Magnetogenetic cell activation using endogenous ferritin

The ability to precisely control the activity of defined cell populations enables studies of their physiological roles and may provide therapeutic applications. While prior studies have shown that magnetic activation of ferritin-tagged ion channels allows cell-specific modulation of cellular activity, the large size of the constructs made the use of adeno-associated virus, AAV, the vector of choice for gene therapy, impractical. In addition, simple means for generating magnetic fields of sufficient strength have been lacking. Toward these ends, we first generated a novel anti-ferritin nanobody that when fused to transient receptor potential cation channel subfamily V member 1, TRPV1, enables direct binding of the channel to endogenous ferritin in mouse and human cells. This smaller construct can be delivered in a single AAV and we validated that it robustly enables magnetically induced cell activation in vitro . In parallel, we developed a simple benchtop electromagnet capable of gating the nanobody-tagged channel in vivo . Finally, we showed that delivering these new constructs by AAV to pancreatic beta cells in combination with the benchtop magnetic field delivery stimulates glucose-stimulated insulin release to improve glucose tolerance in mice in vivo . Together, the novel anti-ferritin nanobody, nanobody-TRPV1 construct and new hardware advance the utility of magnetogenetics in animals and potentially humans.

Human Pancreatic α-Cell Heterogeneity and Trajectory Inference Analysis Using Integrated Single Cell- and Single Nucleus-RNA Sequencing Platforms

Prior studies have shown that pancreatic α-cells can transdifferentiate into β-cells, and that β-cells de-differentiate and are prone to acquire an α-cell phenotype in type 2 diabetes (T2D). However, the specific human α-cell and β-cell subtypes that are involved in α-to-β-cell and β-to-α-cell transitions are unknown. Here, we have integrated single cell RNA sequencing (scRNA-seq) and single nucleus RNA-seq (snRNA-seq) of isolated human islets and human islet grafts and provide additional insight into α-β cell fate switching. Using this approach, we make seven novel observations. 1) There are five different GCG -expressing human α-cell subclusters [α1, α2, α-β-transition 1 (AB-Tr1), α-β-transition 2 (AB-Tr2), and α-β (AB) cluster] with different transcriptome profiles in human islets from non-diabetic donors. 2) The AB subcluster displays multihormonal gene expression, inferred mostly from snRNA-seq data suggesting identification by pre-mRNA expression. 3) The α1, α2, AB-Tr1, and AB-Tr2 subclusters are enriched in genes specific for α-cell function while AB cells are enriched in genes related to pancreatic progenitor and β-cell pathways; 4) Trajectory inference analysis of extracted α- and β-cell clusters and RNA velocity/PAGA analysis suggests a bifurcate transition potential for AB towards both α- and β-cells. 5) Gene commonality analysis identifies ZNF385D, TRPM3, CASR, MEG3 and HDAC9 as signature for trajectories moving towards β-cells and SMOC1, PLCE1, PAPPA2, ZNF331, ALDH1A1, SLC30A8, BTG2, TM4SF4, NR4A1 and PSCK2 as signature for trajectories moving towards α-cells. 6) Remarkably, in contrast to the events in vitro , the AB subcluster is not identified in vivo in human islet grafts and trajectory inference analysis suggests only unidirectional transition from α-to-β-cells in vivo . 7) Analysis of scRNA-seq datasets from adult human T2D donor islets reveals a clear unidirectional transition from β-to-α-cells compatible with dedifferentiation or conversion into α-cells. Collectively, these studies show that snRNA-seq and scRNA-seq can be leveraged to identify transitions in the transcriptional status among human islet endocrine cell subpopulations in vitro , in vivo , in non-diabetes and in T2D. They reveal the potential gene signatures for common trajectories involved in interconversion between α- and β-cells and highlight the utility and power of studying single nuclear transcriptomes of human islets in vivo . Most importantly, they illustrate the importance of studying human islets in their natural in vivo setting.

Single-nucleus RNA sequencing of human pancreatic islets identifies novel gene sets and distinguishes β-cell subpopulations with dynamic transcriptome profiles

BACKGROUND

Single-cell RNA sequencing (scRNA-seq) provides valuable insights into human islet cell types and their corresponding stable gene expression profiles. However, this approach requires cell dissociation that complicates its utility in vivo. On the other hand, single-nucleus RNA sequencing (snRNA-seq) has compatibility with frozen samples, elimination of dissociation-induced transcriptional stress responses, and affords enhanced information from intronic sequences that can be leveraged to identify pre-mRNA transcripts.

