The hypoglycemic sulfonylureas glyburide and tolbutamide inhibit fatty acid oxidation by inhibiting carnitine palmitoyltransferase

Hyperglycemia-driven signaling bridges between diabetes and cancer

Growing epidemiological evidence indicates an association between obesity, type 2 diabetes, and certain cancers, suggesting the existence of common underlying mechanisms in these diseases. Frequent hyperglycemias in type 2 diabetes promote pro-inflammatory responses and stimulate intracellular metabolic flux which rewires signaling pathways and influences the onset and advancement of different types of cancers. Here, we review the provocative impact of hyperglycemia on a subset of interconnected signalling pathways that regulate (i) cell growth and survival, (ii) metabolism adjustments, (iii) protein function modulation in response to nutrient availability (iv) and cell fate and proliferation and which are driven respectively by PI3K (Phosphoinositide 3-kinase), AMPK (AMP-activated protein kinase), O-GlcNAc (O-linked N-acetylglucosamine) and Wnt/β-catenin. Specifically, we will elaborate on their involvement in glucose metabolism, inflammation, and cell proliferation, highlighting their interplay in the pathogenesis of diabetes and cancer. Furthermore, the influence of antineoplastic and antidiabetic drugs on the unbridled cellular pathways will be examined. This review aims to inspire the next molecular studies to understand how type 2 diabetes may lead to certain cancers. This will contribute to personalized medicine and direct better prevention strategies.

Alterations to mitochondrial fatty-acid use in skeletal muscle after chronic exposure to hypoxia depend on metabolic phenotype

We investigated the effects of chronic hypoxia on the maximal use of and sensitivity of mitochondria to different substrates in rat slow-oxidative (soleus, SOL) and fast-glycolytic (extensor digitorum longus, EDL) muscles. We studied mitochondrial respiration in situ in permeabilized myofibers, using pyruvate, octanoate, palmitoyl-carnitine (PC), or palmitoyl-coenzyme A (PCoA). The hypophagia induced by hypoxia may also alter metabolism. Therefore, we used a group of pair-fed rats (reproducing the same caloric restriction, as observed in hypoxic animals), in addition to the normoxic control fed ad libitum. The resting respiratory exchange ratio decreased after 21 days of exposure to hypobaric hypoxia (simulated elevation of 5,500 m). The respiration supported by pyruvate and octanoate were unaffected. In contrast, the maximal oxidative respiratory rate for PCoA, the transport of which depends on carnitine palmitoyltransferase 1 (CPT-1), decreased in the rapid-glycolytic EDL and increased in the slow-oxidative SOL, although hypoxia improved affinity for this substrate in both muscle types. PC and PCoA were oxidized similarly in normoxic EDL, whereas chronic hypoxia limited transport at the CPT-1 step in this muscle. The effects of hypoxia were mediated by caloric restriction in the SOL and by hypoxia itself in the EDL. We conclude that improvements in mitochondrial affinity for PCoA, a physiological long-chain fatty acid, would facilitate fatty-acid use at rest after chronic hypoxia independently of quantitative alterations of mitochondria. Conversely, decreasing the maximal oxidation of PCoA in fast-glycolytic muscles would limit fatty-acid use during exercise.

NEW & NOTEWORTHY Affinity for low concentrations of long-chain fatty acids (LCFA) in mitochondria skeletal muscles increases after chronic hypoxia. Combined with a lower respiratory exchange ratio, this suggests facility for fatty acid utilization at rest. This fuel preference is related to caloric restriction in oxidative muscle and to hypoxia in glycolytic one. In contrast, maximal oxidation for LCFA is decreased by chronic hypoxia in glycolytic muscle and can explain glucose dependence at exercise.

Mortality outcomes of different sulphonylurea drugs: the results of a 14-year cohort study of type 2 diabetic patients

OBJECTIVE

Available data about mortality of type 2 diabetic patients treated with different sulphonylureas are scarce and contradictory.

