Pancreatic islet adaptation in pregnancy and postpartum.

Pancreatic islets, particularly insulin-producing ß-cells, are central regulators of glucose homeostasis capable of responding to a variety of metabolic stressors. Pregnancy is a unique physiological stressor, necessitating the islets to adapt to the complex interplay of maternal and fetal-placental factors influencing the metabolic milieu. In this review we highlight studies defining gestational adaptation mechanisms within maternal islets and emerging studies revealing islet adaptations during the early postpartum and lactation periods. These include adaptations in both ß and in 'non-ß' islet cells. We also discuss insights into how gestational and postpartum adaptation may inform pregnancy-specific and general mechanisms of islet responses to metabolic stress and contribute to investigation of gestational diabetes.

A Visual Diagnosis: Lipodystrophy.

Lipodystrophy syndromes are rare metabolic disorders characterized by local or generalized loss of adipose tissue, resulting in insulin resistance, dyslipidemia, and cosmetic disfiguration. The lipodystrophic phenotype is highly variable, with partial lipodystrophy often missed or misdiagnosed as other diseases from a lack of a proper physical examination and low physician awareness. Correct diagnosis is important for optimal treatment and follow-up strategies in these patients. The use of GLP-1 analogs has not been systematically evaluated in lipodystrophy and could be a potential precision medicine therapy. We aim to make the reader, particularly generalists or endocrinologists outside of tertiary referral centers, aware of the presentation and clinical features of partial lipodystrophy, emphasize the role of a full physical examination in diagnosis, and discuss therapeutic options, including GLP-1-based glycemic management illustrated by our clinical case.

Pancreatic islet cell type-specific transcriptomic changes during pregnancy and postpartum.

Pancreatic ß-cell mass expands during pregnancy and regresses in the postpartum period in conjunction with dynamic metabolic demands on maternal glucose homeostasis. To understand transcriptional changes driving these adaptations in ß-cells and other islet cell types, we performed single-cell RNA sequencing on islets from virgin, late gestation, and early postpartum mice. We identified transcriptional signatures unique to gestation and the postpartum in ß-cells, including induction of the AP-1 transcription factor subunits and other genes involved in the immediate-early response (IEGs). In addition, we found pregnancy and postpartum-induced changes differed within each endocrine cell type, and in endothelial cells and antigen-presenting cells within islets. Together, our data reveal insights into cell type-specific transcriptional changes responsible for adaptations by islet cells to pregnancy and their resolution postpartum.

Localized cytotoxic T cell-associated antigen 4 and antioxidant islet encapsulation alters macrophage signaling and induces regulatory and anergic T cells to enhance allograft survival.

The loss of functional ß-cell mass is a hallmark of type 1 diabetes. Islet transplantation represents a promising alternative approach, but immune-mediated graft destruction remains a major challenge. We sought to use islet encapsulation technologies to improve graft survival and function without systemic immunosuppression. We hypothesized islet encapsulation with nanothin coatings consisting of tannic acid (TA), an antioxidant; poly(N-vinylpyrrolidone) (PVPON), a biocompatible polymer; and cytotoxic T cell-associated antigen 4 immunoglobulin (CTLA-4-Ig), an inhibitory immune receptor, will elicit localized immunosuppression to prolong islet allograft function and suppress effector T cell responses. In the absence of systemic immunosuppression, we demonstrated (PVPON/TA/CTLA-4-Ig)-encapsulated NOD.Rag islet grafts maintain function significantly longer than control IgG-containing (PVPON/TA/IgG) and nonencapsulated controls after transplantation into diabetic C57BL/6 mice. This protection coincided with diminished proinflammatory macrophage responses mediated by signal transducer and activator of transcription 1 signaling, decreased proinflammatory T cell effector responses, and CTLA-4-Ig-specific concomitant increases in anergic CD4+ T cells and regulatory T cells. Our results provide evidence that conjugation of CTLA-4-Ig to (PVPON/TA) coatings can suppress T cell activation, enhance regulatory T cell populations, prolong islet allograft survival, and induce localized immunosuppression after transplantation.

