Synapses of amphids defective (SAD-A) kinase promotes glucose-stimulated insulin secretion through activation of p21-activated kinase (PAK1) in pancreatic β-Cells

Redox Regulation of Brain Selective Kinases BRSK1/2: Implications for Dynamic Control of the Eukaryotic AMPK family through Cys-based mechanisms

In eukaryotes, protein kinase signaling is regulated by a diverse array of post-translational modifications (PTMs), including phosphorylation of Ser/Thr residues and oxidation of cysteine (Cys) residues. While regulation by activation segment phosphorylation of Ser/Thr residues is well understood, relatively little is known about how oxidation of cysteine residues modulate catalysis. In this study, we investigate redox regulation of the AMPK-related Brain-selective kinases (BRSK) 1 and 2, and detail how broad catalytic activity is directly regulated through reversible oxidation and reduction of evolutionarily conserved Cys residues within the catalytic domain. We show that redox-dependent control of BRSKs is a dynamic and multilayered process involving oxidative modifications of several Cys residues, including the formation of intramolecular disulfide bonds involving a pair of Cys residues near the catalytic HRD motif and a highly conserved T-Loop Cys with a BRSK-specific Cys within an unusual CPE motif at the end of the activation segment. Consistently, mutation of the CPE-Cys increases catalytic activity in vitro and drives phosphorylation of the BRSK substrate Tau in cells. Molecular modeling and molecular dynamics simulations indicate that oxidation of the CPE-Cys destabilizes a conserved salt bridge network critical for allosteric activation. The occurrence of spatially proximal Cys amino acids in diverse Ser/Thr protein kinase families suggests that disulfide mediated control of catalytic activity may be a prevalent mechanism for regulation within the broader AMPK family.

BRSK2 in pancreatic β cells promotes hyperinsulinemia-coupled insulin resistance and its genetic variants are associated with human type 2 diabetes

Brain-specific serine/threonine-protein kinase 2 (BRSK2) plays critical roles in insulin secretion and β-cell biology. However, whether BRSK2 is associated with human type 2 diabetes mellitus (T2DM) has not been determined. Here, we report that BRSK2 genetic variants are closely related to worsening glucose metabolism due to hyperinsulinemia and insulin resistance in the Chinese population. BRSK2 protein levels are significantly elevated in β cells from T2DM patients and high-fat diet (HFD)-fed mice due to enhanced protein stability. Mice with inducible β-cell-specific Brsk2 knockout (βKO) exhibit normal metabolism with a high potential for insulin secretion under chow-diet conditions. Moreover, βKO mice are protected from HFD-induced hyperinsulinemia, obesity, insulin resistance, and glucose intolerance. Conversely, gain-of-function BRSK2 in mature β cells reversibly triggers hyperglycemia due to β-cell hypersecretion-coupled insulin resistance. Mechanistically, BRSK2 senses lipid signals and induces basal insulin secretion in a kinase-dependent manner. The enhanced basal insulin secretion drives insulin resistance and β-cell exhaustion and thus the onset of T2DM in mice fed an HFD or with gain-of-function BRSK2 in β cells. These findings reveal that BRSK2 links hyperinsulinemia to systematic insulin resistance via interplay between β cells and insulin-sensitive tissues in the populations carrying human genetic variants or under nutrient-overload conditions.

Regulation of insulin secretion by the post-translational modifications

Post-translational modification (PTM) has a significant impact on cellular signaling and function regulation. In pancreatic β cells, PTMs are involved in insulin secretion, cell development, and viability. The dysregulation of PTM in β cells is clinically associated with the development of diabetes mellitus. Here, we summarized current findings on major PTMs occurring in β cells and their roles in insulin secretion. Our work provides comprehensive insight into understanding the mechanisms of insulin secretion and potential therapeutic targets for diabetes from the perspective of protein PTMs.

AMPKα-like proteins as LKB1 downstream targets in cell physiology and cancer

One of the key events in cancer development is the ability of tumor cells to overcome nutrient deprivation and hypoxia. Among proteins performing metabolic adaptation to the various cellular nutrient conditions, liver kinase B 1 (LKB1) and its main downstream target adenosine monophosphate (AMP)-activated protein kinase α (AMPKα) are important sensors of energy requirements within the cell. Although LKB1 was originally described as a tumor suppressor, given its role in metabolism, it potentially acts as a double-edged sword. AMPKα, a master regulator of cell energy demands, is activated when ATP level drops under a certain threshold, responding accordingly through its downstream targets. Twelve downstream kinase targets of LKB1 have been described as AMPKα-like proteins. This group is comprised of novel (nua) kinase family (NUAK) kinases (NUAK1 and 2) linked to cell cycle progression and ultraviolet (UV)-damage; microtubule affinity regulating kinases (MARKs) (MARK1, MARK2, MARK3, and MARK4) that are involved in cell polarity; salt inducible kinases (SIK) (SIK1, SIK2, also known as Qin-induced kinase or QIK and SIK3) that are implicated in cell metabolism and adipose tissue development and mitotic regulation; maternal embryonic leuzine zipper kinase (MELK) that regulate oocyte maturation; and finally brain selective kinases (BRSKs) (BRSK1 and 2), which have been mainly characterized in the brain due to their role in neuronal polarization. Thus, many efforts have been made in order to harness LKB1 kinase and its downstream targets as a possible therapeutic hub in tumor development and propagation. In this review, we describe LKB1 and its downstream target AMPK summarize major functions of various AMPK-like proteins, while focusing on biological functions of BRSK1 and 2 in different models.

