Nerve growth factor inhibits apoptosis in memory B lymphocytes via inactivation of p38 MAPK, prevention of Bcl-2 phosphorylation, and cytochrome c release

Survival of memory B lymphocytes is tightly linked to the integrity of the Bcl-2 protein and is regulated by a nerve growth factor (NGF) autocrine circuit. In factor-starved memory B cells, the addition of exogenous NGF promptly induced p38 mitogen-activated protein kinase (MAPK), but not c-Jun N-terminal kinase (JNK), dephosphorylation. Conversely, withdrawal of endogenous NGF was followed by p38 MAPK activation and translocation onto mitochondria, whereby it combined with and phosphorylated Bcl-2, as assessed by co-immunoprecipitation and kinase assays in vivo and in vitro. Mitochondria isolated from human memory B cells, then exposed to recombinant p38 MAPK, released cytochrome c, as did mitochondria from Bcl-2-negative MDCK cells loaded with recombinant Bcl-2. Apoptosis induced by NGF neutralization could be blocked by the specific p38 MAPK inhibitor SB203580 or by Bcl-2 mutations in Ser-87 or Thr-56. These data demonstrate that the molecular mechanisms underlying the survival factor function of NGF critically rely upon the continuous inactivation of p38 MAPK, a Bcl-2-modifying enzyme.

NGF-withdrawal induces apoptosis in pancreatic beta cells in vitro

AIMS/HYPOTHESIS

Using primary cultures of human pancreatic islets, purified human pancreatic beta cells and the mouse beta TC6-F7 cell line, we analysed the expression of nerve growth factor, (NGF/NGF) receptors in beta cells. To investigate whether NGF could sub-serve an autocrine antiapoptotic role in beta cells, we studied the effects of NGF withdrawal using a neutralizing monoclonal anti-NGF antibody.

METHODS

The expression of NGF and NGF receptors (gp140(Trk-A) and p75(NTR)) were analysed by RT-PCR and immunofluorescence. Pulse-chase experiments and beta cell/PC12 co-cultures were used to investigate NGF production and secretion from beta cells. Possible apoptosis induced by NGF withdrawal was monitored by phosphatidylserine translocation, nucleosomal formation, DNA laddering and FACS analysis. Involvement of transcription/translation mechanisms were investigated as well as the gp140(Trk-A) required. Finally, signal transduction pathways typically involved in apoptotic mechanisms were analysed by western blot analysis.

RESULTS

We show that NGF and both NGF receptors, gp140(Trk-A) and p75(NTR) are expressed in beta cells where NGF is produced and secreted in a biologically active form. NGF-withdrawal induces beta-cell transcription/translation independent apoptosis but mediated by gp140(Trk-A). Analysis of signal transduction pathways revealed that NGF withdrawal inhibits the PI3-K, protein kinase B (AKT), Bad survival pathway and activates c-Jun kinase (JNK) whereas ERKs and p38 mitogen-activated protein kinase (MAPK) are not affected. Moreover, Bcl-XL, but not Bcl-2 protein expression are reduced.

CONCLUSION/INTERPRETATION

We suggest that the integrity of the NGF/NGF receptor system and NGF bioavailability participate in controlling beta-cell survival in culture which represents a key issue for improving possibilities for transplantations in the treatment of diabetes.

High glucose causes apoptosis in cultured human pancreatic islets of Langerhans: a potential role for regulation of specific Bcl family genes toward an apoptotic cell death program

