Biology subjects

Lutkewitte, A. J.

Publications and source records attributed to Lutkewitte, A. J..

4 recordsLinked to original sources

bioRxiv2021

Silencing alanine transaminase 2 in diabetic liver attenuates hyperglycemia by reducing gluconeogenesis from amino acids

Hepatic gluconeogenesis from amino acids contributes significantly to diabetic hyperglycemia, but the molecular mechanisms involved are incompletely understood. Alanine transaminases (ALT1 and ALT2) catalyze the interconversion of alanine and pyruvate, which is required for gluconeogenesis from alanine. We found that ALT2 was overexpressed in liver of diet-induced obese and db/db mice and that the expression of the gene encoding ALT2 (GPT2) was downregulated following bariatric surgery in people with obesity. The increased hepatic expression of Gpt2 in db/db liver was mediated by activating transcription factor 4; an endoplasmic reticulum stress-activated transcription factor. Hepatocyte-specific knockout of Gpt2 attenuated incorporation of 13C-alanine into newly synthesized glucose by hepatocytes. In vivo Gpt2 knockdown or knockout in liver had no effect on glucose concentrations in lean mice, but Gpt2 suppression alleviated hyperglycemia in db/db mice. These data suggest that ALT2 plays a significant role in hepatic gluconeogenesis from amino acids in diabetes.

physiology↗

bioRxiv2020

Glucolipotoxicity-induced Mig6 desensitizes EGFR signaling and promotes pancreatic beta cell death

A loss of functional beta cell mass is a final etiological event in the development of frank type 2 diabetes (T2D). To preserve or expand beta cells and therefore treat/prevent T2D, growth factors have been considered therapeutically but have largely failed to achieve robust clinical success. The molecular mechanisms preventing the activation of mitogenic signaling pathways from maintaining functional beta cell mass during the development of T2D remain unknown. We speculated that endogenous negative effectors of mitogenic signaling cascades impede beta cell survival/expansion. Thus, we tested the hypothesis that a stress-inducible epidermal growth factor receptor (EGFR) inhibitor, Mitogen-inducible gene 6 (Mig6), regulates beta cell fate in a T2D milieu. To this end, we determined that: 1) glucolipotoxicity (GLT) induces Mig6, thereby blunting EGFR signaling cascades, and 2) Mig6 mediates molecular events regulating beta cell survival/death. We discovered that GLT impairs EGFR activation, and Mig6 is elevated in human islets from T2D donors as well as GLT-treated rodent islets and 832/13 INS-1 beta cells. Mig6 is essential for GLT-induced EGFR desensitization, as Mig6 suppression rescued the GLT-impaired EGFR and ERK1/2 activation. Further, Mig6 mediated EGFR but not insulin-like growth factor-1 receptor nor hepatocyte growth factor receptor activity in beta cells. Finally, we identified that elevated Mig6 augmented beta cell apoptosis, as Mig6 suppression reduced apoptosis during GLT. In conclusion, we established that T2D and GLT induce Mig6 in pancreatic beta cells. The elevated Mig6 desensitizes EGFR signaling and induces beta cell death. Our findings suggest that Mig6 could be a novel therapeutic target for T2D, as blocking Mig6 could possibly enhance mitogenic signaling cascades in a diabetic milieu to promote beta cell survival and prevent beta cell death.

cell biology↗

bioRxiv2020

Mig6 decreases hepatic EGFR activation and survival during saturated fatty acid-induced endoplasmic reticulum stress

