Structure and activity of lipid bilayer within a membrane-protein transporter

Membrane proteins function in native cell membranes, but extraction into isolated particles is needed for many biochemical and structural analyses. Commonly used detergent-extraction methods destroy naturally associated lipid bilayers. Here, we devised a detergent-free method for preparing cell-membrane nanoparticles to study the multidrug exporter AcrB, by cryo-EM at 3.2-Å resolution. We discovered a remarkably well-organized lipid-bilayer structure associated with transmembrane domains of the AcrB trimer. This bilayer patch comprises 24 lipid molecules; inner leaflet chains are packed in a hexagonal array, whereas the outer leaflet has highly irregular but ordered packing. Protein side chains interact with both leaflets and participate in the hexagonal pattern. We suggest that the lipid bilayer supports and harmonizes peristaltic motions through AcrB trimers. In AcrB D407A, a putative proton-relay mutant, lipid bilayer buttresses protein interactions lost in crystal structures after detergent-solubilization. Our detergent-free system preserves lipid-protein interactions for visualization and should be broadly applicable.

Design, Synthesis, and Biological Evaluation of Ester and Ether Derivatives of Antisickling Agent 5-HMF for the Treatment of Sickle Cell Disease

Candidate drugs to counter intracellular polymerization of deoxygenated sickle hemoglobin (Hb S) continue to represent a promising approach to mitigating the primary cause of the pathophysiology associated with sickle cell disease (SCD). One such compound is the naturally occurring antisickling agent, 5-hydroxymethyl-2-furfural (5-HMF), which has been studied in the clinic for the treatment of SCD. As part of our efforts to develop novel efficacious drugs with improved pharmacologic properties, we structurally modified 5-HMF into 12 ether and ester derivatives. The choice of 5-HMF as a pharmacophore was influenced by a combination of its demonstrated attractive hemoglobin modifying and antisickling properties, well-known safety profiles, and its reported nontoxic major metabolites. The derivatives were investigated for their time- and/or dose-dependent effects on important antisickling parameters, such as modification of hemoglobin, corresponding changes in oxygen affinity, and inhibition of red blood cell sickling. The novel test compounds bound and modified Hb and concomitantly increased the protein affinity for oxygen. Five of the derivatives exhibited 1.5- to 4.0-fold higher antisickling effects than 5-HMF. The binding mode of the compounds with Hb was confirmed by X-ray crystallography and, in part, helps explain their observed biochemical properties. Our findings, in addition to the potential therapeutic application, provide valuable insights and potential guidance for further modifications of these (and similar) compounds to enhance their pharmacologic properties.

Nanoconjugated NAP as a Potent and Periphery Selective Mu Opioid Receptor Modulator To Treat Opioid-Induced Constipation

Opioids are the mainstay for cancer and noncancer pain management. However, their use is often associated with multiple adverse effects. Among them, the most common and persistent one is probably opioid-induced constipation (OIC). Periphery selective opioid antagonists may alleviate the symptoms of OIC without compromising the analgesic effects of opioids. Recently our laboratories have identified one novel lead compound, 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-[(4'-pyridyl)acetamido]morphinan (NAP), as a peripherally selective mu opioid receptor ligand carrying subnanomolar affinity to the mu opioid receptor and over 100-folds of selectivity over both the delta and kappa opioid receptors, with reasonable oral availability and half-life, and potential to treat OIC. Nanoparticle-based drug delivery systems are now widely considered due to their technological advantages such as good stability, high carrier capacity, low therapeutic side effects, etc. Herein we report nanoparticle supported NAP as a potential candidate for OIC treatment with improved peripheral selectivity over the original lead compound NAP.

Carnitine palmitoyltransferase 1A functions to repress FoxO transcription factors to allow cell cycle progression in ovarian cancer