Gain-of-function genetic screen of the kinome reveals BRSK2 as an inhibitor of the NRF2 transcription factor

Nuclear factor erythroid 2-related factor 2 (NFE2L2, also known as NRF2) is a transcription factor and master regulator of cellular antioxidant response. Aberrantly high NRF2-dependent transcription is recurrent in human cancer, but conversely NRF2 activity diminishes with age and in neurodegenerative and metabolic disorders. Although NRF2-activating drugs are clinically beneficial, NRF2 inhibitors do not yet exist. Here, we describe use of a gain-of-function genetic screen of the kinome to identify new druggable regulators of NRF2 signaling. We found that the under-studied protein kinase brain-specific kinase 2 (BRSK2) and the related BRSK1 kinases suppress NRF2-dependent transcription and NRF2 protein levels in an activity-dependent manner. Integrated phosphoproteomics and RNAseq studies revealed that BRSK2 drives 5'-AMP-activated protein kinase α2 (AMPK) signaling and suppresses the mTOR pathway. As a result, BRSK2 kinase activation suppresses ribosome-RNA complexes, global protein synthesis and NRF2 protein levels. Collectively, our data illuminate the BRSK2 and BRSK1 kinases, in part by functionally connecting them to NRF2 signaling and mTOR. This signaling axis might prove useful for therapeutically targeting NRF2 in human disease.This article has an associated First Person interview with the first author of the paper.

New Insights into Beta-Cell GLP-1 Receptor and cAMP Signaling

Harnessing the translational potential of the GLP-1/GLP-1R system in pancreatic beta cells has led to the development of established GLP-1R-based therapies for the long-term preservation of beta cell function. In this review, we discuss recent advances in the current research on the GLP-1/GLP-1R system in beta cells, including the regulation of signaling by endocytic trafficking as well as the application of concepts such as signal bias, allosteric modulation, dual agonism, polymorphic receptor variants, spatial compartmentalization of cAMP signaling and new downstream signaling targets involved in the control of beta cell function.

Cdc42: A Novel Regulator of Insulin Secretion and Diabetes-Associated Diseases

Cdc42, a member of the Rho GTPases family, is involved in the regulation of several cellular functions including cell cycle progression, survival, transcription, actin cytoskeleton organization and membrane trafficking. Diabetes is a chronic and metabolic disease, characterized as glycometabolism disorder induced by insulin deficiency related to β cell dysfunction and peripheral insulin resistance (IR). Diabetes could cause many complications including diabetic nephropathy (DN), diabetic retinopathy and diabetic foot. Furthermore, hyperglycemia can promote tumor progression and increase the risk of malignant cancers. In this review, we summarized the regulation of Cdc42 in insulin secretion and diabetes-associated diseases. Organized researches indicate that Cdc42 is a crucial member during the progression of diabetes, and Cdc42 not only participates in the process of insulin synthesis but also regulates the insulin granule mobilization and cell membrane exocytosis via activating a series of downstream factors. Besides, several studies have demonstrated Cdc42 as participating in the pathogenesis of IR and DN and even contributing to promote cancer cell proliferation, survival, invasion, migration, and metastasis under hyperglycemia. Through the current review, we hope to cast light on the mechanism of Cdc42 in diabetes and associated diseases and provide new ideas for clinical diagnosis, treatment, and prevention.

SAD-A, a downstream mediator of GLP-1 signaling, promotes the phosphorylation of Bad S155 to regulate in vitro β-cell functions

The incretin hormone GLP-1 reduces β-cell failure in patients with type 2 diabetes. Previous studies demonstrated that GLP-1 activates SAD-A, a member of the AMPK family, to regulate glucose-stimulated secretion (GSIS), but the underlying mechanisms of SAD-A regulation of β-cell functions remain poorly understood. Here, we propose that activation of SAD-A by GLP-1 promotes the phosphorylation of Bad S155, which in turn positively affects GSIS and β-cell survival. Bad therefore appears to be a downstream molecule of a SAD-A pathway that mediates the GLP-1-triggered reduction in β-cell failure. Knockdown of endogenous SAD-A expression significantly exacerbated in vitro β-cell dysfunction under lipotoxic conditions and promoted lipotoxicity-induced apoptosis, whereas overexpression of SAD-A inhibited β-cell apoptosis. SAD-A silencing increased ER stress and inhibited the autophagic flux, which contributed to β-cell apoptosis. Thus, SAD-A appears to function as a downstream molecule of GLP-1 signaling that results in Bad S155 phosphorylation. This phosphorylation might therefore be involved in the GLP-1-linked protection against β-cell dysfunction and apoptosis.