DESIGN

We evaluated the associations between all-cause and cause-specific mortality and treatments with different sulphonylureas in a retrospective cohort of type 2 diabetic patients from a diabetes clinic.

METHODS

All 1277 patients treated with sulphonylureas during 19961997 were enrolled: 159 patients were treated with tolbutamide, 977 glibenclamide and 141 gliclazide. The baseline data (centralised laboratory parameters, anthropometric data and presence of chronic complications) were abstracted from the clinical records. Information on vital status was collected from demographic files after 14-year follow-up. Adjusted hazard ratios (HR) were estimated with Cox (all-cause mortality) or Fine and Gray models (cause-specific mortality), including several potential confounders.

RESULTS

Five hundred and fifty-six patients died during the follow-up: 262 from cardiovascular causes, 158 from cancer and 136 from other causes. When compared with the glibenclamide users, the gliclazide and tolbutamide users showed a significantly lower cancer mortality (HR=0.30; 95% CI 0.16-0.55, and HR=0.48; 95% CI 0.29-0.79 respectively). These results were strongly confirmed in the 555 patients on sulphonylurea monotherapy. None of the patients who were treated with gliclazide monotherapy died from cancer during the follow-up, and the patients on tolbutamide treatment exhibited a lower cancer mortality than the glibenclamide users (HR=0.40; 95% CI 0.22-0.71). Data did not change after stratification for the duration of sulphonylurea treatment from diabetes diagnosis to the study enrollment.

CONCLUSIONS

Cancer mortality was markedly reduced in the patients on gliclazide and tolbutamide treatment. These results suggest additional benefits for these drugs beyond their blood glucose-lowering effect and strongly advocate for further investigation.

Acute administration of GPR40 receptor agonist potentiates glucose-stimulated insulin secretion in vivo in the rat

Recently, several in vitro studies have shown that GPR40 receptor activation by free fatty acids (FFAs) results in glucose-dependent insulin secretion. However, whether GPR40 receptor activation results in glucose-dependent insulin secretion in vivo in rats is not known. Therefore, we evaluated the effect of synthetic GPR40 receptor agonist (compound 1) on glucose tolerance test (GTT) in fed, fasted, and insulin-resistant rats. In oral GTT, intraperitoneal GTT, and intravenous GTT, GPR40 receptor agonist improved glucose tolerance, which was associated with increase in plasma insulin level. Interestingly, in GTTs, the rise in insulin levels in agonist-treated group was directly proportional to the rate of rise and peak levels of glucose in control group. Although glibenclamide, a widely used insulin secretagogue, improved glucose tolerance in all GTTs, it did not display insulin release in intraperitoneal GTT or intravenous GTT. In the absence of glucose load, GPR40 receptor agonist did not significantly change the plasma insulin concentration, but did decrease the plasma glucose concentration. Fasted rats exhibited impaired glucose-stimulated insulin secretion (GSIS) as compared with fed rats. Compound 1 potentiated GSIS in fasted state but failed to do so in fed state. Suspecting differential pharmacokinetics, a detailed pharmacokinetic evaluation was performed, which revealed the low plasma concentration of compound 1 in fed state. Consequently, we examined the absorption profile of compound 1 at higher doses in fed state; and at a dose at which its absorption was comparable with that in fasted state, we observed significant potentiation of GSIS. Chronic high-fructose (60%) diet feeding resulted in impaired glucose tolerance, which was improved by GPR40 receptor agonist. Therefore, our results demonstrate for the first time that acute GPR40 receptor activation leads to potentiation of GSIS in vivo and improves glucose tolerance even in insulin-resistant condition in rats. Taken together, these results suggest that GPR40 receptor agonists could be potential therapeutic alternatives to sulfonylureas.

