Protein kinase D1 deletion in adipocytes enhances energy dissipation and protects against adiposity

Extracellular glucose and dysfunctional insulin receptor signaling independently upregulate arterial smooth muscle TMEM16A expression

Diabetes alters the function of ion channels responsible for regulating arterial smooth muscle membrane potential, resulting in vasoconstriction. Our prior research demonstrated an elevation of TMEM16A in diabetic arteries. Here, we explored the mechanisms involved in Transmembrane protein 16A (TMEM16A) gene expression. Our data indicate that a Snail-mediated repressor complex regulates arterial TMEM16A gene transcription. Snail expression was reduced in diabetic arteries while TMEM16A expression was upregulated. The TMEM16A promoter contained three canonical E-box sites. Electrophoretic mobility and super shift assays revealed that the -154 nt E-box was the binding site of the Snail repressor complex and binding of the repressor complex decreased in diabetic arteries. High glucose induced a biphasic contractile response in pressurized nondiabetic mouse hindlimb arteries incubated ex vivo. Hindlimb arteries incubated in high glucose also showed decreased phospho-protein kinase D1 and TMEM16A expression. In hindlimb arteries from nondiabetic mice, administration of a bolus dose of glucose activated protein kinase D1 signaling to induce Snail degradation. In both in vivo and ex vivo conditions, Snail expression exhibited an inverse relationship with the expression of protein kinase D1 and TMEM16A. In diabetic mouse arteries, phospho-protein kinase D1 increased while Akt2 and pGSK3β levels declined. These results indicate that in nondiabetic mice, high glucose triggers a transient deactivation of the Snail repressor complex to increase arterial TMEM16A expression independently of insulin signaling. Conversely, insulin resistance activates GSK3β signaling and enhances arterial TMEM16A channel expression. These data have uncovered the Snail-mediated regulation of arterial TMEM16A expression and its dysfunction during diabetes.NEW & NOTEWORTHY The calcium-activated chloride channel, TMEM16A, is upregulated in the diabetic vasculature to cause increased vasoconstriction. In this paper, we have uncovered that the TMEM16A gene expression is controlled by a Snail-mediated repressor complex that uncouples with both insulin-dependent and -independent pathways to allow for upregulated arterial protein expression thereby causing vasoconstriction. The paper highlights the effect of short- and long-term glucose-induced dysfunction of an ion channel expression as a causative factor in diabetic vascular disease.

Protein kinase D (PKD) on the crossroad of lipid absorption, synthesis and utilization

Inappropriate lipid levels in the blood, as well as its content and composition in different organs, underlie multiple metabolic disorders including obesity, non-alcoholic fatty liver disease, type 2 diabetes, and atherosclerosis. Multiple processes contribute to the complex metabolism of triglycerides (TGs), fatty acids (FAs), and other lipid species. These consist of digestion and absorption of dietary lipids, de novo FAs synthesis (lipogenesis), uptake of TGs and FAs by peripheral tissues, TGs storage in the intracellular depots as well as lipid utilization for β-oxidation and their conversion to lipid-derivatives. A majority of the enzymatic reactions linked to lipogenesis, TGs synthesis, lipid absorption, and transport are happening at the endoplasmic reticulum, while β-oxidation takes place in mitochondria and peroxisomes. The Golgi apparatus is a central sorting, protein- and lipid-modifying organelle and hence is involved in lipid metabolism as well. However, the impact of the processes taking part in the Golgi apparatus are often overseen. The protein kinase D (PKD) family (composed of three members, PKD1, 2, and 3) is the master regulator of Golgi dynamics. PKDs are also a sensor of different lipid species in distinct cellular compartments. In this review, we discuss the roles of PKD family members in the regulation of lipid metabolism including the processes executed by PKDs at the Golgi apparatus. We also discuss the role of PKDs-dependent signaling in different cellular compartments and organs in the context of the development of metabolic disorders.

