Type 2 diabetes is a metabolic, chronic disorder characterized by insulin resistance and elevated blood glucose levels. Although a large drug portfolio exists to keep the blood glucose levels under ...control, these medications are not without side effects. More importantly, once diagnosed diabetes is rarely reversible. Dysfunctions in the kidney, retina, cardiovascular system, neurons, and liver represent the common complications of diabetes, which again lack effective therapies that can reverse organ injury. Overall, the molecular mechanisms of how type 2 diabetes develops and leads to irreparable organ damage remain elusive. This review particularly focuses on novel targets that may play role in pathogenesis of type 2 diabetes. Further research on these targets may eventually pave the way to novel therapies for the treatment—or even the prevention—of type 2 diabetes along with its complications.
Type 2 diabetes is a complex metabolic disorder associated with a plethora of complications in peripheral organs. This review article highlights the novel targets that might play role in pathogenesis of type 2 diabetes. Further research on these targets might eventually pave the way to novel therapies for the treatment–or even the prevention–of type 2 diabetes.
The role of the endothelium is not just limited to acting as an inert barrier for facilitating blood transport. Endothelial cells (ECs), through expression of a repertoire of angiocrine molecules, ...regulate metabolic demands in an organ‐specific manner. Insulin flux across the endothelium to muscle cells is a rate‐limiting process influencing insulin‐mediated lowering of blood glucose. Here, we demonstrate that Notch signaling in ECs regulates insulin transport to muscle. Notch signaling activity was higher in ECs isolated from obese mice compared to non‐obese. Sustained Notch signaling in ECs lowered insulin sensitivity and increased blood glucose levels. On the contrary, EC‐specific inhibition of Notch signaling increased insulin sensitivity and improved glucose tolerance and glucose uptake in muscle in a high‐fat diet‐induced insulin resistance model. This was associated with increased transcription of Cav1, Cav2, and Cavin1, higher number of caveolae in ECs, and insulin uptake rates, as well as increased microvessel density. These data imply that Notch signaling in the endothelium actively controls insulin sensitivity and glucose homeostasis and may therefore represent a therapeutic target for diabetes.
Synopsis
Insulin flux from blood plasma to muscle cells across the endothelium is a critical step in insulin‐mediated lowering of blood glucose levels. This study highlights the role of Notch signaling in regulating systemic glucose homeostasis.
Notch signaling activity was higher in endothelial cells (ECs) isolated from obese mice compared to non‐obese mice.
Sustained Notch signaling in ECs lowered insulin sensitivity and increased blood glucose levels in mice.
EC‐specific inhibition of canonical Notch signaling increased insulin sensitivity and improved glucose tolerance in a high‐fat diet‐induced insulin resistance model.
Glucose tolerance improvement was associated with increased transcription of genes involved in caveolae formation (Cav1 and Cavin1), higher number of caveolae in ECs and insulin uptake rates.
Insulin flux from blood plasma to muscle cells across the endothelium is a critical step in insulin‐mediated lowering of blood glucose levels. This study highlights the role of Notch signaling in regulating systemic glucose homeostasis.
Cachexia represents a fatal energy-wasting syndrome in a large number of patients with cancer that mostly results in a pathological loss of skeletal muscle and adipose tissue. Here we show that tumor ...cell exposure and tumor growth in mice triggered a futile energy-wasting cycle in cultured white adipocytes and white adipose tissue (WAT), respectively. Although uncoupling protein 1 (Ucp1)-dependent thermogenesis was dispensable for tumor-induced body wasting, WAT from cachectic mice and tumor-cell-supernatant-treated adipocytes were consistently characterized by the simultaneous induction of both lipolytic and lipogenic pathways. Paradoxically, this was accompanied by an inactivated AMP-activated protein kinase (Ampk), which is normally activated in peripheral tissues during states of low cellular energy. Ampk inactivation correlated with its degradation and with upregulation of the Ampk-interacting protein Cidea. Therefore, we developed an Ampk-stabilizing peptide, ACIP, which was able to ameliorate WAT wasting in vitro and in vivo by shielding the Cidea-targeted interaction surface on Ampk. Thus, our data establish the Ucp1-independent remodeling of adipocyte lipid homeostasis as a key event in tumor-induced WAT wasting, and we propose the ACIP-dependent preservation of Ampk integrity in the WAT as a concept in future therapies for cachexia.
Liver fibrosis is a strong predictor of long-term mortality in individuals with metabolic-associated fatty liver disease; yet, the mechanisms underlying the progression from the comparatively benign ...fatty liver state to advanced non-alcoholic steatohepatitis (NASH) and liver fibrosis are incompletely understood. Using cell-type-resolved genomics, we show that comprehensive alterations in hepatocyte genomic and transcriptional settings during NASH progression, led to a loss of hepatocyte identity. The hepatocyte reprogramming was under tight cooperative control of a network of fibrosis-activated transcription factors, as exemplified by the transcription factor Elf-3 (ELF3) and zinc finger protein GLIS2 (GLIS2). Indeed, ELF3- and GLIS2-controlled fibrosis-dependent hepatokine genes targeting disease-associated hepatic stellate cell gene programs. Thus, interconnected transcription factor networks not only promoted hepatocyte dysfunction but also directed the intra-hepatic crosstalk necessary for NASH and fibrosis progression, implying that molecular “hub-centered” targeting strategies are superior to existing mono-target approaches as currently used in NASH therapy.