SAD-A Promotes Glucose-Stimulated Insulin Secretion Through Phosphorylation and Inhibition of GDIα in Male Islet β Cells

Rho GDP-dissociation inhibitor (GDIα) inhibits glucose-stimulated insulin secretion (GSIS) in part by locking Rho GTPases in an inactive GDP-bound form. The onset of GSIS causes phosphorylation of GDIα at Ser174, a critical inhibitory site for GDIα, leading to the release of Rho GTPases and their subsequent activation. However, the kinase regulator(s) that catalyzes the phosphorylation of GDIα in islet β cells remains elusive. We propose that SAD-A, a member of AMP-activated protein kinase-related kinases that promotes GSIS as an effector kinase for incretin signaling, interacts with and inhibits GDIα through phosphorylation of Ser174 during the onset GSIS from islet β cells. Coimmunoprecipitation and phosphorylation analyses were carried out to identify the physical interaction and phosphorylation site of GDIα by SAD-A in the context of GSIS from INS-1 β cells and primary islets. We identified GDIα directly binds to SAD-A kinase domain and phosphorylated by SAD-A on Ser174, leading to dissociation of Rho GTPases from GDIα complexes. Accordingly, overexpression of SAD-A significantly stimulated GDIα phosphorylation at Ser174 in response to GSIS, which is dramatically potentiated by glucagonlike peptide-1, an incretin hormone. Conversely, SAD-A deficiency, which is mediated by short hairpin RNA transfection in INS-1 cells, significantly attenuated endogenous GDIα phosphorylation at Ser174. Consequently, coexpression of SAD-A completely prevented the inhibitory effect of GDIα on insulin secretion in islets. In summary, glucose and incretin stimulate insulin secretion through the phosphorylation of GDIα at Ser174 by SAD-A, which leads to the activation of Rho GTPases, culminating in insulin exocytosis.

Selection of growth-related genes and dominant genotypes in transgenic Yellow River carp Cyprinus carpio L

Transgenic Yellow River carp is characterized by rapid growth rate and high feed-conversion efficiency and exhibits a great application prospect. However, there is still a significant separation of growth traits in the transgenic Yellow River carp family; as such, growth-related genotypes must be screened for molecular marker-assisted selection. In this study, 23 growth-related candidate genes containing 48 SNP markers were screened through bulked segregant analysis (BSA) among transgenic Yellow River carp family members showing significant separation of growth traits. Then, two growth-related genes (Nos. 17 and 14 genes) were identified through combined genome-wide association study (GWAS) of candidate genes and validation of the full-sibling family approach. Nos. 17 and 14 genes encode BR serine/threonine-protein kinase 2 (BRSK2) and eukaryotic translation-initiation factor 2-alpha kinase 3 (Eif2ak3), respectively. The average body weight of three subgroups carrying the genotypes 17GG, 17GG + 14CC, and 17GG + 14TT of these two genes increased by 27.96, 38.28, and 33.72%, respectively, compared with the controls. The proportion of individuals with body weight > 500 g in these subgroups increased by 19.22, 26.82, and 30.92%, respectively. The results showed that appropriate genotype carriers can be selected from the progeny population through BSA sequencing combined with simplified GWAS analysis. Hence, basic population for breeding can be constructed and transgenic Yellow River carp strains with stable production performance and uniform phenotypic properties can be bred.

AMPK and Friends: Central Regulators of β Cell Biology

If left unchecked, prediabetic hyperglycemia can progress to diabetes and often life-threatening attendant secondary complications. Central to the process of glucose homeostasis are pancreatic β cells, which sense elevations in plasma glucose and additional dietary components and respond by releasing the appropriate quantity of insulin, ensuring the arrest of hepatic glucose output and glucose uptake in peripheral tissues. Given that β cell failure is associated with the transition from prediabetes to diabetes, improved β cell function ('compensation') has a central role in preventing type 2 diabetes mellitus (T2DM). Recent data have shown that both insulin secretion and β cell mass dynamics are regulated by the liver kinase B1-AMP-activated kinase (LKB1-AMPK) pathway and related kinases of the AMPK family; thus, an improved understanding of the biological roles of AMPK in the β cell is now of considerable interest.