METHODS

We obtained nuclear preparations from fresh human islet cells and generated snRNA-seq datasets. We compared these datasets to scRNA-seq output obtained from human islet cells from the same donor. We employed snRNA-seq to obtain the transcriptomic profile of human islets engrafted in immunodeficient mice. In both analyses, we included the intronic reads in the snRNA-seq data with the GRCh38-2020-A library.

RESULTS

First, snRNA-seq analysis shows that the top four differentially and selectively expressed genes in human islet endocrine cells in vitro and in vivo are not the canonical genes but a new set of non-canonical gene markers including ZNF385D, TRPM3, LRFN2, PLUT (β-cells); PTPRT, FAP, PDK4, LOXL4 (α-cells); LRFN5, ADARB2, ERBB4, KCNT2 (δ-cells); and CACNA2D3, THSD7A, CNTNAP5, RBFOX3 (γ-cells). Second, by integrating information from scRNA-seq and snRNA-seq of human islet cells, we distinguish three β-cell sub-clusters: an INS pre-mRNA cluster (β3), an intermediate INS mRNA cluster (β2), and an INS mRNA-rich cluster (β1). These display distinct gene expression patterns representing different biological dynamic states both in vitro and in vivo. Interestingly, the INS mRNA-rich cluster (β1) becomes the predominant sub-cluster in vivo.

CONCLUSIONS

In summary, snRNA-seq and pre-mRNA analysis of human islet cells can accurately identify human islet cell populations, subpopulations, and their dynamic transcriptome profile in vivo.

DYRK1A Inhibitors as Potential Therapeutics for β-Cell Regeneration for Diabetes

According to the World Health Organization (WHO), 422 million people are suffering from diabetes worldwide. Current diabetes therapies are focused on optimizing blood glucose control to prevent long-term diabetes complications. Unfortunately, current therapies have failed to achieve glycemic targets in the majority of people with diabetes. In this context, regeneration of functional insulin-producing human β-cells in people with diabetes through the use of DYRK1A inhibitor drugs has recently received special attention. Several small molecule DYRK1A inhibitors have been identified that induce human β-cell proliferation in vitro and in vivo. Furthermore, DYRK1A inhibitors have also been shown to synergize β-cell proliferation with other classes of drugs, such as TGFβ inhibitors and GLP-1 receptor agonists. In this perspective, we review the status of DYRK1A as a therapeutic target for β-cell proliferation and provide perspectives on technical and scientific challenges for future translational development.

GLP-1 receptor signaling increases PCSK1 and β cell features in human α cells

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that potentiates glucose-stimulated insulin secretion. GLP-1 is classically produced by gut L cells; however, under certain circumstances α cells can express the prohormone convertase required for proglucagon processing to GLP-1, prohormone convertase 1/3 (PC1/3), and can produce GLP-1. However, the mechanisms through which this occurs are poorly defined. Understanding the mechanisms by which α cell PC1/3 expression can be activated may reveal new targets for diabetes treatment. Here, we demonstrate that the GLP-1 receptor (GLP-1R) agonist, liraglutide, increased α cell GLP-1 expression in a β cell GLP-1R-dependent manner. We demonstrate that this effect of liraglutide was translationally relevant in human islets through application of a new scRNA-seq technology, DART-Seq. We found that the effect of liraglutide to increase α cell PC1/3 mRNA expression occurred in a subcluster of α cells and was associated with increased expression of other β cell-like genes, which we confirmed by IHC. Finally, we found that the effect of liraglutide to increase bihormonal insulin+ glucagon+ cells was mediated by the β cell GLP-1R in mice. Together, our data validate a high-sensitivity method for scRNA-seq in human islets and identify a potentially novel GLP-1-mediated pathway regulating human α cell function.