Cancer cells rely on hyperactive de novo lipid synthesis for maintaining malignancy. Recent studies suggest involvement in cancer of fatty acid oxidation, a process functionally opposite to lipogenesis. A mechanistic link from lipid catabolism to oncogenic processes is yet to be established. Carnitine palmitoyltransferase 1 (CPT1) is a rate-limiting enzyme of fatty acid β-oxidation (FAO) that catalyzes the transfer of long-chain acyl group of the acyl-CoA ester to carnitine, thereby shuttling fatty acids into the mitochondrial matrix for β-oxidation. In the present study, we demonstrated that CPT1A was highly expressed in most ovarian cancer cell lines and primary ovarian serous carcinomas. Overexpression of CPT1A correlated with a poor overall survival of ovarian cancer patients. Inactivation of CPT1A decreased cellular ATP levels and induced cell cycle arrest at G0/G1, suggesting that ovarian cancer cells depend on or are addicted to CPT1A-mediated FAO for cell cycle progression. CPT1A deficiency also suppressed anchorage-independent growth and formation of xenografts from ovarian cancer cell lines. The cyclin-dependent kinase inhibitor p21WAF1 (p21) was identified as most consistently and robustly induced cell cycle regulator upon inactivation of CPT1A. Furthermore, p21 was transcriptionally upregulated by the FoxO transcription factors, which were in turn phosphorylated and activated by AMP-activated protein kinase and the mitogen-activated protein kinases JNK and p38. Our results established the oncogenic relevance of CPT1A and a mechanistic link from lipid catabolism to cell cycle regulation, suggesting that CPT1A could be a prognostic biomarker and rational target for therapeutic intervention of cancer.

[Investigation of CXCR4 mediated chemoresistance in nasopharyngeal carcinoma cell line CNE2]

Objective:Since nasopharyngeal carcinoma is easy to develop resistance during cisplatin-based chemotherapy,CXCR4 expression levels were elevated in mang tumors,and the factor to do with tumor metastasis and chemotherapy drug resistance,and so on has a very important link.We established cisplatin-resistant nasopharyngeal carcinoma cell line, named as CNE2/DDP, and investigated the function of CXCR4 in molecular mechanism behind this resistance.Method:CNE2/DDP was firstly build up by increasing concentration of cisplatin. And then afterwards,MTT assay, RNA interference techniques, microRNA overexpresion techniques, quantative PCR and western blotting were applied to analyze the function of CXCR4 and its downstream effectors.Result:①the expression of CXCR4 was increased in CNE2/DDP and downregulation of CXCR4 with CXCR4 siRNA was able to decrease the resistance of CNE/DDP to cisplatin; ②the expression of let-7a was decrease in CNE2/DDP, while the expression of bcl-2 was increased. Upregulation of let-7a via transfection of let-7a mimics could downregulate the expression of bcl-2 and damage the resistance of CNE2/DDP to cisplation;③downregulation of CXCR4 through CXCR4 siRNA transfection was capable of improving the expression of let-7a. Conclusion:We were the first to found that CXCR4 was related to chemoresistance of CNE2/DPP to cisplatin. Meanwhile, we confirmed that CXCR4 affected the expression of bcl-2 through regulating the expression of let-7a to modulate the chemoresistance of CNE2/DPP to cisplatin.

[Study on anti-oxidase in smoking-related laryngeal squamous cell carcinoma]

Objective:To investigate the role of oxidative stress in smoking-related laryngeal squamous carcinoma through detecting the expression of antioxidant enzymes in smoking patients. Method:A total of 138 cases with laryngeal squamous cell carcinoma enrolled in the first hospital affiliated the northern he bei college from 2012 to 2015 and forty five volunteers were conducted. All participants were divided into three groups according to smoking index: group A(heavy smoking, 88 cases of laryngeal cancer patients) and group B(no smoking 50 cases of laryngeal cancer patients) and C group(45 heavy smoking volunteers).Catalase(CAT), glutathione peroxidase(GSH-px) and malondialdehyde(MDA) and the expression of NRF2 in serum, tissue adjacent to carcinoma, and carcinoma tissues from each groups were measured, respectively. Result:①the expression of the CAT and GSH-px in group A were significantly lower than that of group B(P <0.05), but higher than that of group C(P <0.05); ②the MDA level of group A is significantly higher than group B(P <0.05) and C group(P <0.01);③NRF2 was highly expressed in carcinoma tissues, and the expression level was negatively correlated with degree of carcinoma differentiation (P <0.05). Conclusion:Compared with nonsmoking patients, heavy smoking patients with laryngeal cancer were under more severe oxidative stress. NRF2 expression level in patients with laryngeal squamous cell carcinomas was associated with pathological stage.