KATP channel openers have opposite effects on mitochondrial respiration under different energetic conditions

Mitochondrial (m) KATP channel opening has been implicated in triggering cardiac preconditioning. Its consequence on mitochondrial respiration, however, remains unclear. We investigated the effects of two different KATP channel openers and antagonists on mitochondrial respiration under two different energetic conditions. Oxygen consumption was measured for complex I (pyruvate/malate) or complex II (succinate with rotenone) substrates in mitochondria from fresh guinea pig hearts. One of two mKATP channel openers, pinacidil or diazoxide, was given before adenosine diphosphate in the absence or presence of an mKATP channel antagonist, glibenclamide or 5-hydroxydecanoate. Without ATP synthase inhibition, both mKATP channel openers differentially attenuated mitochondrial respiration. Neither mKATP channel antagonist abolished these effects. When ATP synthase was inhibited by oligomycin to decrease [ATP], both mKATP channel openers accelerated respiration for both substrate groups. This was abolished by mKATP channel blockade. Thus, under energetically more physiological conditions, the main effect of mKATP channel openers on mitochondrial respiration is differential inhibition independent of mKATP channel opening. In contrast, under energetically less physiological conditions, mKATP channel opening can be evidenced by accelerated respiration and blockade by antagonists. Therefore, the effects of mKATP channel openers on mitochondrial function likely depend on the experimental conditions and the cell's underlying energetic state.

Rat Liver Carnitine Palmitoyltransferase 1 Forms an Oligomeric Complex within the Outer Mitochondrial Membrane

Carnitine palmitoyltransferase (CPT) 1A catalyzes the rate-limiting step in the transport of long chain acyl-CoAs from cytoplasm to the mitochondrial matrix by converting them to acylcarnitines. Located within the outer mitochondrial membrane, CPT1A activity is inhibited by malonyl-CoA, its allosteric inhibitor. In this study, we investigate for the first time the quaternary structure of rat CPT1A. Chemical cross-linking studies using intact mitochondria isolated from fed rat liver or from Saccharomyces cerevisiae expressing CPT1A show that CPT1A self-assembles into an oligomeric complex. Size exclusion chromatography experiments using solubilized mitochondrial extracts suggest that the fundamental unit of its quaternary structure is a trimer. When studied in blue native-PAGE, the CPT1A hexamer could be observed, however, suggesting that under these native conditions CPT1A trimers might be arranged as dimers. Moreover, the oligomeric state of CPT1A was found unchanged by starvation and by streptozotocin-induced diabetes, conditions characterized by changes in malonyl-CoA sensitivity of CPT1A. Finally, gel filtration analysis of several yeast-expressed chimeric CPTs demonstrates that the first 147 N-terminal residues of CPT1A, encompassing its two transmembrane segments, trigger trimerization independently of its catalytic C-terminal domain. Deletion of residues 1-82, including transmembrane 1, did not abrogate oligomerization, but the latter is limited to a trimer by the presence of the large catalytic C-terminal domain on the cytosolic face of mitochondria. Based on these findings, we proposed that the oligomeric structure of CPT1A would allow the newly formed acylcarnitines to gain direct access into the intermembrane space, hence facilitating substrate channeling.