Hyperlipidemia associated with obesity and type 2 diabetes (T2D) promotes excess hepatic lipid storage (steatosis) and endoplasmic reticulum (ER) stress, thereby reducing hepatic cell proliferation and survival. An important receptor tyrosine kinase controlling liver proliferation and survival is the epidermal growth factor receptor (EGFR). EGFR expression and activation are decreased during steatosis in humans and several animal models of obesity. Therefore, restoring EGFR activation in obesity-induced ER stress and diabetes could restore the livers capacity for survival and regeneration. As an inducible feedback inhibitor of EGFR activity, mitogen-inducible gene 6 (Mig6) is a novel target for enhancing EGFR signaling during diet-induced obesity (DIO) and T2D. Thus, we hypothesized hepatic ER stress induces Mig6 expression and decreases EGFR activation during DIO and diabetes. We identified that Mig6 expression was increased during obesity-induced insulin resistance in C57Bl/6J mice fed a high fat diet. We also discovered that both pharmacological- and fatty acid-driven ER stress increased Mig6 expression and decreased EGF-mediated EGFR activation in primary rat hepatocytes and cell lines. Furthermore, siRNA-mediated Mig6 knockdown restored EGFR signaling and reduced caspase 3/7 activation during ER stress. Therefore, we conclude Mig6 is increased during ER stress in DIO, thereby reducing EGFR activation and enhancing cell death. The implications are the induction of Mig6 during DIO and diabetes may decrease hepatocyte survival, thus hindering cellular repair and regenerative mechanisms.

physiology↗

bioRxiv2020

Antisense oligonucleotides against monoacylglycerol acyltransferase 1 (Mogat1) improve glucose metabolism independently of Mogat1

ObjectiveMonoacylglycerol acyltransferase (MGAT) enzymes catalyze the synthesis of diacylglycerol from monoacylglycerol. Previous work has suggested the importance of MGAT activity in the development of obesity-related hepatic insulin resistance. Indeed, antisense oligonucleotide (ASO)-mediated knockdown of the gene encoding MGAT1, Mogat1, reduced hepatic MGAT activity and improved glucose tolerance and insulin resistance in high fat diet (HFD) fed mice. However, recent work has suggested that some ASOs may have off-target effects on body weight and metabolic parameters via activation of the interferon alpha/beta receptor 1 (IFNAR-1) pathway. MethodsMice with whole-body Mogat1 knockout or a floxed allele for Mogat1 to allow for liver-specific Mogat1-knockout (by either a liver-specific transgenic or adeno-associated virus-driven Cre recombinase) were generated. These mice were placed on a high fat diet and glucose metabolism and insulin sensitivity was assessed after 16 weeks on diet. In some experiments, mice were treated with control or Mogat1 or control ASOs in the presence or absence of IFNAR-1 neutralizing antibody. ResultsGenetic deletion of hepatic Mogat1, either acutely or chronically, did not improve hepatic steatosis, glucose tolerance, or insulin sensitivity in HFD-fed mice. Furthermore, constitutive Mogat1 knockout in all tissues actually exacerbated HFD-induced weight gain, insulin resistance, and glucose intolerance on a HFD. Despite markedly reduced Mogat1 expression, liver MGAT activity was unaffected in all knockout mouse models. Mogat1 overexpression hepatocytes increased liver MGAT activity and TAG content in low-fat fed mice, but did not cause insulin resistance. Interestingly, Mogat1 ASO treatment improved glucose tolerance in both wild-type and Mogat1 null mice, suggesting an off target effect. Inhibition of IFNAR-1 did not block the effect of Mogat1 ASO on glucose homeostasis. ConclusionThese results indicate that genetic loss of Mogat1 does not affect hepatic MGAT activity or metabolic homeostasis on HFD and show that Mogat1 ASOs improve glucose metabolism through effects independent of targeting Mogat1 or activation of IFNAR-1 signaling. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=65 SRC="FIGDIR/small/238535v1_ufig1.gif" ALT="Figure 1"> View larger version (18K): org.highwire.dtl.DTLVardef@18d43c4org.highwire.dtl.DTLVardef@15c1974org.highwire.dtl.DTLVardef@18b396dorg.highwire.dtl.DTLVardef@16745bf_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LIMogat1 liver-specific KO or KD does not improve metabolism in HFD fed mice. C_LIO_LIWhole-body Mogat1-deletion impairs insulin tolerance in HFD fed mice. C_LIO_LIMogat1 ASOs improves whole body metabolism independently of gene knockdown. C_LIO_LIBlockade of the INFR response does not prevent off-target effects of Mogat1 ASOs. C_LI

physiology↗