Rapamycin enhances IFN-γ and IL-4 production in co-culture of gδ T and dendritic cells from mice with lipopolysaccharide-induced acute lung injury

This study aimed to study the role of rapamycin (RAPA) in modulating the interaction between gδ T cells and dendritic cells (DCs) in a lipopolysaccharide (LPS)-induced acute lung injury mouse model. Mice were injected with LPS to establish the acute lung injury model or LPS + RAPA to assess the role of RAPA in modulating cell interactions. Mice were injected with PBS or RAPA alone as controls. gδ T cells and DCs were isolated from all mice and assessed by flow cytometry and fluorescence microscopy. The isolated gδ T cells and DCs were cultured independently or co-cultured to study their interactions. Enzyme-linked immunosorbent assay was performed to assess the expression of the cytokines, namely, interferon (IFN)-γ, interleukin (IL)-4, tumor necrosis factor (TNF)-α and IL-12 in the individually cultured or co-cultured gδ T cells and DCs, and reverse transcription-polymerase chain reaction (RT-PCR) was employed to investigate the levels of relevant mRNAs. Our study found that co-culture of gδ T cells and DCs from mice treated with LPS + RAPA have reduced expression of IFN-γ and IL-4 (but not TNF-α and IL-12) compared to mice treated with LPS only. These results were confirmed by RT-PCR, where the levels of IFN-γ and IL-4 mRNA were also reduced. This study may provide useful information in understanding the interaction between gδ T cells and DCs in the LPS-induced lung injury model in mice.

Chemokine receptor CCR5 antagonist maraviroc: medicinal chemistry and clinical applications

The human immunodeficiency virus (HIV) causes acquired immumodeficiency syndrome (AIDS), one of the worst global pandemic. The virus infects human CD4 T cells and macrophages, and causes CD4 depletion. HIV enters target cells through the binding of the viral envelope glycoprotein to CD4 and the chemokine coreceptor, CXCR4 or CCR5. In particular, the CCR5-utilizing viruses predominate in the blood during the disease course. CCR5 is expressed on the surface of various immune cells including macrophages, monocytes, microglia, dendric cells, and active memory CD4 T cells. In the human population, the CCR5 genomic mutation, CCR5Δ32, is associated with relative resistance to HIV. These findings paved the way for the discovery and development of CCR5 inhibitors to block HIV transmission and replication. Maraviroc, discovered as a CCR5 antagonist, is the only CCR5 inhibitor that has been approved by both US FDA and the European Medicines Agency (EMA) for treating HIV/AIDS patients. In this review, we summarize the medicinal chemistry and clinical studies of Maraviroc.

Design, Synthesis, and Investigation of Novel Nitric Oxide (NO)-Releasing Prodrugs as Drug Candidates for the Treatment of Ischemic Disorders: Insights into NO-Releasing Prodrug Biotransformation and Hemoglobin-NO Biochemistry

We have developed novel nitric oxide (NO)-releasing prodrugs of efaproxiral (RSR13) for their potential therapeutic applications in a variety of diseases with underlying ischemia. RSR13 is an allosteric effector of hemoglobin (Hb) that decreases the protein's affinity for oxygen, thereby increasing tissue oxygenation. NO, because of its vasodilatory property, in the form of ester prodrugs has been found to be useful in managing several cardiovascular diseases by increasing blood flow and oxygenation in ischemic tissues. We synthesized three NO-donor ester derivatives of RSR13 (DD-1, DD-2, and DD-3) by attaching the NO-releasing moieties nitrooxyethyl, nitrooxypropyl, and 1-(pyrrolidin-1-yl)diazen-1-ium-1,2-diolate, respectively, to the carboxylate of RSR13. In vitro studies demonstrated that the compounds released NO in a time-dependent manner upon being incubated with l-cysteine (1.8-9.3%) or human serum (2.3-52.5%) and also reduced the affinity of Hb for oxygen in whole blood (ΔP50 of 4.9-21.7 mmHg vs ΔP50 of 25.4-32.1 mmHg for RSR13). Crystallographic studies showed RSR13, the hydrolysis product of the reaction between DD-1 and deoxygenated Hb, bound to the central water cavity of Hb. Also, the hydrolysis product, NO, was observed exclusively bound to the two α hemes, the first such HbNO structure to be reported, capturing the previously proposed physiological bis-ligated nitrosylHb species. Finally, nitrate was observed bound to βHis97. Ultraperformance liquid chromatography-mass spectrometry analysis of the compounds incubated with matrices used for the various studies demonstrated the presence of the predicted reaction products. Our findings, beyond the potential therapeutic application, provide valuable insights into the biotransformation of NO-releasing prodrugs and their mechanism of action and into hemoglobin-NO biochemistry at the molecular level.

