PAI-1 Deficiency Drives Pulmonary Vascular Smooth Muscle Remodeling and Pulmonary Hypertension.

Pulmonary arterial hypertension (PAH) is a progressive disease characterized by vasoconstriction and remodeling of small pulmonary arteries (PAs). Central to the remodeling process is a switch of pulmonary vascular cells to a proliferative, apoptosis-resistant phenotype. Plasminogen activator inhibitor-1 (PAI-1) is the primary physiological inhibitor of urokinase-type and tissue-type plasminogen activators (uPA and tPA), but its role in PAH is unsettled. Here, we report that: (1) PAI-1 is deficient in remodeled small PAs and in early-passage PA smooth muscle and endothelial cells (PASMCs and PAECs) from subjects with PAH compared to controls; (2) PAI-1-/- mice spontaneously develop pulmonary vascular remodeling associated with up-regulation of mTORC1 signaling, pulmonary hypertension (PH), and right ventricle (RV) hypertrophy; and (3) pharmacological inhibition of uPA in human PAH PASMCs suppresses pro-proliferative mTORC1 and SMAD3 signaling, restores PAI-1 levels, reduces proliferation and induces apoptosis in vitro, and prevents the development of SU5416/hypoxia-induced PH and RV hypertrophy in vivo in mice. These data strongly suggest that down-regulation of PAI-1 in small PAs promotes vascular remodeling and PH due to unopposed activation of uPA and consequent up-regulation of mTOR and TGF-b signaling in PASMCs, and call for further studies to determine the potential benefits of targeting the PAI-1/uPA imbalance to attenuate and/or reverse pulmonary vascular remodeling and PH.

Urokinase-type plasminogen activator (uPA) is critical for progression of tuberous sclerosis complex 2 (TSC2)-deficient tumors.

Lymphangioleiomyomatosis (LAM) is a fatal lung disease associated with germline or somatic inactivating mutations in tuberous sclerosis complex genes (TSC1 or TSC2). LAM is characterized by neoplastic growth of smooth muscle-α-actin-positive cells that destroy lung parenchyma and by the formation of benign renal neoplasms called angiolipomas. The mammalian target of rapamycin complex 1 (mTORC1) inhibitor rapamycin slows progression of these diseases but is not curative and associated with notable toxicity at clinically effective doses, highlighting the need for better understanding LAM's molecular etiology. We report here that LAM lesions and angiomyolipomas overexpress urokinase-type plasminogen activator (uPA). Tsc1-/- and Tsc2-/- mouse embryonic fibroblasts expressed higher uPA levels than their WT counterparts, resulting from the TSC inactivation. Inhibition of uPA expression in Tsc2-null cells reduced the growth and invasiveness and increased susceptibility to apoptosis. However, rapamycin further increased uPA expression in TSC2-null tumor cells and immortalized TSC2-null angiomyolipoma cells, but not in cells with intact TSC. Induction of glucocorticoid receptor signaling or forkhead box (FOXO) 1/3 inhibition abolished the rapamycin-induced uPA expression in TSC-compromised cells. Moreover, rapamycin-enhanced migration of TSC2-null cells was inhibited by the uPA inhibitor UK122, dexamethasone, and a FOXO inhibitor. uPA-knock-out mice developed fewer and smaller TSC2-null lung tumors, and introduction of uPA shRNA in tumor cells or amiloride-induced uPA inhibition reduced tumorigenesis in vivo These findings suggest that interference with the uPA-dependent pathway, when used along with rapamycin, might attenuate LAM progression and potentially other TSC-related disorders.

Urokinase-type Plasminogen Activator (uPA) Promotes Angiogenesis by Attenuating Proline-rich Homeodomain Protein (PRH) Transcription Factor Activity and De-repressing Vascular Endothelial Growth Factor (VEGF) Receptor Expression.