Cdk5 drives formation of heterogeneous pancreatic neuroendocrine tumors.

Pancreatic neuroendocrine tumors (PanNETs) are a heterogeneous population of neoplasms that arise from hormone-secreting islet cells of the pancreas and have increased markedly in incidence over the past four decades. Non-functional PanNETs, which occur more frequently than hormone-secreting tumors, are often not diagnosed until later stages of tumor development and have poorer prognoses. Development of successful therapeutics for PanNETs has been slow, partially due to a lack of diverse animal models for pre-clinical testing. Here, we report development of an inducible, conditional mouse model of PanNETs by using a bi-transgenic system for regulated expression of the aberrant activator of Cdk5, p25, specifically in ß-islet cells. This model produces a heterogeneous population of PanNETs that includes a subgroup of well-differentiated, non-functional tumors. Production of these tumors demonstrates the causative potential of aberrantly active Cdk5 for generation of PanNETs. Further, we show that human PanNETs express Cdk5 pathway components, are dependent on Cdk5 for growth, and share genetic and transcriptional overlap with the INS-p25OE model. The utility of this model is enhanced by the ability to form tumor-derived allografts. This new model of PanNETs will facilitate molecular delineation of Cdk5-dependent PanNETs and the development of new targeted therapeutics.

Partners in Crime: Beta-Cells and Autoimmune Responses Complicit in Type 1 Diabetes Pathogenesis.

Type 1 diabetes (T1D) is an autoimmune disease characterized by autoreactive T cell-mediated destruction of insulin-producing pancreatic beta-cells. Loss of beta-cells leads to insulin insufficiency and hyperglycemia, with patients eventually requiring lifelong insulin therapy to maintain normal glycemic control. Since T1D has been historically defined as a disease of immune system dysregulation, there has been little focus on the state and response of beta-cells and how they may also contribute to their own demise. Major hurdles to identifying a cure for T1D include a limited understanding of disease etiology and how functional and transcriptional beta-cell heterogeneity may be involved in disease progression. Recent studies indicate that the beta-cell response is not simply a passive aspect of T1D pathogenesis, but rather an interplay between the beta-cell and the immune system actively contributing to disease. Here, we comprehensively review the current literature describing beta-cell vulnerability, heterogeneity, and contributions to pathophysiology of T1D, how these responses are influenced by autoimmunity, and describe pathways that can potentially be exploited to delay T1D.

Very Low Vitamin D in a Patient With a Novel Pathogenic Variant in the GC Gene That Encodes Vitamin D-Binding Protein.

Circulating plasma vitamin D metabolites are highly bound to vitamin D-binding protein (DBP), also known as group-specific component or Gc-globulin. DBP, encoded by the GC gene, is a member of the albumin family of globular serum transport proteins. We previously described a homozygous GC gene deletion in a patient with apparent severe vitamin D deficiency, fragility fractures, and ankylosing spondylitis. Here, we report an unrelated patient free of fractures or rheumatologic disease, but with very low 25-hydroxyvitamin D and 1,25-hydroxyvitamin D, as well as undetectable DBP measured by liquid chromatography-tandem mass spectrometry. A whole gene deletion was excluded by microarray, and Sanger sequencing of GC revealed a homozygous pathogenic variant affecting a canonical splice site (c0.702-1G > A). These findings indicate that loss of function variants in GC that eliminate DBP, and severely reduced total circulating vitamin D levels, do not necessarily result in significant metabolic bone disease. Together with our previous report, these cases support the free-hormone hypothesis, and suggest free vitamin D metabolites may serve as preferable indicators of bone and mineral metabolism, particularly when clinical suspicion of DBP deficiency is high.

