Cyclic-adenosine 3',5'-monophosphate-stimulated c-fos gene transcription involves distinct calcium pathways in single beta-cells

Dynamic changes in β-cell [Ca2+] regulate NFAT activation, gene transcription, and islet gap junction communication

OBJECTIVE

Diabetes occurs because of insufficient insulin secretion due to β-cell dysfunction within the islet of Langerhans. Elevated glucose levels trigger β-cell membrane depolarization, action potential generation, and slow sustained free-Ca2+ ([Ca2+]) oscillations, which trigger insulin release. Nuclear factor of activated T-cell (NFAT) is a transcription factor, which is regulated by the increases in [Ca2+] and calceineurin (CaN) activation. NFAT regulation links cell activity with gene transcription in many systems and regulates proliferation and insulin granule biogenesis within the β-cell. However, the link between the regulation of β-cell electrical activity and oscillatory [Ca2+] dynamics with NFAT activation and downstream transcription is poorly understood. Here, we tested whether dynamic changes to β-cell electrical activity and [Ca2+] regulate NFAT activation and downstream transcription.

METHODS

In cell lines, mouse islets, and human islets, including those from donors with type 2 diabetes, we applied both agonists/antagonists of ion channels together with optogenetics to modulate β-cell electrical activity. We measured the dynamics of [Ca2+] and NFAT activation as well as performed whole transcriptome and functional analyses.

RESULTS

Both glucose-induced membrane depolarization and optogenetic stimulation triggered NFAT activation as well as increased the transcription of NFAT targets and intermediate early genes (IEGs). Importantly, slow, sustained [Ca2+] oscillation conditions led to NFAT activation and downstream transcription. In contrast, in human islets from donors with type2 diabetes, NFAT activation by glucose was diminished, but rescued upon pharmacological stimulation of electrical activity. NFAT activation regulated GJD2 expression and increased Cx36 gap junction permeability upon elevated oscillatory [Ca2+] dynamics. However, it is unclear if NFAT directly binds the GJD2 gene to regulate expression.

CONCLUSIONS

This study provides an insight into the specific patterns of electrical activity that regulate NFAT activation, gene transcription, and islet function. In addition, it provides information on how these factors are disrupted in diabetes.

Effects of scopolamine treatment and consequent convulsion development in c‑fos expression in fed, fasted, and refed mice

Fasting, anticholinergics, and seizures affect c‑fos activation in the brain. Additionally, antimuscarinic treated fasted animals develop convulsion soon after re‑feeding. Therefore, we assessed whether c‑fos expression changes in fed, fasting, and refed animals and how scopolamine treatment affects these changes. We further assessed whether there is a change in c‑fos expression after convulsions. For this purpose, BALB/c mice fasted for 1, 3, 6, 12, 24 and 48 h periods were used. The animals were treated with saline or scopolamine. Half\r\nof the animals treated with saline or scopolamine were given food 20 min after injection. All animals were observed for development of convulsions for 30 min. At the end of this period, the brains of all animals were removed, and the percentage of c‑fos active cells in the hypothalamus was determined immunohistochemically. Convulsions occurred within 1‑48 h of fasting, after scopolamine treatment and re‑feeding. Compared to fed animals, c‑fos expression was not significantly changed in those undergoing different fasting periods, but significantly decreased after 12 h fasting. After animals were allowed to eat, c‑fos activation significantly increased in the 1, 3, 6 and 12 refed‑saline groups and decreased in the 48 refed‑saline group. Scopolamine treatment in 1‑24 h fasted animals increased c‑fos expression, but decreased in 48 h fasted animals. Whereas convulsion development in scopolamine‑treated 3, 6, 12 and 24 h refed animals suppressed c‑fos expression. These results demonstrate that re‑feeding and scopolamine treatment induces neuronal activity in the hypothalamus, while scopolamine induced convulsions after food intake suppressed the c‑fos activity.

Cypermethrin, chlorpyrifos, deltamethrin, and imidacloprid exposure up-regulates the mRNA and protein levels of bdnf and c-fos in the brain of adult zebrafish (Danio rerio)

The aim of the present study is to investigate the toxicity effects of frequently used pesticides, involving cypermethrin, deltamethrin, chlorpyrifos and imidacloprid, on the expression of bdnf and c-fos genes in zebrafish brain tissues. Therefore, brain tissues exposed to intoxication was primarily analyzed by indirect immunofluorescence assay. Afterwards, the mRNA transcription levels of BNDF and c-fos genes and the protein levels were measured by qRT-PCR and Western blotting, respectively. The data of the immunofluorescence assay revealed intensive immunopositivity for bdnf and c-fos genes in the tissues exposed to pesticide intoxication in comparison to the control group (p<0.05). Moreover, the transcription levels of BNDF and c-fos genes, and protein levels were elevated following the intoxication (p<0.05, p<0.01, and p<0.001, respectively). These results showed that the exposure to the acute cypermethrin, deltamethrin, chlorpyrifos and imidacloprid intoxication disrupted the normal neuronal activity, resulting in neurotoxic effect, also DNA-binding Increasing c-fos activation, an oncoprotein from the family of the Nuclear Proteins, is also true of the knowledge that these chemicals are oncogenic in zebrafish brain tissues. Thus, the use of these pesticides poses a potential neuronal and oncogenic risk to the non-target organisms.