Lactogens Reduce Endoplasmic Reticulum Stress-Induced Rodent and Human β-Cell Death and Diabetes Incidence in Akita Mice

Diabetes occurs due to a loss of functional β-cells, resulting from β-cell death and dysfunction. Lactogens protect rodent and human β-cells in vitro and in vivo against triggers of β-cell cytotoxicity relevant to diabetes, many of which converge onto a common pathway of endoplasmic reticulum (ER) stress. However, whether lactogens modulate the ER stress pathway is unknown. This study examines whether lactogens can protect β-cells against ER stress and mitigate diabetes incidence in Akita (Ak) mice, a rodent model of ER stress-induced diabetes, akin to neonatal diabetes in humans. We show that lactogens protect INS-1 cells, primary rodent and human β-cells in vitro against two distinct ER stressors, tunicamycin and thapsigargin, through activation of the JAK2/STAT5 pathway. Lactogens mitigate expression of proapoptotic molecules in the ER stress pathway that are induced by chronic ER stress in INS-1 cells and rodent islets. Transgenic expression of placental lactogen in β-cells of Ak mice drastically reduces the severe hyperglycemia, diabetes incidence, hypoinsulinemia, β-cell death, and loss of β-cell mass observed in Ak littermates. These are the first studies in any cell type demonstrating that lactogens modulate the ER stress pathway, causing enhanced β-cell survival and reduced diabetes incidence in the face of chronic ER stress.

Glucose-dependent partitioning of arginine to the urea cycle protects β-cells from inflammation

Chronic inflammation is linked to diverse disease processes, but the intrinsic mechanisms that determine cellular sensitivity to inflammation are incompletely understood. Here, we show the contribution of glucose metabolism to inflammation-induced changes in the survival of pancreatic islet β-cells. Using metabolomic, biochemical and functional analyses, we investigate the protective versus non-protective effects of glucose in the presence of pro-inflammatory cytokines. When protective, glucose metabolism augments anaplerotic input into the TCA cycle via pyruvate carboxylase (PC) activity, leading to increased aspartate levels. This metabolic mechanism supports the argininosuccinate shunt, which fuels ureagenesis from arginine and conversely diminishes arginine utilization for production of nitric oxide (NO), a chief mediator of inflammatory cytotoxicity. Activation of the PC-urea cycle axis is sufficient to suppress NO synthesis and shield cells from death in the context of inflammation and other stress paradigms. Overall, these studies uncover a previously unappreciated link between glucose metabolism and arginine-utilizing pathways via PC-directed ureagenesis as a protective mechanism.

Synthesis and Biological Validation of a Harmine-Based, Central Nervous System (CNS)-Avoidant, Selective, Human β-Cell Regenerative Dual-Specificity Tyrosine Phosphorylation-Regulated Kinase A (DYRK1A) Inhibitor

Recently, our group identified that harmine is able to induce β-cell proliferation both in vitro and in vivo, mediated via the DYRK1A-NFAT pathway. Since, harmine suffers from a lack of selectivity, both against other kinases and CNS off-targets, we therefore sought to expand structure-activity relationships for harmine's DYRK1A activity, to enhance selectivity for off-targets while retaining human β-cell proliferation activity. We carried out optimization of the 9-N-position of harmine to synthesize 29 harmine-based analogs. Several novel inhibitors showed excellent DYRK1A inhibition and human β-cell proliferation capability. An optimized DYRK1A inhibitor, 2-2c, was identified as a novel, efficacious in vivo lead candidate. 2-2c also demonstrates improved selectivity for kinases and CNS off-targets, as well as in vivo efficacy for β-cell proliferation and regeneration at lower doses than harmine. Collectively, these findings demonstrate that 2-2c is a much improved in vivo lead candidate as compared to harmine for the treatment of diabetes.

Circulating Levels of Bone and Inflammatory Markers in Gestational Diabetes Mellitus