Urokinase-type plasminogen activator (uPA) regulates angiogenesis and vascular permeability through proteolytic degradation of extracellular matrix and intracellular signaling initiated upon its binding to uPAR/CD87 and other cell surface receptors. Here, we describe an additional mechanism by which uPA regulates angiogenesis. Ex vivo VEGF-induced vascular sprouting from Matrigel-embedded aortic rings isolated from uPA knock-out (uPA(-/-)) mice was impaired compared with vessels emanating from wild-type mice. Endothelial cells isolated from uPA(-/-) mice show less proliferation and migration in response to VEGF than their wild type counterparts or uPA(-/-) endothelial cells in which expression of wild type uPA had been restored. We reported previously that uPA is transported from cell surface receptors to nuclei through a mechanism that requires its kringle domain. Intranuclear uPA modulates gene transcription by binding to a subset of transcription factors. Here we report that wild type single-chain uPA, but not uPA variants incapable of nuclear transport, increases the expression of cell surface VEGF receptor 1 (VEGFR1) and VEGF receptor 2 (VEGFR2) by translocating to the nuclei of ECs. Intranuclear single-chain uPA binds directly to and interferes with the function of the transcription factor hematopoietically expressed homeodomain protein or proline-rich homeodomain protein (HHEX/PRH), which thereby lose their physiologic capacity to repress the activity of vehgr1 and vegfr2 gene promoters. These studies identify uPA-dependent de-repression of vegfr1 and vegfr2 gene transcription through binding to HHEX/PRH as a novel mechanism by which uPA mediates the pro-angiogenic effects of VEGF and identifies a potential new target for control of pathologic angiogenesis.

The imidazoline compound RX871024 promotes insulinoma cell death independent of AMP-activated protein kinase inhibition.

We have previously shown that the insulinotropic imidazoline compound RX871024 induces death of insulinoma MIN6 cells, an effect involving stimulation of c-Jun N-terminal kinase (JNK) and caspase 3. It has also been reported that AMP-activated protein kinase (AMPK) activates JNK and induces ß-cell death. Here we show that RX871024, but not another insulinotropic imidazoline compound (BL11282), suppressed AMPK activity in MIN6 cells. The inhibitory effect of RX871024 on AMPK was supported by the observation that the imidazoline induced lipid droplet formation in the cytoplasm of MIN6 cells. This reflects stimulation of anabolic pathways and inhibition of catabolic pathways in the cell that happen under conditions when AMPK is inhibited. Activation of AMPK by 5-aminoimidazole-4-carboxamide riboside (AICAR) elevated basal and cytokine-induced death in primary ß-cells and in insulinoma MIN6 cells. RX871024 aggravated AICAR-induced insulinoma MIN6 cell death regardless of the presence of pro-inflammatory cytokines. The specific cytotoxic effect of imidazoline compound RX871024 on insulinoma cell death but not primary ß-cell death is independent of its action on AMPK and may suggest the possibility of using this type of compound in the treatment of insulinomas.

A GLP-1 receptor agonist conjugated to an albumin-binding domain for extended half-life.

Glucagon-like peptide 1 (GLP-1) and related peptide agonists have been extensively investigated for glycaemic control in Type 2 diabetes, and may also have therapeutic applications for other diseases. Due to the short half-life (t1/2 < 2 min) of the endogenous peptide, caused by proteolytic degradation and renal clearance, different strategies for half-life extension and sustained release have been explored. In the present study, conjugates between a GLP-1 analogue and a 5 kDa albumin-binding domain (ABD) derived from streptococcal protein G have been chemically synthesized and evaluated. ABD binds with high affinity to human serum albumin, which is highly abundant in plasma and functions as a drug carrier in the circulation. Three different GLP-1-ABD conjugates, with the two peptides connected by linkers of two, four, and six PEG units, respectively, were synthesized and tested in mouse pancreatic islets at high (11 mM) and low (3 mM) glucose concentration. Insulin release upon stimulation was shown to be glucose-dependent, showing no significant difference between the three different GLP-1-ABD conjugates and unconjugated GLP-1 analogue. The biological activity, in combination with the high affinity binding to albumin, make the GLP-1-ABD conjugates promising GLP-1 receptor agonists expected to show extended in vivo half-life.

Packaging of supplemented urokinase into naked alpha-granules of in vitro -grown megakaryocytes for targeted therapeutic delivery.