Exploration on natural product anibamine side chain modification toward development of novel CCR5 antagonists and potential anti-prostate cancer agents

Prostate cancer is one of the leading causes of death among males in the world. Prostate cancer cells have been shown to express upregulated chemokine receptor CCR5, a G protein-coupled receptor (GPCR) that relates to the inflammation process. Anibamine, a natural product containing a pyridine ring and two aliphatic side chains, was shown to carry a binding affinity of 1 μM at CCR5 as an antagonist with potential anti-cancer activity. However, it is not drug-like according to the Lipinski's rule of five mainly due to its two long aliphatic side chains. In our effort to improve its drug-like property, a series of anibamine derivatives were designed and synthesized by placement of aromatic side chains through an amide linkage to the pyridine ring. The newly synthesized compounds were tested for their CCR5 affinity and antagonism, and potential anti-proliferation activity against prostate cancer cell lines. Basal cytotoxicity was finally studied for compounds showing potent anti-proliferation activity. It was found that compounds with hydrophobic substitutions on the aromatic systems seemed to carry more promising CCR5 binding and prostate cancer cell proliferation inhibition activities.

Three branches of phospholipase C signaling pathway promote hepatocyte growth in rat liver regeneration

In general, the phospholipase C (PLC) signaling pathway is involved in many physiological activities, including cell growth. However, little is known regarding how the PLC signaling pathway participates in regulating hepatocyte (HC) growth during liver regeneration (LR). To further explore the influence of the PLC signaling pathway on HCs at the cellular level, HCs of high purity and vitality were isolated using Percoll density-gradient centrifugation after partial hepatectomy. The genes of the PLC signaling pathway and target genes of transcription factors in the pathway were obtained by searching the pathways and transcription factor databases, and changes in gene expression of isolated HCs were examined using the Rat Genome 230 2.0 Microarray. The results suggested that various genes involved in the pathway (including 151 known genes and 39 homologous genes) and cell growth (including 262 known genes and 37 homologous genes) were associated with LR. Subsequently, the synergetic effect of these genes in LR was analyzed using a mathematical model (Et) according to their expression profiles. The results showed that the Et values of G protein-coupled receptor/PLC, integrin/PLC, and growth factor receptor/PLC branches of the PLC pathway were all significantly strengthened during the progression and termination phases of LR. The synergetic effect of target genes, in parallel with target gene-related cell growth, was also enhanced during whole rat LR, suggesting the potential positive effect of PLC on HC growth. The present data indicate that the PLC signaling pathway may promote HC growth through 3 mechanisms during rat LR after partial hepatectomy.

Kinetic isotope effects support the twisted amide mechanism of Pin1 peptidyl-prolyl isomerase

The Pin1 peptidyl-prolyl isomerase catalyzes isomerization of pSer/pThr-Pro motifs in regulating the cell cycle. Peptide substrates, Ac-Phe-Phe-phosphoSer-Pro-Arg-p-nitroaniline, were synthesized in unlabeled form, and with deuterium-labeled Ser-d3 and Pro-d7 amino acids. Kinetic data were collected as a function of Pin1 concentration to measure kinetic isotope effects (KIEs) on catalytic efficiency (kcat/Km). The normal secondary (2°) KIE value measured for the Ser-d3 substrate (kH/kD = 1.6 ± 0.2) indicates that the serine carbonyl does not rehybridize from sp(2) to sp(3) in the rate-determining step, ruling out a nucleophilic addition mechanism. The normal 2° KIE can be explained by hyperconjugation between Ser α-C-H/D and C═O and release of steric strain upon rotation of the amide bond from cis to syn-exo. The inverse 2° KIE value (kH/kD = 0.86 ± 0.08) measured for the Pro-d7 substrate indicates rehybridization of the prolyl nitrogen from sp(2) to sp(3) during the rate-limiting step of isomerization. No solvent kinetic isotope was measured by NMR exchange spectroscopy (kH2O/kD2O = 0.92 ± 0.12), indicating little or no involvement of exchangeable protons in the mechanism. These results support the formation of a simple twisted amide transition state as the mechanism for peptidyl prolyl isomerization catalyzed by Pin1. A model of the reaction mechanism is presented using crystal structures of Pin1 with ground state analogues and an inhibitor that resembles a twisted amide transition state.