Pak1 mediates the stimulatory effect of insulin and curcumin on hepatic ChREBP expression

Insulin can stimulate hepatic expression of carbohydrate-responsive element-binding protein (ChREBP). As recent studies revealed potential metabolic beneficial effects of ChREBP, we asked whether its expression can also be regulated by the dietary polyphenol curcumin. We also aimed to determine mechanisms underlying ChREBP stimulation by insulin and curcumin. The effect of insulin on ChREBP expression was assessed in mouse hepatocytes, while the effect of curcumin was assessed in mouse hepatocytes and with curcumin gavage in mice. Chemical inhibitors for insulin signaling molecules were utilized to identify involved signaling molecules, and the involvement of p21-activated protein kinase 1 (Pak1) was determined with its chemical inhibitor and Pak1-/- hepatocytes. We found that both insulin and curcumin-stimulated ChREBP expression in Akt-independent but MEK/ERK-dependent manner, involving the inactivation of the transcriptional repressor Oct-1. Aged Pak1-/- mice showed reduced body fat volume. Pak1 inhibition or its genetic deletion attenuated the stimulatory effect of insulin or curcumin on ChREBP expression. Our study hence suggests the existence of a novel signaling cascade Pak1/MEK/ERK/Oct-1 for both insulin and curcumin in exerting their glucose-lowering effect via promoting hepatic ChREBP production, supports the recognition of beneficial functions of ChREBP, and brings us a new overview on dietary polyphenols.

The incretin hormone GLP-1 and mechanisms underlying its secretion

Glucagon-like peptide-1 (GLP-1) is a cell type-specific post-translational product of proglucagon. It is encoded by the proglucagon gene and released primarily from intestinal endocrine L-cells in response to hormonal, neuronal, and nutritional stimuli. In addition to serving as an incretin in mediating the effect of meal consumption on insulin secretion, GLP-1 exerts other functions in pancreatic islets, including regulation of β-cell proliferation and protection of β-cells against metabolic stresses. Furthermore, GLP-1 exerts numerous other functions in extrapancreatic organs, whereas brain GLP-1 signaling controls satiety. Herein we review the discovery of two incretins and the development of GLP-1-based drugs. We also describe the development of cellular models for studying mechanisms underlying GLP-1 secretion over the past 30 years. However, the main content of this review is a summary of studies on the exploration of mechanisms underlying GLP-1 secretion. We not only summarize studies conducted over the past three decades on elucidating the role of nutritional components and hormonal factors in regulating GLP-1 secretion, but also present a few very recent studies showing the possible role of dietary polyphenols. Finally, the emerging role of gut microbiota in metabolic homeostasis with the potential implication on GLP-1 secretion is discussed.

Hepatic functions of GLP-1 and its based drugs: current disputes and perspectives

GLP-1 and its based drugs possess extrapancreatic metabolic functions, including that in the liver. These direct hepatic metabolic functions explain their therapeutic efficiency for subjects with insulin resistance. The direct hepatic functions could be mediated by previously assumed "degradation" products of GLP-1 without involving canonic GLP-1R. Although GLP-1 analogs were created as therapeutic incretins, extrapancreatic functions of these drugs, as well as native GLP-1, have been broadly recognized. Among them, the hepatic functions are particularly important. Postprandial GLP-1 release contributes to insulin secretion, which represses hepatic glucose production. This indirect effect of GLP-1 is known as the gut-pancreas-liver axis. Great efforts have been made to determine whether GLP-1 and its analogs possess direct metabolic effects on the liver, as the determination of the existence of direct hepatic effects may advance the therapeutic theory and clinical practice on subjects with insulin resistance. Furthermore, recent investigations on the metabolic beneficial effects of previously assumed "degradation" products of GLP-1 in the liver and elsewhere, including GLP-128-36 and GLP-132-36, have drawn intensive attention. Such investigations may further improve the development and the usage of GLP-1-based drugs. Here, we have reviewed the current advancement and the existing controversies on the exploration of direct hepatic functions of GLP-1 and presented our perspectives that the direct hepatic metabolic effects of GLP-1 could be a GLP-1 receptor-independent event involving Wnt signaling pathway activation.

Structure and inhibition analysis of the mouse SAD-B C-terminal fragment

The SAD (synapses of amphids defective) kinases, including SAD-A and SAD-B, play important roles in the regulation of neuronal development, cell cycle, and energy metabolism. Our recent study of mouse SAD-A identified a unique autoinhibitory sequence (AIS), which binds at the junction of the kinase domain (KD) and the ubiquitin-associated (UBA) domain and exerts autoregulation in cooperation with UBA. Here, we report the crystal structure of the mouse SAD-B C-terminal fragment including the AIS and the kinase-associated domain 1 (KA1) at 2.8 Å resolution. The KA1 domain is structurally conserved, while the isolated AIS sequence is highly flexible and solvent-accessible. Our biochemical studies indicated that the SAD-B AIS exerts the same autoinhibitory role as that in SAD-A. We believe that the flexible isolated AIS sequence is readily available for interaction with KD-UBA and thus inhibits SAD-B activity.

