The molecular mechanism of EGF receptor activation in pancreatic beta-cells by thyrotropin-releasing hormone

β-Arrestin2 Is Critically Involved in the Differential Regulation of Phosphosignaling Pathways by Thyrotropin-Releasing Hormone and Taltirelin

In recent years, thyrotropin-releasing hormone (TRH) and its analogs, including taltirelin (TAL), have demonstrated a range of effects on the central nervous system that represent potential therapeutic agents for the treatment of various neurological disorders, including neurodegenerative diseases. However, the molecular mechanisms of their actions remain poorly understood. In this study, we investigated phosphosignaling dynamics in pituitary GH1 cells affected by TRH and TAL and the putative role of β-arrestin2 in mediating these effects. Our results revealed widespread alterations in many phosphosignaling pathways involving signal transduction via small GTPases, MAP kinases, Ser/Thr- and Tyr-protein kinases, Wnt/β-catenin, and members of the Hippo pathway. The differential TRH- or TAL-induced phosphorylation of numerous proteins suggests that these ligands exhibit some degree of biased agonism at the TRH receptor. The different phosphorylation patterns induced by TRH or TAL in β-arrestin2-deficient cells suggest that the β-arrestin2 scaffold is a key factor determining phosphorylation events after TRH receptor activation. Our results suggest that compounds that modulate kinase and phosphatase activity can be considered as additional adjuvants to enhance the potential therapeutic value of TRH or TAL.

The forgotten effects of thyrotropin-releasing hormone: Metabolic functions and medical applications

Thyrotropin-releasing hormone (TRH) causes a variety of thyroidal and non-thyroidal effects, the best known being the feedback regulation of thyroid hormone levels. This was employed in the TRH stimulation test, which is currently little used. The role of TRH as a cancer biomarker is minor, but exaggerated responses to TSH and prolactin levels in breast cancer led to the hypothesis of a potential role for TRH in the pathogenesis of this disease. TRH is a rapidly degraded peptide with multiple targets, limiting its suitability as a biomarker and drug candidate. Although some studies reported efficacy in neural diseases (depression, spinal cord injury, amyotrophic lateral sclerosis, etc.), therapeutic use of TRH is presently restricted to spinocerebellar degenerative disease. Regulation of TRH production in the hypothalamus, patterns of expression of TRH and its receptor in the body, its role in energy metabolism and in prolactin secretion are addressed in this review.

Maternal subclinical hypothyroidsm and gestational diabetes mellitus: a meta-analysis

OBJECTIVE

The association between subclinical hypothyroidism (SCH) and gestational diabetes mellitus (GDM) is controversial. This review evaluates whether the risk of GDM is different in pregnant women with SCH compared to euthyroid pregnant women.

METHODS

A computerized search of the MEDLINE and EMBASE databases was conducted from their inceptions to July 2013 and was complemented with the perusal of the reference sections of the retrieved articles. Prespecified criteria were applied to assess eligibility, and standard meta-analytic methodology was employed for evidence synthesis.

RESULTS

Six cohort studies, reporting data on 35,350 pregnant women (1,216 women with SCH), were identified. The risk of GDM in pregnant women with SCH was found to be substantially higher compared to euthyroid pregnant women (5 studies, pooled unadjusted odds ratio [OR]: 1.35, 95% confidence interval [CI]: 1.05-1.75, I2: 41%, Harbord test P = .44). Similarly, the risk of GDM was estimated to be significantly higher in pregnant women with SCH when using adjusted estimates (3 studies, pooled adjusted OR: 1.39, 95% CI: 1.07-1.79, I2: 0%). Neither finding remained significant in sensitivity analyses.

CONCLUSION

A modestly increased risk of GDM might be present in pregnant women with SCH compared to euthyroid pregnant women. Assuming a 5% baseline risk of GDM and that SCH increases the risk of GDM by 50% (in odds) compared to a euthyroid population, then there would be 1 extra case of GDM in every 43 pregnant women with SCH. This preliminary finding warrants further investigation.

Human Bone Marrow Subpopulations Sustain Human Islet Function and Viability In vitro

AIMS

Allogeneic bone marrow (BM) has been shown to support human islet survival and function in long-term culture by initiating human islet vascularization and β-cell regeneration. Various BM subpopulations may play different roles in human islet functions and survival. In this paper we investigated the effects of BM and its subpopulations, endothelial progenitor cells (E) and mesenchymal (M) cells on human islet's β-cell function and regeneration.

STUDY DESIGN

Isolation and identification of subpopulations from human bone marrow and culture with allogeneic human islet to investigate effects of different cell population on human islet function and regeneration.

