Yuk Man Leung

Heavy metals, islet function and diabetes development

Heavy metals have been known to possess many adverse health effects for a long time. Uncontrolled industrialization breaks out heavy metal pollution in the world. Heavy metal pollutants damage organ functions and disrupt physiological homeostasis. Diabetes mellitus is growing in prevalence worldwide. Several studies have indicated that the deficiency and efficiency of some essential trace metals may play a role in the islet function and development of diabetes mellitus. Some toxic metals have also been shown to be elevated in biological samples of diabetes mellitus patients. In the present work, we review the important roles of heavy metals in islet function and diabetes development in which the in vitro, in vivo, or human evidences are associated with exposure to zinc, arsenic, cadmium, mercury, and nickel. Through this work, we summarize the evidence which suggests that some heavy metals may play an important role in diabetes mellitus as environmental risk factors.

SNAREing voltage-gated K+ and ATP-sensitive K+ channels : Tuning β-cell excitability with syntaxin-1A and other exocytotic proteins

The three SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins, syntaxin, SNAP25 (synaptosome-associated protein of 25 kDa), and synaptobrevin, constitute the minimal machinery for exocytosis in secretory cells such as neurons and neuroendocrine cells by forming a series of complexes prior to and during vesicle fusion. It was subsequently found that these SNARE proteins not only participate in vesicle fusion, but also tether with voltage-dependent Ca2+ channels to form an excitosome that precisely regulates calcium entry at the site of exocytosis. In pancreatic islet β-cells, ATP-sensitive K+ (KATP) channel closure by high ATP concentration leads to membrane depolarization, voltage-dependent Ca2+ channel opening, and insulin secretion, whereas subsequent opening of voltage-gated K+ (Kv) channels repolarizes the cell to terminate exocytosis. We have obtained evidence that syntaxin-1A physically interacts with Kv2.1 (the predominant Kv in β-cells) and the sulfonylurea recep...

Rhynchophylline from Uncaria rhynchophylla functionally turns delayed rectifiers into A-type K+ channels.

Rhynchophylline (1), a neuroprotective agent isolated from the traditional Chinese medicinal herb Uncaria rhynchophylla, was shown to affect voltage-gated K(+) (Kv) channel slow inactivation in mouse neuroblastoma N2A cells. Extracellular 1 (30 microM) accelerated the slow decay of Kv currents and shifted the steady-state inactivation curve to the left. Intracellular dialysis of 1 did not accelerate the slow current decay, suggesting that this compound acts extracellularly. In addition, the percent blockage of Kv currents by this substance was independent of the degree of depolarization and the intracellular K(+) concentration. Therefore, 1 did not appear to directly block the outer channel pore, with the results obtained suggesting that it drastically accelerated Kv channel slow inactivation. Interestingly, 1 also shifted the activation curve to the left. This alkaloid also strongly accelerated slow inactivation and caused a left shift of the activation curve of Kv1.2 channels heterologously expressed in HEK293 cells. Thus, this compound functionally turned delayed rectifiers into A-type K(+) channels.

Dynamin Is Functionally Coupled to Insulin Granule Exocytosis

The insulin granule integral membrane protein marker phogrin-green fluorescent protein was co-localized with insulin in Min6B1 β-cell secretory granules but did not undergo plasma membrane translocation following glucose stimulation. Surprisingly, although expression of a dominant-interfering dynamin mutant (Dyn/K44A) inhibited transferrin receptor endocytosis, it had no effect on phogringreen fluorescent protein localization in the basal or secretagogue-stimulated state. By contrast, co-expression of Dyn/K44A with human growth hormone as an insulin secretory marker resulted in a marked inhibition of human growth hormone release by glucose, KCl, and a combination of multiple secretagogues. Moreover, serial pulse depolarization stimulated an increase in cell surface capacitance that was also blocked in cells expressing Dyn/K44A. Similarly, small interference RNA-mediated knockdown of dynamin resulted in marked inhibition of glucose-stimulated insulin secretion. Together, these data suggest the presence of a selective kiss and run mechanism of insulin release. Moreover, these data indicate a coupling between endocytosis and exocytosis in the regulation of β-cell insulin secretion.

