Assessing Cellular Stress and Inflammation in Discrete Oxytocin-secreting Brain Nuclei in the Neonatal Rat Before and After First Colostrum Feeding

The goal of this protocol is to isolate oxytocin-receptor rich brain nuclei in the neonatal brain before and after first colostrum feeding. The expression ...

Columbia University

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.

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

Our recent findings of a weaning-related pattern of oxytocin (OT) and OT receptor (OTR) expression in the rat enteric nervous system and in villus-crypt enterocytes, together with the known high level and stability of OT in breast milk support that OT may play a role in gut function and development. We previously described a biphasic dose-response of the PI3K/Akt pathway in gut cells treated with OT. Activation peaked at 62.5 nM OT (30 min) and coincided with OTR internalization. Here we use automated Western blotting to further explore OT-elicited changes in Akt and pAkt(T308), as well as in downstream substrates p70 S6 kinase-1 (S6K1) and eIF-4E binding protein 1 (4E-BP1). Relative to fresh growth medium (FGM) alone, our results showed OT in FGM reduced the abundance and phosphorylation of S6K1 and the phosphorylation of 4E-BP1, both substrates of mammalian target of rapamycin complex 1 (mTORC1). Phosphorylation of mTORC1 regulator, Raptor(S792), was increased by high and low OT concentrations, with predicted inhibitory effects on mTORC1. OT thus downregulates anabolic effects induced by FGM activity catalyzed by mTORC1. OT is a regulator of the PI3K/Akt/mTORC1 pathway in Caco2BB cells and may modulate translation in gut cells.

Oxytocin modulates mTORC1 pathway in the gut.

We have shown that oxytocin receptor (OTR) expression in neonatal rat enterocytes is robust from birth to weaning, but OTR function during this period is unknown. We previously reported that oxytocin (OT) stimulation of Caco2BB cells (enterocytes in vitro) inhibits the mammalian target of rapamycin complex 1 (mTORC1) signaling. The unfolded protein response (UPR) is known to protectively reduce translation during endoplasmic reticulum (ER) stress. Because the mTORC1 pathway is linked to cellular stress, we investigated markers of UPR in OT-stimulated Caco2BB cells. We report that OT modulates several factors involved in sensing and translation of ER stress. High OT (62.5 nM) reduced translation initiation factor 4E-BP1 phosphorylation (Ser65), which is known to inhibit cap-dependent translation via its rate-limiting eukaryotic translation initiation factor 4E (eIF4E). Importantly, high OT increased phosphorylation of eukaryotic translation initiation factor 2a (eIF2a) phospho-Ser51, which inhibits eIF2a. High OT also increased protein kinase RNA-like endoplasmic reticulum kinase phosphorylation, a sensor of ER stress and a kinase of eIF2a. Both high and low OT activated inositol requiring enzyme1 (IRE1), which generates the transcription factor X-box binding protein 1 (XBP1) and induces the UPR. We also show that OT modulates XBP1 splicing and induces tribbles 3 (TRIB3; a negative regulator of Akt and protein involved in autophagy) and immunoglobulin binding protein (BiP; ER-chaperone). Taken together, these results indicate that OT modulates sensors of ER stress and autophagy. These findings support our hypothesis that transiently elevated OTR expression in neonatal gut may serve a protective function during a critical postnatal developmental period. 

Oxytocin modulates markers of the unfolded protein response in Caco2BB gut cells.

The neuropeptide neuromodulator and hormone oxytocin (OT) activates signaling pathways involved in mRNA translation in response to endoplasmic reticulum stress and reduces inflammation associated with experimental colitis in rats. The anti-inflammatory effects of OT may serve a vital role in the development, survival and function of newborn-type enterocytes during microbial gut colonization, which coincides with the milk suckling period when OT receptor expression peaks in the gut. Furthermore, mice deficient in the OT receptor have abnormal gut structure and function, underscoring OT's developmental importance.

Oxytocin opposes effects of bacterial endotoxin on ER-stress signaling in Caco2BB gut cells.

Little is known about the role of oxytocin (OT) in colostrum during early gut colonization. We previously showed that transient OT receptor (OTR) expression on newborn rat enterocytes coincides with the milk-suckling period, and that OT activates endoplasmic reticulum stress sensors in cultured enterocytes. Here, we explored whether colostrum-OT attenuates stress in newborn villi primed and unprimed by colostrum by measuring levels of stress markers including BiP (an ER chaperone), eIF2a (translation initiation factor), and pPKR (eIF2a kinase). We also measured two inflammation-signaling proteins NF-κB and its inhibitor IκB. To test the impact of colostrum on autophagy, we measured a marker of autophagy initiation, LC3A. Colostrum increased inactive p-eIF2a, p-PKR and IκB and reduced p-IκB, BiP and LC3A. LPS increased and OT decreased p-IkB. BiP (GRP78) was higher in unprimed than primed villi. Together, these data suggest that colostrum OT attenuates the impact of inflammation on postnatal gut villi and that OT enhances autophagy to protect against amino acid insufficiency-induced stress during the interval between birth and the first feeding.

Benjamin Y. Klein

Department of Psychiatry

Columbia University

Assessing Cellular Stress and Inflammation in Discrete Oxytocin-secreting Brain Nuclei in the Neonatal Rat Before and After First Colostrum Feeding

Benjamin Y. Klein

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Columbia University

Assessing Cellular Stress and Inflammation in Discrete Oxytocin-secreting Brain Nuclei in the Neonatal Rat Before and After First Colostrum Feeding

Department of Psychiatry