Myricetin suppresses p21-activated kinase 1 in human breast cancer MCF-7 cells through downstream signaling of the β-catenin pathway

As a main active compound in the bark of waxberry (Myrica rubra), myricetin is a macrocyclic diarylheptanoid, and can trigger the apoptosis of HeLa and PC3 cells. The aim of the present study was to elucidate the anticancer effect of myricetin on human breast cancer MCF-7 cells and to explore the possible mechanisms of action. MCF-7 cells were treated with different concentrations of myricetin (0-80 µM) for 12, 24 and 48 h. In the present study, we found that myricetin suppressed the cell viability of the MCF-7 cells at least partly through the induction of apoptosis as determined by MTT assay and flow cytometry. Western blot analysis revealed that myricetin effectively suppressed the protein expression of p21-activated kinase 1 (PAK1), MEK and phosphorylated extracellular mitogen-activated protein kinase (ERK1/2). In addition, treatment of myricetin activated glycogen synthase kinase-3β (GSK3β) and Bax protein expression, and inhibited β-catenin/cyclin D1/proliferating cell nuclear antigen (PCNA)/survivin and promoted caspase-3 activity in the MCF-7 cells. These results demonstrated that myricetin suppressed the cell viability of human breast cancer MCF-7 cells through PAK1/MEK/ERK/GSK3β/β-catenin/cyclin D1/PCNA/survivin/Bax-caspase-3 signaling.

Cardiolipin remodeling by TAZ/tafazzin is selectively required for the initiation of mitophagy

Tafazzin (TAZ) is a phospholipid transacylase that catalyzes the remodeling of cardiolipin, a mitochondrial phospholipid required for oxidative phosphorylation. Mutations of TAZ cause Barth syndrome, which is characterized by mitochondrial dysfunction and dilated cardiomyopathy, leading to premature death. However, the molecular mechanisms underlying the cause of mitochondrial dysfunction in Barth syndrome remain poorly understood. Here we investigated the role of TAZ in regulating mitochondrial function and mitophagy. Using primary mouse embryonic fibroblasts (MEFs) with doxycycline-inducible knockdown of Taz, we showed that TAZ deficiency in MEFs caused defective mitophagosome biogenesis, but not other autophagic processes. Consistent with a key role of mitophagy in mitochondria quality control, TAZ deficiency in MEFs also led to impaired oxidative phosphorylation and severe oxidative stress. Together, these findings provide key insights on mitochondrial dysfunction in Barth syndrome, suggesting that pharmacological restoration of mitophagy may provide a novel treatment for this lethal condition.

Molecular control of fission yeast cytokinesis

Cytokinesis gives rise to two independent daughter cells at the end of the cell division cycle. The fission yeast Schizosaccharomyces pombe has emerged as one of the most powerful systems to understand how cytokinesis is controlled molecularly. Like in most eukaryotes, fission yeast cytokinesis depends on an acto-myosin based contractile ring that assembles at the division site under the control of spatial cues that integrate information on cell geometry and the position of the mitotic apparatus. Cytokinetic events are also tightly coordinated with nuclear division by the cell cycle machinery. These spatial and temporal regulations ensure an equal cleavage of the cytoplasm and an accurate segregation of the genetic material in daughter cells. Although this model system has specificities, the basic mechanisms of contractile ring assembly and function deciphered in fission yeast are highly valuable to understand how cytokinesis is controlled in other organisms that rely on a contractile ring for cell division.

Structural insight into the mechanism of synergistic autoinhibition of SAD kinases

The SAD/BRSK kinases participate in various important life processes, including neural development, cell cycle and energy metabolism. Like other members of the AMPK family, SAD contains an N-terminal kinase domain followed by the characteristic UBA and KA1 domains. Here we identify a unique autoinhibitory sequence (AIS) in SAD kinases, which exerts autoregulation in cooperation with UBA. Structural studies of mouse SAD-A revealed that UBA binds to the kinase domain in a distinct mode and, more importantly, AIS nestles specifically into the KD-UBA junction. The cooperative action of AIS and UBA results in an ‘αC-out' inactive kinase, which is conserved across species and essential for presynaptic vesicle clustering in C. elegans. In addition, the AIS, along with the KA1 domain, is indispensable for phospholipid binding. Taken together, these data suggest a model for synergistic autoinhibition and membrane activation of SAD kinases.

The SAD kinases contain a UBA domain that binds to the kinase domain and has a role in autoinhibition and allosteric activation of the AMPK homoenzyme. Here, the authors identify an autoinhibitory sequence in SAD and show that the UBA domain synergistically functions as an autoinhibitory domain.

Molecular control of the Wee1 regulatory pathway by the SAD kinase Cdr2

Cell growth and division are tightly coordinated to maintain cell size constant during successive cell cycles. In Schizosaccharomyces pombe, the SAD kinase Cdr2 regulates the cell size at division and the positioning of the division plane. Cdr2 forms nodes on the medial cortex containing factors that constitute an inhibitory pathway for Wee1. This pathway is regulated by polar gradients of the DYRK kinase Pom1, and involves a direct inhibitor of Wee1, the SAD kinase Cdr1. Cdr2 also interacts with the anillin Mid1, which defines the division plane, and with additional components of the medial cortical nodes, including Blt1, which participate in the mitotic-promoting and cytokinetic functions of nodes. Here, we show that the interaction of Cdr2 with Wee1 and Mid1 requires the UBA domain of Cdr2, which is necessary for its kinase activity. In contrast, Cdr1 associates with the C-terminus of Cdr2, which is composed of basic and KA-1 lipid-binding domains. Mid1 also interacts with the C-terminus of Cdr2 and might bridge the N- and C-terminal domains, whereas Blt1 associates with the central spacer region. We propose that the association of Cdr2 effectors with different domains might constrain Cdr1 and Wee1 spatially to promote Wee1 inhibition upon Cdr2 kinase activation.

