Metabolic functions of atypical protein kinase C: "good" and "bad" as defined by nutritional status

The PI3K/Akt signaling axis and type 2 diabetes mellitus (T2DM): From mechanistic insights into possible therapeutic targets

Type 2 diabetes mellitus (T2DM) is an immensely debilitating chronic disease that progressively undermines the well-being of various bodily organs and, indeed, most patients succumb to the disease due to post-T2DM complications. Although there is evidence supporting the activation of the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway by insulin, which is essential in regulating glucose metabolism and insulin resistance, the significance of this pathway in T2DM has only been explored in a few studies. The current review aims to unravel the mechanisms by which different classes of PI3Ks control the metabolism of glucose; and also to discuss the original data obtained from international research laboratories on this topic. We also summarized the role of the PI3K/Akt signaling axis in target tissues spanning from the skeletal muscle to the adipose tissue and liver. Furthermore, inquiries regarding the impact of disrupting this axis on insulin function and the development of insulin resistance have been addressed. We also provide a general overview of the association of impaired PI3K/Akt signaling pathways in the pathogenesis of the most prevalent diabetes-related complications. The last section provides a special focus on the therapeutic potential of this axis by outlining the latest advances in active compounds that alleviate diabetes via modulation of the PI3K/Akt pathway. Finally, we comment on the future research aspects in which the field of T2DM therapies using PI3K modulators might be developed.

Vanillic acid mitigates hyperinsulinemia induced ER stress mediated altered calcium homeostasis, MAMs distortion and surplus lipogenesis in HepG2 cells

Hyperinsulinemia (HI) induced insulin resistance (IR) and associated pathologies are the burning and unsolvable issues in diabetes treatment. The cellular, molecular and biochemical events associated with HI are not yet elucidated. Similarly, no focused research on designing therapeutic strategies with natural products for attenuation of HI are seen in literature. Keeping this in mind we planned the present study to evaluate the alterations occurring at ER/Ca²⁺ homeostasis/mitochondria associated endoplasmic reticulum membranes (MAMs) in HepG2 cells during HI and to evaluate the possible beneficial effect of vanillic acid (VA) to mitigate the complications. An in vitro model of HI was established by treating HepG2 cells with human insulin (1 μM) for 24 h. Then, ER stress, Ca²⁺ homeostasis, MAMs, IR and hepatic lipogenesis were studied at protein level. Various proteins critical to ER, Ca²⁺ homeostasis and MAMs such as p-IRE-1α, ATF6, p-PERK, p-eIF2α, CHOP, XBP1, p-CAMKII, InsP3R, SERCA, JNK, GRP78, VDAC, Cyp D, GRP75, MFN2, PTEN and mTORC were studied and found altered significantly causing ER stress, defect in Ca²⁺ movements and distortion of MAMs. The decreased expression of IRS2 and an unaltered expression of IRS1 confirmed the development of selective insulin resistance in hepatocytes during HI and this was the crucial factor for the progression of the hepatic lipid accumulation. We found simultaneous treatment of VA is beneficial up to a certain extent to protect HepG2 cells from the adverse effect of HI via its antioxidant, antilipogenic, mitochondrial and ER protection properties.

Membrane lipid raft homeostasis is directly linked to neurodegeneration

Age-associated neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD) and Alzheimer's disease (AD) are an unmet health need, with significant economic and societal implications, and an ever-increasing prevalence. Membrane lipid rafts (MLRs) are specialised plasma membrane microdomains that provide a platform for intracellular trafficking and signal transduction, particularly within neurons. Dysregulation of MLRs leads to disruption of neurotrophic signalling and excessive apoptosis which mirrors the final common pathway for neuronal death in ALS, PD and AD. Sphingomyelinase (SMase) and phospholipase (PL) enzymes process components of MLRs and therefore play central roles in MLR homeostasis and in neurotrophic signalling. We review the literature linking SMase and PL enzymes to ALS, AD and PD with particular attention to attractive therapeutic targets, where functional manipulation has been successful in preclinical studies. We propose that dysfunction of these enzymes is upstream in the pathogenesis of neurodegenerative diseases and to support this we provide new evidence that ALS risk genes are enriched with genes involved in ceramide metabolism (P=0.019, OR = 2.54, Fisher exact test). Ceramide is a product of SMase action upon sphingomyelin within MLRs, and it also has a role as a second messenger in intracellular signalling pathways important for neuronal survival. Genetic risk is necessarily upstream in a late age of onset disease such as ALS. We propose that manipulation of MLR structure and function should be a focus of future translational research seeking to ameliorate neurodegenerative disorders.

Identifying Glycemic Variability in Diabetes Patient Cohorts and Evaluating Disease Outcomes

Glycemic variability (GV) is an obstacle to effective blood glucose control and an autonomous risk factor for diabetes complications. We, therefore, explored sample data of patients with diabetes mellitus who maintained better amplitude of glycemic fluctuations and compared their disease outcomes with groups having poor control. A retrospective study was conducted using electronic data of patients having hemoglobin A1C (HbA1c) values with five recent time points from Think Whole Person Healthcare (TWPH). The control variability grid analysis (CVGA) plot and coefficient of variability (CV) were used to identify and cluster glycemic fluctuation. We selected important variables using LASSO. Chi-Square, Fisher’s exact test, Bonferroni chi-Square adjusted residual analysis, and multivariate Kruskal–Wallis tests were used to evaluate eventual disease outcomes. Patients with very high CV were strongly associated (p < 0.05) with disorders of lipoprotein (p = 0.0014), fluid, electrolyte, and acid–base balance (p = 0.0032), while those with low CV were statistically significant for factors influencing health status such as screening for other disorders (p = 0.0137), long-term (current) drug therapy (p = 0.0019), and screening for malignant neoplasms (p = 0.0072). Reducing glycemic variability may balance alterations in electrolytes and reduce differences in lipid profiles, which may assist in strategies for managing patients with diabetes mellitus.

Synthesis and structure-activity relationships of thieno[2,3-d]pyrimidines as atypical protein kinase C inhibitors to control retinal vascular permeability and cytokine-induced edema

Studies demonstrate that small molecule targeting of atypical protein kinase C (aPKC) may provide an effective means to control vascular permeability, prevent edema, and reduce inflammation providing novel and important alternatives to anti-VEGF therapies for certain blinding eye diseases. Based on a literature tricyclic thieno[2,3-d]pyrimidine lead (1), an ATP-competitive inhibitor of the aPKC iota (ι) and aPKC zeta (ζ) isoforms, we have synthesized a small series of compounds in 1-2 steps from a readily available chloro intermediate. A single pyridine congener was also made using 2D NMR to assign regiochemistry. Within the parent pyrimidine series, a range of potencies was observed against aPKCζ whereas the pyridine congener was inactive. Selected compounds were also tested for their effect toward VEGF-induced permeability in BREC cells. The most potent of these (7l) was further assayed against the aPKCι isoform and showed a favorable selectivity profile against a panel of 31 kinases, including kinases from the AGC superfamily, with a focus on PKC isoforms and kinases previously shown to affect permeability. Further testing of 7l in a luciferase assay in HEK293 cells showed an ability to prevent TNF-α induced NFκB activation while not having any effect on cell survival. Intravitreal administration of 7l to the eye yielded a complete reduction in permeability in a test to determine whether the compound could block VEGF- and TNFα-induced permeability across the retinal vasculature in a rat model. The compound in mice displayed good microsomal stability and in plasma moderate exposure (AUC and C_max), low clearance, a long half-life and high oral bioavailability. With IV dosing, higher levels were observed in the brain and eye relative to plasma, with highest levels in the eye by either IV or PO dosing. With a slow oral absorption profile, 7l accumulates in the eye to maintain a high concentration after dosing with higher levels than in plasma. Compound 7l may represent a class of aPKC inhibitors for further investigation.

Advances in physiological functions and mechanisms of (-)-epicatechin

(-)-Epicatechin (EC) is a flavanol easily obtained through the diet and is present in tea, cocoa, vegetables, fruits, and cereals. Recent studies have shown that EC protects human health and exhibits prominent anti-oxidant and anti-inflammatory activities, enhances muscle performance, improves symptoms of cardiovascular and cerebrovascular diseases, prevents diabetes, and protects the nervous system. With the development of modern medical and biotechnology research, the mechanisms of action associated with EC toward various chronic diseases are becoming more apparent, and the pharmacological development and utilization of EC has been increasingly clarified. Currently, there is no comprehensive systematic introduction to the effects of EC and its mechanisms of action. This review presents the latest research progress and the role of EC in the prevention and treatment of various chronic diseases and its protective health effects and provides a theoretical basis for future research on EC.