Type 2 diabetes is characterized by insulin resistance and inadequate insulin secretion. In the advanced stages of the disease, beta-cell dysfunction worsens and insulin therapy may be necessary to achieve satisfactory metabolic control. Studies in autopsies found decreased beta-cell mass in pancreas of people with type 2 diabetes. Apoptosis, a constitutive program of cell death modulated by the Bcl family genes, has been implicated in loss of beta-cells in animal models of type 2 diabetes. In this study, we compared the effect of 5 days' culture in high glucose concentration (16.7 mmol/l) versus normal glucose levels (5.5 mmol/l) or hyperosmolar control (mannitol 11 mmol/l plus glucose 5 mmol/l) on the survival of human pancreatic islets. Apoptosis, analyzed by flow cytometry and electron and immunofluorescence microscopy, was increased in islets cultured in high glucose (HG5) as compared with normal glucose (NG5) or hyperosmolar control (NG5+MAN5). We also analyzed by reverse transcriptase-polymerase chain reaction and Western blotting the expression of the Bcl family genes in human islets cultured in normal glucose or high glucose. The antiapoptotic gene Bcl-2 was unaffected by glucose change, whereas Bcl-xl was reduced upon treatment with HG5. On the other hand, proapoptotic genes Bad, Bid, and Bik were overexpressed in the islets maintained in HG5. To define the pancreatic localization of Bcl proteins, we performed confocal immunofluorescence analysis on human pancreas. Bad and Bid were specifically expressed in beta-cells, and Bid was also expressed, although at low levels, in the exocrine pancreas. Bik and Bcl-xl were expressed in other endocrine islet cells as well as in the exocrine pancreas. These data suggest that in human islets, high glucose may modulate the balance of proapoptotic and antiapoptotic Bcl proteins toward apoptosis, thus favoring beta-cell death.

Neurotrophins and neurotrophin receptors mRNAs expression in pancreatic islets and insulinoma cell lines

CONTEXT

It is worth noting that islets and betaTC6-F7 cells share a common pattern of expression of neurotrophins and neurotrophin receptors. Recently, several studies have hypothesized a role for nerve growth factor in pancreatic development and maturation, suggesting that nerve growth factor may be a survival factor for pancreatic beta-cells.

OBJECTIVE

The aim of the present study was to investigate the pattern of expression of neurotrophins and their relative receptors both in rat pancreatic islets and in a wide panel of insulinoma cell lines.

MAIN OUTCOME MEASURES

A semi-quantitative reverse-transcription polymerase chain reaction analysis was performed on ribonucleic acids extracted from these cells.

RESULTS

Reverse transcription-polymerase chain reaction analysis demonstrates that brain-derived neurotrophic factor, as well as neurotrophins 3 and 4, are expressed both in islets and in all insulinoma cells, while nerve growth factor is expressed only in islets, betaTC6-F7 cells and, at a low level, in RIN 1046-38 cells. Receptors protein tyrosine kinase A and C are ubiquitously expressed both in islets and insulinoma cells. Tyrosine kinase B is absent in HIT-T15 cells.

CONCLUSIONS

These data indicate that betaTC6-F7 cells are a suitable model for studying the role of neurotrophins in the survival of beta-cells.

Molecular and functional characterization of pituitary adenylate cyclase-activating polypeptide (PACAP-38)/vasoactive intestinal polypeptide receptors in pancreatic beta-cells and effects of PACAP-38 on components of the insulin secretory system

It has been previously demonstrated that pituitary adenylate cyclase-activating polypeptide (PACAP) regulates insulin secretion. PACAP exerts its biological action by binding to at least three different receptor subtypes coupled to different signal transduction mechanisms. The signaling pathways underlying the insulinotropic effect of PACAP involve mainly the activation of adenylate cyclase to form cAMP, which directly and indirectly, through increased intracellular Ca2+, stimulates insulin exocytosis. In the present study we have characterized the functional and molecular expression of PACAP/vasoactive intestinal polypeptide receptors isoforms and subtypes and its isoforms in a beta-cell line and in isolated rat pancreatic islets. Although insulinoma cells express the messenger RNA encoding PAC1 (-R and -hop variants), VPAC1 and VPAC2, binding experiments indicate the preponderance of PAC1 over VPAC 1-2 receptors. We have also shown that the main signaling pathway of PACAP in beta-cells is mediated by adenylate cyclase, whereas the inositol 1,4,5-trisphosphate pathway is almost inactive. Furthermore, we have demonstrated that PACAP exerts long-term effects on beta-cells, such as transcriptional regulation of the insulin gene and genes of the glucose-sensing system (GLUT1 and hexokinase 1).