New insights concerning the molecular basis for defective glucoregulation in soluble adenylyl cyclase knockout mice

Recently published findings indicate that a knockout (KO) of soluble adenylyl cyclase (sAC, also known as AC-10) gene expression in mice leads to defective glucoregulation that is characterized by reduced pancreatic insulin secretion and reduced intraperitoneal glucose tolerance. Summarized here are current concepts regarding the molecular basis for this phenotype, with special emphasis on the potential role of sAC as a determinant of glucose-stimulated insulin secretion. Highlighted is new evidence that in pancreatic beta cells, oxidative glucose metabolism stimulates mitochondrial CO₂production that in turn generates bicarbonate ion (HCO(3)(-)). Since HCO(3)(-) binds to and directly stimulates the activity of sAC, we propose that glucose-stimulated cAMP production in beta cells is mediated not simply by transmembrane adenylyl cyclases (TMACs), but also by sAC. Based on evidence that sAC is expressed in mitochondria, there exists the possibility that beta-cell glucose metabolism is linked to mitochondrial cAMP production with consequent facilitation of oxidative phosphorylation. Since sAC is also expressed in the cytoplasm, sAC catalyzed cAMP production may activate cAMP sensors such as PKA and Epac2 to control ion channel function, intracellular Ca²⁺ handling, and Ca²⁺-dependent exocytosis. Thus, we propose that the existence of sAC in beta cells provides a new and unexpected explanation for previously reported actions of glucose metabolism to stimulate cAMP production. It seems possible that alterations of sAC activity might be of importance when evaluating new strategies for the treatment of type 2 diabetes (T2DM), or when evaluating why glucose metabolism fails to stimulate insulin secretion in patients diagnosed with T2DM. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

p21-Activated protein kinases and their emerging roles in glucose homeostasis

p21-Activated protein kinases (PAKs) are centrally involved in a plethora of cellular processes and functions. Their function as effectors of small GTPases Rac1 and Cdc42 has been extensively studied during the past two decades, particularly in the realms of cell proliferation, apoptosis, and hence tumorigenesis, as well as cytoskeletal remodeling and related cellular events in health and disease. In recent years, a large number of studies have shed light onto the fundamental role of group I PAKs, most notably PAK1, in metabolic homeostasis. In skeletal muscle, PAK1 was shown to mediate the function of insulin on stimulating GLUT4 translocation and glucose uptake, while in pancreatic β-cells, PAK1 participates in insulin granule localization and vesicle release. Furthermore, we demonstrated that PAK1 mediates the cross talk between insulin and Wnt/β-catenin signaling pathways and hence regulates gut proglucagon gene expression and the production of the incretin hormone glucagon-like peptide-1 (GLP-1). The utilization of chemical inhibitors of PAK and the characterization of Pak1(-/-) mice enabled us to gain mechanistic insights as well as to assess the overall contribution of PAKs in metabolic homeostasis. This review summarizes our current understanding of PAKs, with an emphasis on the emerging roles of PAK1 in glucose homeostasis.

YES, a Src family kinase, is a proximal glucose-specific activator of cell division cycle control protein 42 (Cdc42) in pancreatic islet β cells

Second-phase insulin secretion sustains insulin release in the face of hyperglycemia associated with insulin resistance, requiring the continued mobilization of insulin secretory granules to the plasma membrane. Cdc42, the small Rho family GTPase recognized as the proximal glucose-specific trigger to elicit second-phase insulin secretion, signals downstream to activate the p21-activated kinase (PAK1), which then signals to Raf-1/MEK/ERK to induce filamentous actin (F-actin) remodeling, to ultimately mobilize insulin granules to the plasma membrane. However, the steps required to initiate Cdc42 activation in a glucose-specific manner in β cells have remained elusive. Toward this, we identified the involvement of the Src family kinases (SFKs), based upon the ability of SFK inhibitors to block glucose-stimulated Cdc42 and PAK1 activation events as well as the amplifying pathway of glucose-stimulated insulin release, in MIN6 β cells. Indeed, subsequent studies performed in human islets revealed that SFK phosphorylation was induced only by glucose and within 1 min of stimulation before the activation of Cdc42 at 3 min. Furthermore, pervanadate treatment validated the phosphorylation event to be tyrosine-specific. Although RT-PCR showed β cells to express five different SFK proteins, only two of these, YES and Fyn kinases, were found localized to the plasma membrane, and of these two, only YES kinase underwent glucose-stimulated tyrosine phosphorylation. Immunodetection and RNAi analyses further established YES kinase as a proximal glucose-specific signal in the Cdc42-signaling cascade. Identification of YES kinase provides new insight into the mechanisms underlying the sustainment of insulin secretion via granule mobilization/replenishment and F-actin remodeling.