Clinical significance of atypical protein kinase C (PKCι and PKCζ) and its relationship with yes-associated protein in lung adenocarcinoma

BACKGROUND

Protein kinase C iota (PKCι) and protein kinase C zeta (PKCζ) are two atypical protein kinase (aPKC) enzymes that contribute to cell proliferation and cancer development. The Hippo/YAP pathway is commonly disrupted and upregulated in cancers. Herein, the expression patterns and clinical relevance of PKCι and PKCζ are evaluated in relation to YAP, a downstream effector of Hippo, in lung adenocarcinoma (LAC). The protein and mRNA expression levels of PKCι, PKCζ, YAP, and their phosphorylated forms, namely p-PKCι, p-PKCζ and p-YAP, are evaluated in relation to clinicopathological factors, including patient survival.

METHODS

A total of 200 primary LAC tissue samples were examined by immunohistochemistry for PKCι, p-PKCι, PKCζ, p-PKCζ, YAP, and p-YAP protein expression. Sixty pairs of LAC and non-neoplastic lung tissue samples were assessed for PRKCI, PRKCZ, and YAP mRNA levels. PKCι, p-PKCι, PKCζ, and p-PKCζ protein expression were evaluated by Western blot analysis in the PC9 and PC9/GR LAC cell lines with YAP modulation.

RESULTS

LAC demonstrated cytoplasmic PKCι, p-PKCι, PKCζ, and p-PKCζ immunostaining patterns. Positive aPKC protein expressions were related with poor patient survival. Especially, increased p-PKCι protein expression was significantly correlated with higher pathological stage and shortened overall survival. YAP overexpression contributes phosphorylation of PKCι and PKCζ protein expression in the LAC cell line.

CONCLUSIONS

PKCι and PKCζ are related to YAP in LAC. PKCι and PKCζ play distinct roles in LAC; specifically, p-PKCι overexpression is suggested to underlie factors that indicate a poor prognosis.

A role for phosphatidylcholine and phosphatidylethanolamine in hepatic insulin signaling

Phosphatidylethanolamine N-methyltransferase (PEMT) is an important enzyme in hepatic phosphatidylcholine (PC) biosynthesis. Pemt^-/- mice fed a high-fat diet are protected from obesity and whole-body insulin resistance. However, Pemt^-/- mice develop severe nonalcoholic steatohepatitis (NASH). Because NASH is often associated with hepatic insulin resistance, we investigated whether the increased insulin sensitivity in Pemt^-/- mice was restricted to nonhepatic tissues or whether the liver was also insulin sensitive. Strikingly, the livers of Pemt^-/- mice compared with those of Pemt^+/+ mice were not insulin resistant, despite elevated levels of hepatic triacylglycerols and diacylglycerols, as well as increased hepatic inflammation and fibrosis. Endogenous glucose production was lower in Pemt^-/- mice under both basal and hyperinsulinemic conditions. Experiments in primary hepatocytes and hepatoma cells revealed improved insulin signaling in the absence of PEMT, which was not due to changes in diacylglycerols, ceramides, or gangliosides. On the other hand, the phospholipid composition in hepatocytes seems critically important for insulin signaling such that lowering the PC:phosphatidylethanolamine (PE) ratio improves insulin signaling. Thus, treatments to reduce the PC:PE ratio in liver may protect against the development of hepatic insulin resistance.-Van der Veen, J. N., Lingrell, S., McCloskey, N., LeBlond, N. D., Galleguillos, D., Zhao, Y. Y., Curtis, J. M., Sipione, S., Fullerton, M. D., Vance, D. E., Jacobs, R. L. A role for phosphatidylcholine and phosphatidylethanolamine in hepatic insulin signaling.

Combined Hyperglycemia- and Hyperinsulinemia-Induced Insulin Resistance in Adipocytes Is Associated With Dual Signaling Defects Mediated by PKC-ζ

A hyperglycemic and hyperinsulinemic environment characteristic of type 2 diabetes causes insulin resistance. In adipocytes, defects in both insulin sensitivity and maximum response of glucose transport have been demonstrated. To investigate the molecular mechanisms, freshly isolated rat adipocytes were incubated in control (5.6 mM glucose, no insulin) and high glucose (20 mM)/high insulin (100 nM) (HG/HI) for 18 hours to induce insulin resistance. Insulin-resistant adipocytes manifested decreased sensitivity of glucose uptake associated with defects in insulin receptor substrate (IRS)-1 Tyr phosphorylation, association of p85 subunit of phosphatidylinositol-3-kinase, Akt Ser473 and Thr308 phosphorylation, accompanied by impaired glucose transporter 4 translocation. In contrast, protein kinase C (PKC)-ζ activity was augmented by chronic HG/HI. Inhibition of PKC-ζ with a specific cell-permeable peptide reversed the signaling defects and insulin sensitivity of glucose uptake. Transfection of dominant-negative, kinase-inactive PKC-ζ blocked insulin resistance, whereas constitutively active PKC-ζ recapitulated the defects. The HG/HI incubation was associated with stimulation of IRS-1 Ser318 and Akt Thr34 phosphorylation, targets of PKC-ζ. Transfection of IRS-1 S318A and Akt T34A each partially corrected insulin signaling, whereas combined transfection of both completely normalized insulin signaling. In vivo hyperglycemia/hyperinsulinemia in rats for 48 hours similarly resulted in activation of PKC-ζ and increased phosphorylation of IRS-1 Ser318 and Akt Thr34. These data indicate that impairment of insulin signaling by chronic HG/HI is mediated by dual defects at IRS-1 and Akt mediated by PKC-ζ.

Splenectomy after partial hepatectomy accelerates liver regeneration in mice by promoting tight junction formation via polarity protein Par 3-aPKC

AIMS

Several experimental studies have demonstrated that removal of the spleen accelerates liver regeneration after partial hepatectomy. While the mechanism of splenectomy promotes liver regeneration by the improvement of the formation of tight junction and the establishment of hepatocyte polarity is still unknown.

MAIN METHODS

We analyzed the cytokines, genes and proteins expression between 70% partial hepatectomy mice (PHx) and simultaneous 70% partial hepatectomy and splenectomy mice (PHs) at predetermined timed points.

KEY FINDINGS

Compared with the PHx group mice, splenectomy accelerated hepatocyte proliferation in PHs group. The expression of Zonula occludens-1 (ZO-1) indicated that splenectomy promotes the formation of tight junction during liver regeneration. TNF-α, IL-6, HGF, TSP-1 and TGF-β1 were essential factors for the formation of tight junction and the establishment of hepatocytes polarity in liver regeneration. After splenectomy, Partitioning defective 3 homolog (Par 3) and atypical protein kinase C (aPKC) regulate hepatocyte localization and junctional structures in regeneration liver.

SIGNIFICANCE

Our data suggest that the time course expression of TNF-α, IL-6, HGF, TSP-1, and TGF-β1 and the change of platelets take part in liver regeneration. Combination with splenectomy accelerates liver regeneration by improvement of the tight junction formation which may help to establish hepatocyte polarity via Par 3-aPKC. This may provide a clue for us that splenectomy could accelerate liver regeneration after partial hepatectomy of hepatocellular carcinoma and living donor liver transplantation.

A mathematical model of the impact of insulin secretion dynamics on selective hepatic insulin resistance

Physiological insulin secretion exhibits various temporal patterns, the dysregulation of which is involved in diabetes development. We analyzed the impact of first-phase and pulsatile insulin release on glucose and lipid control with various hepatic insulin signaling networks. The mathematical model suggests that atypical protein kinase C (aPKC) undergoes a bistable switch-on and switch-off, under the control of insulin receptor substrate 2 (IRS2). The activation of IRS1 and IRS2 is temporally separated due to the inhibition of IRS1 by aPKC. The model further shows that the timing of aPKC switch-off is delayed by reduced first-phase insulin and reduced amplitude of insulin pulses. Based on these findings, we propose a sequential model of postprandial hepatic control of glucose and lipid by insulin, according to which delayed aPKC switch-off contributes to selective hepatic insulin resistance, which is a long-standing paradox in the field.