PLACE AND DURATION OF STUDY

Department of Medicine, Center for Stem Cell & Diabetes Research, RWMC, Providence, RI, USA, between 2010 - 2014.

METHODOLOGY

Human islets were distributed from Integrated Islet Distribution Program (IIDP) and human bone marrow (BM) was harvested by Bone marrow transplantation center at Roger Williams Hospital. BM subpopulation was identified cell surface markers through Fluorescence-activated cell sorting, applied in flow cytometry (FACS), islet function was evaluated by human ELISA kit and β cell regeneration was evaluated by three methods of Cre-Loxp cell tracing, β cell sorting and RT-PCR for gene expression.

RESULTS

Four different BM and seven different islet donates contributed human tissues. We observed islet β-cell having self regeneration capability in short term culture (3∼5 days) using a Cre-Loxp cell tracing. BM and its subtype E, M have similar benefits on β cell function during co-culture with human islet comparison to islet only. However, only whole BM enables to sustain the capability of islet β-cell self regeneration resulting in increasing β cell population while single E and M individual do not significantly affect on that. Mechanism approach to explore β-cell self regeneration by evaluating transcription factor expressions, we found that BM significantly increases the activations of β-cell regeneration relative transcription factors, the LIM homeodomain protein (Isl1), homologue to zebrafish somite MAF1 (MAFa), the NK-homeodomain factor 6.1 (NKX6.1), the paired box family factors 6 (PAX6), insulin promoter factor 1 (IPF1) and kinesin family member 4A (KIF4a).

CONCLUSION

These results suggest that BM and its derived M and E cells enable to support human islet β-cell function. However, only BM can sustain the capability of β-cell self regeneration through initiating β-cell transcriptional factors but not individual E and M cells suggesting pure E and M cells less supportive for islet long-term survival in vitro.

Cellular functions regulated by phosphorylation of EGFR on Tyr845

The Src gene product (Src) and the epidermal growth factor receptor (EGFR) are prototypes of oncogene products and function primarily as a cytoplasmic non-receptor tyrosine kinase and a transmembrane receptor tyrosine kinase, respectively. The identification of Src and EGFR, and the subsequent extensive investigations of these proteins have long provided cutting edge research in cancer and other molecular and cellular biological studies. In 1995, we reported that the human epidermoid carcinoma cells, A431, contain a small fraction of Src and EGFR in which these two kinase were in physical association with each other, and that Src phosphorylates EGFR on tyrosine 845 (Y845) in the Src-EGFR complex. Y845 of EGFR is located in the activation segment of the kinase domain, where many protein kinases contain kinase-activating autophosphorylation sites (e.g., cAMP-dependent protein kinase, Src family kinases, transmembrane receptor type tyrosine kinases) or trans-phosphorylation sites (e.g., cyclin-dependent protein kinase, mitogen-activated protein kinase, Akt protein kinase). A number of studies have demonstrated that Y845 phosphorylation serves an important role in cancer as well as normal cells. Here we compile the experimental facts involving Src phosphorylation of EGFR on Y845, by which cell proliferation, cell cycle control, mitochondrial regulation of cell metabolism, gamete activation and other cellular functions are regulated. We also discuss the physiological relevance, as well as structural insights of the Y845 phosphorylation.

Secreted phospholipase A2-IIA-induced a phenotype of activated microglia in BV-2 cells requires epidermal growth factor receptor transactivation and proHB-EGF shedding

BACKGROUND

Activation of microglia, the primary component of the innate immune response in the brain, is a hallmark of neuroinflammation in neurodegenerative disorders, including Alzheimer's disease (AD) and other pathological conditions such as stroke or CNS infection. In response to a variety of insults, microglial cells produce high levels of inflammatory cytokines that are often involved in neuronal injury, and play an important role in the recognition, engulfment, and clearance of apoptotic cells and/or invading microbes. Secreted phospholipase A2-IIA (sPLA2-IIA), an enzyme that interacts with cells involved in the systemic immune/inflammatory response, has been found up-regulated in the cerebrospinal fluid and brain of AD patients. However, despite several approaches, its functions in mediating CNS inflammation remain unknown. In the present study, the role of sPLA2-IIA was examined by investigating its direct effects on microglial cells.

METHODS

Primary and immortalized microglial cells were stimulated by sPLA2-IIA in order to characterize the cytokine-like actions of the phospholipase. The hallmarks of activated microglia analyzed include: mitogenic response, phagocytic capabilities and induction of inflammatory mediators. In addition, we studied several of the potential molecular mechanisms involved in those events.