Methylmercury chloride induces alveolar type II epithelial cell damage through an oxidative stress-related mitochondrial cell death pathway

Mercury, one of the widespread pollutants in the world, induces oxidative stress and dysfunction in many cell types. Alveolar type II epithelial cells are known to be vulnerable to oxidative stress. Alveolar type II epithelial cells produce and secrete surfactants to maintain morphological organization, biophysical functions, biochemical composition, and immunity in lung tissues. However, the precise action and mechanism of mercury on alveolar type II epithelial cell damage remains unclear. In this study, we investigate the effect and possible mechanism of methylmercury chloride (MeHgCl) on the human lung invasive carcinoma cell line (Cl1-0) and mouse lung tissue. Cl1-0 cells were exposed to MeHgCl (2.5-10 microM) for 24-72 h. The results showed a decrease in cell viability and an increase in malondialdehyde (MDA) level and ROS production at 72 h after MeHgCl exposure in a dose-dependent manner. Caspase-3 activity, sub-G1 contents and annexin-V binding were dramatically enhanced in Cl1-0 cells treated with MeHgCl. MeHgCl could also activate Bax, release cytochrome c, and cleave poly(ADP-Ribose) polymerase (PARP), and decrease surfactant proteins mRNA levels. Moreover, in vivo study showed that mercury contents of blood and lung tissues were significantly increased after MeHgCl treatment in mice. The MDA levels in plasma and lung tissues were also dramatically raised after MeHgCl treatment. Lung tissue sections of MeHgCl-treated mice showed pathological fibrosis as compared with vehicle control. The mRNA levels of proteins in apoptotic signaling, including p53, mdm-2, Bax, Bad, and caspase-3 were increased in mice after exposure to MeHgCl. In addition, the mRNA levels of surfactant proteins (SPs), namely, SP-A, SP-B, SP-C, and SP-D (alveolar epithelial cell functional markers) were significantly decreased. These results suggest that MeHgCl activates an oxidative stress-induced mitochondrial cell death in alveolar epithelial cells.

Isobolographic analysis of interaction between nisoxetine‐ and mepivacaine‐induced spinal blockades in rats

Although nisoxetine has been shown to elicit cutaneous (peripheral) anesthesia, spinal (central) anesthesia with nisoxetine was not exposed. The aim of this study was to examine spinal anesthesia of nisoxetine and its influence on the antinociceptive action of mepivacaine. We compared nisoxetine with an established local anesthetic mepivacaine for spinal anesthesia after rats were intrathecally injected with drugs. The drugs were spinally administered alone as well as in combination, and their potencies were compared via dose–response curves and isobolographic analysis. We showed that nisoxetine, as well as mepivacaine elicited spinal anesthesia in dose‐dependent manners. On a 50% effective dose (ED50) basis, the spinal block effect of nisoxetine in motor function, proprioception, and nociception [0.99 (0.91–1.10), 0.85 (0.76–0.95), 0.82 (0.74–0.89)] was more potent (P < 0.05) than that of mepivacaine [1.28 (1.21–1.34), 1.14 (1.07–1.22), 0.99 (0.93–1.05)], respectively. Furthermore, the nociceptive/sensory blockade (ED50) was greater than the motor blockade in both nisoxetine and mepivacaine groups (P < 0.05). Saline group (vehicle) produced no spinal anesthesia. Coadministration of nisoxetine with mepivacaine displayed an additive effect. Our data reported nisoxetine produced significant anesthesia at spinal level, and additive interaction with the local anesthetic, mepivacaine. Intrathecal nisoxetine elicited more potent spinal anesthesia than mepivacaine.

Cutaneous analgesia after subcutaneous injection of memantine and amantadine and their systemic toxicity in rats.