Dysregulation of insulin secretion dynamics plays a role in diabetes development. Here, the authors build a mathematical model of hepatic insulin signaling and propose a sequential model of post-meal control of glucose and lipids, according to which delayed aPKC suppression would contribute to selective hepatic insulin resistance.

A proof-of-concept trial of protein kinase C iota inhibition with auranofin for the paclitaxel-induced acute pain syndrome

PURPOSE

Paclitaxel causes the paclitaxel-induced acute pain (PIAP) syndrome. Based on preclinical data, we hypothesized that the protein kinase C (PKC) iota inhibitor, auranofin (a gold salt used for other pain conditions), palliates this pain.

METHODS

In a randomized, double-blinded manner, patients who had suffered this syndrome were assigned a one-time dose of auranofin 6 mg orally on day #2 of the chemotherapy cycle (post-paclitaxel) versus placebo. Patients completed the Brief Pain Inventory and a pain diary on days 2 through 8 and at the end of the cycle. The primary endpoint was pain scores, as calculated by area under the curve, in response to "Please rate your pain by circling the one number that best describes your pain at its worse in the last 24 hours."

RESULTS

Thirty patients were enrolled. For the primary endpoint, mean area under the curve of 55 units (standard deviation 19) and 61 units (standard deviation 22) were observed in auranofin-treated and placebo-exposed patients, respectively (p = 0.44). On day 8 and at the end of the cycle, pain scores in auranofin-treated patients were more favorable, although differences were not statistically significant.

CONCLUSIONS

In the dose schedule studied, auranofin did not palliate the PIAP syndrome, but delayed beneficial trends suggest further study for this indication.

Differential hepatic distribution of insulin receptor substrates causes selective insulin resistance in diabetes and obesity

Hepatic insulin signalling involves insulin receptor substrates (Irs) 1/2, and is normally associated with the inhibition of gluconeogenesis and activation of lipogenesis. In diabetes and obesity, insulin no longer suppresses hepatic gluconeogenesis, while continuing to activate lipogenesis, a state referred to as 'selective insulin resistance'. Here, we show that 'selective insulin resistance' is caused by the differential expression of Irs1 and Irs2 in different zones of the liver. We demonstrate that hepatic Irs2-knockout mice develop 'selective insulin resistance', whereas mice lacking in Irs1, or both Irs1 and Irs2, develop 'total insulin resistance'. In obese diabetic mice, Irs1/2-mediated insulin signalling is impaired in the periportal zone, which is the primary site of gluconeogenesis, but enhanced in the perivenous zone, which is the primary site of lipogenesis. While hyperinsulinaemia reduces Irs2 expression in both the periportal and perivenous zones, Irs1 expression, which is predominantly in the perivenous zone, remains mostly unaffected. These data suggest that 'selective insulin resistance' is induced by the differential distribution, and alterations of hepatic Irs1 and Irs2 expression.

Gentiopicroside and sweroside from Veratrilla baillonii Franch. induce phosphorylation of Akt and suppress Pck1 expression in hepatoma cells

The use of phytochemicals and herbal medicines has accompanied human history. Advances in modern biomedical sciences have allowed us to investigate the functional mechanisms of herbal medicines and phytochemicals. Veratrilla baillonii Franch. has long been used as a medicinal herb in southwestern China. Here, we analyzed the effects of an ethanol extract from V. baillonii (VBFE) on the expression levels of the cytosolic form of the phosphoenolpyruvate carboxykinase gene (Pck1) mRNA and components of the insulin signalling cascade in HL1C hepatoma cells. Compared with the insulin control, VBFE treatment inhibited the expression of Pck1 mRNA in a dose-dependent manner. This was associated with the phosphorylation of Akt and Erk1/2 in a time-dependent manner. Further analysis of the purified components of VBFE indicated that gentiopicroside and sweroside from VBFE, alone and in combination, suppressed Pck1 expression and induced Akt and Erk1/2 phosphorylation. In conclusion, gentiopicroside and sweroside suppress Pck1 expression and induce phosphorylation of components in the insulin signalling cascade. This is the first study to demonstrate that gentiopicroside and sweroside show insulin-mimicking effects on the regulation of Pck1 expression. Further studies are warranted to explore the potential of gentiopicroside and sweroside in the control of blood glucose in animals.

Conditional knockout of polarity complex (atypical) PKCι reveals an anti-inflammatory function mediated by NF-κB

Atypical PKC, Par6, and Par3 constitute a conserved complex signaling cell asymmetry. In contrast to its role in other tissues, atypical PKC inhibits NF-κB activation in epithelia and may function in maintaining low levels of inflammation in addition to establishing apicobasal polarity.

The conserved proteins of the polarity complex made up of atypical PKC (aPKC, isoforms ι and ζ), Par6, and Par3 determine asymmetry in several cell types, from Caenorhabditis elegans oocytes to vertebrate epithelia and neurons. We previously showed that aPKC is down-regulated in intestinal epithelia under inflammatory stimulation. Further, expression of constitutively active PKCι decreases NF-κB activity in an epithelial cell line, the opposite of the effect reported in other cells. Here we tested the hypothesis that aPKC has a dual function in epithelia, inhibiting the NF-κB pathway in addition to having a role in apicobasal polarity. We achieved full aPKC down-regulation in small intestine villi and colon surface epithelium using a conditional epithelium-specific knockout mouse. The results show that aPKC is dispensable for polarity after cell differentiation, except for known targets, including ROCK and ezrin, claudin-4 expression, and barrier permeability. The aPKC defect resulted in increased NF-κB activity, which could be rescued by IKK and ROCK inhibitors. It also increased expression of proinflammatory cytokines. In contrast, expression of anti-inflammatory IL-10 decreased. We conclude that epithelial aPKC acts upstream of multiple mechanisms that participate in the inflammatory response in the intestine, including, but not restricted to, NF-κB.

S100B protein in skeletal muscle regeneration: regulation of myoblast and macrophage functions

Not available.

Insulin Signaling in Liver and Adipose Tissues in Periparturient Dairy Cows Supplemented with Dietary Nicotinic Acid

The glucose homeostasis in dairy cattle is very well controlled, in line with the metabolic adaptation during the periparturient period. Former studies showed that nicotinic acid (NA) lowered plasma non-esterified fatty acids (NEFA) concentrations and increased insulin sensitivity in dairy cows. Thus, the purpose of this study was to investigate whether the expression of proteins involved in hepatic and adipose insulin signaling and protein expression of hepatic glucose transporter 2 (GLUT2) were affected by dietary NA and dietary concentrate intake in periparturient dairy cows. Twenty pluriparous German Holstein cows were fed with the same diet from about 21 days before the expected calving date (d-21) to calving. After calving, cows were randomly assigned in 4 groups and fed with diets different in concentrate proportion (“HC” with 60:40% or “LC” with 30:70% concentrate-to-roughage ratio) and supplemented with NA (24 g/day) (NA) or without (CON) until d21. Biopsy samples were taken from the liver, subcutaneous (SCAT) and retroperitoneal (RPAT) adipose tissues at d-21 and d21. Protein expression of insulin signaling molecules (insulin receptor (INSR), phosphatidylinositol-3-kinase (PI3K), protein kinase Cζ (PKCζ)) and hepatic GLUT2 was measured by Western Blotting. The ratio of protein expression at d21/at d-21 was calculated and statistically evaluated for the effects of time and diet. Cows in HC had significantly higher dietary energy intake than cows in LC. In RPAT a decrease in PI3K and PKCζ expression was found in all groups, irrespectively of diet. In the liver, the GLUT2 expression was significantly lower in cows in NA compared with cows in CON. In conclusion, insulin signaling might be decreased in RPAT over time without any effect of diet. NA was able to modulate hepatic GLUT2 expression, but its physiological role is unclear.