Lactation improves pancreatic ß cell mass and function through serotonin production.

Pregnancy imposes a substantial metabolic burden on women through weight gain and insulin resistance. Lactation reduces the risk of maternal postpartum diabetes, but the mechanisms underlying this benefit are unknown. Here, we identified long-term beneficial effects of lactation on ß cell function, which last for years after the cessation of lactation. We analyzed metabolic phenotypes including ß cell characteristics in lactating and non-lactating humans and mice. Lactating and non-lactating women showed comparable glucose tolerance at 2 months after delivery, but after a mean of 3.6 years, glucose tolerance in lactated women had improved compared to non-lactated women. In humans, the disposition index, a measure of insulin secretory function of ß cells considering the degree of insulin sensitivity, was higher in lactated women at 3.6 years after delivery. In mice, lactation improved glucose tolerance and increased ß cell mass at 3 weeks after delivery. Amelioration of glucose tolerance and insulin secretion were maintained up to 4 months after delivery in lactated mice. During lactation, prolactin induced serotonin production in ß cells. Secreted serotonin stimulated ß cell proliferation through serotonin receptor 2B in an autocrine and paracrine manner. In addition, intracellular serotonin acted as an antioxidant to mitigate oxidative stress and improved ß cell survival. Together, our results suggest that serotonin mediates the long-term beneficial effects of lactation on female metabolic health by increasing ß cell proliferation and reducing oxidative stress in ß cells.

Serotonin Regulates Adult ß-Cell Mass by Stimulating Perinatal ß-Cell Proliferation.

A sufficient ß-cell mass is crucial for preventing diabetes, and perinatal ß-cell proliferation is important in determining the adult ß-cell mass. However, it is not yet known how perinatal ß-cell proliferation is regulated. Here, we report that serotonin regulates ß-cell proliferation through serotonin receptor 2B (HTR2B) in an autocrine/paracrine manner during the perinatal period. In ß-cell-specific Tph1 knockout (Tph1 ßKO) mice, perinatal ß-cell proliferation was reduced along with the loss of serotonin production in ß-cells. Adult Tph1 ßKO mice exhibited glucose intolerance with decreased ß-cell mass. Disruption of Htr2b in ß-cells also resulted in decreased perinatal ß-cell proliferation and glucose intolerance in adulthood. Growth hormone (GH) was found to induce serotonin production in ß-cells through activation of STAT5 during the perinatal period. Thus, our results indicate that GH-GH receptor-STAT5-serotonin-HTR2B signaling plays a critical role in determining the ß-cell mass by regulating perinatal ß-cell proliferation, and defects in this pathway affect metabolic phenotypes in adults.

Prolactin Receptor Signaling Regulates a Pregnancy-Specific Transcriptional Program in Mouse Islets.

Pancreatic ß-cells undergo profound hyperplasia during pregnancy to maintain maternal euglycemia. Failure to reprogram ß-cells into a more replicative state has been found to underlie susceptibility to gestational diabetes mellitus (GDM). We recently identified a requirement for prolactin receptor (PRLR) signaling in the metabolic adaptations to pregnancy, where ß-cell-specific PRLR knockout (ßPRLRKO) mice exhibit a metabolic phenotype consistent with GDM. However, the underlying transcriptional program that is responsible for the PRLR-dependent metabolic adaptations during gestation remains incompletely understood. To identify PRLR signaling gene regulatory networks and target genes within ß-cells during pregnancy, we performed a transcriptomic analysis of pancreatic islets isolated from either ßPRLRKO mice or littermate controls in late gestation. Gene set enrichment analysis identified forkhead box protein M1 and polycomb repressor complex 2 subunits, Suz12 and enhancer of zeste homolog 2 (Ezh2), as novel candidate regulators of PRLR-dependent ß-cell adaptation. Gene ontology term pathway enrichment revealed both established and novel PRLR signaling target genes that together promote a state of increased cellular metabolism and/or proliferation. In contrast to the requirement for ß-cell PRLR signaling in maintaining euglycemia during pregnancy, PRLR target genes were not induced following high-fat diet feeding. Collectively, the current study expands our understanding of which transcriptional regulators and networks mediate gene expression required for islet adaptation during pregnancy. The current work also supports the presence of pregnancy-specific adaptive mechanisms distinct from those activated by nutritional stress.