Triazolo[4,5-d]pyrimidin-5-amines based ERK3 inhibitors fail to demonstrate selective effects on adipocyte function

Extracellular signal-regulated kinase 3 (ERK3 also designated MAPK6 - mitogen-activated protein kinase 6) is a ubiquitously expressed kinase participating in the regulation of a broad spectrum of physiological and pathological processes. Targeted inhibition of the kinase may allow the development of novel treatment strategies for a variety of types of cancer and somatic pathologies, as well as preserving metabolic health, combat obesity and diabetes. We chose and synthesized three triazolo [4,5-d]pyrimidin-5-amines proposed previously as putative ERK3 inhibitors to assess their selectivity and biological effects in terms of metabolic state impact in living cells. As it was previously shown that ERK3 is a major regulator of lipolysis in adipocytes, we focused on this process. Our new results indicate that in addition to the previously identified lipolytic enzyme ATGL, ERK3 also regulates hormone-sensitive lipase (HSL) and monoglyceride lipase (MGL). Moreover, this kinase also promotes the abundance of fatty acid synthase (FASN) as well as protein kinase cAMP-activated catalytic subunit alpha (PKACα). To investigate various effects of putative ERK3 inhibitors on lipolysis, we utilized different adipocyte models. We demonstrated that molecules exhibit lipolysis-modulating effects; however, the effects of triazolo [4,5-d]pyrimidin-5-amines based inhibitors on lipolysis are not dependent on ERK3. Subsequently, we revealed a wide range of the compounds' possible targets using a machine learning-based prediction. Therefore, the tested compounds inhibit ERK3 in vitro, but the biological effect of this inhibition is significantly overlapped and modified by some other molecular events related to the non-selective binding to other targets.

Platelet-derived lipids promote insulin secretion of pancreatic β cells

Hyperreactive platelets are commonly observed in diabetic patients indicating a potential link between glucose homeostasis and platelet reactivity. This raises the possibility that platelets may play a role in the regulation of metabolism. Pancreatic β cells are the central regulators of systemic glucose homeostasis. Here, we show that factor(s) derived from β cells stimulate platelet activity and platelets selectively localize to the vascular endothelium of pancreatic islets. Both depletion of platelets and ablation of major platelet adhesion or activation pathways consistently resulted in impaired glucose tolerance and decreased circulating insulin levels. Furthermore, we found platelet-derived lipid classes to promote insulin secretion and identified 20-Hydroxyeicosatetraenoic acid (20-HETE) as the main factor promoting β cells function. Finally, we demonstrate that the levels of platelet-derived 20-HETE decline with age and that this parallels with reduced impact of platelets on β cell function. Our findings identify an unexpected function of platelets in the regulation of insulin secretion and glucose metabolism, which promotes metabolic fitness in young individuals.

Membrane trafficking in breast cancer progression: protein kinase D comes into play

Protein kinase D (PKD) is a serine/threonine kinase family that controls important cellular functions, most notably playing a key role in the secretory pathway at the trans-Golgi network. Aberrant expression of PKD isoforms has been found mainly in breast cancer, where it promotes various cellular processes such as growth, invasion, survival and stem cell maintenance. In this review, we discuss the isoform-specific functions of PKD in breast cancer progression, with a particular focus on how the PKD controlled cellular processes might be linked to deregulated membrane trafficking and secretion. We further highlight the challenges of a therapeutic approach targeting PKD to prevent breast cancer progression.

PKN1 Kinase: A Key Player in Adipocyte Differentiation and Glucose Metabolism

Adipocyte dysfunction is the driver of obesity and correlates with insulin resistance and the onset of type 2 diabetes. Protein kinase N1 (PKN1) is a serine/threonine kinase that has been shown to contribute to Glut4 translocation to the membrane and glucose transport. Here, we evaluated the role of PKN1 in glucose metabolism under insulin-resistant conditions in primary visceral adipose tissue (VAT) from 31 patients with obesity and in murine 3T3-L1 adipocytes. In addition, in vitro studies in human VAT samples and mouse adipocytes were conducted to investigate the role of PKN1 in the adipogenic maturation process and glucose homeostasis control. We show that insulin-resistant adipocytes present a decrease in PKN1 activation levels compared to nondiabetic control counterparts. We further show that PKN1 controls the adipogenesis process and glucose metabolism. PKN1-silenced adipocytes present a decrease in both differentiation process and glucose uptake, with a concomitant decrease in the expression levels of adipogenic markers, such as PPARγ, FABP4, adiponectin and CEBPα. Altogether, these results point to PKN1 as a regulator of key signaling pathways involved in adipocyte differentiation and as an emerging player of adipocyte insulin responsiveness. These findings may provide new therapeutic approaches for the management of insulin resistance in type 2 diabetes.