Mechanisms for greater insulin-stimulated glucose uptake in normal and insulin-resistant skeletal muscle after acute exercise

Enhanced skeletal muscle and whole body insulin sensitivity can persist for up to 24-48 h after one exercise session. This review focuses on potential mechanisms for greater postexercise and insulin-stimulated glucose uptake (ISGU) by muscle in individuals with normal or reduced insulin sensitivity. A model is proposed for the processes underlying this improvement; i.e., triggers initiate events that activate subsequent memory elements, which store information that is relayed to mediators, which translate memory into action by controlling an end effector that directly executes increased insulin-stimulated glucose transport. Several candidates are potential triggers or memory elements, but none have been conclusively verified. Regarding potential mediators in both normal and insulin-resistant individuals, elevated postexercise ISGU with a physiological insulin dose coincides with greater Akt substrate of 160 kDa (AS160) phosphorylation without improved proximal insulin signaling at steps from insulin receptor binding to Akt activity. Causality remains to be established between greater AS160 phosphorylation and improved ISGU. The end effector for normal individuals is increased GLUT4 translocation, but this remains untested for insulin-resistant individuals postexercise. Following exercise, insulin-resistant individuals can attain ISGU values similar to nonexercising healthy controls, but after a comparable exercise protocol performed by both groups, ISGU for the insulin-resistant group has been consistently reported to be below postexercise values for the healthy group. Further research is required to fully understand the mechanisms underlying the improved postexercise ISGU in individuals with normal or subnormal insulin sensitivity and to explain the disparity between these groups after similar exercise.

Contraction stimulates muscle glucose uptake independent of atypical PKC

Exercise increases skeletal muscle glucose uptake, but the underlying mechanisms are only partially understood. The atypical protein kinase C (PKC) isoforms λ and ζ (PKC‐λ/ζ) have been shown to be necessary for insulin‐, AICAR‐, and metformin‐stimulated glucose uptake in skeletal muscle, but not for treadmill exercise‐stimulated muscle glucose uptake. To investigate if PKC‐λ/ζ activity is required for contraction‐stimulated muscle glucose uptake, we used mice with tibialis anterior muscle‐specific overexpression of an empty vector (WT), wild‐type PKC‐ζ (PKC‐ζ^WT), or an enzymatically inactive T410A‐PKC‐ζ mutant (PKC‐ζ^T410A). We also studied skeletal muscle‐specific PKC‐λ knockout (MλKO) mice. Basal glucose uptake was similar between WT, PKC‐ζ^WT, and PKC‐ζ^T410A tibialis anterior muscles. In contrast, in situ contraction‐stimulated glucose uptake was increased in PKC‐ζ^T410A tibialis anterior muscles compared to WT or PKC‐ζ^WT tibialis anterior muscles. Furthermore, in vitro contraction‐stimulated glucose uptake was greater in soleus muscles of MλKO mice than WT controls. Thus, loss of PKC‐λ/ζ activity increases contraction‐stimulated muscle glucose uptake. These data clearly demonstrate that PKC‐λ/ζ activity is not necessary for contraction‐stimulated glucose uptake.

BMI-related progression of atypical PKC-dependent aberrations in insulin signaling through IRS-1, Akt, FoxO1 and PGC-1α in livers of obese and type 2 diabetic humans

Information on insulin resistance in human liver is limited. In mouse diet-induced obesity (DIO), hepatic insulin resistance initially involves: lipid+insulin-induced activation of atypical protein kinase C (aPKC); elevated Akt activity/activation but selective impairment of compartmentalized Akt-dependent FoxO1 phosphorylation; and increases in gluconeogenic and lipogenic enzymes. In advanced stages, e.g., in hepatocytes of type 2 diabetes (T2D) humans, insulin activation of insulin receptor substrate-1(IRS-1) and Akt fails, further increasing FoxO1-dependent gluconeogenic/lipogenic enzyme expression. Increases in hepatic PGC-1α also figure prominently, but uncertainly, in this scheme. Here, we examined signaling factors in liver samples harvested from human transplant donors with increasing BMI, 20→25→30→35→40→45. We found, relative to lean (BMI=20-25) humans, obese (BMI>30) humans had all abnormalities seen in early mouse DIO, but, surprisingly, at all elevated BMI levels, had decreased insulin receptor-1 (IRS-1) levels, decreased Akt activity, and increased expression/abundance of aPKC-ι and PGC-1α. Moreover, with increasing BMI, there were: progressive increases in aPKC activity and PKC-ι expression/abundance; progressive decreases in IRS-1 levels, Akt activity and FoxO1 phosphorylation; progressive increases in expression/abundance of PGC-1α; and progressive increases in gluconeogenic and lipogenic enzymes. Remarkably, all abnormalities reached T2D levels at higher BMI levels. Most importantly, both "early" and advanced abnormalities were largely reversed by 24-hour treatment of T2D hepatocytes with aPKC inhibitor. We conclude: hepatic insulin resistance in human obesity is: advanced; BMI-correlated; and sequentially involves increased aPKC-activating ceramide; increased aPKC levels and activity; decreases in IRS-1 levels, Akt activity, and FoxO1 phosphorylation; and increases in expression/abundance of PGC-1α and gluconeogenic and lipogenic genes.

Atypical PKC: a target for treating insulin-resistant disorders of obesity, the metabolic syndrome and type 2 diabetes mellitus

INTRODUCTION

The prevalence of obesity, the metabolic syndrome and type 2 diabetes mellitus have reached pandemic levels. Present therapies do not directly target the key factor responsible for the insulin resistance that underlies the development of these syndromes.

AREAS COVERED

This review focuses on hepatic atypical PKC (aPKC) as a key target for treating these disorders. It reviews data obtained from multiple experimental mouse models of obesity and type 2 diabetes, and hepatocytes of type 2 diabetic humans.

EXPERT OPINION

The review shows that hepatic aPKC is excessively activated by diet-derived lipids and by insulin itself in hyperinsulinemic states. It also shows how excessively activated hepatic aPKC increases expression of gluconeogenic, lipogenic and proinflammatory factors that underlie the development of glucose intolerance, insulin resistance, obesity, hepatosteatosis and hyperlipidemia. Most importantly, the review shows how the selective inhibition of hepatic aPKC by a variety of means, including expression of inhibitory forms of aPKC, genetic deletion of aPKC and use of several newly developed small-molecular-weight chemical agents result in correction of hepatic abnormalities, such as excessive expression of gluconeogenic, lipogenic and proinflammatory factors, and correction or improvement in clinical abnormalities (glucose intolerance, obesity, hepatosteatosis and hyperlipidemia).

Higher protein kinase C ζ in fatty rat liver and its effect on insulin actions in primary hepatocytes

We previously showed the impairment of insulin-regulated gene expression in the primary hepatocytes from Zucker fatty (ZF) rats, and its association with alterations of hepatic glucose and lipid metabolism. However, the molecular mechanism is unknown. A preliminary experiment shows that the expression level of protein kinase C ζ (PKCζ), a member of atypical PKC family, is higher in the liver and hepatocytes of ZF rats than that of Zucker lean (ZL) rats. Herein, we intend to investigate the roles of atypical protein kinase C in the regulation of hepatic gene expression. The insulin-regulated hepatic gene expression was evaluated in ZL primary hepatocytes treated with atypical PKC recombinant adenoviruses. Recombinant adenovirus-mediated overexpression of PKCζ, or the other atypical PKC member PKCι/λ, alters the basal and impairs the insulin-regulated expressions of glucokinase, sterol regulatory element-binding protein 1c, the cytosolic form of phosphoenolpyruvate carboxykinase, the catalytic subunit of glucose 6-phosphatase, and insulin like growth factor-binding protein 1 in ZL primary hepatocytes. PKCζ or PKCι/λ overexpression also reduces the protein level of insulin receptor substrate 1, and the insulin-induced phosphorylation of AKT at Ser473 and Thr308. Additionally, PKCι/λ overexpression impairs the insulin-induced Prckz expression, indicating the crosstalk between PKCζ and PKCι/λ. We conclude that the PKCζ expression is elevated in hepatocytes of insulin resistant ZF rats. Overexpressions of aPKCs in primary hepatocytes impair insulin signal transduction, and in turn, the down-stream insulin-regulated gene expression. These data suggest that elevation of aPKC expression may contribute to the hepatic insulin resistance at gene expression level.