RESULTS

The direct exposure of microglial cells to sPLA2-IIA stimulated, in a time- and dose-dependent manner, their phagocytic and proliferative capabilities. sPLA2-IIA also triggered the synthesis of the inflammatory proteins COX-2 and TNFα. In addition, EGFR phosphorylation and shedding of the membrane-anchored heparin-binding EGF-like growth factor (pro-HB-EGF) ectodomain, as well as a rapid activation/phosphorylation of the classical survival proteins ERK, P70S6K and rS6 were induced upon sPLA2-IIA treatment. We further demonstrated that the presence of an EGFR inhibitor (AG1478), a matrix metalloproteinase inhibitor (GM6001), an ADAM inhibitor (TAPI-1), and a HB-EGF neutralizing antibody abrogated the phenotype of activated microglia induced by the sPLA2-IIA.

CONCLUSION

These results support the hypothesis that sPLA2-IIA may act as a potent modulator of microglial functions through its ability to induce EGFR transactivation and HB-EGF release. Accordingly, pharmacological modulation of EGFR might be a useful tool for treating neuroinflammatory diseases characterized by sPLA2-IIA accumulation.

Heparin-binding EGF-like growth factor (HB-EGF) mediates 5-HT-induced insulin resistance through activation of EGF receptor-ERK1/2-mTOR pathway

Although an inverse correlation between insulin sensitivity and the level of Gq/11-coupled receptor agonists, such as endothelin-1, thrombin, and 5-hydroxytryptamine (5-HT), has been reported, its precise mechanism remains unclear. In this report, we provide evidence that 5-HT induced production of heparin-binding epidermal growth factor-like growth factor (HB-EGF) and caused insulin resistance in 3T3-L1 adipocytes, primary adipocytes, and C2C12 myotubes. In 3T3-L1 adipocytes, 5-HT stimulated HB-EGF production by promoting metalloproteinase-dependent shedding of transmembrane protein pro-HB-EGF. HB-EGF then bound and tyrosine-phosphorylated EGF receptors, which activated the mammalian target of rapamycin pathway through ERK1/2 phosphorylation. Mammalian target of rapamycin activation caused serine phosphorylation of insulin receptor substrate-1, which attenuated insulin-stimulated tyrosine phosphorylation of insulin receptor substrate-1 and glucose uptake. Pharmacological inhibition of either Gq/11-coupled receptors or metalloproteinases, as well as either inhibition or knockdown of HB-EGF or Gαq/11, restored insulin signal transduction impaired by 5-HT. Inhibition of metalloproteinase activity also abolished HB-EGF production and subsequent EGF receptor activation by other Gq/11-coupled receptor agonists known to cause insulin resistance, such as endothelin-1 and thrombin. These results suggest that transactivation of the EGF receptor through HB-EGF processing plays a pivotal role in 5-HT-induced insulin resistance.

Human BM stem cells initiate angiogenesis in human islets in vitro

BM stem cells may have regenerative effects on islet function through angiogenesis. Human islets (100islet equivalent/mL) were cultured alone (control) or co-cultured (experimental group) with whole human BM (1 × 10(6) cells/mL) for 210 days. A protein array measuring angiogenesis factors found upregulated (experimental vs control, day 210) proteins levels of VEGF-a (535 vs 2 pg/mL), PDGF (280.79 vs 0 pg/mL), KGF (939 vs 8 pg/mL), TIMP-1 (4592 vs 4332 pg/mL) and angiogenin (506 vs 97 pg/mL). Lower protein levels of angiopoietin-2 (5 vs 709 pg/mL) were observed. Depletion of pro-angiogenesis factors in co-culture decreased the effects of BM-induced islet vascularization. Depletion of VEGF-a, eKGF and PDGF significantly reduced islet vascularization but individual depletion of KGF and PDGF had less effects overall on vessel formation. BM-induced vascularization showed significant endothelial cell distribution. Islet vascularization was linked to islet growth. A decrease in islet size indicated poor vascularization. Insulin release was evident in the tissues generated from human islet-BM co-culture throughout the entire culture period. Significant increase in insulin (28.66-fold vs control) and glucagon (24.4-fold vs control) gene expression suggest BM can induce endocrine cell regeneration. In conclusion, BM promotes human islet tissue regeneration via regulation of angiogenesis factors.