Protein kinase D1 (Prkd1) deletion in brown adipose tissue leads to altered myogenic gene expression after cold exposure, while thermogenesis remains intact

Brown adipose tissue (BAT) has in recent times been rediscovered in adult humans, and together with work from preclinical models, has shown to have the potential of providing a variety of positive metabolic benefits. These include lower plasma glucose, improved insulin sensitivity, and reduced susceptibility to obesity and its comorbidities. As such, its continued study could offer insights to therapeutically modulate this tissue to improve metabolic health. It has been reported that adipose-specific deletion of the gene for protein kinase D1 (Prkd1) in mice enhances mitochondrial respiration and improves whole-body glucose homeostasis. We sought to determine whether these effects were mediated specifically through brown adipocytes using a Prkd1 brown adipose tissue (BAT) Ucp1-Cre-specific knockout mouse model, Prkd1^BKO . We unexpectedly observed that upon both cold exposure and β₃ -AR agonist administration, Prkd1 loss in BAT did not alter canonical thermogenic gene expression or adipocyte morphology. We took an unbiased approach to assess whether other signaling pathways were affected. RNA from cold-exposed mice was subjected to RNA-Seq analysis. These studies revealed that myogenic gene expression is altered in Prkd1^BKO BAT after both acute and extended cold exposure. Given that brown adipocytes and skeletal myocytes share a common precursor cell lineage expressing myogenic factor 5 (Myf5), these data suggest that loss of Prkd1 in BAT may alter the biology of mature brown adipocytes and preadipocytes in this depot. The data presented herein clarify the role of Prkd1 in BAT thermogenesis and present new avenues for the further study of Prkd1 function in BAT.

Clusterin negatively modulates mechanical stress-mediated ligamentum flavum hypertrophy through TGF-β1 signaling

Ligamentum flavum hypertrophy (LFH) is a major cause of lumbar spinal canal stenosis (LSCS). The pathomechanisms for LFH have not been fully elucidated. Isobaric tags for relative and absolute quantitation (iTRAQ) technology, proteomics assessments of human ligamentum flavum (LF), and successive assays were performed to explore the effect of clusterin (CLU) upregulation on LFH pathogenesis. LFH samples exhibited higher cell positive rates of the CLU, TGF-β1, α-SMA, ALK5 and p-SMAD3 proteins than non-LFH samples. Mechanical stress and TGF-β1 initiated CLU expression in LF cells. Notably, CLU inhibited the expression of mechanical stress-stimulated and TGF-β1-stimulated COL1A2 and α-SMA. Mechanistic studies showed that CLU inhibited mechanical stress-stimulated and TGF-β1-induced SMAD3 activities through suppression of the phosphorylation of SMAD3 and by inhibiting its nuclear translocation by competitively binding to ALK5. PRKD3 stabilized CLU protein by inhibiting lysosomal distribution and degradation of CLU. CLU attenuated mechanical stress-induced LFH in vivo. In summary, the findings showed that CLU attenuates mechanical stress-induced LFH by modulating the TGF-β1 pathways in vitro and in vivo. These findings imply that CLU is induced by mechanical stress and TGF-β1 and inhibits LF fibrotic responses via negative feedback regulation of the TGF-β1 pathway. These findings indicate that CLU is a potential treatment target for LFH.

The protein clusterin regulates the body’s response to lower back pain induced by mechanical stress and could be a target for treatments. Lower back pain is common and is exacerbated by our upright stance. A major cause of the pain is excessive cell growth (hypertrophy) in the ligaments between vertebrae. This growth narrows the spinal canal and compresses nerves. Using a unique mouse model bred to walk upright, Zhongmin Zhang and Liang Wang at Southern Medical University in Guangzhou, China, and co-workers showed that clusterin, a protein involved in regulation of cell survival, can reduce the hypertrophy caused by mechanical stresses, and could be used in back pain treatments. Clusterin regulates the activity of the growth factor TGF-β1, which plays a role in synthesizing new tissues after injury, but can spur excessive growth.

