Differential effect of insulin and elevated glucose level on adenosine handling in rat T lymphocytes

High glucose impairs ATP formation on the surface of human peripheral blood B lymphocytes

Diabetes-associated lymphocyte dysfunction may be attributed to the direct effect of hyperglycemia, but the impact of glucose concentration on B cell functionality is not fully resolved. Since, adenosine 5'-triphosphate (ATP) and its metabolite adenosine are the core constituents of the purinergic signaling network involved in regulation of immune response we aimed to investigate the impact of high glucose concentration on ATP outflow and metabolism on B cell surface. Purified human peripheral blood B cells cultured at high glucose (25 mM) concentration released significantly less ATP (~60%) comparing to cells cultured in low glucose (5mM) concentration. We observed that high glucose altered ATP hydrolysis on B cell surface due to increased activity of nucleoside triphosphate diphosphohydrolase-1 (NTPDase-1/CD39). In the presence of 10 μM [(3)H]AMP and 100 μM ATP significant quantities of [(3)H]ADP and [(3)H]ATP were generated, although the AMP to ADP phosphorylation potential of B cells cultured in high glucose decreased significantly. The flow cytometry analysis revealed that the level of ecto-adenylate kinase 1β (AK1β) on surface of B cells cultured in high glucose decreased significantly. Inhibition of NTPDase1/CD39 activity with 100 μM ARL67156 resulted in decreased cell viability, although significantly more viable cells retained in the culture media containing low glucose compared to high glucose media. Selective inhibition of P2X7 purinergic receptor irrespective of glucose concentration completely protected B cells against the ARL 67156-induced cell death. We assume that high glucose-induced alteration of ATP handling on B cell surface might contribute to impaired functionality of B cells in diabetes.

High glucose concentration impairs ATP outflow and immunoglobulin production by human peripheral B lymphocytes: involvement of P2X7 receptor

AIMS/HYPOTHESIS

Patients with diabetes are more prone to bacterial infections mostly due to hyperglycemia-induced suppression of immune cells function. B lymphocytes by secreting antibodies inhibit microbial replication, but the impact of high glucose concentration on humoral immune response is not fully resolved. The aim of this work was to investigate the effect of high glucose concentration on B cells response to stimulation with a bacterial antigen and autocrine regulation.

METHODS

Purified human peripheral blood B cells were cultured at different glucose concentrations and stimulated in vitro with Staphylococcus aureus Cowan I (SAC) plus IL-2. B cells proliferation, differentiation and IgM expression were analyzed by flow cytometry. B cell ATP release and involvement of P2 purinergic receptors in regulation of IgM secretion was assessed.

RESULTS

B cells cultured at 25 mM glucose in response to SAC stimulation released significantly less (≈ 55%) IgM comparing to cells maintained in 5mM glucose. Under resting and stimulatory conditions B cells released significant quantities of ATP to the culture media, but ATP level decreased when B cells were maintain in high glucose. SAC-induced B cell IgM release was totally blocked by highly selective antagonist (Az11645373) of P2X7 receptor. IgM secretion increased in the presence of potent P2X7 receptor agonist (BzATP), but this effect was abolished by high glucose concentration.

CONCLUSIONS/INTERPRETATION

High glucose concentration impairs B cell function by suppression of P2X7 receptor-dependent IgM release in response to in vitro bacterial antigen stimulation. This alteration may greatly contribute to the impaired humoral immune response in diabetics.

High glucose suppresses expression of equilibrative nucleoside transporter 1 (ENT1) in rat cardiac fibroblasts through a mechanism dependent on PKC-zeta and MAP kinases