Anti-diabetic actions of glucagon-like peptide-1 on pancreatic beta-cells

Glucagon-like peptide-1 (GLP-1), an incretin hormone, is released from intestinal L-cells in response to nutrients. GLP-1 lowers blood glucose levels by stimulating insulin secretion from pancreatic beta-cells in a glucose-dependent manner. In addition, GLP-1 slows gastric emptying, suppresses appetite, reduces plasma glucagon, and stimulates glucose disposal, which are beneficial for glucose homeostasis. Therefore, incretin-based therapies such as GLP-1 receptor agonists and inhibitors of dipeptidyl peptidase IV, an enzyme which inactivates GLP-1, have been developed for treatment of diabetes. This review outlines our knowledge of the actions of GLP-1 on insulin secretion and biosynthesis, beta-cell proliferation and regeneration, and protection against beta-cell damage, as well as the involvement of recently discovered signaling pathways of GLP-1 action, mainly focusing on pancreatic beta-cells.

Signaling mechanisms of glucose-induced F-actin remodeling in pancreatic islet β cells

The maintenance of whole-body glucose homeostasis is critical for survival, and is controlled by the coordination of multiple organs and endocrine systems. Pancreatic islet β cells secrete insulin in response to nutrient stimuli, and insulin then travels through the circulation promoting glucose uptake into insulin-responsive tissues such as liver, skeletal muscle and adipose. Many of the genes identified in human genome-wide association studies of diabetic individuals are directly associated with β cell survival and function, giving credence to the idea that β-cell dysfunction is central to the development of type 2 diabetes. As such, investigations into the mechanisms by which β cells sense glucose and secrete insulin in a regulated manner are a major focus of current diabetes research. In particular, recent discoveries of the detailed role and requirements for reorganization/remodeling of filamentous actin (F-actin) in the regulation of insulin release from the β cell have appeared at the forefront of islet function research, having lapsed in prior years due to technical limitations. Recent advances in live-cell imaging and specialized reagents have revealed localized F-actin remodeling to be a requisite for the normal biphasic pattern of nutrient-stimulated insulin secretion. This review will provide an historical look at the emergent focus on the role of the actin cytoskeleton and its regulation of insulin secretion, leading up to the cutting-edge research in progress in the field today.

SAD-A kinase controls islet β-cell size and function as a mediator of mTORC1 signaling

The mammalian target of rapamycin (mTOR) plays an important role in controlling islet β-cell function. However, the underlying molecular mechanisms remain poorly elucidated. Synapses of amphids defective kinase-A (SAD-A) is a 5' adenosine monophosphate-activated protein kinase-related protein kinase that is exclusively expressed in pancreas and brain. In this study, we investigated a role of the kinase in regulating pancreatic β-cell morphology and function as a mediator of mTOR complex 1 (mTORC1) signaling. We show that global SAD-A deletion leads to defective glucose-stimulated insulin secretion and petite islets, which are reminiscent of the defects in mice with global deletion of ribosomal protein S6 kinase 1, a downstream target of mTORC1. Consistent with these findings, selective deletion of SAD-A in pancreas decreased islet β-cell size, whereas SAD-A overexpression significantly increased the size of mouse insulinomas cell lines β-cells. In direct support of SAD-A as a unique mediator of mTORC1 signaling in islet β-cells, we demonstrate that glucose dramatically stimulated SAD-A protein translation in isolated mouse islets, which was potently inhibited by rapamycin, an inhibitor of mTORC1. Moreover, the 5'-untranslated region of SAD-A mRNA is highly structured and requires mTORC1 signaling for its translation initiation. Together, these findings identified SAD-A as a unique pancreas-specific effector protein of mTORC1 signaling.

Ras-related C3 botulinum toxin substrate 1 (RAC1) regulates glucose-stimulated insulin secretion via modulation of F-actin

AIMS/HYPOTHESIS

The small G-protein ras-related C3 botulinum toxin substrate 1 (RAC1) plays various roles in mammalian cells, such as in the regulation of cytoskeletal organisation, cell adhesion, migration and morphological changes. The present study examines the effects of RAC1 ablation on pancreatic beta cell function.

METHODS

Isolated islets from pancreatic beta cell-specific Rac1-knockout (betaRac1(-/-)) mice and RAC1 knockdown INS-1 insulinoma cells treated with small interfering RNA were used to investigate insulin secretion and cytoskeletal organisation in pancreatic beta cells.

RESULTS

BetaRac1(-/-) mice showed decreased glucose-stimulated insulin secretion, while there were no apparent differences in islet morphology. Isolated islets from the mice had blunted insulin secretion in response to high glucose levels. In RAC1 knockdown INS-1 cells, insulin secretion was also decreased in response to high glucose levels, consistent with the phenotype of betaRac1(-/-) mice. Even under high glucose levels, RAC1 knockdown INS-1 cells remained intact with F-actin, which inhibits the recruitment of the insulin granules, resulting in an inhibition of insulin secretion.