Gestational diabetes mellitus (GDM) can cause short- and long-term complications to the mother and fetus. While the precise mechanisms in preserving glucose balance in a healthy pregnancy are unknown, various growth factors and hormones have been implicated or associated with GDM risk in humans or rodents, including prolactin, tumor necrosis factor alpha (TNFα), osteoprotegerin (OPG), hepatocyte growth factor (HGF), and receptor activator of nuclear factor-kappa B ligand (RANKL). We aimed to evaluate the relationship of these and other protein markers in women with GDM. In this cross-sectional study, blood samples were collected from pregnant women with GDM and with normal glucose tolerance (NGT) at the 24- to 32-week obstetrical visit, during the 1-h oral glucose challenge test or 3-h oral glucose tolerance test. Blood plasma was analyzed for RANKL, OPG, prolactin, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), HGF, plasminogen activator inhibitor type 1 (PAI-1), and TNFα. Forty-six women with NGT and 47 women with GDM were included (mean ± standard deviation maternal age 31.6 ± 5.7, mean ± standard deviation gestational age 28.1 ± 2.2 weeks). Groups were similar in terms of age, body mass index, gestational age, and race/ethnicity. Serum levels of OPG, prolactin, TRAIL, HGF, PAI-1, and TNFα were similar in both groups. RANKL was lower in GDM subjects (p = 0.019). Contrary to previous reports in the literature, we found a lower serum RANKL level in women with GDM. Further investigation is needed to determine whether there are suitable serum markers for diagnosing GDM or determining prognosis or severity.

Time-dependent changes in glucose and insulin regulation during intermittent hypoxia and continuous hypoxia

Hypoxia manifests in many forms including the short repetitive intermittent hypoxia (IH) of sleep apnoea and the continuous hypoxia (CH) of altitude, both of which may impact metabolic function. Based on our own previous studies and the available literature, we hypothesized that whereas acute exposure to IH and CH would lead to comparable metabolic dysfunction, with longer-term exposure, metabolism would normalize to a greater extent with CH than IH. Studies were conducted in lean C57BL/6J mice exposed to either IH or CH for 1 day or 4 weeks and compared to either intermittent air (IA) or unhandled (UN) controls, respectively. We utilized the frequently sampled intravenous glucose tolerance test and minimal model analyses to determine insulin-dependent (insulin sensitivity; S (I)) and insulin-independent (glucose effectiveness; S (g)) glucose disposal, as well as the insulin response to glucose (acute insulin response to glucose; AIR(g)). Our data show that 1-day exposure impaired the glucose tolerance and caused reductions in S (g) and AIR(g) in both the IH and CH groups, but only IH caused a significant decrease in S (I) (7.5 ± 2.7 vs. 17.0 ± 5.3 μU ml(-1) min(-1); p < 0.05). After 4-week exposure, there was evidence of metabolic adaptation in both hypoxic groups, however, in the CH group, there was a supranormal increase in S (I) relative to both UN and IH groups. We conclude that in lean mice, the marked metabolic dysfunction that occurs with acute exposure to hypoxia is reversed to a greater extent with chronic CH exposure than chronic IH exposure.

Sex-specific effect of estrogen sulfotransferase on mouse models of type 2 diabetes

Estrogen sulfotransferase (EST), the enzyme responsible for the sulfonation and inactivation of estrogens, plays an important role in estrogen homeostasis. In this study, we showed that induction of hepatic Est is a common feature of type 2 diabetes. Loss of Est in female mice improved metabolic function in ob/ob, dexamethasone-, and high-fat diet-induced mouse models of type 2 diabetes. The metabolic benefit of Est ablation included improved body composition, increased energy expenditure and insulin sensitivity, and decreased hepatic gluconeogenesis and lipogenesis. This metabolic benefit appeared to have resulted from decreased estrogen deprivation and increased estrogenic activity in the liver, whereas such benefit was abolished in ovariectomized mice. Interestingly, the effect of Est was sex-specific, as Est ablation in ob/ob males exacerbated the diabetic phenotype, which was accounted for by the decreased islet β-cell mass and failure of glucose-stimulated insulin secretion in vivo. The loss of β-cell mass in ob/ob males deficient in Est was associated with increased macrophage infiltration and inflammation in white adipose tissue. Our results revealed an essential role of EST in energy metabolism and the pathogenesis of type 2 diabetes. Inhibition of EST, at least in females, may represent a novel approach to manage type 2 diabetes.

