PI3K/Akt responses to oxytocin stimulation in Caco2BB gut cells.

Recently, we discovered oxytocin receptor (OTR) expression in the developing gut villus epithelium that emerges in villus-crypt junctions after weaning. Oxytocin (OT) and OTR regulate many physiological functions in various tissues; however, their function in gut epithelium is unknown. We explored responses of PI3K and Akt phosphoisoforms to OT stimuli in the Caco2BB human gut cell line. In Caco2BB cells, PI3K and pAkt levels peaked at 62.5  nM OT. At higher concentrations, PI3K decreased more gradually than pAkt(S473) suggesting that the pAkt(S473) response is separate from PI3K. At ≤7.8  nM OT, pAkt(T308) increased while pAkt(S473) decreased. Using a specific OTR antagonist, we demonstrated that responses of pAkt(T308) to OT depend on OTR in contrast to the partial OTR-dependence of the pAkt(S473) response. Differential pAkt phosphoisoform responses included pAkt phosphoserine 473 persistently free of phosphothreonine 308. The reduction in PI3K after 62.5  nM OT for 30  min coincided with OTR internalization. The PI3K/Akt activation profile was somewhat different in other cell lines (MCF-7 breast cancer cells, HT29 gut cells), which have PI3K activating mutations, that were examined to establish experimental parameters. In Caco2BB cells, the divergent effects of OT upon pAkt phosphoisoforms suggests separate sub-pathways; pAkt (T308) activation depends on OTR via the PI3K pathway and pAkt(S473) presumably results from its specific kinase mTORC2 (mammalian target of rapamycin complex 2). Thus, OT may modulate gut cell functions downstream of mTOR complexes (e.g., translation control as suggested by others in uterine cells). We will next explore OT-stimulated kinase activities downstream of mTOR related to pAkt phosphoisoforms.