Differential tissue response to growth hormone in mice.

Growth hormone (GH) has been shown to act directly on multiple tissues throughout the body. Historically, it was believed that GH acted directly in the liver and only indirectly in other tissues via insulin-like growth hormone 1 (IGF-1). Despite extensive work to describe GH action in individual tissues, a comparative analysis of acute GH signaling in key metabolic tissues has not been performed. Herein, we address this knowledge gap. Acute tissue response to human recombinant GH was assessed in mice by measuring signaling via phospho-STAT5 immunoblotting. STAT5 activation is an easily and reliably detected early marker of GH receptor engagement. We found differential tissue sensitivities; liver and kidney were equally GH-sensitive and more sensitive than white adipose tissue, heart, and muscle (gastrocnemius). Gastrocnemius had the greatest maximal response compared to heart, liver, white adipose tissue, and whole kidney. Differences in maximum responsiveness were positively correlated with tissue STAT5 abundance, while differences in sensitivity were not explained by differences in GH receptor levels. Thus, GH sensitivity and responsiveness of distinct metabolic tissues differ and may impact physiology and disease.

Piecing together the puzzle of pancreatic islet adaptation in pregnancy.

Pregnancy places acute demands on maternal physiology, including profound changes in glucose homeostasis. Gestation is characterized by an increase in insulin resistance, counterbalanced by an adaptive increase in pancreatic ß cell production of insulin. Failure of normal adaptive responses of the islet to increased maternal and fetal demands manifests as gestational diabetes mellitus (GDM). The gestational changes and rapid reversal of islet adaptations following parturition are at least partly driven by an anticipatory program rather than post-factum compensatory adaptations. Here, I provide a comprehensive review of the cellular and molecular mechanisms underlying normal islet adaptation during pregnancy and how dysregulation may lead to GDM. Emerging areas of interest and understudied areas worthy of closer examination in the future are highlighted.

Gestational Diabetes Mellitus From Inactivation of Prolactin Receptor and MafB in Islet ß-Cells.

ß-Cell proliferation and expansion during pregnancy are crucial for maintaining euglycemia in response to increased metabolic demands placed on the mother. Prolactin and placental lactogen signal through the prolactin receptor (PRLR) and contribute to adaptive ß-cell responses in pregnancy; however, the in vivo requirement for PRLR signaling specifically in maternal ß-cell adaptations remains unknown. We generated a floxed allele of Prlr, allowing conditional loss of PRLR in ß-cells. In this study, we show that loss of PRLR signaling in ß-cells results in gestational diabetes mellitus (GDM), reduced ß-cell proliferation, and failure to expand ß-cell mass during pregnancy. Targeted PRLR loss in maternal ß-cells in vivo impaired expression of the transcription factor Foxm1, both G1/S and G2/M cyclins, tryptophan hydroxylase 1 (Tph1), and islet serotonin production, for which synthesis requires Tph1. This conditional system also revealed that PRLR signaling is required for the transient gestational expression of the transcription factor MafB within a subset of ß-cells during pregnancy. MafB deletion in maternal ß-cells also produced GDM, with inadequate ß-cell expansion accompanied by failure to induce PRLR-dependent target genes regulating ß-cell proliferation. These results unveil molecular roles for PRLR signaling in orchestrating the physiologic expansion of maternal ß-cells during pregnancy.

Mifepristone Treatment of Cushing's Syndrome in a Pediatric Patient.