Our prior finding that uPA endogenously expressed and stored in the platelets of transgenic mice prevented thrombus formation without causing bleeding, prompted us to develop a potentially clinically relevant means of generating anti-thrombotic human platelets in vitro from CD34 + hematopoietic cell-derived megakaryocytes. CD34 + -megakaryocytes internalize and store in α-granules single-chain uPA (scuPA) and a uPA variant modified to be plasmin-resistant, but thrombin-activatable, (uPAT). Both uPAs co-localized with internalized factor V (FV), fibrinogen and plasminogen, low-density lipoprotein receptor-related protein 1 (LRP1), and interferon-induced transmembrane protein 3 (IFITM3), but not with endogenous von Willebrand factor (VWF). Endocytosis of uPA by CD34 + -\megakaryocytes was mediated in part via LRP1 and αIIbß3. scuPA-containing megakaryocytes degraded endocytosed intragranular FV, but not endogenous VWF, in the presence of internalized plasminogen, whereas uPAT-megakaryocytes did not significantly degrade either protein. We used a carotid-artery injury model in NOD-scid IL2rγnull (NSG) mice homozygous for VWF R1326H (a mutation switching binding VWF specificity from mouse to human glycoprotein IbmlIX) to test whether platelets derived from scuPA-MKs or uPAT-Mks would prevent thrombus formation. NSG/VWF R1326H mice exhibited a lower thrombotic burden after carotid artery injury compared to NSG mice unless infused with human platelets or MKs, whereas intravenous injection of either uPA-containing megakaryocytes into NSG/VWF R1326H generated sufficient uPA-containing human platelets to lyse nascent thrombi. These studies suggest the potential to deliver uPA or potentially other ectopic proteins within platelet α-granules from in vitro- generated megakaryocytes. Key points: Unlike platelets, in vitro-grown megakaryocytes can store exogenous uPA in its α-granules.uPA uptake involves LRP1 and αIIbß3 receptors and is functionally available from activated platelets.

Suppressor of cytokine signaling-1 inhibits caspase activation and protects from cytokine-induced beta cell death.

Pancreatic beta cell damage caused by proinflammatory cytokines interleukin-1beta (IL-1beta), interferon-gamma (IFNgamma) and tumor necrosis factor-alpha (TNFalpha) is a key event in the pathogenesis of type 1 diabetes. The suppressor of cytokine signaling-1 (SOCS-1) blocks IFNgamma-induced signaling and prevents diabetes in the non-obese diabetic mouse. Here, we investigated if SOCS-1 overexpression in primary beta cells provides protection from cytokine-induced islet cell dysfunction and death. We demonstrate that SOCS-1 does not prevent increase in NO production and decrease in glucose-stimulated insulin secretion in the presence of IL-1beta, IFNgamma, TNFalpha. However, it decreases the activation of caspase-3, -8 and -9, and thereby, promotes a robust protection from cytokine-induced beta cell death. Our data suggest that SOCS-1 overexpression may not be sufficient in preventing all the biological activities of IFNgamma in beta cells. In summary, we show that interference with IFNgamma signal transduction pathways by SOCS-1 inhibits cytokine-stimulated pancreatic beta cell death.

Proteins in the insulin-secreting cell line MIN6 bind the imidazoline compound BL11282.

The imidazoline BL11282 stimulates insulin release and alters islet proteomes. Subcellular fractions of MIN6 cells showed that the membrane fraction exhibited binding to BL11282 on a Biacore chip and to BL11282-labelled magnetic beads. Bound material extracted from the beads showed a approximately 50 kDa differential band upon SDS-PAGE and a weaker 100 kDa band. The former was sensitive to competitive removal by preincubation of the fraction with BL11282, then highlighting the approximately 100 kDa band. Masspectrometric analysis revealed the approximately 50 kDa band to be EF1A and the approximately 100 kDa band to be glucose regulated P94, both of interest in insulin synthesis and secretion.

Insulinotropic compounds decrease endothelial cell survival.