Recently it was demonstrated that the elevated concentration of glucose but not lack of insulin is responsible for suppression of equilibrative nucleoside transporter (ENT1) in diabetic rat cardiac fibroblasts (CFs). The present study was undertaken to determine the signaling pathway utilized by glucose to regulate the expression of ENT1 in the primary culture of rat CFs. Pretreatment of CFs with Go 6983, an isozyme non-selective PKC inhibitor, prevented the high glucose (25 mM) effect on ENT1 mRNA level and nitrobenzylthioinosine (NBTI)-sensitive adenosine uptake. Similar effect was observed with a cell-permeable PKC-zeta pseudosubstrate, whereas Go 6976 a selective inhibitor of Ca(2+)-dependent PKC-alpha and PKC-beta isozymes had little effect on high glucose-induced suppression of ENT1 mRNA level. Incubation of CFs with nitric oxide (NO) donors (SNAPE, SNP) or NO synthase inhibitors (L-NAME, L-NMMA) prior to exposition of CFs to high glucose did not change the glucose effect on ENT1 mRNA level. The high glucose-induced suppression of ENT1 expression was blocked by PD9859 (an inhibitor of MEK), whereas neither wortmannin (an inhibitor of PI3K) nor rapamycin (an inhibitor of mTOR) affected the glucose action on ENT1 transcript level. Highly effective in preventing the high glucose effect on ENT1 mRNA level were GW 5074 (an inhibitor of Raf kinase) and SB 203580 (selective p38 MAPK inhibitor). These findings indicate that high glucose suppresses the expression of ENT1 in CFs by NO independent manner involving the signaling through PKC-zeta, Raf-1, MEK, and p38 MAPK pathways.

Insulin restores glucose inhibition of adenosine transport by increasing the expression and activity of the equilibrative nucleoside transporter 2 in human umbilical vein endothelium

L-Arginine transport and nitric oxide (NO) synthesis (L-arginine/NO pathway) are stimulated by insulin, adenosine or elevated extracellular D-glucose in human umbilical vein endothelial cells (HUVEC). Adenosine uptake via the human equilibrative nucleoside transporters 1 (hENT1) and 2 (hENT2) has been proposed as a mechanism regulating adenosine plasma concentration, and therefore its vascular effects in human umbilical veins. Thus, altered expression and/or activity of hENT1 or hENT2 could lead to abnormal physiological plasma adenosine level. We have characterized insulin effect on adenosine transport in HUVEC cultured in normal (5 mM) or high (25 mM) D-glucose. Insulin (1 nM) increased overall adenosine transport associated with higher hENT2-, but lower hENT1-mediated transport in normal D-glucose. Insulin increased hENT2 protein abundance in normal or high D-glucose, but reduced hENT1 protein abundance in normal D-glucose. Insulin did not alter the reduced hENT1 protein abundance, but blocked the reduced hENT1 and hENT2 mRNA expression induced by high D-glucose. Insulin effect on hENT1 mRNA expression in normal D-glucose was blocked by N(G)-nitro-L-arginine methyl ester (L-NAME, NO synthase inhibitor) and mimicked by S-nitroso-N-acetyl-L,D-penicillamine (SNAP, NO donor). L-NAME did not block insulin effect on hENT2 expression. In conclusion, insulin stimulation of overall adenosine transport results from increased hENT2 expression and activity via a NO-independent mechanism. These findings could be important in hyperglycemia-associated pathological pregnancies, such as gestational diabetes, where plasma adenosine removal by the endothelium is reduced, a condition that could alter the blood flow from the placenta to the fetus affecting fetus growth and development.

Reduced ability to release adenosine by diabetic rat cardiac fibroblasts due to altered expression of nucleoside transporters

Adenosine produced by cardiac cells is known to attenuate the proliferation of cardiac fibroblasts (CFs), inhibit collagen synthesis, and protect the myocardium against ischaemic and reperfusion injury. Diabetic patients' hearts exhibit ventricular hypertrophy and demonstrate reduced tolerance to hypoxia or ischaemia. In this study, we characterize the effects of glucose and insulin on processes that determine the release of adenosine from CFs. We showed that during ATP depletion, rat CFs cultured in the absence of insulin release significantly less adenosine compared to cells grown in the presence of insulin. Moreover, under both conditions the quantity of released adenosine depends on glucose concentration. We demonstrate that this is due to altered expression of nucleoside transporters. High glucose (25 mm) induced 85% decrease in nucleoside transporter ENT1 mRNA levels. Decrease of the insulin level below 10(-11) m resulted in over 3-fold increase in the nucleoside transporter CNT2 mRNA content. Measurements of adenosine transport in CFs cultured in the presence of 5 mm glucose and 10 nm insulin showed that the bidirectional equilibrative adenosine transport accounted for 70% of the overall adenosine uptake. However, cells grown in the presence of high glucose (25 mm) demonstrated 65% decrease of the bidirectional equilibrative adenosine transport. Experiments on CFs cultured in the absence of insulin showed that the unidirectional Na(+)-dependent adenosine uptake rose in these cells more than 4-fold. These results indicate that the development of diabetes may result in an increased uptake of interstitial adenosine by CFs, and reduction of the ability of these cells to release adenosine during ATP deprivation.