Targeting ERK3/MK5 complex for treatment of obesity and diabetes

Kinases represent one of the largest druggable families of proteins. Importantly, many kinases are aberrantly activated/de-activated in multiple organs during obesity, which contributes to the development of diabetes and associated diseases. Previous results indicate that the complex between Extracellular-regulated kinase 3 (ERK3) and Mitogen-Activated Protein Kinase (MAPK)-activated protein kinase 5 (MK5) suppresses energy dissipation and promotes fatty acids (FAs) output in adipose tissue and, therefore promotes obesity and diabetes. However, the therapeutic potential of targeting this complex at the systemic level has not been fully explored. Here we applied a translational approach to target the ERK3/MK5 complex in mice. Importantly, deletion of ERK3 in the whole body or administration of MK5-specific inhibitor protects against obesity and promotes insulin sensitivity. Finally, we show that the expression of ERK3 and MK5 correlates with the degree of obesity and that ERK3/MK5 complex regulates energy dissipation in human adipocytes. Altogether, we demonstrate that ERK3/MK5 complex can be targeted in vivo to preserve metabolic health and combat obesity and diabetes.

Development and evaluation of a simultaneous strategy for pyrimidine metabolome quantification in multiple biological samples

Pyrimidines are critical nutrients and key biomolecules in nucleic acid biosynthesis and carbohydrate and lipid metabolism. Here, we proposed the concept of the pyrimidine metabolome, which covers 14 analytes in pyrimidine de novo and salvage synthetic pathways, and established a novel analytical strategy with ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) to efficiently illustrate pyrimidine transient distribution and dynamic balance. The lower limits of quantification (LLOQs) of all analytes were less than 10 ng/mL. Acceptable inter- and intra-day relative deviation (<15%) was detected, and good stability was obtained under different storage conditions. Metabolomics analysis revealed pyrimidine metabolic diversity in the plasma and brain among species, and a visualization strategy exhibited that pyrimidine biosynthetic metabolism is quite active in brain. Distinct metabolic features were also observed in cells with pyrimidine metabolomic disorders during proliferation and apoptosis. Absolute concentrations of pyrimidine metabolites in different bio-samples offered reference data for future pyrimidine studies.

Small-Molecule Inhibitor Targeting Protein Kinase D: A Potential Therapeutic Strategy

The protein kinase D (PKD) family is a family of serine-threonine kinases that are members of the calcium/calmodulin-dependent kinase (CaMK) superfamily. PKDs have been increasingly implicated in multiple pivotal cellular processes and pathological conditions. PKD dysregulation is associated with several diseases, including cancer, inflammation, and obesity. Over the past few years, small-molecule inhibitors have emerged as alternative targeted therapy with fewer adverse side effects than currently available chemotherapy, and these specifically targeted inhibitors limit non-specific toxicities. The successful development of PKD inhibitors would significantly suppress the growth and proliferation of various cancers and inhibit the progression of other diseases. Various PKD inhibitors have been studied in the preclinical setting. In this context, we summarize the PKD inhibitors under investigation and their application for different kinds of diseases.

A phosphoproteomic approach reveals that PKD3 controls PKA-mediated glucose and tyrosine metabolism

Protein kinase D3 (PKD3) regulates hepatic metabolism in a PKA-dependent manner and reveals many other putative PKD3 targets in the liver.

Members of the protein kinase D (PKD) family (PKD1, 2, and 3) integrate hormonal and nutritional inputs to regulate complex cellular metabolism. Despite the fact that a number of functions have been annotated to particular PKDs, their molecular targets are relatively poorly explored. PKD3 promotes insulin sensitivity and suppresses lipogenesis in the liver of animals fed a high-fat diet. However, its substrates are largely unknown. Here we applied proteomic approaches to determine PKD3 targets. We identified more than 300 putative targets of PKD3. Furthermore, biochemical analysis revealed that PKD3 regulates cAMP-dependent PKA activity, a master regulator of the hepatic response to glucagon and fasting. PKA regulates glucose, lipid, and amino acid metabolism in the liver, by targeting key enzymes in the respective processes. Among them the PKA targets phenylalanine hydroxylase (PAH) catalyzes the conversion of phenylalanine to tyrosine. Consistently, we showed that PKD3 is activated by glucagon and promotes glucose and tyrosine levels in hepatocytes. Therefore, our data indicate that PKD3 might play a role in the hepatic response to glucagon.