Thyrotropin-releasing hormone (TRH) reverses hyperglycemia in rat

Hyperglycemia in thyrotropin-releasing hormone (TRH) null mice indicates that TRH is involved in the regulation of glucose homeostasis. Further, TRH levels in the pancreas peak during the stages of late embryonic and early neonatal beta cell development. These observations are consistent in linking TRH to islet cell proliferation and differentiation. In this study, we examined the effect of TRH administration in damaged pancreatic rat (streptozotocin, STZ) to determine whether TRH could improve damaged pancreatic beta cells function. We hypothesize that TRH is able to reverse STZ-induced hyperglycemia by increasing pancreatic islet insulin content, preventing apoptosis, and potentially induce islet regeneration. It was found that following intra-peritoneal (ip) injection, TRH (10 microg/kg body weight (bwt)) reverses STZ (65 mg/kg bwt)-induced hyperglycemia (TRH given 3 days after STZ injection). Increased circulating insulin levels and insulin content in extracted pancreas suggests that TRH reversed STZ-induced hyperglycemia through improving pancreatic islet beta cell function. Further studies show a significantly lower level of apoptosis in islets treated with TRH as well as the presence of proliferation marker nestin and Brdu, suggesting that the TRH has the potential to prevent apoptosis and stimulate islet proliferation.

Ginseng on hyperglycemia: effects and mechanisms

It has been reported that American ginseng attenuates hyperglycemia and may present itself as a supplement to diabetes therapy. However, the lack of standardization in the usage of ginseng root leads to inconclusive results when applied to diabetes treatment. The mechanisms of American ginseng root in the treatment of diabetes remains a mystery. This greatly limits the effective utilization of American ginseng in facilitating diabetic therapy. Initiating studies have shown that American ginseng increases insulin production and reduces cell death in pancreatic β-cells. Also, studies have revealed American ginseng's ability to decrease blood glucose in type II diabetes patients as well as in streptozotocin-induced diabetic animals (STZ-diabetic mice). These data suggest that effects of ginseng in improving hyperglycemia may alter mitochondrial function as well as apoptosis cascades to ensure cell viability in pancreatic islet cells. This review briefly summarizes current knowledge of ginseng components and clinical studies related to diabetes. Further research will be needed to explore and identify the component(s) of ginseng, which may be responsible for the beneficial effects observed in animal studies which could then be extrapolated to human islets.

Allogeneic bone marrow supports human islet beta cell survival and function over six months

In this study, we have established a new strategy increasing human islet longevity utilizing allogeneic whole bone marrow (BM) co-cultured with human islets. The cultured islets' function and survival have been evaluated by analysis of insulin secretion in response to high-glucose-challenge, morphological evaluation of cell growth. Human islet only culture failed to reveal evidence of long term survival, growth or function in terms of insulin release or insulin response to glucose challenge. These results indicate that BM increases islet survival and function with the eventual formation of pancreatic endocrine tissue capable of sustaining beta cell fuction.

Tyrosine phosphorylation of p145met mediated by EGFR and Src is required for serum-independent survival of human bladder carcinoma cells

Here we address the molecular mechanism of serum-independent survival and growth of human bladder carcinoma cell line 5637. Serum starvation promoted tyrosine phosphorylation of a 145-kDa protein and activation of the tyrosine kinase Src and the receptor for epidermal growth factor (EGFR) over a slow time course (>8 hours). The phosphorylated 145-kDa protein was identified as the beta-subunit of c-Met/hepatocyte growth factor (HGF) receptor, p145(met), in which tyrosine residues 1003, 1234, and 1235 were phosphorylated. Inhibitors of Src (PP2, SU6656) or EGFR (AG99), but not p145(met) (K252a), effectively blocked tyrosine phosphorylation of p145(met) and promoted cell death accompanied by activation of caspase-like proteases. Conditioned medium from the serum-starved 5637 cells or purified EGF readily promoted the activation of Src and EGFR, and tyrosine phosphorylation of p145(met) in normally grown 5637 cells, suggesting that autocrine signaling of EGFR ligands is responsible for signal transduction events in serum-starved cells. Consistent with this idea, a monoclonal antibody against EGFR that would interfere with the ligand binding to EGFR blocked tyrosine phosphorylation events and promoted the caspase activation and cell death in serum-free conditions. Such apoptotic cell death was also induced by pretreatment of cells with a high concentration of HGF that downregulated endogenous p145(met). Nevertheless, Cu2+ ions, competitive inhibitors for HGF-binding to p145(met), did not show any effect on cellular functions in serum-free conditions. These results suggest that the serum-independent growth of 5637 cells involves the transmembrane signaling cascade via EGFR ligand(s) (but not HGF), EGFR, Src and p145(met).