Free fatty acids block glucose-induced β-cell proliferation in mice by inducing cell cycle inhibitors p16 and p18

Pancreatic β-cell proliferation is infrequent in adult humans and is not increased in type 2 diabetes despite obesity and insulin resistance, suggesting the existence of inhibitory factors. Free fatty acids (FFAs) may influence proliferation. In order to test whether FFAs restrict β-cell proliferation in vivo, mice were intravenously infused with saline, Liposyn II, glucose, or both, continuously for 4 days. Lipid infusion did not alter basal β-cell proliferation, but blocked glucose-stimulated proliferation, without inducing excess β-cell death. In vitro exposure to FFAs inhibited proliferation in both primary mouse β-cells and in rat insulinoma (INS-1) cells, indicating a direct effect on β-cells. Two of the fatty acids present in Liposyn II, linoleic acid and palmitic acid, both reduced proliferation. FFAs did not interfere with cyclin D2 induction or nuclear localization by glucose, but increased expression of inhibitor of cyclin dependent kinase 4 (INK4) family cell cycle inhibitors p16 and p18. Knockdown of either p16 or p18 rescued the antiproliferative effect of FFAs. These data provide evidence for a novel antiproliferative form of β-cell glucolipotoxicity: FFAs restrain glucose-stimulated β-cell proliferation in vivo and in vitro through cell cycle inhibitors p16 and p18. If FFAs reduce proliferation induced by obesity and insulin resistance, targeting this pathway may lead to new treatment approaches to prevent diabetes.

Plasma membrane subdomain compartmentalization contributes to distinct mechanisms of ceramide action on insulin signaling

OBJECTIVE

Ceramide is now recognized as a negative regulator of insulin signaling by impairing protein kinase B (PKB)/Akt activation. In different cells, two distinct mechanisms have been proposed to mediate ceramide inhibition of PKB/Akt: one involving atypical protein kinase C zeta (PKCzeta) and the other the protein phosphatase-2 (PP2A). We hypothesized that ceramide action through PKCzeta or PP2A might depend on plasma membrane (PM) structural organization and especially on caveolin-enriched domain (CEM) abundance.

RESEARCH DESIGN AND METHODS

We have used different PKCzeta mutant constructs or the PP2A inhibitor, okadaic acid (OKA), to selectively inhibit PKCzeta- and PP2A-dependent pathways in cells expressing different caveolin-1 levels and evaluated the impact of insulin and ceramide on PKB/Akt activity in different PM subdomains.

RESULTS

Although the PKCzeta-mediated negative effect of ceramide on insulin-stimulated PKB/Akt was dominant in adipocytes, a ceramide action through PP2A outside CEMs, prevented by OKA, was also unraveled. To test the importance of CEM to direct ceramide action through the PKCzeta pathway, we treated 3T3-L1 preadipocytes devoid of CEMs with ceramide and we saw a shift of the lipid-negative action on PKB/Akt to a PP2A-mediated mechanism. In fibroblasts with low CEM abundance, the ceramide-activated PP2A pathway dominated, but could be shifted to a ceramide-activated PKCzeta pathway after caveolin-1 overexpression.

CONCLUSIONS

Our results show that ceramide can switch from a PKCzeta-dependent mechanism to a PP2A pathway, acting negatively on PKB/Akt, and hence revealing a critical role of CEMs of the PM in this process.

Marginal mass islet transplantation with autologous mesenchymal stem cells promotes long-term islet allograft survival and sustained normoglycemia

Allogeneic islet transplantation is an option to treat diabetes however there are obstacles that are limiting its clinical use. We have examined whether mesenchymal stem cells (MSC) improve islet graft survival and whether such therapy allows for better graft acceptance with reduced requirement for immunosuppression. In vitro-expanded syngeneic bone marrow-derived MSC were co-transplanted with islets into omental pouch in a rat model of streptozotocin-induced diabetes. Marginal mass syngeneic islet transplantation into the omentum with MSC promoted sustained normoglycemia. Interestingly, allogeneic islets +MSC, but not islets alone, with short-term use of immunosuppression enhanced long-term islet graft survival, insulin expression in the grafts and induced normal serum insulin levels and normoglycemia. T cells from recipients transplanted with allogeneic islets +MSC produced low levels of IFN-gamma and TNF-alpha upon ex-vivo activation, and this transplantation protocol promoted the generation of IL-10-secreting CD4(+) T cells. These data encourage further preclinical and eventually, clinical MSC-based islet transplantation to improve the outcome of allogeneic islet transplantation in the treatment of diabetes.