Cyclohexyl ketone inhibitors of Pin1 dock in a trans-diaxial cyclohexane conformation

Cyclohexyl ketone substrate analogue inhibitors (Ac–pSer-Ψ[C = OCH]-Pip–tryptamine) of Pin1, the cell cycle regulatory peptidyl-prolyl isomerase (PPIase), were designed and synthesized as potential electrophilic acceptors for the Pin1 active site Cys113 nucleophile to test a proposed nucleophilic addition-isomerization mechanism. Because they were weak inhibitors, models of all three stereoisomers were docked into the active site of Pin1. Each isomer consistently minimized to a trans-diaxial cyclohexane conformation. From this, we hypothesize that Pin1 stretches substrates into a trans-pyrrolidine conformation to lower the barrier to isomerization. Our reduced amide inhibitor of Pin1 adopted a similar trans-pyrrolidine conformation in the crystal structure. The molecular model of 1, which mimics the l-Ser-l-Pro stereochemistry, in the Pin1 active site showed a distance of 4.4 Å, and an angle of 31° between Cys113-S and the ketone carbon. The computational models suggest that the mechanism of Pin1 PPIase is not likely to proceed through nucleophilic addition.

A reduced-amide inhibitor of Pin1 binds in a conformation resembling a twisted-amide transition state

The mechanism of the cell cycle regulatory peptidyl prolyl isomerase (PPIase), Pin1, was investigated using reduced-amide inhibitors designed to mimic the twisted-amide transition state. Inhibitors, R-pSer-Ψ[CH(2)N]-Pro-2-(indol-3-yl)ethylamine, 1 [R = fluorenylmethoxycarbonyl (Fmoc)] and 2 (R = Ac), of Pin1 were synthesized and bioassayed. Inhibitor 1 had an IC(50) value of 6.3 μM, which is 4.5-fold better for Pin1 than our comparable ground-state analogue, a cis-amide alkene isostere-containing inhibitor. The change of Fmoc to Ac in 2 improved aqueous solubility for structural determination and resulted in an IC(50) value of 12 μM. The X-ray structure of the complex of 2 bound to Pin1 was determined to 1.76 Å resolution. The structure revealed that the reduced amide adopted a conformation similar to the proposed twisted-amide transition state of Pin1, with a trans-pyrrolidine conformation of the prolyl ring. A similar conformation of substrate would be destabilized relative to the planar amide conformation. Three additional reduced amides, with Thr replacing Ser and l- or d-pipecolate (Pip) replacing Pro, were slightly weaker inhibitors of Pin1.

Convergent synthesis of alpha-ketoamide inhibitors of Pin1

A convergent synthesis of alpha-ketoamide inhibitors of Pin1 is described. An alpha-hydroxyorthothioester derivative of Ser was reacted directly with an amine synthon. The reaction was catalyzed by HgO and HgCl(2) to form alpha-hydroxyamide. Thus, hydrolysis and coupling were combined in one step with 80% yield. Two diastereomers of a phospho-Ser-Pro alpha-ketoamide analogue were synthesized. The IC(50) values of 100 and 200 microM were surprisingly weak for Pin1 peptidyl prolyl isomerase.

Pin1 as an anticancer drug target

Pin1 specifically catalyzes the cis/trans isomerization of phospho-Ser/Thr-Pro bonds and plays an important role in many cellular events through the effects of conformational change on the function of its biological substrates, including cell division cycle 25 C (Cdc25C), c-Jun and p53. Pin1 is overexpressed in many human cancer tissues, including breast, prostate and lung cancer. Its expression correlates with cyclin D1 levels, which contribute to cell transformation. Overexpression of Pin1 promotes tumor growth, while inhibition of Pin1 causes tumor cell apoptosis. Pin1 plays an important role in oncogenesis and therefore may serve as an effective anticancer target. Many inhibitors of Pin1 have been discovered, including several classes of designed inhibitors (alkene isosteres, reduced amides, indanyl ketones) and natural products (juglone, pepticinnamin E analogues, PiB and its derivatives obtained from a library screen). Pin1 inhibitors could be used as a novel type of anticancer drug by blocking cell cycle progression. Therefore, Pin1 represents a new diagnostic and therapeutic anticancer drug target.