OBJECTIVES: Hyperglycemia induces damage of vascular endothelial cells leading to diabetic complications. We investigated the effects of insulinotropic compounds and elevated glucose on endothelial cells in the absence or presence of vascular endothelial growth factor (VEGF). RESULTS: Human umbilical vein endothelial cells (HUVECs) were treated with glibenclamide, repaglinide and insulinotropic imidazolines at high glucose concentration in the presence or absence of VEGF and viability, proliferation and nitric oxide production were measured. Hyperglycemia inhibited pro-survival effects of VEGF on endothelial cells. Glibenclamide and repaglinide decreased HUVEC viability at elevated glucose concentration in the absence but not in the presence of VEGF, without affecting HUVEC proliferation. Repaglinide also had some positive influence on HUVEC function elevating NO production in the presence of VEGF. Imidazolines showed different activities on endothelial cell survival. Efaroxan diminished HUVEC viability at elevated glucose concentration in the presence, however not in the absence of VEGF, while RX871024 decreased HUVEC survival regardless of the presence of VEGF. SIGNIFICANCE OF THE STUDY: Our data demonstrate an important interplay between the actual insulinotropic compounds, VEGF and ambient glucose concentration affecting the survival of the vascular endothelial cells. Consequently, this interplay needs to be taken into consideration when designing novel oral antidiabetic compounds.

Polyphosphate/platelet factor 4 complexes can mediate heparin-independent platelet activation in heparin-induced thrombocytopenia.

Heparin-induced thrombocytopenia (HIT) is a thrombotic disorder initiated by antibodies to complexes between platelet factor 4 (PF4) and heparin. The risk of recurrent thromboembolism persists after heparin is cleared and platelet activation leading to release of PF4 has dissipated. We asked whether antigenic complexes between polyphosphates and PF4 released from activated platelets might intensify or sustain the prothrombotic phenotype of HIT. PF4 forms stable, ultralarge complexes with polyphosphates of various sizes, including those released from platelets, which are recognized by the HIT-like monoclonal KKO, an immunoglobulin G2bκ monoclonal heparin/PF4 binding antibody, and by human HIT antibodies. KKO helps to protect PF4/polyphosphate complexes from degradation by phosphatases. Complement is activated when HIT antibodies bind to PF4/polyphosphate complexes and PF4 reverses the inhibition of complement by polyphosphates. Polyphosphates and PF4 are stored primarily in separate granules in resting platelets, but they colocalize when the cells are activated. Platelets activated by subaggregating doses of thrombin receptor activating peptide release polyphosphates and PF4, which form antigenic complexes that allow KKO to further activate platelets in the absence of heparin and exogenous PF4. These studies suggest that thrombin- or immune complex-mediated release of endogenous antigenic PF4/polyphosphate complexes from platelets may augment the prothrombotic risk of HIT and perpetuate the risk of thrombosis after heparin has been discontinued.

Calcium has a permissive role in interleukin-1beta-induced c-jun N-terminal kinase activation in insulin-secreting cells.

The c-jun N-terminal kinase (JNK) signaling pathway mediates IL-1beta-induced apoptosis in insulin-secreting cells, a mechanism relevant to the destruction of pancreatic beta-cells in type 1 and 2 diabetes. However, the mechanisms that contribute to IL-1beta activation of JNK in beta-cells are largely unknown. In this study, we investigated whether Ca(2+) plays a role for IL-1beta-induced JNK activation. In insulin-secreting rat INS-1 cells cultured in the presence of 11 mm glucose, combined pharmacological blockade of L- and T-type Ca(2+) channels suppressed IL-1beta-induced in vitro phosphorylation of the JNK substrate c-jun and reduced IL-1beta-stimulated activation of JNK1/2 as assessed by immunoblotting. Inhibition of IL-1beta-induced in vitro kinase activity toward c-jun after collective L- and T-type Ca(2+) channel blockade was confirmed in primary rat and ob/ob mouse islets and in mouse betaTC3 cells. Ca(2+) influx, specifically via L-type but not T-type channels, contributed to IL-1beta activation of JNK. Activation of p38 and ERK in response to IL-1beta was also dependent on L-type Ca(2+) influx. Membrane depolarization by KCl, exposure to high glucose, treatment with Ca(2+) ionophore A23187, or exposure to thapsigargin, an inhibitor of sarco(endo)plasmic reticulum Ca(2+) ATPase, all caused an amplification of IL-1beta-induced JNK activation in INS-1 cells. Finally, a chelator of intracellular free Ca(2+) [bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid-acetoxymethyl], an inhibitor of calmodulin (W7), and inhibitors of Ca(2+)/calmodulin-dependent kinase (KN62 and KN93) partially reduced IL-1beta-stimulated c-jun phosphorylation in INS-1 or betaTC3 cells. Our data suggest that Ca(2+) plays a permissive role in IL-1beta activation of the JNK signaling pathway in insulin-secreting cells.