Display omitted
•Advanced NASH is accompanied by partial loss of hepatocyte identity•NASH-induced hepatokines associate with fibrosis-linked genes in hepatic stellate cells•A cooperative transcription factor network drives hepatocyte genomic reprogramming in NASH•Fibrosis-activated ELF3 and GLIS2 promote intra-hepatic crosstalk and liver fibrosis
The mechanisms that drive the advanced stages of metabolic-associated fatty liver disease (MALFD) are still largely unknown. Using a cell type-resolved genomics approach, Loft et al. identified a fibrosis-activated hepatocyte transcription factor network that contributes to the loss of hepatocyte identity and dictates intra-hepatic cross-talk during the progression of MALFD.
Maladaptive insulin signaling is a key feature in the pathogenesis of severe metabolic disorders, including obesity and diabetes. Enhancing insulin sensitivity represents a major goal in the ...treatment of patients affected by diabetes. Here, we identify transforming growth factor-β1 stimulated clone 22 D4 (TSC22D4) as a novel interaction partner for protein kinase B/Akt1, a critical mediator of insulin/phosphatidylinositol 3-kinase signaling pathway. While energy deprivation and oxidative stress promote the TSC22D4-Akt1 interaction, refeeding mice or exposing cells to glucose and insulin impairs this interaction, which relies on an intrinsically disordered region (D2 domain) within TSC22D4. Functionally, the interaction with TSC22D4 reduces basal phosphorylation of Akt and its downstream targets during starvation, thereby promoting insulin sensitivity. Genetic, liver-specific reconstitution experiments in mice demonstrate that the interaction between TSC22D4 and Akt1 improves glucose handling and insulin sensitivity. Overall, our findings postulate a model whereby TSC22D4 acts as an environmental sensor and interacts with Akt1 to regulate insulin signaling and glucose metabolism.
Members of the lipocalin protein family serve as biomarkers for kidney disease and acute phase inflammatory reactions, and are under preclinical development for the diagnosis and therapy of ...allergies. However, none of the lipocalin family members has made the step into clinical development, mostly due to their complex biological activity and the lack of in-depth mechanistic knowledge. Here, we show that the hepatokine lipocalin 13 (LCN13) triggers glucose-dependent insulin secretion and cell proliferation of primary mouse islets. However, inhibition of endogenous LCN13 expression in lean mice did not alter glucose and lipid homeostasis. Enhanced hepatic secretion of LCN13 in either diet-induced or genetic obesity led to no discernible impact on systemic glucose and lipid metabolism, neither in preventive nor therapeutic setting. Of note, loss or forced LCN13 hepatic secretion did not trigger any compensatory regulation of related lipocalin family members. Together, these data are in stark contrast to the suggested gluco-regulatory and therapeutic role of LCN13 in obesity, and imply complex regulatory steps in LCN13 biology at the organismic level mitigating its principal insulinotropic effects.
Obesity-related insulin resistance represents the core component of the metabolic syndrome, promoting glucose intolerance, pancreatic beta cell failure and type 2 diabetes. Efficient and safe insulin ...sensitization and glucose control remain critical therapeutic aims to prevent diabetic late complications Here, we identify transforming growth factor beta-like stimulated clone (TSC) 22 D4 as a molecular determinant of insulin signalling and glucose handling. Hepatic TSC22D4 inhibition both prevents and reverses hyperglycaemia, glucose intolerance and insulin resistance in diabetes mouse models. TSC22D4 exerts its effects on systemic glucose homeostasis-at least in part-through the direct transcriptional regulation of the small secretory protein lipocalin 13 (LCN13). Human diabetic patients display elevated hepatic TSC22D4 expression, which correlates with decreased insulin sensitivity, hyperglycaemia and LCN13 serum levels. Our results establish TSC22D4 as a checkpoint in systemic glucose metabolism in both mice and humans, and propose TSC22D4 inhibition as an insulin sensitizing option in diabetes therapy.
The innate immune kinase TBK1 initiates inflammatory responses to combat infectious pathogens by driving production of type I interferons. TBK1 also controls metabolic processes and promotes ...oncogene‐induced cell proliferation and survival. Here, we demonstrate that TBK1 activates mTOR complex 1 (mTORC1) directly. In cultured cells, TBK1 associates with and activates mTORC1 through site‐specific mTOR phosphorylation (on S2159) in response to certain growth factor receptors (i.e., EGF‐receptor but not insulin receptor) and pathogen recognition receptors (PRRs) (i.e., TLR3; TLR4), revealing a stimulus‐selective role for TBK1 in mTORC1 regulation. By studying cultured macrophages and those isolated from genome edited mTOR S2159A knock‐in mice, we show that mTOR S2159 phosphorylation promotes mTORC1 signaling, IRF3 nuclear translocation, and IFN‐β production. These data demonstrate a direct mechanistic link between TBK1 and mTORC1 function as well as physiologic significance of the TBK1‐mTORC1 axis in control of innate immune function. These data unveil TBK1 as a direct mTORC1 activator and suggest unanticipated roles for mTORC1 downstream of TBK1 in control of innate immunity, tumorigenesis, and disorders linked to chronic inflammation.