A Review of Clinical Pathway Data of 1663 Total Knee Arthroplasties in a Tertiary Institution in Singapore

Introduction and Objectives: A total knee arthroplasty (TKA) clinical pathway database has been used in our institution since the year 2000. The primary aim of this study was to review the patient epidemiology, postoperative complications and factors influencing hospital length of stay following TKA. The clinical outcomes and cost-savings between elective and same day admissions for TKA patients were also reviewed. Materials and Methods: The study cohort retrieved from the database comprised 1371 patients (1663 knees) who underwent total knee replacement over a 6-year time period. The following variables were reviewed: epidemiological data, admission data (elective admission [EA] versus same day admission [SDA]), hospital length of stay (LOS), and complication rates. Results: The mean age of patients undergoing TKA is 65.2 years (range, 22 to 90). Osteoarthritis was the main surgical indication in 96% of the study cohort. Overall, there was a gradual decline in the hospital length of stay for the study cohort for the 6-year time period. The overall complication rate was 2% and the 3 most common complications were deep vein thrombosis, pulmonary embolism and urinary tract infection. Conclusion: With an increasing elderly population there will be an annual increase in the number of TKAs. In our local population TKAs are performed primarily for the Chinese female in the 7th decade. The overall complication rate of TKA remains low with a mortality rate of <1%. Key words: Complications, Epidemiology, Total knee arthroplasty

A review of clinical pathway data of 1,663 total knee arthroplasties in a tertiary institution in Singapore

INTRODUCTION AND OBJECTIVES

A total knee arthroplasty (TKA) clinical pathway database has been used in our institution since the year 2000. The primary aim of this study was to review the patient epidemiology, postoperative complications and factors influencing hospital length of stay following TKA. The clinical outcomes and cost-savings between elective and same day admissions for TKA patients were also reviewed.

MATERIALS AND METHODS

The study cohort retrieved from the database comprised 1,371 patients (1,663 knees) who underwent total knee replacement over a 6-year time period. The following variables were reviewed: epidemiological data, admission data (elective admission [EA] versus same day admission [SDA]), hospital length of stay (LOS), and complication rates.

RESULTS

The mean age of patients undergoing TKA is 65.2 years (range, 22 to 90). Osteoarthritis was the main surgical indication in 96% of the study cohort. Overall, there was a gradual decline in the hospital length of stay for the study cohort for the 6-year time period. The overall complication rate was 2% and the 3 most common complications were deep vein thrombosis, pulmonary embolism and urinary tract infection.

CONCLUSION

With an increasing elderly population there will be an annual increase in the number of TKAs. In our local population TKAs are performed primarily for the Chinese female in the 7th decade. The overall complication rate of TKA remains low with a mortality rate of <1%.

Gluteal compartment syndrome following abdominal aortic aneurysm repair: a case report

Compartment syndromes occur when the elevated tissue pressure within a confined limb's myofascial compartment exceeds capillary pressure, with subsequent neurovascular compromise. In order to reduce disability and the consequences of ensuring ischemia, it is essential for early recognition and intervention. This is more commonly recognized in the calf. We report an unusual case of gluteal compartment syndrome after abdominal aortic aneurysm (AAA) repair.

Clear cell sarcoma of the rectus sheath

We report a 35-year-old Chinese woman with clear cell sarcoma of the rectus sheath aponeurosis presenting as a tender anterior abdominal mass. She was treated with wide local excision. Local recurrence and distant metastasis occurred within two months of the onset of the complaint. Clear cell sarcoma is a rare cancer with a propensity for slow progressive invasion. They occur most commonly in the extremities, and the majority of patients are young adults. This case report demonstrates an unusual site of occurrence for clear cell sarcoma.