Autologous bone marrow-derived rat mesenchymal stem cells promote PDX-1 and insulin expression in the islets, alter T cell cytokine pattern and preserve regulatory T cells in the periphery and induce sustained normoglycemia

Cell-based therapies offer considerable promise for prevention or cure of diabetes. We explored the potential of autologous, self-renewing, mesenchymal stem cells (MSC) as a clinically-applicable approach to promote glucose homeostasis. In vitro-expanded syngeneic bone marrow-derived MSC were administered following or prior to diabetes induction into a rat model of streptozotocin-induced beta cell injury. MSC were CD45(-)/CD44(+)/CD54(+)/CD90(+)/CD106(+). MSC spontaneously secreted IL-6, HGF, TGF-beta1 and expressed high levels of SDF-1 and low levels of VEGF, IL-1beta and PGE(2), but no EGF, insulin or glucagon. MSC homed to the pancreas and this therapy allowed for enhanced insulin secretion and sustained normoglycemia. Interestingly, immunohistochemistry demonstrated that, the islets from MSC-treated rats expressed high levels of PDX-1 and that these cells were also positive for insulin staining. In addition, peripheral T cells from MSC-treated rats exhibited a shift toward IL-10/IL-13 production and higher frequencies of CD4(+)/CD8(+) Foxp3(+) T cells compared to the PBS-treated rats. These data suggest that the bioactive factors secreted by MSC establish a tissue microenvironment that supports beta cell activation/survival in the pancreas. In addition, because of anti-inflammatory and immunoregulatory effects of MSC on T cells, this work can lead to clinical trial of autologous MSC to prevent/cure type-1 diabetes.

Intermittent hypoxia reverses the diurnal glucose rhythm and causes pancreatic beta-cell replication in mice

Obstructive sleep apnoea (OSA) and type 2 diabetes frequently co-exist and potentially interact haemodynamically and metabolically. However, the confounding effects of obesity have obscured the examination of any independent or interactive effects of the hypoxic stress of OSA and the hyperglycaemia of type 2 diabetes on haemodynamic and metabolic outcomes. We have developed a chronically catheterized, unhandled, lean murine model to examine the effects of intermittent hypoxic (IH) exposure and exogenous glucose infusion on the diurnal pattern of arterial blood pressure and blood glucose, as well as pancreatic beta-cell growth and function. Four experimental groups of adult male C57BL/J mice were exposed to 80 h of (1) either IH (nadir of inspired oxygen 5-6% at 60 cycles h(-1) for 12 h during light period) or intermittent air (IA; control) and (2) continuous infusion of either 50% dextrose or saline (control). IH exposure during saline infusion caused a sustained increase in arterial blood pressure of 10 mmHg (P < 0.0001), reversed the normal diurnal rhythm of blood glucose (P < 0.03), doubled corticosterone levels (P < 0.0001), and increased replication of pancreatic beta-cells from 1.5 +/- 0.3 to 4.0 +/- 0.8% bromodeoxyuridine (BrdU)-positive) beta-cells. The combined stimulus of IH exposure and glucose infusion attenuated the hypertension, exacerbated the reversed diurnal glucose rhythm, and produced the highest rates of apoptosis in beta-cells, without any additive effects on beta-cell replication. We conclude that, in contrast to the development of sustained hypertension, IH impaired glucose homeostasis only during periods of hypoxic exposure. IH acted as a stimulus to pancreatic beta-cell replication, but the presence of hyperglycaemia may increase the hypoxic susceptibility of beta-cells. This model will provide a basis for future mechanistic studies as well as assessing the metabolic impact of common comorbities in OSA, including obesity, insulin resistance and type 2 diabetes.

Intermittent hypoxia causes insulin resistance in lean mice independent of autonomic activity

RATIONALE AND OBJECTIVES

Although many clinical physiology and epidemiology studies show an association between obstructive sleep apnea (OSA) and markers of insulin resistance, no causal pathway has been established. The purpose of the current study was to determine if the intermittent hypoxia (IH) stimulus that characterizes OSA causes insulin resistance in the absence of obesity. Furthermore, we assessed the impact of IH on specific metabolic function in liver and muscle. Finally, we examined the potential mechanistic role of the autonomic nervous system (ANS) in mediating insulin resistance in response to IH.