Common mechanisms of Alzheimer's disease and ischemic stroke: the role of protein kinase C in the progression of age-related neurodegeneration

Ischemic stroke and Alzheimer's disease (AD), despite being distinct disease entities, share numerous pathophysiological mechanisms such as those mediated by inflammation, immune exhaustion, and neurovascular unit compromise. An important shared mechanistic link is acute and chronic changes in protein kinase C (PKC) activity. PKC isoforms have widespread functions important for memory, blood-brain barrier maintenance, and injury repair that change as the body ages. Disease states accelerate PKC functional modifications. Mutated forms of PKC can contribute to neurodegeneration and cognitive decline. In some cases the PKC isoforms are still functional but are not successfully translocated to appropriate locations within the cell. The deficits in proper PKC translocation worsen stroke outcome and amyloid-β toxicity. Cross talk between the innate immune system and PKC pathways contribute to the vascular status within the aging brain. Unfortunately, comorbidities such as diabetes, obesity, and hypertension disrupt normal communication between the two systems. The focus of this review is to highlight what is known about PKC function, how isoforms of PKC change with age, and what additional alterations are consequences of stroke and AD. The goal is to highlight future therapeutic targets that can be applied to both the treatment and prevention of neurologic disease. Although the pathology of ischemic stroke and AD are different, the similarity in PKC responses warrants further investigation, especially as PKC-dependent events may serve as an important connection linking age-related brain injury.

Hepatic insulin resistance in ob/ob mice involves increases in ceramide, aPKC activity, and selective impairment of Akt-dependent FoxO1 phosphorylation

Pathogenesis of insulin resistance in leptin-deficient ob/ob mice is obscure. In another form of diet-dependent obesity, high-fat-fed mice, hepatic insulin resistance involves ceramide-induced activation of atypical protein kinase C (aPKC), which selectively impairs protein kinase B (Akt)-dependent forkhead box O1 protein (FoxO1) phosphorylation on scaffolding protein, 40 kDa WD(tryp-x-x-asp)-repeat propeller/FYVE protein (WD40/ProF), thereby increasing gluconeogenesis. Resultant hyperinsulinemia activates hepatic Akt and mammalian target of rapamycin C1, and further activates aPKC; consequently, lipogenic enzyme expression increases, and insulin signaling in muscle is secondarily impaired. Here, in obese minimally-diabetic ob/ob mice, hepatic ceramide and aPKC activity and its association with WD40/ProF were increased. Hepatic Akt activity was also increased, but Akt associated with WD40/ProF was diminished and accounted for reduced FoxO1 phosphorylation and increased gluconeogenic enzyme expression. Most importantly, liver-selective inhibition of aPKC decreased aPKC and increased Akt association with WD40/ProF, thereby restoring FoxO1 phosphorylation and reducing gluconeogenic enzyme expression. Additionally, lipogenic enzyme expression diminished, and insulin signaling in muscle, glucose tolerance, obesity, hepatosteatosis, and hyperlipidemia improved. In conclusion, hepatic ceramide accumulates in response to CNS-dependent dietary excess irrespective of fat content; hepatic insulin resistance is prominent in ob/ob mice and involves aPKC-dependent displacement of Akt fromWD40/ProF and subsequent impairment of FoxO1 phosphorylation and increased expression of hepatic gluconeogenic and lipogenic enzymes; and hepatic alterations diminish insulin signaling in muscle.

Akt-dependent phosphorylation of hepatic FoxO1 is compartmentalized on a WD40/ProF scaffold and is selectively inhibited by aPKC in early phases of diet-induced obesity

Initiating mechanisms that impair gluconeogenic enzymes and spare lipogenic enzymes in diet-induced obesity (DIO) are obscure. Here, we examined insulin signaling to Akt and atypical protein kinase C (aPKC) in liver and muscle and hepatic enzyme expression in mice consuming a moderate high-fat (HF) diet. In HF diet-fed mice, resting/basal and insulin-stimulated Akt and aPKC activities were diminished in muscle, but in liver, these activities were elevated basally and were increased by insulin to normal levels. Despite elevated hepatic Akt activity, FoxO1 phosphorylation, which diminishes gluconeogenesis, was impaired; in contrast, Akt-dependent phosphorylation of glycogenic GSK3β and lipogenic mTOR was elevated. Diminished Akt-dependent FoxO1 phosphorylation was associated with reduced Akt activity associated with scaffold protein WD40/Propeller/FYVE (WD40/ProF), which reportedly facilitates FoxO1 phosphorylation. In contrast, aPKC activity associated with WD40/ProF was increased. Moreover, inhibition of hepatic aPKC reduced its association with WD40/ProF, restored WD40/ProF-associated Akt activity, restored FoxO1 phosphorylation, and corrected excessive expression of hepatic gluconeogenic and lipogenic enzymes. Additionally, Akt and aPKC activities in muscle improved, as did glucose intolerance, weight gain, hepatosteatosis, and hyperlipidemia. We conclude that Akt-dependent FoxO1 phosphorylation occurs on the WD/Propeller/FYVE scaffold in liver and is selectively inhibited in early DIO by diet-induced increases in activity of cocompartmentalized aPKC.

Discovery and optimization of 1,3,5-trisubstituted pyrazolines as potent and highly selective allosteric inhibitors of protein kinase C-ζ

There is increasing evidence that the atypical protein kinase C, PKCζ, might be a therapeutic target in pulmonary and hepatic inflammatory diseases. However, targeting the highly conserved ATP-binding pocket in the catalytic domain held little promise to achieve selective inhibition. In the present study, we introduce 1,3,5-trisubstituted pyrazolines as potent and selective allosteric PKCζ inhibitors. The rigid scaffold offered many sites for modification, all acting as hot spots for improving activity, and gave rise to sharp structure-activity relationships. Targeting of PKCζ in cells was confirmed by reporter gene assay, transfection assays, and Western blotting. The strongly reduced cell-free and cellular activities toward a PIF-pocket mutant of PKCζ suggested that the inhibitors most likely bound to the PIF-pocket on the kinase catalytic domain. Thus, using a rigidification strategy and by establishing and optimizing multiple molecular interactions with the binding site, we were able to significantly improve the potency of the previously reported PKCζ inhibitors.

Hepatic Atypical Protein Kinase C: An Inherited Survival-Longevity Gene that Now Fuels Insulin-Resistant Syndromes of Obesity, the Metabolic Syndrome and Type 2 Diabetes Mellitus

This review focuses on how insulin signals to metabolic processes in health, why this signaling is frequently deranged in Western/Westernized societies, how these derangements lead to, or abet development of, insulin-resistant states of obesity, the metabolic syndrome and type 2 diabetes mellitus, and what our options are for restoring insulin signaling, and glucose/lipid homeostasis. A central theme in this review is that excessive hepatic activity of an archetypal protein kinase enzyme, “atypical” protein kinase C (aPKC), plays a critically important role in the development of impaired glucose metabolism, systemic insulin resistance, and excessive hepatic production of glucose, lipids and proinflammatory factors that underlie clinical problems of glucose intolerance, obesity, hepatosteatosis, hyperlipidemia, and, ultimately, type 2 diabetes. The review suggests that normally inherited genes, in particular, the aPKC isoforms, that were important for survival and longevity in times of food scarcity are now liabilities in times of over-nutrition. Fortunately, new knowledge of insulin signaling mechanisms and how an aberration of excessive hepatic aPKC activation is induced by over-nutrition puts us in a position to target this aberration by diet and/or by specific inhibitors of hepatic aPKC.

PKCλ haploinsufficiency prevents diabetes by a mechanism involving alterations in hepatic enzymes

Tissue-specific knockout (KO) of atypical protein kinase C (aPKC), PKC-λ, yields contrasting phenotypes, depending on the tissue. Thus, whereas muscle KO of PKC-λ impairs glucose transport and causes glucose intolerance, insulin resistance, and liver-dependent lipid abnormalities, liver KO and adipocyte KO of PKC-λ increase insulin sensitivity through salutary alterations in hepatic enzymes. Also note that, although total-body (TB) homozygous KO of PKC-λ is embryonic lethal, TB heterozygous (Het) KO (TBHetλKO) is well-tolerated. However, beneath their seemingly normal growth, appetite, and overall appearance, we found in TBHetλKO mice that insulin receptor phosphorylation and signaling through insulin receptor substrates to phosphatidylinositol 3-kinase, Akt and residual aPKC were markedly diminished in liver, muscle, and adipose tissues, and glucose transport was impaired in muscle and adipose tissues. Furthermore, despite these global impairments in insulin signaling, other than mild hyperinsulinemia, glucose tolerance, serum lipids, and glucose disposal and hepatic glucose output in hyperinsulinemic clamp studies were normal. Moreover, TBHetλKO mice were protected from developing glucose intolerance during high-fat feeding. This metabolic protection (in the face of impaired insulin signaling) in HetλKO mice seemed to reflect a deficiency of PKC-λ in liver with resultant 1) increases in FoxO1 phosphorylation and decreases in expression of hepatic gluconeogenic enzymes and 2) diminished expression of hepatic lipogenic enzymes and proinflammatory cytokines. In keeping with this postulate, adenoviral-mediated supplementation of hepatic PKC-λ induced a diabetic state in HetλKO mice. Our findings underscore the importance of hepatic PKC-λ in provoking abnormalities in glucose and lipid metabolism.