The role of mitochondria in protection of the heart by preconditioning

A prolonged period of ischaemia followed by reperfusion irreversibly damages the heart. Such reperfusion injury (RI) involves opening of the mitochondrial permeability transition pore (MPTP) under the conditions of calcium overload and oxidative stress that accompany reperfusion. Protection from MPTP opening and hence RI can be mediated by ischaemic preconditioning (IP) where the prolonged ischaemic period is preceded by one or more brief (2–5 min) cycles of ischaemia and reperfusion. Following a brief overview of the molecular characterisation and regulation of the MPTP, the proposed mechanisms by which IP reduces pore opening are reviewed including the potential roles for reactive oxygen species (ROS), protein kinase cascades, and mitochondrial potassium channels. It is proposed that IP-mediated inhibition of MPTP opening at reperfusion does not involve direct phosphorylation of mitochondrial proteins, but rather reflects diminished oxidative stress during prolonged ischaemia and reperfusion. This causes less oxidation of critical thiol groups on the MPTP that are known to sensitise pore opening to calcium. The mechanisms by which ROS levels are decreased in the IP hearts during prolonged ischaemia and reperfusion are not known, but appear to require activation of protein kinase Cε, either by receptor-mediated events or through transient increases in ROS during the IP protocol. Other signalling pathways may show cross-talk with this primary mechanism, but we suggest that a role for mitochondrial potassium channels is unlikely. The evidence for their activity in isolated mitochondria and cardiac myocytes is reviewed and the lack of specificity of the pharmacological agents used to implicate them in IP is noted. Some K⁺ channel openers uncouple mitochondria and others inhibit respiratory chain complexes, and their ability to produce ROS and precondition hearts is mimicked by bona fide uncouplers and respiratory chain inhibitors. IP may also provide continuing protection during reperfusion by preventing a cascade of MPTP-induced ROS production followed by further MPTP opening. This phase of protection may involve survival kinase pathways such as Akt and glycogen synthase kinase 3 (GSK3) either increasing ROS removal or reducing mitochondrial ROS production.

Targeted expression of Kir6.2 in mitochondria confers protection against hypoxic stress

Selective K(+) transport in the inner mitochondrial membrane has been attributed to at least three different channel types: ATP-sensitive, Ca(2+)-regulated and voltage-dependent K(+) channels. Studies utilizing their selective modulators have suggested that an increased activity of these channels plays an important role in the cellular protection from metabolic stress. However, direct evidence for this effect is largely absent, and recent findings on the lack of specificity for several channel openers and blockers have questioned the actual contribution of the mitochondrial K(+) channels in the preservation of cellular viability. In order to directly investigate the role of enhanced mitochondrial K(+) uptake in cellular protection, we selectively expressed the inward rectifying K(+) channel Kir6.2 in the mitochondria of HEK293 and HL-1 cells. Targeted Kir6.2 expression was achieved by cloning the Kir6.2 gene in pCMV/mito/GFP vector and the proper trafficking to mitochondria was confirmed by colocalization studies and Western blot. An increased K(+) influx to mitochondria overexpressing Kir6.2, as evidenced by using the K(+)-sensitive PBFI AM fluorescent dye, substantially improved the cellular viability after hypoxic stress, which was assessed by lactate dehydrogenase (LDH) release. In parallel, monitoring of mitochondrial Ca(2+) during stress, via the specific indicator rhod-2, revealed a significant attenuation of Ca(2+) accumulation in mitochondria overexpressing K(+) channels. This effect was abolished in mitochondria expressing an inactive mutant of Kir6.2. Mitochondria expressing Kir6.2 K(+) channel also exhibited a significant degree of depolarization that became even more pronounced during the stress. In conclusion, this study provides the first non-pharmacological evidence that an increased K(+) influx to mitochondria protects against hypoxic stress by preventing detrimental effects of Ca(2+) overload.

Glibenclamide inhibits islet carnitine palmitoyltransferase 1 activity, leading to PKC-dependent insulin exocytosis

Hypoglycemic sulfonylureas such as glibenclamide have been widely used to treat type 2 diabetic patients for 40 yr, but controversy remains about their mode of action. The widely held view is that they promote rapid insulin exocytosis by binding to and blocking pancreatic beta-cell ATP-dependent K+ (KATP) channels in the plasma membrane. This event stimulates Ca2+ influx and sets in motion the exocytotic release of insulin. However, recent reports show that >90% of glibenclamide-binding sites are localized intracellularly and that the drug can stimulate insulin release independently of changes in KATP channels and cytoplasmic free Ca2+. Also, glibenclamide specifically and progressively accumulates in islets in association with secretory granules and mitochondria and causes long-lasting insulin secretion. It has been proposed that nutrient insulin secretagogues stimulate insulin release by increasing formation of malonyl-CoA, which, by blocking carnitine palmitoyltransferase 1 (CPT-1), switches fatty acid (FA) catabolism to synthesis of PKC-activating lipids. We show that glibenclamide dose-dependently inhibits beta-cell CPT-1 activity, consequently suppressing FA oxidation to the same extent as glucose in cultured fetal rat islets. This is associated with enhanced diacylglycerol (DAG) formation, PKC activation, and KATP-independent glibenclamide-stimulated insulin exocytosis. The fat oxidation inhibitor etomoxir stimulated KATP-independent insulin secretion to the same extent as glibenclamide, and the action of both drugs was not additive. We propose a mechanism in which inhibition of CPT-1 activity by glibenclamide switches beta-cell FA metabolism to DAG synthesis and subsequent PKC-dependent and KATP-independent insulin exocytosis. We suggest that chronic CPT inhibition, through the progressive islet accumulation of glibenclamide, may explain the prolonged stimulation of insulin secretion in some diabetic patients even after drug removal that contributes to the sustained hypoglycemia of the sulfonylurea.