Cushing's syndrome (CS) in the pediatric population is challenging to diagnose and treat. Although next-generation medical therapies are emerging for adults with CS, none are currently approved or used in children. Here we describe the first use of mifepristone, a glucocorticoid receptor antagonist, to treat CS in a pediatric subject. The patient, a 14-year-old girl with an 18-month history of metastatic neuroendocrine carcinoma, suffered from fatigue, profound myopathy, irritability, and depression. She was found to have hypertension, hypokalemia, and worsening control of her preexisting type 1 diabetes. In this report, we detail our clinical evaluation that confirmed CS caused by an ectopic adrenocorticotropic hormone secreting tumor. Surgical and radiation therapies were not pursued because of her poor functional status and limited life expectancy, and medical treatment of CS was indicated for symptom relief. Mifepristone treatment provided rapid improvement in glycemic control, insulin resistance, and hypertension as well as significant diminishment of her myopathy and fatigue. Hypokalemia was managed with an oral potassium replacement and dose escalation of spironolactone; no other significant adverse effects were observed. Despite successful palliation of Cushing's signs and symptoms, the patient died of progression of her cancer. This case demonstrates the safety and efficacy of mifepristone treatment in a pediatric patient with symptomatic, ectopic CS. We conclude that, in appropriate pediatric patients with CS, glucocorticoid receptor antagonism with mifepristone should be considered to control the effects of hypercortisolism and to improve quality of life.

Mouse and human resistins impair glucose transport in primary mouse cardiomyocytes, and oligomerization is required for this biological action.

The adipocytokine resistin impairs glucose tolerance and insulin sensitivity in rodents. Here, we examined the effect of resistin on glucose uptake in isolated adult mouse cardiomyocytes. Murine resistin reduced insulin-stimulated glucose uptake, establishing the heart as a resistin target tissue. Notably, human resistin also impaired insulin action in mouse cardiomyocytes, providing the first evidence that human and mouse resistin homologs have similar functions. Resistin is a cysteine-rich molecule that circulates as a multimer of a dimeric form dependent upon a single intermolecular disulfide bond, which, in the mouse, involves Cys26; mutation of this residue to alanine (C26A) produces a monomeric molecule that appears to be bioactive in the liver. Remarkably, unlike native resistin, monomeric C26A resistin had no effect on basal or insulin-stimulated glucose uptake in mouse cardiomyocytes. Resistin impairs glucose uptake in cardiomyocytes by mechanisms that involve altered vesicle trafficking. Thus, in cardiomyocytes, both mouse and human resistins directly impair glucose transport; and in contrast to effects on the liver, these actions of resistin require oligomerization.

Regulation of fasted blood glucose by resistin.

The association between obesity and diabetes supports an endocrine role for the adipocyte in maintaining glucose homeostasis. Here we report that mice lacking the adipocyte hormone resistin exhibit low blood glucose levels after fasting, due to reduced hepatic glucose production. This is partly mediated by activation of adenosine monophosphate-activated protein kinase and decreased expression of gluconeogenic enzymes in the liver. The data thus support a physiological function for resistin in the maintenance of blood glucose during fasting. Remarkably, lack of resistin diminishes the increase in post-fast blood glucose normally associated with increased weight, suggesting a role for resistin in mediating hyperglycemia associated with obesity.

Resistin: molecular history and prognosis.

Obesity and diabetes have reached epidemic proportions worldwide. The antidiabetic thiazolidinedione (TZD) drugs are insulin-sensitizing agents now widely used in the treatment of type 2 diabetes. TZDs are ligands for the nuclear hormone receptor peroxisome proliferator activated receptor gamma, which is a master regulator of adipogenesis and adipocyte metabolism. The molecular mechanisms by which TZDs improve insulin sensitivity have not been fully identified. Here we consider a novel secreted factor first identified as a TZD-suppressible gene in mouse adipocytes, called resistin, and discuss what is currently known about resistin regulation and function in mouse and human.