METHODS AND RESULTS

Hyperinsulinemic euglycemic clamps were conducted and whole-body insulin sensitivity, hepatic glucose output, and muscle-specific glucose utilization assessed in conscious, chronically instrumented adult male C57BL/6J mice exposed to (1) IH (achieving a nadir of Fi(O(2)) = 5-6% at 60 cycles/h for 9 h), (2) intermittent air as a control, (3) IH with ANS blockade (hexamethonium), or (4) IA with ANS blockade. IH decreased whole-body insulin sensitivity compared with intermittent air (38.8 +/- 2.7 vs. 49.4 +/- 1.5 mg/kg/min, p < 0.005) and reduced glucose utilization in oxidative muscle fibers, but did not cause a change in hepatic glucose output. Furthermore, the reduction in whole-body insulin sensitivity during IH was not restored by ANS blockade.

CONCLUSION

We conclude that IH can cause acute insulin resistance in otherwise lean, healthy animals, and that the response is associated with decreased glucose utilization of oxidative muscle fibers, but that it occurs independently of activation of the ANS.

Leptin increases proliferation of human steosarcoma cells through activation of PI(3)-K and MAPK pathways

BACKGROUND

Serum leptin levels are strongly and directly related to fat body mass (FBM). Bone mineral density (BMD) increases with FBM, and obesity has a protective effect against osteoporosis. We have previously demonstrated that leptin therapy has a significant effect in preventing ovariectomy-induced bone loss in rats and leptin also exerts direct osteogenic effects in vitro. To obtain a better understanding of the physiology and pharmacology of leptin in bone metabolism, we evaluated the leptin-induced signal transduction pathways and proliferative response in the human osteosarcoma cell line Saos-2.

MATERIAL/METHODS

Saos-2 cell lines were used. Leptin receptor common form (OB-Ra) and long form (OB-Rb) were detected by RT-PCR and immunocytochemistry. PI(3)-K activity was immunoprecipitated using antibodies directed against tyrosine-phosphorylated proteins and IRS-1. The activated form of p42/p44 MAPK was investigated in cytosolic extracts of confluent Saos-2 in response to leptin.

RESULTS

In this study, we tested the hypothesis that leptin might be a mediator linking obesity and bone cell proliferation. We found that Saos-2 cells expressed OB-Ra and OB-Rb. Leptin (10 nmol/L - 2 umol/L) caused a significant increase in the activation of PI (3)-K that was accompanied by an increase in cell proliferation dose dependently based on the [3H]-thymidine incorporation. The specific PI (3)-K inhibitors LY294002 and wortmannin blocked leptin-induced cell proliferation. Interestingly, leptin activated MAPK and the specific MAPK-inhibitor PD98059 blocked DNA synthesis induced by leptin.

CONCLUSIONS

Our data support the hypothesis that leptin may increase bone mass by stimulating osteoblast proliferation through activation of the P1 (3)-K and MAPK signaling pathways.

Short-term, high-dose parathyroid hormone-related protein as a skeletal anabolic agent for the treatment of postmenopausal osteoporosis

PTH-related protein (PTHrP) is homologous with PTH. PTH, an effective anabolic agent for treating osteoporosis, has been shown to stimulate both bone resorption by osteoclasts and bone formation by osteoblasts. We examined whether PTHrP might share anabolic properties in osteoporosis. A 3-month double-blind, prospective, placebo-controlled, randomized clinical trial was performed in 16 healthy postmenopausal women with osteoporosis. All received calcium and vitamin D, and all continued their prior hormone replacement therapy. One group also received daily sc PTHrP (6.56 microg/kg x d, or approximately 400 microg/d), and the other group received placebo injections. The PTHrP group displayed a 4.7% increase in lumbar spine bone mineral density (BMD) and also demonstrated an increase in osteoblastic bone formation, as assessed using serum osteocalcin measurements. In contrast, there was no increase in bone-specific alkaline phosphatase and collagen-1 propeptide or either of two markers of osteoclastic bone resorption, N-telopeptide, or deoxypyridinoline. One subject in the placebo group withdrew from the study, but there were no significant adverse events in the PTHrP group. PTHrP administered sc in high doses for only 3 months appears to be a potent anabolic agent, producing a 4.7% increase in lumbar spine BMD. This compares very favorably to available antiresorptive drugs for osteoporosis and is similar to the increases in BMD at this early time point reported for PTH. Despite the high doses, PTHrP was well tolerated. Larger clinical trials are required to confirm these results and fully assess the anabolic potential of PTHrP in osteoporosis.