The purpose of the study is to find subcutaneous equianalgesic doses of memantine, amantadine and bupivacaine and use these doses to quantify the cardiovascular and central nervous system toxicity of these agents after intravenous administration. Memantine, amantadine and bupivacaine, a local anesthetic, in a dose-related fashion were determined for cutaneous analgesia by a block of the cutaneous trunci muscle reflex in rats, and equipotent doses were calculated. Following rapid intravenous infusion of equianalgesic bupivacaine, memantine, amantadine and saline (vehicle) in rats, we observed the onset time of seizure, apnea and impending death, and monitored mean arterial blood pressure and heart rate. Memantine and amantadine elicited dose-dependent cutaneous analgesia. At the 50% effective dose (ED(50)), the rank of potencies was bupivacaine [1.8 (1.7-2.0)]>memantine [19.1 (17.6-21.8)]>amantadine [36.1 (32.0-40.3)] (P<0.05). On ED(25), ED(50) and ED(75) basis, the duration caused by bupivacaine was similar to that caused by memantine or amantadine. At equianalgesic doses, the infusion time of memantine or amantadine required to induce seizure, impending death, and apnea was longer than that of bupivacaine during rapid intravenous infusion (P<0.01). The decreasing slope in mean arterial blood pressure and heart rate was slower with memantine and amantadine when compared with bupivacaine at equivalent doses (P<0.01). Our data showed that memantine and amantadine (i) were equal to bupivacaine at producing durations of cutaneous analgesia but (ii) were less likely than bupivacaine to cause cardiovascular and central nervous system toxicity.

Suppression of voltage-gated Na+ channels and neuronal excitability by imperatorin

Imperatorin is a naturally occurring furocoumarin compound isolated from plants such as Angelica archangelica and Cnidium monnieri. It has multiple pharmacological effects including anticonvulsant effects. Here we determined the effects of imperatorin on voltage-gated Na(+) channels (VGSC) using whole-cell patch clamp techniques in differentiated neuronal NG108-15 cells. We showed that imperatorin inhibited VGSC; such inhibition did not show state-dependence. Imperatorin caused a left shift in the steady-state inactivation curve without affecting activation gating. The inhibition of VGSC by imperatorin displayed a mild frequency-dependence. Imperatorin was also shown to inhibit VGSC and action potential amplitude without affecting voltage-gated K(+) channels in rat hippocampal CA1 neurons. In conclusion, our results suggest that imperatorin dampens neuronal excitability by inhibiting VGSC.

Parthenolide-Induced Cytotoxicity in H9c2 Cardiomyoblasts Involves Oxidative Stress

BACKGROUND Cardiac cellular injury as a consequence of ischemia and reperfusion involves nuclear factor-κB (NF-κ B), amongst other factors, and NF-κ B inhibitors could substantially reduce myocardial infarct size. Parthenolide, a sesquiterpene lactone compound which could inhibit NF-κ B, has been shown to ameliorate myocardial reperfusion injury but may also produce toxic effects in cardiomyocytes at high concentrations. The aim of this study was to examine the cytotoxic effects of this drug on H9c2 cardiomyoblasts, which are precursor cells of cardiomyocytes. METHODS Cell viability and apoptosis were examined by MTT and TUNEL assay, respectively, and protein expression was analyzed by western blot. Reactive oxygen species (ROS) production was measured using DCFH-DA as dye. Cytosolic Ca(2+) concentration and mitochondrial membrane potential were measured microfluorimetrically using, respectively, fura 2 and rhodamine 123 as dyes. RESULTS Parthenolide caused apoptosis at 30 μ M, as judged by TUNEL assay and Bax and cytochrome c translocation. It also caused collapse of mitochondrial membrane potential and endoplasmic reticulum stress. Parthenolide triggered ROS formation, and vitamin C (antioxidant) partially alleviated parthenolide-induced cell death. CONCLUSIONS The results suggested that parthenolide at high concentrations caused cytotoxicity in cardiomyoblasts in part by inducing oxidative stress, and demonstrated the imperative for cautious and appropriate use of this agent in cardioprotection. KEY WORDS Cardiomyoblast; Endoplasmic reticulum stress; Oxidative stress; Parthenolide; Reperfusion injury.

Pancreatic Islet α Cell Commands Itself: Secrete More Glucagon!

Glucagon release by pancreatic alpha cells is stimulated by low glucose through unclear mechanisms. In this issue of Cell Metabolism, Cabrera et al. (2008) show that glutamate released from alpha cells acts on glutamate receptors in a positive autocrine fashion that drives glucagon secretion during small physiological fluctuations of blood glucose.