Two generations of insulinotropic imidazoline compounds.

The imidazoline RX871024 increased basal- and glucose-stimulated insulin release in vitro and in vivo. The compound inhibited activity of ATP-sensitive K(+) channels as well as voltage-gated K(+) channels, which led to membrane depolarization, an increase in the cytosolic Ca(2+) concentration ([Ca(2+)](i)), and insulin release. Importantly, RX871024 also enhanced the insulinotropic effect of glucose in cells with clamped [Ca(2+)](i) but in the presence of high ATP and Ca(2+)concentration inside the cell. We believe that the latter effect on insulin exocytosis was at least in part mediated by a rise in diacylglycerol, which then activated protein kinase C (PKC) and increased the generation of arachidonic acid (AA) metabolites. Activation of both the PKC and AA pathways resulted in potentiation of glucose effects on insulin secretion. Unlike RX871024, the novel imidazoline BL11282 did not block ATP-dependent K(+) channels, but similarly to RX871024, it stimulated insulin secretion in depolarized or permeabilized islets. Accordingly, BL11282 did not influence glucose and insulin levels under basal conditions either in vitro or in vivo, but it markedly enhanced the insulinotropic effects of glucose. BL11282 restored the impaired insulin response to glucose in islets from spontaneously diabetic GK rats. We conclude that BL11282 belongs to a new class of insulinotropic compounds that demonstrate a strong glucose-dependent effect on insulin exocytosis.

On-line monitoring of apoptosis in insulin-secreting cells.

Apoptosis was monitored in intact insulin-producing cells both with microfluorometry and with two-photon laser scanning microscopy (TPLSM), using a fluorescent protein based on fluorescence resonance energy transfer (FRET). TPLSM offers three-dimensional spatial information that can be obtained relatively deep in tissues. This provides a potential for future in vivo studies of apoptosis. The cells expressed a fluorescent protein (C-DEVD-Y) consisting of two fluorophores, enhanced cyan fluorescent protein (ECFP) and enhanced yellow fluorescent protein (EYFP), linked by the amino acid sequence DEVD selectively cleaved by caspase-3-like proteases. FRET between ECFP and EYFP in C-DEVD-Y could therefore be monitored on-line as a sensor of caspase-3 activation. The relevance of using caspase-3 activation to indicate beta-cell apoptosis was demonstrated by inhibiting caspase-3-like proteases with Z-DEVD-fmk and thereby showing that caspase-3 activation was needed for high-glucose-and cytokine-induced apoptosis in the beta-cell and for staurosporine-induced apoptosis in RINm5F cells. In intact RINm5F cells expressing C-DEVD-Y and in MIN6 cells expressing the variant C-DEVD-Y2, FRET was lost at 155 +/- 23 min (n = 9) and 257 +/- 59 min (n = 4; mean +/- SE) after activation of apoptosis with staurosporine (6 micromol/l), showing that this method worked in insulin-producing cells.

Pancreatic Islet Survival and Engraftment Is Promoted by Culture on Functionalized Spider Silk Matrices.