Protein Kinase D2 drives chylomicron-mediated lipid transport in the intestine and promotes obesity

Lipids are the most energy-dense components of the diet, and their overconsumption promotes obesity and diabetes. Dietary fat content has been linked to the lipid processing activity by the intestine and its overall capacity to absorb triglycerides (TG). However, the signaling cascades driving intestinal lipid absorption in response to elevated dietary fat are largely unknown. Here, we describe an unexpected role of the protein kinase D2 (PKD2) in lipid homeostasis. We demonstrate that PKD2 activity promotes chylomicron-mediated TG transfer in enterocytes. PKD2 increases chylomicron size to enhance the TG secretion on the basolateral side of the mouse and human enterocytes, which is associated with decreased abundance of APOA4. PKD2 activation in intestine also correlates positively with circulating TG in obese human patients. Importantly, deletion, inactivation, or inhibition of PKD2 ameliorates high-fat diet-induced obesity and diabetes and improves gut microbiota profile in mice. Taken together, our findings suggest that PKD2 represents a key signaling node promoting dietary fat absorption and may serve as an attractive target for the treatment of obesity.

A PKD-MFF signaling axis couples mitochondrial fission to mitotic progression

Mitochondria are highly dynamic organelles subjected to fission and fusion events. During mitosis, mitochondrial fission ensures equal distribution of mitochondria to daughter cells. If and how this process can actively drive mitotic progression remains largely unknown. Here, we discover a pathway linking mitochondrial fission to mitotic progression in mammalian cells. The mitochondrial fission factor (MFF), the main mitochondrial receptor for the Dynamin-related protein 1 (DRP1), is directly phosphorylated by Protein Kinase D (PKD) specifically during mitosis. PKD-dependent MFF phosphorylation is required and sufficient for mitochondrial fission in mitotic but not in interphasic cells. Phosphorylation of MFF is crucial for chromosome segregation and promotes cell survival by inhibiting adaptation of the mitotic checkpoint. Thus, PKD/MFF-dependent mitochondrial fission is critical for the maintenance of genome integrity during cell division.

Adipose-specific knockout of Protein Kinase D1 suppresses de novo lipogenesis in mice via SREBP1c-dependent signaling

Having healthy adipose tissue is essential for metabolic health, as excessive adipose tissue in the body can cause its dysregulation and driving chronic metabolic diseases. Protein kinase D1 (PKD1) is considered to be a key kinase in signal transduction, which regulates multiple cellular functions, but its physiological functions in adipose are still not fully understood. This study aimed at elucidating the function of adipocyte PKD1 on lipogenesis. From RNA-Sequencing data, we found that the fatty acid biosynthesis pathway in white adipose tissue lacking PKD1 was significantly affected. Critical rate-limiting enzymes for de novo lipogenesis in adipocytes, such as FASN, ACCα, and SCD1, were significantly repressed after deleting PKD1 in vivo and in vitro. Further studies revealed that blockade of PKD1 significantly increased phosphorylation of SREBP1c at serine 372 site. Co-immunoprecipitation analysis showed that PKD1 interacts with SREBP1c in vitro and in vivo. Importantly, overexpression of SREBP1c reversed the inhibition of FASN and ACCα expression caused by PKD1 silencing. Together, adipocyte PKD1 promotes de novo lipogenesis via SREBP1c-dependent manner in visceral white adipose tissue and might provide a new target for the development of anti-obesity therapies.

The role of protein kinase D (PKD) in intracellular nutrient sensing and regulation of adaptive responses to the obese environment

Obesity is associated with ectopic accumulation of lipids, which is implicated in the development of insulin resistance, type 2 diabetes mellitus and cardiovascular disease. As the global prevalence of obesity continues to rise, it is becoming increasingly important to understand the underlying cellular mechanisms of this disease. Protein kinase D (PKD) is an intracellular signalling kinase with well characterized roles in intracellular vesicle transport and secretion, cancer cell proliferation and cardiac hypertrophy. However, emerging evidence also highlights PKD as a novel nutrient sensor. PKD activation is mediated by the accumulation of the lipid intermediate diacylglycerol, and PKD activity in the liver, heart and adipose tissue increases upon feeding. In obesity, PKD signalling is linked to reduced insulin signalling and dysfunction in adipose tissue, liver and heart, whilst in the pancreas, PKD is essential for the compensatory increase in glucose-stimulated insulin secretion from β-cells during obesity. Collectively, these studies reveal aspects of PKD signalling that are involved in the tissue-specific responses to obesity. This review summarizes the emerging evidence suggesting that PKD plays an important role in regulating the adaptive response to the obese environment.