Insulin receptor signaling in normal and insulin-resistant states

In the wake of the worldwide increase in type-2 diabetes, a major focus of research is understanding the signaling pathways impacting this disease. Insulin signaling regulates glucose, lipid, and energy homeostasis, predominantly via action on liver, skeletal muscle, and adipose tissue. Precise modulation of this pathway is vital for adaption as the individual moves from the fed to the fasted state. The positive and negative modulators acting on different steps of the signaling pathway, as well as the diversity of protein isoform interaction, ensure a proper and coordinated biological response to insulin in different tissues. Whereas genetic mutations are causes of rare and severe insulin resistance, obesity can lead to insulin resistance through a variety of mechanisms. Understanding these pathways is essential for development of new drugs to treat diabetes, metabolic syndrome, and their complications.

Impairment of insulin-stimulated glucose transport and ERK activation by adipocyte-specific knockout of PKC-λ produces a phenotype characterized by diminished adiposity and enhanced insulin suppression of hepatic gluconeogenesis

Tissue-specific knockout (KO) of atypical protein kinase C-λ (PKC-λ) impairs insulin-stimulated glucose transport in muscle (M) and lipid synthesis in liver (L), thereby producing insulin resistance in MλKO mice and insulin-hypersensitivity in LλKO mice. Here, we generated mice with KO of PKC-λ in adipocytes, i.e., AλKO mice. In isolated adipocytes of AλKO mice, insulin-stimulated aPKC activity and glucose transport were diminished, as were ERK levels and activity. Insulin-stimulated glucose transport and insulin activation of ERK in adipocytes of wild-type mice were similarly inhibited by acute inhibition of PKC-λ with a highly-specific chemical inhibitor. With impairments in glucose transport and ERK activation, AλKO mice had diminished adiposity and serum leptin levels. In addition, AλKO mice had normal glucose tolerance and insulin hypersensitivity owing to enhanced suppression of hepatic glucose output, which apparently reflected increases in Akt activity and FoxO1 phosphorylation, and subsequent decreases in expression of gluconeogenic phosphoenolpyruvate carboxykinase. We conclude that: PKC-λ is required for insulin-stimulated glucose transport and ERK signaling in mouse adipocytes; and diminution of these processes is attended by leanness and therefore hypoleptinemia. How these and perhaps other PKC-λ-dependent processes communicate to liver and improve insulin suppression of hepatic gluconeogenesis remains unclear.

Metformin action in human hepatocytes: coactivation of atypical protein kinase C alters 5'-AMP-activated protein kinase effects on lipogenic and gluconeogenic enzyme expression

AIMS/HYPOTHESIS

Atypical protein kinase C (aPKC) levels and activity are elevated in hepatocytes of individuals with type 2 diabetes and cause excessive increases in the levels of lipogenic and gluconeogenic enzymes; aPKC inhibitors largely correct these aberrations. Metformin improves hepatic gluconeogenesis by activating 5'-AMP-activated protein kinase (AMPK). However, metformin also activates aPKC in certain tissues; in the liver, this activation could amplify diabetic aberrations and offset the positive effects of AMPK. In this study, we examined whether metformin activates aPKC in human hepatocytes and the metabolic consequences of any such activation.

METHODS

We compared protein kinase activities and alterations in lipogenic and gluconeogenic enzyme levels during activity of the AMPK activators metformin and AICAR, relative to those of an aPKC-ι inhibitor, in hepatocytes from non-diabetic and type 2 diabetic human organ donors.

RESULTS

Metformin and 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR) activated aPKC at concentrations comparable with those required for AMPK activation. Moreover, both agents increased lipogenic enzyme levels by an aPKC-dependent mechanism. Thus, whereas insulin- and diabetes-dependent increases in lipogenic enzyme levels were reversed by aPKC inhibition, such levels were increased in hepatocytes from non-diabetic donors and remained elevated in hepatocytes from diabetic donors following metformin and AICAR treatment. In addition, whereas aPKC inhibition diminished gluconeogenic enzyme levels in the absence and presence of insulin in hepatocytes from both non-diabetic and diabetic donors, metformin and AICAR increased gluconeogenic enzyme levels in hepatocytes from non-diabetic individuals, but nevertheless diminished gluconeogenic enzyme levels in insulin-treated hepatocytes from diabetic donors.

CONCLUSIONS/INTERPRETATION

Metformin and AICAR activate aPKC together with AMPK in human hepatocytes. Activation of aPKC increases lipogenic enzyme levels and alters gluconeogenic enzyme levels, and therefore appears to offset the positive effects of AMPK.

FGF21 suppresses hepatic glucose production through the activation of atypical protein kinase Cι/λ

Fibroblast growth factor 21 (FGF21) has been identified as a potent and robust metabolic regulator. Administration of recombinant FGF21 protein to rodents and rhesus monkeys exerts strong anti-diabetic effects. Previous studies have demonstrated that FGF21 inhibits glucose output in the rat H4IIE hepatoma cell line. We performed pharmacological studies to investigate the mechanisms by which FGF21 regulates glucose production in these cells. We found that both insulin and FGF21 suppressed gene expression of G6Pase and PEPCK. Accordingly, glucose production was inhibited. The FGF21 effects were phosphoinositide 3-kinase (PI3K)-dependent, and, unlike insulin, Akt-independent. Additionally, we found that FGF21 induced PKCι/λ phosphorylation in a PI3K-dependent manner; and that a non-isoform selective PKC inhibitor blocked FGF21 inhibition of glucose production, while an inhibitor of classical and novel PKC isoforms had no effect on FGF21 inhibitory activity. Furthermore, hepatic PKCι/λ phosphorylation was upregulated in FGF21-treated diabetic db/db mice.These data support the proposition that FGF21 inhibits hepatic glucose production by the PI3K-dependent activation of PKCι/λ.

Phosphorylation and recruitment of BAF60c in chromatin remodeling for lipogenesis in response to insulin

Fatty acid and triglyceride synthesis is induced in response to feeding and insulin. This lipogenic induction involves coordinate transcriptional activation of lipogenic enzymes, including fatty acid synthase and glycerol-3-phosphate acyltransferase. We recently reported the importance of USF-1 phosphorylation and subsequent acetylation in insulin-induced lipogenic gene activation. Here, we show that Brg1/Brm-associated factor (BAF) 60c is a specific chromatin remodeling component for lipogenic gene transcription in liver. In response to insulin, BAF60c is phosphorylated at S247 by atypical PKCζ/λ, which causes translocation of BAF60c to the nucleus and allows a direct interaction of BAF60c with USF-1 that is phosphorylated by DNA-PK and acetylated by P/CAF. Thus, BAF60c is recruited to form the lipoBAF complex to remodel chromatin structure and to activate lipogenic genes. Consequently, BAF60c promotes lipogenesis in vivo and increases triglyceride levels, demonstrating its role in metabolic adaption to activate the lipogenic program in response to feeding and insulin.