Transplantation of pancreatic islets is one approach for treatment of diabetes, however, hampered by the low availability of viable islets. Islet isolation leads to disruption of the environment surrounding the endocrine cells, which contributes to eventual cell death. The reestablishment of this environment is vital, why we herein investigated the possibility of using recombinant spider silk to support islets in vitro after isolation. The spider silk protein 4RepCT was formulated into three different formats; 2D-film, fiber mesh and 3D-foam, in order to provide a matrix that can give the islets physical support in vitro. Moreover, cell-binding motifs from laminin were incorporated into the silk protein in order to create matrices that mimic the natural cell environment. Pancreatic mouse islets were thoroughly analyzed for adherence, necrosis and function after in vitro maintenance on the silk matrices. To investigate their suitability for transplantation, we utilized an eye model which allows in vivo imaging of engraftment. Interestingly, islets that had been maintained on silk foam during in vitro culture showed improved revascularization. This coincided with the observation of preserved islet architecture with endothelial cells present after in vitro culture on silk foam. Selected matrices were further evaluated for long-term preservation of human islets. Matrices with the cell-binding motif RGD improved human islet maintenance (from 36% to 79%) with preserved islets architecture and function for over 3 months in vitro. The islets established cell-matrix contacts and formed vessel-like structures along the silk. Moreover, RGD matrices promoted formation of new, insulin-positive islet-like clusters that were connected to the original islets via endothelial cells. On silk matrices with islets from younger donors (<35 year), the amount of newly formed islet-like clusters found after 1 month in culture were almost double compared to the initial number of islets added.

Glucose metabolites inhibit protein phosphatases and directly promote insulin exocytosis in pancreatic beta-cells.

In human type 2 diabetes mellitus, loss of glucose-sensitive insulin secretion is an early pathogenetic event. Glucose is the cardinal physiological stimulator of insulin secretion from the pancreatic beta-cell, but the mechanisms involved in glucose sensing are not fully understood. Specific ser/thr protein phosphatase (PPase) inactivation by okadaic acid promotes Ca(2+) entry and insulin exocytosis in the beta-cell. We now show that glycolytic and Krebs cycle intermediates, whose concentrations increase upon glucose stimulation, not only dose dependently inhibit ser/thr PPase enzymatic activities, but also directly promote insulin exocytosis from permeabilized beta-cells. Thus, fructose-1,6-bisphosphate, phosphoenolpyruvate, 3-phosphoglycerate, citrate, and oxaloacetate inhibit PPases and significantly enhance insulin exocytosis, nonadditive to that of okadaic acid, at micromolar Ca2+ concentrations. In contrast, the effect of GTP is potentiated by okadaic acid, suggesting that the action of GTP does not require PPase inactivation. We conclude that specific glucose metabolites and GTP inhibit beta-cell PPase activities and directly stimulate Ca2+-independent insulin exocytosis. The glucose metabolites, but not GTP, seem to require PPase inactivation for their stimulatory effect on exocytosis. Thus, an increase in phosphorylation state, through inhibition of protein dephosphorylation by metabolic intermediates, may be a novel regulatory mechanism linking glucose sensing to insulin exocytosis in the beta-cell.

RX871024 reduces NO production but does not protect against pancreatic beta-cell death induced by proinflammatory cytokines.

The imidazoline compound RX871024 reduces IL-1beta-induced NO production thereby protecting against IL-1beta-induced beta-cell apoptosis. The aim of this study was to evaluate whether imidazolines RX871024 and efaroxan protect beta-cells against death in the presence of a combination of the cytokines IL-1beta, IFNgamma, and TNFalpha. To address this issue, experiments involving different methods for detection of cell death, different concentrations of the cytokines, and a variety of conditions of preparation and culturing of ob/ob mouse islets and beta-cells have been carried out. Thoroughly performed experiments have not been able to demonstrate a protective effect of RX871024 and efaroxan on beta-cell death induced by the combination of cytokines. However, the inhibitory effect of RX871024 on NO production in ob/ob mouse islets and beta-cells was still observed in the presence of all three cytokines and correlated with the decrease in p38 MAPK phosphorylation. Conversely, efaroxan did not affect cytokine-induced NO production. Our data indicate that a combination of pro-inflammatory cytokines IL-1beta, IFNgamma, and TNFalpha, conditions modelling those that take place in type 1 diabetes, induces pancreatic beta-cell death that does not directly correlate with NO production and cannot be counteracted with imidazoline compounds.

Monomeric G-protein, Rhes, is not an imidazoline-regulated protein in pancreatic beta-cells.