Inhibition of carnitine palmitoyltransferase in the rat small intestine reduces export of triacylglycerol into the lymph

Following digestion of dietary triacylglycerol (TAG), intestinal epithelial cells absorb fatty acids and monoacylglycerols that are resynthesized into TAG by enzymes located on the endoplasmic reticulum (ER). A study in rat liver (Abo-Hashema, K. A., M. H. Cake, G. W. Power, and D. J. Clarke. 1999. Evidence for TAG synthesis in the lumen of microsomes via a lipolysis-esterification pathway involving carnitine acyltransferases. J. Biol. Chem. 274: 35577-35582) showed that there is a carnitine-dependent ER lumenal synthesis of TAG. We wanted to test the hypothesis that a similar pathway was present in rat intestine by utilizing etomoxir, a specific inhibitor of carnitine palmitoyltransferase (CPT). Intraduodenal infusion of etomoxir inhibited CPT activity in the ER by 69%. Etomoxir did not affect either the uptake of intraduodenally infused [3H]glyceryltrioleate by the intestinal mucosa or the production of mucosal [3H]TAG, excluding the possibility that etomoxir interfered with TAG absorption or synthesis. Etomoxir did not inhibit protein synthesis, glucose, cholesterol or palmitate absorption or metabolism, or ATP concentrations. Etomoxir substantially (74%) diminished lymph TAG output from intralumenally infused glyceryltrioleate. In conclusion, these data strongly support the hypothesis that an ER CPT system exists and is necessary for processing dietary TAG into chylomicrons. The significant reduction in lymphatic output of chylomicron TAG on etomoxir treatment suggests that the major source of chylomicron TAG is a diacylglyceroltransferase on the lumenal surface of the ER.

Persistent hypoglycemia is induced by tolbutamide administration in broiler chickens fed a low-carbohydrate diet

With a view to gaining an insight into the regulatory mechanism of blood glucose concentrations specific to the chicken, an experimental induction of hypoglycemia was conducted by single or sequential administration of tolbutamide in broiler chickens fed a standard or low-carbohydrate diet. A single dosing of tolbutamide at levels of 25-200 mg/kg body weight decreased plasma glucose concentrations for 2 to 8 h after the dosing in chickens fed either diet. No significant rise in plasma insulin concentration was observed for 2 to 24 h after the single dosing of tolbutamide in chickens on either diet, with the exception of a significant rise when chickens on the standard diet received 100 mg tolbutamide. However, a transient increase of plasma insulin concentration was observed only in the 20 min immediately after the single dosing. Persistent hypoglycemia that was sustained for 5 days, with no significant changes in plasma insulin concentration, was induced by sequential dosing (3 times per day for 5 days, every 8 h) of tolbutamide (100 and 200 mg/kg body weight) in chickens fed the low-carbohydrate diet. In these chickens, the consistently low concentration of plasma glucose, with small diurnal variations, was evidenced by the determination of plasma glucose every 3 h in day 4/5 of the tolbutamide dosing. In chickens fed the standard diet, on the other hand, the low plasma glucose concentrations for 5 days were accompanied by significant diurnal fluctuations. Chickens with persistent hypoglycemia showed slight decreases in plasma non-esterified fatty acids (NEFA) concentration and only slight changes in blood D-3-hydroxybutyrate (3HB) concentration. The present study shows that the persistent hypoglycemia with normoinsulinemia, in the main, is induced by tolbutamide dosing in chickens fed a low-carbohydrate diet, and that the blood concentrations of NEFA and 3HB, alternatives of energy source in animals, are only slightly changed or not at all in hypoglycemic chickens.

