Vitamin D actions in neurons require the PI3K pathway for both enhancing insulin signaling and rapid depolarizing effects

Authors: TEIXEIRA, SILVANIA - Children'S Nutrition Research Center (CNRC); HARRISON, KEISHA - Children'S Nutrition Research Center (CNRC); UZODIKE, MUNACHISMO - Rice University; RAJAPAKSHE, KIMAL - Imcb: Institute Of Molecular And Cell Biology; COARFA, CRISTIAN - Imcb: Institute Of Molecular And Cell Biology; HE, YANLIN - Children'S Nutrition Research Center (CNRC); XU, YONG - Children'S Nutrition Research Center (CNRC); SISLEY, STEPHANIE - Children'S Nutrition Research Center (CNRC)

Submitted to: The Journal of Steroid Biochemistry and Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/29/2020
Publication Date: 5/11/2020
Citation: Teixeira, S., Harrison, K., Uzodike, M., Rajapakshe, K., Coarfa, C., He, Y., Xu, Y., Sisley, S. 2020. Vitamin D actions in neurons require the PI3K pathway for both enhancing insulin signaling and rapid depolarizing effects. The Journal of Steroid Biochemistry and Molecular Biology. 200:105690. https://doi.org/10.1016/j.jsbmb.2020.105690.
DOI: https://doi.org/10.1016/j.jsbmb.2020.105690

Interpretive Summary: Low vitamin D levels are associated with diabetes but how the two are linked is not clear. We previously published that vitamin D can act in the brain to improve blood glucose levels. While we also showed that the vitamin D receptor was required in a specific area of the brain (paraventricular hypothalamus) to help control blood glucose levels, it was not clear how vitamin D or its receptor could work in the cell to change circulating glucose levels. It is important to understand how vitamin D may work in the brain in order to determine better ways of using vitamin D for diabetes treatment or prevention. In our previous study, we showed that vitamin D can improve insulin sensitivity in the body. Since insulin action in the brain is also important for regulating blood sugar levels, we hypothesized that vitamin D may improve insulin action in the brain. In this study we found that vitamin D administration into the brain changed the expression of many different genes within the hypothalamus of the brain. Many of these gene are important for insulin action and indeed we found that vitamin D increased "Insulin Signaling Pathway" genes when we performed pathway analysis of our results. We also found that vitamin D improved insulin action in a hypothalamic cell line. In order to improve insulin action, the PI3 kinase was required since vitamin D had no effect on insulin action when combined with a PI3 kinase inhibitor. These previous actions of vitamin D in cell culture took 2-24 hours and occurred likely through the increase expression levels of key genes within the Insulin/PI3 kinase pathway. However, this study also showed that vitamin D could activate hypothalamic neurons rapidly which surprisingly, also required the PI3 kinase. Thus, our research shows that vitamin D can improve insulin action in neurons and can rapidly activate neurons and both actions of vitamin D require the PI3 kinase. This work suggests that vitamin D may act in the brain to help augment insulin action. These findings have a significant impact for the development and treatment of diabetes as people with type 2 diabetes have insulin resistance and thus, the action of vitamin D in the brain may be very beneficial.

Technical Abstract: Despite correlations between low vitamin D levels and diabetes incidence/severity, supplementation with vitamin D has not been widely effective in improving glucose parameters. This may be due to a lack of knowledge regarding how low vitamin D levels physiologically affect glucose homeostasis. We have previously shown that the brain may be a critical area for vitamin D-mediated action on peripheral glucose levels. However, the mechanisms for how vitamin D acts in the brain are unknown. We utilized a multimodal approach to determine the mechanisms by which vitamin D may act in the brain. We first performed an unbiased search (RNA-sequencing) for pathways affected by vitamin D. Vitamin D (125-dihydroxyvitamin D3; 1,25D3) delivered directly into the third ventricle of obese animals differentially regulated multiple pathways, including the insulin signaling pathway. The insulin signaling pathway includes PI3K, which is important in the brain for glucose regulation. Since others have shown that vitamin D acts through the PI3K pathway in non-neuronal cells (muscle and bone), we hypothesized that vitamin D may act in neurons through a PI3K-dependent pathway. In a hypothalamic cellculture model (GT1-7 cells), we demonstrate that 1,25D3 increased phosphorylation of Akt in the presence of insulin. However, this was blocked with pre-treatment of wortmannin, a PI3K inhibitor. 1,25D3 increased gene transcription of several genes within the PI3K pathway, including Irs2 and p85, without affecting expression of InsR or Akt. Since we had previously shown that 1,25D3 has signiffcant effects on neuronal function, we also tested if the PI3K pathway could mediate rapid actions of vitamin D. We found that 1,25D3 increased the firing frequency of neurons through a PI3K-dependent mechanism. Collectively, these data support that vitamin D enhances insulin signaling and neuronal excitability through PI3K dependent processes which involve both transcriptional and membrane-initiated signaling events.