The monomeric G-protein, Rhes, is a candidate imidazoline-regulated molecule involved in mediating the insulin secretory response to efaroxan [S.L. Chan, L.K. Monks, H. Gao, P. Deaville, N.G. Morgan, Identification of the monomeric G-protein, Rhes, as an efaroxan-regulated protein in the pancreatic beta-cell, Br. J. Pharmacol. 136 (1) (2002) 31-36]. This suggestion was based on observations regarding changes in Rhes mRNA expression in rat islets and pancreatic beta-cells after prolonged culture with efaroxan, leading to desensitization of the insulin response to the compound. To verify this report, we have evaluated the effects of the imidazoline compounds efaroxan and BL11282 on Rhes mRNA expression in isolated rat pancreatic islets maintained in conditions identical to those used by Chan et al. The results demonstrate that desensitization of the insulin response to efaroxan, or to another imidazoline, BL11282, does not change Rhes mRNA expression levels. Transfection of MIN6 cells with plasmids containing Rhes or Rhes-antisense also does not alter efaroxan- or BL11282-induced insulin secretion. Together, these data do not support the hypothesis that Rhes is an imidazoline-regulated protein.

Inositol hexakisphosphate promotes dynamin I- mediated endocytosis.

Membrane homeostasis is maintained by exocytosis and endocytosis. The molecular mechanisms regulating the interplay between these two processes are not clear. Inositol hexakisphosphate (InsP(6)) is under metabolic control and serves as a signal in the pancreatic beta cell stimulus-secretion coupling by increasing Ca(2+)-channel activity and insulin exocytosis. We now show that InsP(6) also promotes dynamin I-mediated endocytosis in the pancreatic beta cell. This effect of InsP(6) depends on calcineurin-induced dephosphorylation and is accounted for by both activation of protein kinase C and inhibition of the phosphoinositide phosphatase synaptojanin and thereby formation of phosphatidylinositol 4,5-bisphosphate. In regulating both exocytosis and endocytosis, InsP(6) thus may have an essential integral role in membrane trafficking.

Insulinotropic activity of the imidazoline derivative RX871024 in the diabetic GK rat.

The insulinotropic activity of the imidazoline derivative RX871024 was compared in pancreatic islets from nondiabetic Wistar rats and spontaneously diabetic Goto-Kakizaki (GK) rats. RX871024 significantly stimulated insulin secretion in islets from both animal groups. The insulinotropic activity of RX871024 was higher than that of the sulfonylurea glibenclamide. This difference was more pronounced in islets from GK rats compared with Wistar rat islets. More importantly, RX871024 substantially improved glucose sensitivity in diabetic beta-cells, whereas glibenclamide stimulated insulin secretion about twofold over a broad range of glucose concentrations in nondiabetic and diabetic rats. RX871024 induced a faster increase in cytosolic free Ca(2+) concentration and faster inhibition of ATP-dependent K(+) channel activity in GK rat islets compared with Wistar rat islets. RX871024 also induced a more pronounced increase in diacylglycerol concentration in GK rat islets. These data support the idea that imidazoline compounds can form the basis for the development of novel drugs for treatment of type 2 diabetes, which can restore glucose sensitivity in diabetic beta-cells.

Removal of Ca2+ channel beta3 subunit enhances Ca2+ oscillation frequency and insulin exocytosis.

An oscillatory increase in pancreatic beta cell cytoplasmic free Ca2+ concentration, [Ca2+]i, is a key feature in glucose-induced insulin release. The role of the voltage-gated Ca2+ channel beta3 subunit in the molecular regulation of these [Ca2+]i oscillations has now been clarified by using beta3 subunit-deficient beta cells. beta3 knockout mice showed a more efficient glucose homeostasis compared to wild-type mice due to increased glucose-stimulated insulin secretion. This resulted from an increased glucose-induced [Ca2+]i oscillation frequency in beta cells lacking the beta3 subunit, an effect accounted for by enhanced formation of inositol 1,4,5-trisphosphate (InsP3) and increased Ca2+ mobilization from intracellular stores. Hence, the beta3 subunit negatively modulated InsP3-induced Ca2+ release, which is not paralleled by any effect on the voltage-gated L type Ca2+ channel. Since the increase in insulin release was manifested only at high glucose concentrations, blocking the beta3 subunit in the beta cell may constitute the basis for a novel diabetes therapy.