Laminarin counteracts diet-induced obesity associated with glucagon-like peptide-1 secretion

Laminarin, a type of β-glucan isolated from brown seaweeds, exhibits verity of physiological activities, which include immunology modulation and antitumor function. To investigate the effect of laminarin on energy homeostasis, mice were orally administrated with laminarin to test food intake, fat deposition, and glucose homeostasis. Chronically, laminarin treatment significantly decreases high-fat-diet-induced body weight gain and fat deposition and reduces blood glucose level and glucose tolerance. Acutely, laminarin enhances serum glucagon-like peptide-1 (GLP-1) content and the mRNA expression level of proglucagon and prohormone convertase 1 in ileum. Subsequently, laminarin suppresses the food intake of mice, the hypothalamic AgRP neuron activity, and AgRP expression but activates pancreatic function. Furthermore, laminarin-induced appetite reduction was totally blocked by Exendin (9-39), a specific competitive inhibitor of GLP-1 receptor. Then, STC-1 cells were adopted to address the underlying mechanism, by which laminarin promoted GLP-1 secretion in vitro. Results showed that laminarin dose-dependently promoted GLP-1 secretion and c-Fos protein expression in STC-1 cells, which were independent of Dectin-1 and CD18. Interestingly, BAPTA-AM, a calcium-chelating agent, potently attenuated laminarin-induced [Ca²⁺]_i elevation, c-Fos expression, and GLP-1 secretion. In summary, our data support that laminarin counteracts diet-induced obesity and stimulates GLP-1 secretion via [Ca²⁺]_i; this finding provides an experimental basis for laminarin application to treat obesity and maintain glucose homeostasis.

A rapid interference between glucocorticoids and cAMP-activated signalling in hypothalamic neurones prevents binding of phosphorylated cAMP response element binding protein and glucocorticoid receptor at the CRE-Like and composite GRE sites of thyrotrophin-releasing hormone gene promoter

Glucocorticoids or cAMP increase, within minutes, thyrotrophin-releasing hormone (TRH) transcription in hypothalamic primary cultures, although this effect is prevented if cells are simultaneously incubated with both drugs. Rat TRH promoter contains a CRE site at -101/-94 bp and a composite GRE element (cGRE) at -218/-197 bp. Nuclear extracts of hypothalamic cells incubated with 8Br-cAMP or dexamethasone, and not their combination, bind to oligonucleotides containing the CRE or cGRE sequences. Adjacent to CRE are Sp/Krüppel response elements, and flanking the GRE half site, two AP1 binding sites. The present study aimed to identify the hypothalamic transcription factors that bind to these sites. We verified that the effects of glucocorticoid were not mimicked by corticosterone-bovine serum albumin. Footprinting and chromatin immunoprecipitation (ChIP) assays were used to examine the interaction of cAMP- and glucocorticoid-mediated regulation of TRH transcription at the CRE and cGRE regions of the TRH promoter. Nuclear extracts from hypothalamic cells incubated for 1 h with cAMP or glucocorticoids protected CRE. The GRE half site was recognised by nuclear proteins from cells stimulated with glucocorticoids and, for the adjacent AP-1 sites, by nuclear proteins from cells stimulated with cAMP or phorbol esters. Protection of CRE or cGRE was lost if cells were coincubated with dexamethasone and 8Br-cAMP. ChIP assays revealed phospho-CREB, c-Jun, Sp1, c-Fos and GR antibodies bound the TRH promoter of cells treated with cAMP or glucocorticoids; anti:RNA-polymerase II immunoprecipitated TRH promoter in a similar proportion as anti:pCREB or anti:GR. Recruitment of pCREB, SP1 or GR was lost when cells were exposed simultaneously to 8Br-cAMP and glucocorticoids. The data show that while pCREB and Sp1 bind to CRE-2, or GR to cGRE of the TRH promoter, the mutual antagonism between cAMP and glucocorticoid signalling, which prevent their binding to TRH promoter, could serve as a mechanism by which glucocorticoids rapidly suppress cAMP and noradrenaline-stimulated TRH transcription.

Diminished phosphodiesterase-8B potentiates biphasic insulin response to glucose

cAMP activates multiple signal pathways, crucial for the pancreatic beta-cells function and survival and is a major potentiator of insulin release. A family of phosphodiesterases (PDEs) terminate the cAMP signals. We examined the expression of PDEs in rat beta-cells and their role in the regulation of insulin response. Using RT-PCR and Western blot analyses, we identified PDE3A, PDE3B, PDE4B, PDE4D, and PDE8B in rat islets and in INS-1E cells and several possible splice variants of these PDEs. Specific depletion of PDE3A with small interfering (si) RNA (siPDE3A) led to a small (67%) increase in the insulin response to glucose in INS-1E cells but not rat islets. siPDE3A had no effect on the glucagon-like peptide-1 (10 nmol/liter) potentiated insulin response in rat islets. Depletion in PDE8B levels in rat islets using similar technology (siPDE8B) increased insulin response to glucose by 70%, the potentiation being of similar magnitude during the first and second phase insulin release. The siPDE8B-potentiated insulin response was further increased by 23% when glucagon-like peptide-1 was included during the glucose stimulus. In conclusion, PDE8B is expressed in a small number of tissues unrelated to glucose or fat metabolism. We propose that PDE8B, an 3-isobutyl-1-methylxanthine-insensitive cAMP-specific phosphodiesterase, could prove a novel target for enhanced insulin response, affecting a specific pool of cAMP involved in the control of insulin granule trafficking and exocytosis. Finally, we discuss evidence for functional compartmentation of cAMP in pancreatic beta-cells.