GLUT2 and glucokinase expression is coordinately regulated by sulfonylurea

In the present study we examined the effect of sulfonylurea on the expression of the glucose transporter GLUT2 and the glucose phosphorylating enzyme Glucokinase (GK) in betaTC6-F7 cells; furthermore, we studied the modifications induced by sulfonylurea on glucose-responsiveness and -sensitivity. Results demonstrate that sulfonylurea increases GLUT2 and GK mRNA expression after 24 h in a dose-dependent manner. On the contrary, after 48 and 72 h a time-dependent reduction of both GLUT2 and GK mRNA occurs. GLUT2 and GK protein expression follow the same modifications. Therefore, GLUT2 and GK are coordinately regulated by sulfonylurea, probably by a common mechanism. Glucose-induced insulin release is increased by sulfonylurea as well as glucose sensitivity. Our study suggests that short-term effect of sulfonylurea increases while long-term effect reduces the expression of glucose sensing elements. The long-term inhibitory effect on glucose sensing elements would explain the reduced insulin secretion occurring after chronic sulfonylurea treatment.

Corticosterone treatment differentially affects adrenocorticoid receptors expression and binding in the hippocampus and spinal cord of the rat

We have studied the immediate and long-term effects of high doses of corticosterone (CORT) on mRNA expression and binding properties of mineralocorticoid receptor and glucocorticoid receptor in the hippocampus and spinal cord of rats. Animals were treated with corticosterone (10 mg/d subcutaneously) for 21 consecutive days, and mineralocorticoid and glucocorticoid receptors were studied either 24 h or 2 wk after the last injection. Major results show that corticosterone treatment reduces mineralocorticoid and glucocorticoid receptor maximum binding capacity (Bmax) in both the hippocampus and spinal cord and that this reduction is partially reversed after cessation of treatment. With respect to mRNA expression, in the hippocampus recovery after cessation of treatment is complete. By contrast, in the spinal cord, mineralocorticoid receptor mRNA expression is irreversibly increased after treatment, but the glucocorticoid receptor mRNA level remains unaffected during and after treatment. Thus, these data suggest the presence of distinct regulatory mechanisms for adrenocorticoid receptors in rat brain and spinal cord, in response to long-term exposure to high levels of circulating corticosterone and after recovery from treatment.

The effects of pregnancy steroids on adaptation of beta cells to pregnancy involve the pancreatic glucose sensor glucokinase

Pregnancy is associated with adaptive changes including increased number and size of beta cells and enhanced gap-junctional coupling among beta cells, increased glucose-induced insulin response and decreased glucose stimulation threshold. The role exerted by pregnancy steroids and lactogenic hormones in the development of islets upregulation during pregnancy has been widely investigated. In the present study we studied the possibility that pregnancy steroids induce functional modifications of beta cells involving the expression and function of glucokinase. Our results indicate that estradiol and progesterone do not influence significantly glucokinase mRNA expression, while they induce a dose-dependent and time-dependent increase of glucokinase activity in RIN 1046-38 cells. The increased enzymatic activity results in an increased glucose-induced insulin release. Therefore it is possible to hypothesize that pregnancy steroids influence glucokinase expression in beta cells at a post-transcriptional level and that this effect contributes to the development of hyperinsulinemia during pregnancy.

Distribution of adrenocorticoid receptors in the rat CNS measured by competitive PCR and cytosolic binding

Combined quantitative polymerase chain reaction (PCR) and cytosolic binding assay techniques are used to measure mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) mRNA, Kd, and Bmax in various rat central nervous system (CNS) regions, namely amygdala, hypothalamus, hippocampus, cortex, pituitary, and cervical, thoracic, and lumbar spinal cord. Two internal standards (i.s.) cDNA were cloned for quantitative PCR purposes. The i.s. templates differed from the respective wild-type (wt) templates for a single base-pair mutation introduced by PCR that generated a unique restriction site, thus allowing amplification products arising from coamplification of wt and i.s. to be distinguished. Results show that cerebellum, which displayed average Bmax values for both receptors, contained the highest level of MR and GR mRNA. Hippocampus also had a high level of MR mRNA. Low mRNA content was found in the hypothalamus for MR and GR as well as in the cortex for GR. High Bmax values for both MR and GR were found in the lumbar spinal cord, despite a modest mRNA content. The lowest Bmax values were found in the cortex for both receptors. It is, therefore, concluded that mRNA content and Bmax are not closely correlated in the rat CNS. These data suggest a differential regulation of various adrenocorticoid receptor isoforms. Moreover, this quantitative PCR method is very sensitive and can be used to assay small amounts of material in order to obtain absolute measurements of mRNA expression.

