Search

Low 25-hydroxyvitamin D levels correlate with the prevalence of diabetes; however, the mechanisms remain uncertain. Here, we show that nutritional deprivation–responsive mechanisms regulate vitamin D metabolism. Both fasting and diabetes suppressed hepatic cytochrome P450 (CYP) 2R1, the main vitamin D 25-hydroxylase responsible for the first bioactivation step. Overexpression of coactivator peroxisome proliferator–activated receptor γ coactivator 1-α (PGC-1α), induced physiologically by fasting and pathologically in diabetes, resulted in dramatic downregulation of CYP2R1 in mouse hepatocytes in an estrogen-related receptor α (ERRα)–dependent manner. However, PGC-1α knockout did not prevent fasting-induced suppression of CYP2R1 in the liver, indicating that additional factors contribute to the CYP2R1 repression. Furthermore, glucocorticoid receptor (GR) activation repressed the liver CYP2R1, suggesting GR involvement in the regulation of CYP2R1. GR antagonist mifepristone partially prevented CYP2R1 repression during fasting, suggesting that glucocorticoids and GR contribute to the CYP2R1 repression during fasting. Moreover, fasting upregulated the vitamin D catabolizing CYP24A1 in the kidney through the PGC-1α-ERRα pathway. Our study uncovers a molecular mechanism for vitamin D deficiency in diabetes and reveals a novel negative feedback mechanism that controls crosstalk between energy homeostasis and the vitamin D pathway.

Pregnane X receptor (PXR) is a nuclear receptor that senses chemical environment and is activated by numerous clinically used drugs and environmental contaminants. Previous studies have indicated that several drugs known to activate PXR appear to induce glucose intolerance. We now aimed to reveal the role of PXR in drug-induced glucose intolerance and characterize the mechanisms involved. We used PXR knockout mice model to investigate the significance of this nuclear receptor in the regulation of glucose tolerance. PXR ligand pregnenolone-16ɑ-carbonitrile (PCN) impaired glucose tolerance in the wildtype mice but not in the PXR knockout mice. Furthermore, DNA microarray and bioinformatics analysis of differentially expressed genes and glucose metabolism relevant pathways in PCN treated primary hepatocytes indicated that PXR regulates genes involved in glucose uptake. PCN decreased the expression of glucose transporter 2 (GLUT2) in mouse liver and in the wildtype mouse hepatocytes but not in the PXR knockout cells. Data mining of published chromatin immunoprecipitation-sequencing results indicate that Glut2 gene is a direct PXR target. Furthermore, PCN induced internalization of GLUT2 protein from the plasma membrane to the cytosol in the liver in vivo and repressed glucose uptake in the primary hepatocytes. Our results indicate that the activation of PXR impairs glucose tolerance and thus PXR represents a novel diabetogenic pathway. PXR activation dysregulates GLUT2 function by two different mechanisms. These findings may partly explain the diabetogenic effects of medications and environmental contaminants.

Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and AMP-activated protein kinase (AMPK) are major factors regulating energy homeostasis. In this study, we aimed to investigate how energy flux affects several hepatic functions mediated by these factors. We define a novel role of PGC-1α and AMPK as modulators of the immune system in the liver. We show that PGC-1α is involved in the regulation of a cluster of genes related to the immune system, most importantly Interleukin 1 receptor antagonist (IL1Rn). Since PGC-1α is responsive to energetic stress associated with fasting or physical exercise, the same stimuli promote IL1Rn in hepatocytes. We identify AMPK as an independent inducer of IL1Rn and hypothesise that it could account for the anti-inflammatory effect of the antidiabetic drug metformin. We also demonstrate that metformin reduces expression of Sirtuin 3 (SIRT3) in hepatocytes and promotes acetylation of mitochondrial protein. We suggest that this mechanism, in spite of increased mitochondrial biogenesis, contributes to reduced ATP synthesis in metformin-treated samples. In addition, we demonstrate that Pregnane X receptor (PXR) is induced in the liver during fasting and by PGC-1α in hepatocytes. Furthermore, we describe a negative regulatory mechanism involving SIRT1, activated by pyruvate and interfering with PXR signaling. We show that SIRT1 attenuates PGC-1α-mediated co-activation of PXR and its target genes, i.e. Cyp3a11, with possible implications for drug and xenobiotic metabolism. In conclusion, we demonstrate how energetic stress affects various hepatic functions mediated by PGC-1α and AMPK. Moreover, we describe SIRT1 and metformin as factors capable of modulating this response., Tiivistelmä Peroksisomiproliferaattori-aktivoituvan reseptori gamman koaktivaattori 1α (PGC-1α) ja AMP:n aktivoima proteiinikinaasi (AMPK) ovat keskeisiä energiametabolian säätelijöitä. Tässä tutkimuksessa oli tavoitteena selvittää kuinka energiataso vaikuttaa useisiin, näiden tekijöiden säätelemiin maksan toimintoihin. Osoitamme että PGC-1α ja AMPK tekijöillä on ennestään tuntematon merkitys immuunijärjestelmän säätelyssä maksassa. Näytämme myös, että PGC-1α säätelee joukkoa geenejä, joiden tehtävä liittyy immuunijärjestelmään, tärkeimpänä Interleukiini 1 reseptori antagonistia (IL1Rn). Paastoon ja fyysiseen aktiivisuuteen liittyvä energiastressi aktivoi PGC-1α:aa ja näiden samojen stimuluksien havaittiin lisäävän myös IL1Rn tasoa hepatosyyteissä. Havaitsimme AMPK:n olevan itsenäinen IL1Rn indusori ja hypoteesimme mukaan tämä voi välittää diabeteslääkkeenä käytettävän metformiinin anti-inflammatorisia vaikutuksia. Osoitamme myös, että metformiini alentaa Sirtuiini (SIRT) 3:n ekspressiota maksasoluissa ja lisää mitokondriaalisten proteiinien asetylaatiota. Uskomme tämän mekanismin, huolimatta lisääntyneestä mitokondrioiden biogeneesistä, myötävaikuttavan vähentyneeseen ATP synteesiin metformiinikäsitellyissä näytteissä. Lisäksi osoitamme, että paasto ja PGC-1α indusoivat Pregnaani X reseptorin (PXR) ilmentymistä maksasoluissa. Kuvaamme myös PXR signalointiin vaikuttavan ja pyruvaatin aktivoiman, SIRT1:n välitteisen, negatiivisen säätelymekanismin. SIRT1 estää PGC-1α välitteistä PXR koaktivaatiota ja kohdegeenien, kuten Cyp3a11, aktivaatiota, millä voidaan olettaa olevan merkitystä lääkeaineiden ja vierasaineiden metaboliaan. Yhteenvetona osoitamme, että energiastressi PGC-1α:n ja AMPK:n välittämänä vaikuttaa useisiin maksan toimintoihin. Lisäksi näytämme, että SIRT1 ja metformiini voivat moduloida näitä vaikutuksia.