Novel atypical PKC inhibitors prevent vascular endothelial growth factor-induced blood-retinal barrier dysfunction

Pro-inflammatory cytokines and growth factors such as VEGF (vascular endothelial growth factor) contribute to the loss of the BRB (blood-retinal barrier) and subsequent macular oedema in various retinal pathologies. VEGF signalling requires PKCβ [conventional PKC (protein kinase C)] activity; however, PKCβ inhibition only partially prevents VEGF-induced endothelial permeability and does not affect pro-inflammatory cytokine-induced permeability, suggesting the involvement of alternative signalling pathways. In the present study, we provide evidence for the involvement of aPKC (atypical PKC) signalling in VEGF-induced endothelial permeability and identify a novel class of inhibitors of aPKC that prevent BRB breakdown in vivo. Genetic and pharmacological manipulations of aPKC isoforms were used to assess their contribution to endothelial permeability in culture. A chemical library was screened using an in vitro kinase assay to identify novel small-molecule inhibitors, and further medicinal chemistry was performed to delineate a novel pharmacophore. We demonstrate that aPKC isoforms are both sufficient and required for VEGF-induced endothelial permeability. Furthermore, these specific, potent, non-competitive, small-molecule inhibitors prevented VEGF-induced tight junction internalization and retinal endothelial permeability in response to VEGF in both primary culture and in rodent retina. The results of the present study suggest that aPKC inhibition with 2-amino-4-phenyl-thiophene derivatives may be developed to preserve the BRB in retinal diseases such as diabetic retinopathy or uveitis, and the BBB (blood-brain barrier) in the presence of brain tumours.

Role of adipose and hepatic atypical protein kinase C lambda (PKCλ) in the development of obesity and glucose intolerance

PKCλ, an atypical member of the multifunctional protein kinase C family, has been implicated in the regulation of insulin-stimulated glucose transport and of the intracellular immune response. To further elucidate the role of this cellular regulator in diet-induced obesity and insulin resistance, we generated both liver (PKC-Alb) and adipose tissue (PKC-Ap2) specific knockout mice. Body weight, fat mass, food intake, glucose homeostasis and energy expenditure were evaluated in mice maintained on either chow or high fat diet (HFD). Ablation of PKCλ from the adipose tissue resulted in mice that were indistinguishable from their wild-type littermates. However, PKC-Alb mice were resistant to diet-induced obesity (DIO). Surprisingly this DIO resistance was not associated with either a reduction in caloric intake or an increase in energy expenditure as compared with their wild-type littermates. Furthermore, these mice displayed an improvement in glucose tolerance. When maintained on chow diet, these mice were similar to wild types in respect to body weight and fat mass, yet insulin sensitivity was impaired compared with wt littermates. Taken together these data suggest that hepatic PKCλ is modulating insulin-mediated glucose turnover and response to high fat diet feeding, thus offering a deeper understanding of an important target for anti-obesity therapeutics.

Regulation of insulin sensitivity by serine/threonine phosphorylation of insulin receptor substrate proteins IRS1 and IRS2

The insulin receptor substrate proteins IRS1 and IRS2 are key targets of the insulin receptor tyrosine kinase and are required for hormonal control of metabolism. Tissues from insulin-resistant and diabetic humans exhibit defects in IRS-dependent signalling, implicating their dysregulation in the initiation and progression of metabolic disease. However, IRS1 and IRS2 are regulated through a complex mechanism involving phosphorylation of >50 serine/threonine residues (S/T) within their long, unstructured tail regions. In cultured cells, insulin-stimulated kinases (including atypical PKC, AKT, SIK2, mTOR, S6K1, ERK1/2 and ROCK1) mediate feedback (autologous) S/T phosphorylation of IRS, with both positive and negative effects on insulin sensitivity. Additionally, insulin-independent (heterologous) kinases can phosphorylate IRS1/2 under basal conditions (AMPK, GSK3) or in response to sympathetic activation and lipid/inflammatory mediators, which are present at elevated levels in metabolic disease (GRK2, novel and conventional PKCs, JNK, IKKβ, mPLK). An emerging view is that the positive/negative regulation of IRS by autologous pathways is subverted/co-opted in disease by increased basal and other temporally inappropriate S/T phosphorylation. Compensatory hyperinsulinaemia may contribute strongly to this dysregulation. Here, we examine the links between altered patterns of IRS S/T phosphorylation and the emergence of insulin resistance and diabetes.

Restoration of defective L-type Ca2+ current in cardiac myocytes of type 2 diabetic db/db mice by Akt and PKC-ι

Diabetes is associated with an increased risk of heart failure and the development of a cardiomyopathy whose etiology is only partially understood. Ca entry through the voltage-dependent L-type Ca channel CaV1.2 initiates the contractile cycle in cardiac myocytes. Decreased cardiac contractility and depressed CaV1.2 function have been reported in obese type 2 diabetic db/db mice. Here, we demonstrate that a reduction in phosphoinositide 3-kinase (PI3K) signaling is a major contributor to the altered function of CaV1.2 in db/db cardiac myocytes. Using the whole-cell patch clamp technique, we determined that intracellular infusion of cardiac myocytes from db/db mice with phosphatidylinositol 3,4,5-trisphosphate (PIP3), the second messenger produced by PI3K, increased the L-type Ca current (ICa,L) density nearly to the level seen in wild-type cells. PIP3 also reversed the positive shift in the voltage dependence of the steady-state current activation observed in db/db myocytes. Infusion of protein kinases that act downstream of PI3K also affected ICa,L. Akt1 and Akt2 were as effective as PIP3 in restoring the ICa,L density in db/db myocytes but did not affect the voltage dependence of current activation. The infusion of atypical PKC-ι (the human homolog of mouse PKC-λ) caused a small but significant increase in the ICa,L density and completely reversed the shift in voltage dependence of steady-state current activation. These results indicate that a defect in PI3K/PIP3/Akt/PKC-λ signaling is mainly responsible for the depressed CaV1.2 function in the hearts of db/db mice with type 2 diabetes.

The problem of establishing relationships between hepatic steatosis and hepatic insulin resistance

Excessive deposition of fat in the liver (hepatic steatosis) is frequently accompanied by hepatic insulin resistance. Whether this correlation is due to a causal relationship between the conditions has been the subject of considerable debate, and the literature abounds with conflicting data and theories. Here we provide a perspective by defining the problem and its challenges, analyzing the possible causative relationships, and drawing some conclusions.

Insulin signalling in hepatocytes of humans with type 2 diabetes: excessive production and activity of protein kinase C-ι (PKC-ι) and dependent processes and reversal by PKC-ι inhibitors

AIMS/HYPOTHESIS

We examined the role of protein kinase C-ι (PKC-ι) in mediating alterations in the abundance of enzymes in hepatocytes of type 2 diabetic humans that contribute importantly to the development of lipid and carbohydrate abnormalities in type 2 diabetes.

METHODS

We examined (1) insulin signalling in isolated hepatocytes of non-diabetic and type 2 diabetic humans and (2) the effects of two newly developed small molecule PKC-ι inhibitors on aberrant signalling and downstream processes.

RESULTS

In contrast with PKC-ι deficiency in diabetic muscle, which diminishes glucose transport, PKC-ι in diabetic hepatocytes was overproduced and overactive, basally and after insulin treatment, and, moreover, was accompanied by increased abundance of PKC-ι-dependent lipogenic, proinflammatory and gluconeogenic enzymes. Heightened PKC-ι activity most likely reflected heightened activity of IRS-2-dependent phosphatidylinositol 3-kinase (PI3K), as IRS-1 levels and IRS-1/PI3K activity were markedly diminished. Importantly, insulin-stimulated PKC-ι abundance and its overabundance in diabetic hepatocytes was reversed in vitro by both insulin deprivation and PKC-ι inhibitors; this suggested operation of an insulin-driven, feed-forward/positive-feedback mechanism. In contrast with PKC-ι, protein kinase B (Akt2) activity and activation by insulin was diminished, apparently reflecting IRS-1 deficiency. Treatment of diabetic hepatocytes with PKC-ι/λ inhibitors diminished abundance of lipogenic, proinflammatory and gluconeogenic enzymes.

CONCLUSIONS/INTERPRETATION

Our findings suggest that a vicious cycle of PKC-ι overactivity and overproduction exists in hepatocytes of humans with type 2 diabetes and contributes importantly to maintaining overactivity of lipogenic, proinflammatory and gluconeogenic pathways, which underlies the lipid and carbohydrate abnormalities in type 2 diabetes.