Synopsis
TBK1, an IKK‐related kinase that drives interferon production as well cancer cell proliferation and survival, phosphorylates mTOR to activate mTORC1 in response to EGF and innate immune agonists, suggesting unanticipated roles for mTORC1 downstream of TBK1 in control of innate immunity and tumorigenesis.
TBK1 interacts with mTORC1 and phosphorylates mTOR on S2159 to increase its catalytic activity.
Cells lacking TBK1 or expressing a mTOR S2159A allele exhibit reduced mTORC1 signaling in response to EGF‐receptor and TLR3/4 activation.
Primary macrophages derived from genome edited mTOR S2159A mice exhibit reduced mTORC1 signaling in response to TLR3/4 activation.
Primary macrophages treated with rapamycin as well as those derived from mTORS2159A mice produce reduced levels of IFN‐β due to impaired nuclear translocation of the transcription factor IRF3.
Innate immune kinase TBK1‐dependent activation of mTORC1 occurs in response to pathogen recognition and EGF receptor activation and drives interferon production, thus highlighting the role of mTOR for innate immunity.
Non-alcoholic fatty liver disease (NAFLD), non-alcoholic steatohepatitis (NASH) and liver fibrosis emerge as progressive liver diseases that accompany metabolic syndrome usually characterized by ...obesity, insulin resistance and type 2 diabetes. Currently no FDA approved treatments exist for the treatment of NASH and liver fibrosis, which requires a better knowledge of the underlying molecular mechanisms.
TSC22D4 belongs to the TSC-22 protein family, the members of which are regulated by inflammatory and stress signals. Interestingly, patients with type 2 diabetes, with NAFLD as well as with NASH all have elevated levels of hepatic TSC22D4 expression. Previous studies with targeted deletion of TSC22D4 specifically in hepatocytes showed that TSC22D4 not only acts as a critical controller of diabetic hyperglycemia, but also contributes to NAFLD/NASH progression. To gain better insight into the development of progressive liver diseases, here we studied the function of TSC22D4 in hepatic stellate cells (HSCs), which play a key role in the pathogenesis of liver fibrosis. Our results indicated that TSC22D4 contributes to TGFβ1-mediated activation of HSCs and promotes their proliferation and migration. RNA-Sequencing analysis revealed that TSC22D4 initiates transcriptional events associated with HSC activation. Overall, our findings establish TSC22D4 as a key hub in the development of liver fibrosis, acting across different cellular compartments. Combinatorial TSC22D4 targeting in both hepatocytes and HSC may thus show superior efficacy against progressive liver disease.
•TSC22D4 acts downstream of TGFβ1 signaling in hepatic stellate cells (HSCs).•TSC22D4 promotes TGFβ1 induced proliferation and migration in HSCs.•RNA-Seq data reveal that TSC22D4 induces the expression of profibrogenic genes.
Increased pro-inflammatory signaling is a hallmark of metabolic dysfunction in obesity and diabetes. Although both inflammatory and energy substrate handling processes represent critical layers of ...metabolic control, their molecular integration sites remain largely unknown. Here, we identify the heterodimerization interface between the α and β subunits of transcription factor GA-binding protein (GAbp) as a negative target of tumor necrosis factor alpha (TNF-α) signaling. TNF-α prevented GAbpα and β complex formation via reactive oxygen species (ROS), leading to the non-energy-dependent transcriptional inactivation of AMP-activated kinase (AMPK) β1, which was identified as a direct hepatic GAbp target. Impairment of AMPKβ1, in turn, elevated downstream cellular cholesterol biosynthesis, and hepatocyte-specific ablation of GAbpα induced systemic hypercholesterolemia and early macro-vascular lesion formation in mice. As GAbpα and AMPKβ1 levels were also found to correlate in obese human patients, the ROS-GAbp-AMPK pathway may represent a key component of a hepato-vascular axis in diabetic long-term complications.
Display omitted
•TNF-α-induced ROS formation diminishes hepatic GAbp transcription factor function•Impaired hepatic GAbp function results in transcriptional inactivation of AMPK•AMPK deficiency increases hepatic cholesterol secretion•Hypercholesterolemia upon GAbp inhibition induces atherosclerotic lesion formation
Inflammatory signaling contributes to metabolic disease progression in obesity and diabetes. Niopek et al. identify the transcription factor GAbp to be inactivated in the liver by TNF-α-dependent oxidative stress. Inactivation of GAbp increases cholesterol levels through impaired hepatic AMPK function, contributing to macro-vascular lesion formation as a diabetic long-term complication.
PDF