Adenovirus-mediated hepatocyte growth factor expression in mouse islets improves pancreatic islet transplant performance and reduces beta cell death

Hepatocyte growth factor (HGF) increases beta cell proliferation and function in rat insulin promoter (RIP)-targeted transgenic mice. RIP-HGF mouse islets also function superiorly to normal islets in a transplant setting. Here, we aimed to determine whether viral gene transfer of the HGF gene into mouse islets ex vivo could enhance the performance of normal islets in a streptozotocin-diabetic severe combined immunodeficient mouse marginal islet mass model in which 300 uninfected or adenovirus (Adv) LacZ-transduced islet equivalents were insufficient to correct hyperglycemia. In dramatic contrast, 300 AdvHGF-transduced islet equivalents promptly (day 1) and significantly (p < 0.01) decreased random non-fasting blood glucose levels, from 351 +/- 20 mg/dl to an average of 191 +/- 7 mg/dl over 8 weeks. At day 1 post-transplant, beta cell death was significantly (p < 0.05) decreased, and the total insulin content was significantly (p < 0.05) increased in AdvHGF-transduced islets containing grafts. This anti-beta cell death action of HGF was independently confirmed in RIP-HGF mice and in INS-1 cells, both treated with streptozotocin. Activation of the phosphatidylinositol 3-kinase/Akt intracellular-signaling pathway appeared to be involved in this beta cell protective effect of HGF in vitro. In summary, adenoviral delivery of HGF to murine islets ex vivo improves islet transplant survival and blood glucose control in a subcapsular renal graft model in immuno-incompetent diabetic mice.

Parathyroid hormone-related protein in the pancreatic islet and the cardiovascular system

Parathyroid hormone-related protein was discovered as the causative agent responsible for the common paraneoplastic syndrome, humoral hypercalcemia of malignancy. It is now clear that the PTHrP gene is expressed in virtually every cell and tissue in the body at some point in development or adult life and that the peptide is critical for normal life. Two of the tissues that produce PTHrP are the insulin-producing beta cells of the pancreatic islet and the vascular smooth muscle cells of the arterial wall. In this review, the physiologic roles of PTHrP in the islet and in the arterial wall are explored. PTHrP is a classical neuroendocrine prohormone that undergoes extensive post-translational processing to yield a family of daughter peptides that are the mature secretory forms of the peptide. In addition to its ability to act as a traditional endocrine, paracrine, or autocrine factor, PTHrP appears to be able to act as an "intracrine" factor as well, directly entering the nucleus after translation and stimulating proliferation, apoptosis, and perhaps other cellular responses as well. The cell biology underlying this phenomenon is also explored herein.

Progressive pancreatic islet hyperplasia in the islet-targeted, parathyroid hormone-related protein-overexpressing mouse

PTH-related protein (PTHrP) is a paracrine/autocrine factor produced in most cell types in the body. Its functions include the regulation of cell cycle, of differentiation, of apoptosis, and of developmental events. One of the cells which produces PTHrP is the pancreatic beta cell. We have previously described a transgenic mouse model of targeted overexpression of PTHrP in the beta cell, the RIP-PTHrP mouse. These studies showed that PTHrP overexpression markedly increased islet mass and insulin secretion and resulted in hypoglycemia. Those studies were limited to RIP-PTHrP mice of 8-12 weeks of age. In the current report, we demonstrate that PTHrP overexpression induces a progressive increase in islet mass over the life of the RIP-PTHrP mouse, and that, in contrast to some other models of targeted PTHrP overexpression, the phenotype is not developmental, but occurs postnatally. The marked increase in islet mass is not associated with a measurable increase in beta cell replication rates. A further slowing in the normally low islet apoptosis rate could not be demonstrated in the RIP-PTHrP islet. Thus, the marked increase in islet mass in the RIP-PTHrP mouse is unexplained in mechanistic terms. Finally, RIP-PTHrP mice are resistant to the diabetogenic effects of streptozotocin. The mechanisms responsible for the increase in islet mass in the RIP-PTHrP mouse likely lie in either very subtle changes in islet turnover or in early steps in islet differentiation and development. The ability of PTHrP to increase islet mass and function, as well as its ability to attenuate the diabetogenic effects of streptozotocin, indicate that further study of PTHrP on islet development and function are important and may lead to therapeutic strategies in diabetes mellitus.