primary coupling of orexin receptors to Ca2+ influxallows them to couple to other signal pathways; in the absenceof coupling to Ca2+ influx, orexin receptors can act as signalintegrators by taking advantage of other Ca2+ influx pathways.

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