Insulin regulation of beta-cell function involves a feedback loop on SERCA gene expression, Ca(2+) homeostasis, and insulin expression and secretion

The insulin receptor signaling pathway is present in beta-cells and is believed to be important in beta-cell function. We show here that insulin directly regulates beta-cell function in isolated rodent islets. Long-term insulin treatment caused a sustained increase in [Ca(2+)](i) and enhanced glucose-stimulated insulin secretion in rat islets, but failed to increase insulin content. Chronic activation of insulin receptor signaling by IRS-1 overexpression in the beta-cell inhibited gene expression of SERCA3, an endoplasmic reticulum Ca(2+)-ATPase. Insulin gene transcription was stimulated by insulin receptor signaling and insulin mimetic compound (L-783 281) in a glucose- and Grb2-dependent manner. Thus, beta-cell SERCA3 is a target for insulin regulation, which implies that beta-cell Ca(2+) homeostasis is regulated in an autocrine feedback loop by insulin. This study identifies a novel regulatory pathway of insulin secretion at the molecular level with two main components: (1) regulation of intracellular Ca(2+) homeostasis via SERCA3 and (2) regulation of insulin gene expression.

Insulin receptor substrate 1-induced inhibition of endoplasmic reticulum Ca2+ uptake in beta-cells. Autocrine regulation of intracellular ca2+ homeostasis and insulin secretion

To understand the role of the insulin receptor pathway in beta-cell function, we have generated stable beta-cells (betaIRS1-A) that overexpress by 2-fold the insulin receptor substrate-1 (IRS-1) and compared them to vector-expressing controls. IRS-1 overexpression dramatically increased basal cytosolic Ca2+ levels from 81 to 278 nM, but it did not affect Ca2+ response to glucose. Overexpression of the insulin receptor also caused an increase in cytosolic Ca2+. Increased cytosolic Ca2+ was due to inhibition of Ca2+ uptake by the endoplasmic reticulum, because endoplasmic reticulum Ca2+ uptake and content were reduced in betaIRS1-A cells. Fractional insulin secretion was significantly increased 2-fold, and there was a decrease in betaIRS1-A insulin content and insulin biosynthesis. Steady-state insulin mRNA levels and glucose-stimulated ATP were unchanged. High IRS-1 levels also reduced beta-cell proliferation. These data demonstrate a direct link between the insulin receptor signaling pathway and the Ca2+-dependent pathways regulating insulin secretion of beta-cells. We postulate that during regulated insulin secretion, released insulin binds the beta-cell insulin receptor and activates IRS-1, thus further increasing cytosolic Ca2+ by reducing Ca2+ uptake. We suggest the existence of a novel pathway of autocrine regulation of intracellular Ca2+ homeostasis and insulin secretion in the beta-cell of the endocrine pancreas.

Insulin receptor signaling in the beta-cell influences insulin gene expression and insulin content: evidence for autocrine beta-cell regulation

The insulin receptor (IR) is expressed by insulin-secreting beta-cells, but its cellular function is unknown. We transfected betaTC6-F7 beta-cells with cDNAs encoding either wild-type or mutant kinase-inactive (A/K1018) IRs, and by fluorescence-activated cell sorting generated polyclonal beta-cell lines that overexpressed each receptor type at levels two- to fourfold higher than the parental cells. Beta-cells overexpressing wild-type IRs had a proportional increase in insulin-stimulated tyrosine kinase activity; no change occurred in beta-cells expressing the mutant IR. We observed a threefold increase in cellular insulin content in beta-cells that overexpressed the wild-type IR, as determined by radioimmunoassay. This increase occurred despite a fivefold elevated rate of both basal and 10 mmol/l glucose-induced insulin secretion. Fractional insulin secretion (percentage of total cell insulin releasable at 10 mmol/l glucose) was unchanged in beta-cells overexpressing the wild-type IR compared with the parental beta-cell line. Insulin content and insulin secretion were unaffected by overexpression of kinase-inactive IRs. Steady-state insulin mRNA levels were elevated twofold in the beta-cells overexpressing the wild-type IR and unchanged in the beta-cells expressing the kinase-inactive receptor, as determined by Northern blot analysis. The rate of insulin mRNA degradation measured in the presence of 5 microg/ml actinomycin D was not significantly affected in either cell line. In the absence of glucose, the basal level of (pro)insulin biosynthesis in the beta-cells overexpressing the wild-type IR increased significantly (61%) compared with the beta-cells transfected with the kinase-inactive IR. These data indicate that IR signaling can regulate insulin gene transcription and can modulate the steady-state insulin content of beta-cells.