Correction of metabolic abnormalities in a rodent model of obesity, metabolic syndrome, and type 2 diabetes mellitus by inhibitors of hepatic protein kinase C-ι

Excessive activity of hepatic atypical protein kinase (aPKC) is proposed to play a critical role in mediating lipid and carbohydrate abnormalities in obesity, the metabolic syndrome, and type 2 diabetes mellitus. In previous studies of rodent models of obesity and type 2 diabetes mellitus, adenoviral-mediated expression of kinase-inactive aPKC rapidly reversed or markedly improved most if not all metabolic abnormalities. Here, we examined effects of 2 newly developed small-molecule PKC-ι/λ inhibitors. We used the mouse model of heterozygous muscle-specific knockout of PKC-λ, in which partial deficiency of muscle PKC-λ impairs glucose transport in muscle and thereby causes glucose intolerance and hyperinsulinemia, which, via hepatic aPKC activation, leads to abdominal obesity, hepatosteatosis, hypertriglyceridemia, and hypercholesterolemia. One inhibitor, 1H-imidazole-4-carboxamide, 5-amino-1-[2,3-dihydroxy-4-[(phosphonooxy)methyl]cyclopentyl-[1R-(1a,2b,3b,4a)], binds to the substrate-binding site of PKC-λ/ι, but not other PKCs. The other inhibitor, aurothiomalate, binds to cysteine residues in the PB1-binding domains of aPKC-λ/ι/ζ and inhibits scaffolding. Treatment with either inhibitor for 7 days inhibited aPKC, but not Akt, in liver and concomitantly improved insulin signaling to Akt and aPKC in muscle and adipocytes. Moreover, both inhibitors diminished excessive expression of hepatic, aPKC-dependent lipogenic, proinflammatory, and gluconeogenic factors; and this was accompanied by reversal or marked improvements in hyperglycemia, hyperinsulinemia, abdominal obesity, hepatosteatosis, hypertriglyceridemia, and hypercholesterolemia. Our findings highlight the pathogenetic importance of insulin signaling to hepatic PKC-ι in obesity, the metabolic syndrome, and type 2 diabetes mellitus and suggest that 1H-imidazole-4-carboxamide, 5-amino-1-[2,3-dihydroxy-4-[(phosphonooxy)methyl]cyclopentyl-[1R-(1a,2b,3b,4a)] and aurothiomalate or similar agents that selectively inhibit hepatic aPKC may be useful treatments.

Molecular basis of signaling specificity of insulin and IGF receptors: neglected corners and recent advances

Insulin and insulin-like growth factor (IGF) receptors utilize common phosphoinositide 3-kinase/Akt and Ras/extracellular signal-regulated kinase signaling pathways to mediate a broad spectrum of "metabolic" and "mitogenic" responses. Specificity of insulin and IGF action in vivo must in part reflect expression of receptors and responsive pathways in different tissues but it is widely assumed that it is also determined by the ligand binding and signaling mechanisms of the receptors. This review focuses on receptor-proximal events in insulin/IGF signaling and examines their contribution to specificity of downstream responses. Insulin and IGF receptors may differ subtly in the efficiency with which they recruit their major substrates (IRS-1 and IRS-2 and Shc) and this could influence effectiveness of signaling to "metabolic" and "mitogenic" responses. Other substrates (Grb2-associated binder, downstream of kinases, SH2Bs, Crk), scaffolds (RACK1, β-arrestins, cytohesins), and pathways (non-receptor tyrosine kinases, phosphoinositide kinases, reactive oxygen species) have been less widely studied. Some of these components appear to be specifically involved in "metabolic" or "mitogenic" signaling but it has not been shown that this reflects receptor-preferential interaction. Very few receptor-specific interactions have been characterized, and their roles in signaling are unclear. Signaling specificity might also be imparted by differences in intracellular trafficking or feedback regulation of receptors, but few studies have directly addressed this possibility. Although published data are not wholly conclusive, no evidence has yet emerged for signaling mechanisms that are specifically engaged by insulin receptors but not IGF receptors or vice versa, and there is only limited evidence for differential activation of signaling mechanisms that are common to both receptors. Cellular context, rather than intrinsic receptor activity, therefore appears to be the major determinant of whether responses to insulin and IGFs are perceived as "metabolic" or "mitogenic."

An introduction to phosphoinositides

Phosphoinositides (PIs) are minor components of cellular membranes that play critical regulatory roles in several intracellular functions. This chapter describes the main enzymes regulating the turnover of each of the seven PIs in mammalian cells and introduces to some of their intracellular functions and to some evidences of their involvement in human diseases. Due to the complex interrelation between the distinct PIs and the plethora of functions that they can regulate inside a cell, this chapter is not meant to be a comprehensive coverage of all aspects of PI signalling but rather an introduction to this complex signalling field. For more details of their regulation/functions and extensive description of their intracellular roles, more detailed reviews are suggested on each single topic.

Nerve growth factor enhances the excitability of rat sensory neurons through activation of the atypical protein kinase C isoform, PKMζ

Our previous work showed that nerve growth factor (NGF) increased the excitability of small-diameter capsaicin-sensitive sensory neurons by activating the p75 neurotrophin receptor and releasing sphingolipid-derived second messengers. Whole cell patch-clamp recordings were used to establish the signaling pathways whereby NGF augments action potential (AP) firing (i.e., sensitization). Inhibition of MEK1/2 (PD-98059), PLC (U-73122, neomycin), or conventional/novel isoforms of PKC (bisindolylmaleimide I) had no effect on the sensitization produced by NGF. Pretreatment with a membrane-permeable, myristoylated pseudosubstrate inhibitor of atypical PKCs (aPKCs: PKMζ, PKCζ, and PKCλ/ι) blocked the NGF-induced increase in AP firing. Inhibitors of phosphatidylinositol 3-kinase (PI3K) also blocked the sensitization produced by NGF. Isolated sensory neurons were also treated with small interfering RNA (siRNA) targeted to PKCζ. Both Western blots and quantitative real-time PCR established that PKMζ, but neither full-length PKCζ nor PKCλ/ι, was significantly reduced after siRNA exposure. Treatment with these labeled siRNA prevented the NGF-induced enhancement of excitability. Furthermore, consistent with the high degree of catalytic homology for aPKCs, internal perfusion with active recombinant PKCζ or PKCι augmented excitability, recapitulating the sensitization produced by NGF. Internal perfusion with recombinant PKCζ suppressed the total potassium current and enhanced the tetrodotoxin-resistant sodium current. Pretreatment with the myristoylated pseudosubstrate inhibitor blocked the increased excitability produced by ceramide or internal perfusion with recombinant PKCζ. These results demonstrate that NGF leads to the activation of PKMζ that ultimately enhances the capacity of small-diameter capsaicin-sensitive sensory neurons to fire APs through a PI3K-dependent signaling cascade.

Anthocyanin inhibits high glucose-induced hepatic mtGPAT1 activation and prevents fatty acid synthesis through PKCζ

Mitochondrial acyl-CoA:glycerol-sn-3-phosphate acyltransferase 1 (mtGPAT1) controls the first step of triacylglycerol (TAG) synthesis and is critical to the understanding of chronic metabolic disorders such as primary nonalcoholic fatty liver disease (NAFLD). Anthocyanin, a large group of polyphenols, was negatively correlated with hepatic lipid accumulation, but its impact on mtGPAT1 activity and NAFLD has yet to be determined. Hepatoma cell lines and KKAy mice were used to investigate the impact of anthocyanin on high glucose-induced mtGPAT1 activation and hepatic steatosis. Treatment with anthocyanin cyanidin-3-O-β-glucoside (Cy-3-g) reduced high glucose-induced GPAT1 activity through the prevention of mtGPAT1 translocation from the endoplasmic reticulum to the outer mitochondrial membrane (OMM), thereby suppressing intracellular de novo lipid synthesis. Cy-3-g treatment also increased protein kinase C ζ phosphorylation and membrane translocation in order to phosphorylate the mtF0F1-ATPase β-subunit, reducing its enzymatic activity and thus inhibiting mtGPAT1 activation. In vivo studies further showed that Cy-3-g treatment significantly decreases hepatic mtGPAT1 activity and its presence in OMM isolated from livers, thus ameliorating hepatic steatosis in diabetic KKAy mice. Our findings reveal a novel mechanism by which anthocyanin regulates lipogenesis and thereby inhibits hepatic steatosis, suggesting its potential therapeutic application in diabetes and related steatotic liver diseases.