Inhibition Lysosomal Degradation of Clusterin by Protein Kinase D3 Promotes Triple-Negative Breast Cancer Tumor Growth

Triple negative breast cancer (TNBC), with its lack of targeted therapies, shows the worst mortality rate among all breast cancer subtypes. Clusterin (CLU) is implicated to play important oncogenic roles in cancer via promoting various downstream oncogenic pathways. Here, protein kinase D3 (PRKD3) is defined to be a key regulator of CLU in promoting TNBC tumor growth. Mechanically, PRKD3 with kinase activity binding to CLU is critical for CLU protein stability via inhibiting CLU's lysosomal distribution and degradation. CLU and PRKD3 protein level are significantly elevated and positively correlated in collected TNBC tumor samples. CLU silencer (OGX-011) and PRKDs inhibitor (CRT0066101) can both result in impressive tumor growth suppression in vitro and in vivo, suggesting targeting CLU and its key regulator-PRKD3 are promisingly efficient against TNBC. Finally, secreted CLU (sCLU) is found to be elevated in serums from TNBC patients and reduced in serum from TNBC murine models post OGX-011 and/or CRT0066101 treatment, suggesting serum sCLU is a promising blood-based biomarker for clinical management of TNBC. Taken together, this study provides a thorough molecular basis as well as preclinical evidences for targeting CLU pathway as a new promising strategy against TNBC via revealing PRKD3 as the key regulator of CLU in TNBC.

The protein kinase D1-mediated inflammatory pathway is involved in olanzapine-induced impairment of skeletal muscle insulin signaling in rats

AIMS

Skeletal muscle insulin resistance (SMIR) contributes to the metabolic syndrome. Mounting evidence has demonstrated that the second generation antipsychotic olanzapine causes SMIR. The present study sought to investigate the molecular mechanisms underlying olanzapine-induced SMIR.

MAIN METHODS

Male rats were given olanzapine (5 mg/kg, by a gavage method) for consecutive eight weeks. Plasma glucose and insulin concentrations were determined enzymatically or by ELISA. Gene/protein expression was analyzed by Real-Time PCR, Western blot and/or immunohistochemistry.

KEY FINDINGS

Olanzapine increased fasting plasma insulin concentration, and decreased glucose clearance during insulin tolerance test in rats. In skeletal muscle, it decreased protein expression of membrane glucose transporter (GLUT) 4, the ratio of membrane to total GLUT4, and total insulin receptor substrate 1 (IRS1). However, it increased protein phosphorylation of Ser³⁰⁷ in IRS1, Y⁶⁰⁷ in phosphoinositide 3-kinase p85α and Ser³⁰⁷ in AKT. These results indicate olanzapine-induced impairment of skeletal muscle insulin signaling. Mechanistically, olanzapine upregulated mRNA expression of TNFα, IL6 and IL1β, and protein phosphorylation of both IκB kinase (IKK)α/β and nuclear factor (NF)κB p65. Furthermore, it increased protein phosphorylation of Ser^485/491 in AMPKα2, whereas it decreased AMPKα2 activity. More importantly, both Western blot and immunohistochemical analyses revealed that olanzapine increased protein phosphorylation of Ser^744/748 in protein kinase D1 (PKD1).

SIGNIFICANCE

The present results suggest that the PKD1-mediated inflammatory pathway is involved in olanzapine-induced impairment of skeletal muscle insulin signaling in rats. Our findings may go new insight into the mechanisms underlying olanzapine-induced SMIR.

Lipin1 Is Involved in the Pathogenesis of Diabetic Encephalopathy through the PKD/Limk/Cofilin Signaling Pathway

Diabetic encephalopathy is a type of central diabetic neuropathy resulting from diabetes mainly manifested as cognitive impairments. However, its underlying pathogenesis and effective treatment strategies remain unclear. In the present study, we investigated the effect of Lipin1, a phosphatidic acid phosphatase enzyme, on the pathogenesis of diabetic encephalopathy. We found that in vitro, Lipin1 exerts protective effects on high glucose-induced reductions of PC12 cell viability, while in vivo, Lipin1 is downregulated within the CA1 hippocampal region in a type I diabetes rat model. Increased levels of Lipin1 within the CA1 region are accompanied with protective effects including amelioration of dendritic spine and synaptic deficiencies, phosphorylation of the synaptic plasticity-related proteins, LIM kinase 1 (p-limk1) and cofilin, as well as increases in the synthesis of diacylglycerol (DAG), and the expression of phosphorylated protein kinase D (p-PKD). These effects are associated with the rescue of cognitive disorders as shown in this rat model of diabetes. In contrast, knockdown of Lipin1 within the CA1 region enhanced neuronal abnormalities and the genesis of cognitive impairment in rats. These results suggest that Lipin1 may exert neuroprotective effects involving the PKD/Limk/Cofilin signaling pathway and may serve as a potential therapeutic target for diabetic encephalopathy.