CONCLUSIONS/INTERPRETATION

In RAC1-deficient pancreatic beta cells, F-actin acts as a barrier for insulin granules and reduces glucose-stimulated insulin secretion.

SAD-A potentiates glucose-stimulated insulin secretion as a mediator of glucagon-like peptide 1 response in pancreatic β cells

Type 2 diabetes is characterized by defective glucose-stimulated insulin secretion (GSIS) from pancreatic β cells, which can be restored by glucagon-like peptide 1 (GLP-1), an incretin hormone commonly used for the treatment of type 2 diabetes. However, molecular mechanisms by which GLP-1 affects glucose responsiveness in islet β cells remain poorly understood. Here we investigated a role of SAD-A, an AMP-activated protein kinase (AMPK)-related kinase, in regulating GSIS in mice with conditional SAD-A deletion. We show that selective deletion of SAD-A in pancreas impaired incretin's effect on GSIS, leading to glucose intolerance. Conversely, overexpression of SAD-A significantly enhanced GSIS and further potentiated GLP-1's effect on GSIS from isolated mouse islets. In support of SAD-A as a mediator of incretin response, SAD-A is expressed exclusively in pancreas and brain, the primary targeting tissues of GLP-1 action. Additionally, SAD-A kinase is activated in response to stimulation by GLP-1 through cyclic AMP (cAMP)/Ca(2+)-dependent signaling pathways in islet β cells. Furthermore, we identified Thr443 as a key autoinhibitory phosphorylation site which mediates SAD-A's effect on incretin response in islet β cells. Consequently, ablation of Thr443 significantly enhanced GLP-1's effect on GSIS from isolated mouse islets. Together, these findings identified SAD-A kinase as a pancreas-specific mediator of incretin response in islet β cells.

Depletion of PAK1 enhances ubiquitin-mediated survivin degradation in pancreatic β-cells

Functional β-cell mass deficiency in diabetes results from imbalanced β-cell death and replication, and decreased PAK1 protein levels in human islets from donors with type 2 diabetes implicates a possible role for PAK1 in maintaining β-cell mass. Here, we aim to address the linkage between PAK1 and Survivin, a protein essential for β-cell replication. PAK1 knockout (KO) mouse islets exhibited decreased expression of Survivin protein. MIN6 β-cells with siRNA-mediated suppression of PAK1 also had decreased Survivin protein and exhibited an increased level of ubiquitinated-Survivin. However, no significant changes in Survivin mRNA were found in islets from PAK1 KO mice and PAK1-depleted MIN6 β-cells. The decreased Survivin level in MIN6 cells subjected to hyperglycemic stress was prevented by expression of exogenous PAK1. Moreover, overexpressing Survivin restored proliferation of β-cells that was impaired by the loss of PAK1. These data implicate a role for PAK1 in regulating Survivin protein stability in the β-cell and suggest PAK1 as a potential molecular target for the restoration of β-cell mass.

A p21-activated kinase (PAK1) signaling cascade coordinately regulates F-actin remodeling and insulin granule exocytosis in pancreatic β cells

Human islet studies implicate an important signaling role for the Cdc42 effector protein p21-activated kinase (PAK1) in the sustained/second-phase of insulin secretion. Because human islets from type 2 diabetic donors lack ∼80% of normal PAK1 protein levels, the mechanistic requirement for PAK1 signaling in islet function was interrogated. Similar to MIN6 β cells, human islets elicited glucose-stimulated PAK1 activation that was sensitive to the PAK1 inhibitor, IPA3. Given that sustained insulin secretion has been correlated with glucose-induced filamentous actin (F-actin) remodeling, we tested the hypothesis that a Cdc42-activated PAK1 signaling cascade is required to elicit F-actin remodeling to mobilize granules to the cell surface. Live-cell imaging captured the glucose-induced cortical F-actin remodeling in MIN6 β cells; IPA3-mediated inhibition of PAK1 abolished this remodeling. IPA3 also ablated glucose-stimulated insulin granule accumulation at the plasma membrane, consistent with its role in sustained/second-phase insulin release. Both IPA3 and a selective inhibitor of the Cdc42 GTPase, ML-141, blunted the glucose-stimulated activation of Raf-1, suggesting Raf-1 to be downstream of Cdc42→PAK1. IPA3 also inhibited MEK1/2 activation, implicating the MEK1/2→ERK1/2 cascade to occur downstream of PAK1. Importantly, PD0325901, a new selective inhibitor of MEK1/2→ERK1/2 activation, impaired F-actin remodeling and the sustained/amplification pathway of insulin release. Taken together, these data suggest that glucose-mediated activation of Cdc42 leads to activation of PAK1 and prompts activation of its downstream targets Raf-1, MEK1/2 and ERK1/2 to elicit F-actin remodeling and recruitment of insulin granules to the plasma membrane to support the sustained phase of insulin release.