Effect of biotin on glucokinase activity, mRNA expression and insulin release in cultured beta-cells

Biotin is known to influence hepatic glucokinase (GK) expression both at a transcriptional and at a translational level. The aim of the present paper was to investigate the effect of biotin on pancreatic GK. For this purpose, RIN1046-38 cells were cultured in the presence of different biotin concentrations for different times; there-after, GK mRNA expression, GK activity and insulin release were studied. Results demonstrated that biotin has a biphasic effect on GK mRNA expression, being stimulatory after short-term treatment and inhibitory after longterm treatment. GK activity was increased after long-term treatment. Insulin release was not affected by biotin treatment. These data suggest that biotin may influence glucose metabolism also by acting directly at the level of beta-cells.

Quantitative analysis of pancreatic glucokinase gene expression in cultured beta cells by competitive polymerase chain reaction

Regulation of glucokinase (GK) gene expression in pancreatic beta cells has been poorly investigated, both due to low abundance of the gene and to difficulties in cells isolation. The present study describes the establishment of a competitive RT-PCR method for quantitative analysis of GK gene. The method has been applied to the analysis of GK mRNA expression RIN 1046-38 cells. We have monitored modifications of GK mRNA expression after different periods of time in culture and we have studied the effect induced by dexamethasone (DEX) treatment. We show that the method is very sensitive and requires very low amount of RNA. Data demonstrate that GK mRNA expression in RIN cells is reduced as a function of passages in culture and that the reduction is positively correlated with the decrease of insulin responsiveness observed in high passages cells. DEX treatment inhibits GK mRNA expression in RIN cells in a dose-dependent and time-dependent manner.

Differential regulation of adrenocorticoid receptors in the hippocampus and spinal cord of adrenalectomized rats

Using multiple polymerase chain reaction assay and cytosolic receptor binding assay we studied type I, mineralocorticoid receptor (MR), and type II, glucocorticoid receptor (GR), adrenocorticoid receptors expression in rat hippocampus and spinal cord, at various times after adrenalectomy: 12 hr, 24 hr, 3 days, and 1 week. Analysis of the data demonstrates that in hippocampus the expression of MR and GR mRNA was not significantly affected by adrenalectomy. On the contrary, Bmax of MR was significantly increased at each time post-surgery, with only slight modifications of Kd. Bmax and Kd for GR showed a significant increase after 3 days and 1 week. In the spinal cord, MR mRNA was increased 12 hr after adrenalectomy, reaching a maximum at 3 days. Bmax of MR was also significantly increased after 3 days, whereas its Kd remained unchanged for the entire duration of the the study. Both GR mRNA and binding parameters were poorly affected by adrenalectomy. The results of the present experiments demonstrate that the absence of adrenocortical hormones influences differentially MR and GR expression in hippocampus and spinal cord, suggesting the existence of various and independent mechanisms of regulation of adrenocorticoid receptor.

Distribution of glucocorticoid receptor mRNA in the rat spinal cord

Using in situ hybridization (ISH), we studied the distribution of rat glucocorticoid receptors (GR) mRNA in rat spinal cord. mRNA encoding for GR was abundant throughout the white matter and a clear pattern of distribution was detected within the grey matter. In the grey matter mRNA was primarily localized in the ventral horn, where motoneurones were strongly labelled. In the dorsal horn, the distribution appears more diffuse but the superficial layers (I and II) clearly exhibited a shigher signal. We conclude that, in rat spinal cord, GR are present in both glial and neuronal cells. In particular, both somatosensory and motor pathways contain GR.

DBI mRNA is expressed in endocrine pancreas and its post-translational product DBI(33-50) inhibits insulin release

It has been previously demonstrated that DBI is present in endocrine pancreas and it is able to inhibit insulin release in isolated rat islets. Its mechanism of action has been investigated, demonstrating the possible involvement of cAMP and ATP-dependent K(+) channels. DB1(33-50), a post-translational product of DBI, is also able to inhibit insulin release, but its action has not been characterized. In the present study, we have investigated the presence of DBI mRNA in pancreas, islets and cultured ß cells. The possible mechanism of action of DBI(33-50) and the involvement of BZ/GABA(A) receptors has been studied.