Murine in vitro cellular models to better understand adipogenesis and its potential applications

Adipogenesis has been extensively studied using in vitro models of cellular differentiation, enabling long-term regulation of fat cell metabolism in human adipose tissue (AT) material. Many studies promote the idea that manipulation of this process could potentially reduce the prevalence of obesity and its related diseases. It has now become essential to understand the molecular basis of fat cell development to tackle this pandemic disease, by identifying therapeutic targets and new biomarkers. This review explores murine cell models and their applications for study of the adipogenic differentiation process in vitro. We focus on the benefits and limitations of different cell line models to aid in interpreting data and selecting a good cell line model for successful understanding of adipose biology.

Diacylglycerol-evoked activation of PKC and PKD isoforms in regulation of glucose and lipid metabolism: a review

Protein kinase C (PKC) and Protein kinase D (PKD) isoforms can sense diacylglycerol (DAG) generated in the different cellular compartments in various physiological processes. DAG accumulates in multiple organs of the obese subjects, which leads to the disruption of metabolic homeostasis and the development of diabetes as well as associated diseases. Multiple studies proved that aberrant activation of PKCs and PKDs contributes to the development of metabolic diseases. DAG-sensing PKC and PKD isoforms play a crucial role in the regulation of metabolic homeostasis and therefore might serve as targets for the treatment of metabolic disorders such as obesity and diabetes.

The adrenergic-induced ERK3 pathway drives lipolysis and suppresses energy dissipation

In this study, El-Merahbi et al. investigated new regulators of lipolysis, and using a high-throughput screen identified the extracellular-regulated kinase 3 (ERK3) in lipolysis regulation. They identified a downstream target of the ERK3/MK5 pathway, the transcription factor FOXO1, which promotes expression of the major lipolytic enzyme ATGL, and provide evidence that targeted deletion of ERK3 in mouse adipocytes inhibits lipolysis.

Obesity-induced diabetes affects >400 million people worldwide. Uncontrolled lipolysis (free fatty acid release from adipocytes) can contribute to diabetes and obesity. To identify future therapeutic avenues targeting this pathway, we performed a high-throughput screen and identified the extracellular-regulated kinase 3 (ERK3) as a hit. We demonstrated that β-adrenergic stimulation stabilizes ERK3, leading to the formation of a complex with the cofactor MAP kinase-activated protein kinase 5 (MK5), thereby driving lipolysis. Mechanistically, we identified a downstream target of the ERK3/MK5 pathway, the transcription factor FOXO1, which promotes the expression of the major lipolytic enzyme ATGL. Finally, we provide evidence that targeted deletion of ERK3 in mouse adipocytes inhibits lipolysis, but elevates energy dissipation, promoting lean phenotype and ameliorating diabetes. Thus, ERK3/MK5 represents a previously unrecognized signaling axis in adipose tissue and an attractive target for future therapies aiming to combat obesity-induced diabetes.

The kinase PKD3 provides negative feedback on cholesterol and triglyceride synthesis by suppressing insulin signaling

Hepatic activation of protein kinase C (PKC) isoforms by diacylglycerol (DAG) promotes insulin resistance and contributes to the development of type 2 diabetes (T2D). The closely related protein kinase D (PKD) isoforms act as effectors for DAG and PKC. Here, we showed that PKD3 was the predominant PKD isoform expressed in hepatocytes and was activated by lipid overload. PKD3 suppressed the activity of downstream insulin effectors including the kinase AKT and mechanistic target of rapamycin complex 1 and 2 (mTORC1 and mTORC2). Hepatic deletion of PKD3 in mice improved insulin-induced glucose tolerance. However, increased insulin signaling in the absence of PKD3 promoted lipogenesis mediated by SREBP (sterol regulatory element-binding protein) and consequently increased triglyceride and cholesterol content in the livers of PKD3-deficient mice fed a high-fat diet. Conversely, hepatic-specific overexpression of a constitutively active PKD3 mutant suppressed insulin-induced signaling and caused insulin resistance. Our results indicate that PKD3 provides feedback on hepatic lipid production and suppresses insulin signaling. Therefore, manipulation of PKD3 activity could be used to decrease hepatic lipid content or improve hepatic insulin sensitivity.