Molecular and cellular characterization of the GABAA receptor in the rat pancreas

In the present study, we characterize the molecular structure of the GABAA receptor in pancreas, islets, alpha and beta cells, and in RIN 1046-38 cells. Using the polymerase chain reaction and specific primers for 11 out of the 15 subunits known so far, that may contribute to the composition of the GABAA receptors, we demonstrate that pancreas and its cellular components, as well RIN 1046-38 cells, might contain a GABAA receptor resulting from all the possible combinations in a pentameric configuration of the subtypes alpha 1, alpha 2, alpha 3 of the alpha subunit family, beta 1, beta 2, beta 3 subtypes of the beta subunit family, delta subunit and gamma 2 subtype of the gamma subunit family. The presence of the gamma 2 subunit renders the GABAA receptors potentially sensitive to allosteric modulators.

Regional distribution in the rat central nervous system of a mRNA encoding a portion of the cardiac sodium/calcium exchanger isolated from cerebellar granule neurons

The cardiac Na+/Ca2+ exchanger is a bidirectional electrogenic ion transporter that exchanges three Na+ ions for each Ca2+ ion and plays a critical role in returning sarcolemma Ca2+ concentrations to their resting levels. Because of the importance that the Na+/Ca2+ exchanger may play in maintaining neuronal Ca2+ homeostasis in the central nervous system, we subcloned a 456 bp portion of the Na+/Ca2+ exchanger cDNA from RNA isolated from primary cultures of rat cerebellar granule neurons using the polymerase chain reaction (PCR). This cDNA fragment was sequenced and shown to share 91.4% sequence identity with the human and 88% sequence identity with the canine cardiac Na+/Ca2+ exchangers. The PCR amplification product was used to analyze the distribution of this portion of the Na+/Ca2+ exchanger mRNA in various regions of the CNS by both Northern blotting and in situ hybridization histochemistry. The Northern analysis showed that the rank order of abundance of this mRNA was: hippocampus > cortex > cerebellum > hypothalamus > midbrain > striatum. The in situ hybridization data indicated that the corresponding mRNA containing this portion of the exchanger was present in numerous brain regions including multiple cortical layers, the hippocampus, septal nuclei, various thalamic nuclei, cerebellum, hypothalamus, olfactory bulb, brainstem, in various regions of the thoracic spinal cord and to a lesser extent in the striatum. The differential distribution of the mRNA as revealed by the in situ hybridization pattern suggests that either additional molecular variants exist or that different Na+/Ca2+ exchange mechanisms may be operative in those cell types that contain low amounts of this fragment of the exchanger mRNA.

Developmental expression of the alpha 6 GABAA receptor subunit mRNA occurs only after cerebellar granule cell migration

Using a competitive polymerase chain reaction (PCR) and appropriate internal standards, we have analyzed absolute amounts of the alpha 6 GABAA receptor subunit mRNA in the postnatally developing cerebellum and neocortex. The PCR data have shown that absolute amounts of the alpha 6 receptor subunit mRNA in the cerebellum increase dramatically (nearly 100-fold) during the second postnatal week, reaching maximal levels by postnatal day 21 (1 fmol/microgram total RNA). The absolute amount of the alpha 6 GABAA receptor subunit mRNA in the cortex at postnatal day 1 was 2 amol/microgram total RNA and increased to 7 amol/micrograms total RNA by postnatal day 14. No further increase in alpha 6 mRNA expression in the adult cortex was observed. Microscopic analysis of emulsion coated and counterstained sections indicated that alpha 6 GABAA receptor subunit mRNA labeling was only detected in the internal granule cell layer and not in either the external granule cell layer or in migrating granule cells. The alpha 1 GABAA receptor subunit mRNA increased in the cerebellar cortex with a similar temporal profile, although its distribution extended to additional cell types (Purkinje cells, stellate/basket cells and possibly cerebellar astrocytes). The temporal expression of these two GABAA receptor subunit mRNAs is coincident with the formation of synaptic contacts in the granule cell dendrites suggesting that afferent pathways innervating these neurons following cell migration may play a critical role in increasing the expression of mRNAs encoding the alpha 1 and alpha 6 GABAA receptor subunits.