Energy metabolism and substrates oxidative patterns in type 2 diabetic patients treated with sulphonylurea alone or in combination with metformin

OBJECTIVE

To compare substrates oxidative patterns in type 2 diabetic patients treated with sulphonylurea alone or in combination with metformin.

METHODS

Plasma glucose (PG), plasma insulin (PI) and substrates oxidation rates measured by indirect calorimetry were compared during a test day at 8.00 a.m. (before breakfast), 11.00 a.m. (before the lunch), 2.00 p.m. (2 h after the lunch) and at 5.00 p.m. in 56 diabetic patients treated with diet (group C, n = 14), sulphonylurea (group S, n = 14) or with a sulphonylurea-metformin combination (group SM, n = 28).

RESULTS

The three groups were comparable for age, body mass index (b.m.i.), body composition and PG levels. Mean glucose oxidation (Gox) was increased since mean lipid oxidation (Lox) was decreased in group S in comparison both with group C (3.02+/-0.08 vs. 2.62+/-0.08 g/min/kg/10(3), p < 0.05; 0.53+/-0.04 vs. 0.88+/-0.09 g/min/kg/10(3), p < 0.01). Mean Lox was also decreased in group S in comparison with group SM (0.88+/-0.06 vs. 0.53+/-0.04 g/min1/kg1/10(3), p < 0.0001) whereas the difference in Gox between these latter two groups was only significant in the basal state (1.94+/-0.17 vs. 2.47+/-0.17 g/min1/kg1/10(3), p < 0.05). Mean respiratory quotient (RQ) was increased in group S (0.90+/-0.01) in comparison both with group C (0.86+/-0.01, p < 0.001) and with group SM (0.86+/-0.01, p < 0.001). Mean energy expenditure was lower in group S than in group SM (21.4+/-0.6 vs. 23.6+/-0.6 kcal/min/kg/10(3), p < 0.05). Substrates oxidative patterns, RQ values and energy expenditure were similar in group C and in group SM.

CONCLUSIONS

When compared to patients treated with a sulphonylurea-metformin bitherapy, patients treated with a sulphonylurea monotherapy have a shift in their ratio of fat to carbohydrate oxidation that could make body weight loss more difficult in this latter group.

Topology of hepatic mitochondrial carnitine palmitoyltransferase I

Our earlier work using intact mitochondria and isolated mitochondrial outer membranes confirms the observations of Murthy and Pande that CPT-I is located on the mitochondrial outer membranes and supports the notion that this enzyme has a malonyl-CoA binding domain facing the cytosol and an acyl-CoA binding domain facing the inter membrane space. Our data also suggests that coenzyme A binds at the active site of CPT-I, as does acyl-CoA, 2-bromopalmitoyl-CoA, and (+)-hemipalmitoylcarnitinium, but malonyl-CoA does not bind at that site. Inhibition of CPT-I at the malonyl-CoA binding site by HPG and Ro 25-0187, which have no CoA moiety, contributes to a resolution of this question in that the CoA itself is not essential for the binding of malonyl-CoA to its regulatory site, but the dicarbonyl function which is present in malonyl-CoA, HPG, and Ro 25-0187 is absolutely essential. Our re-evaluation of the topology of hepatic mitochondrial CPT-I confirms the original observations that this enzyme has at least two different binding domains, one domain binding malonyl-CoA, HPG, and Ro-25-187 and the other domain binding acyl-CoA and other inhibitors of CPT-I. Furthermore, the malonyl-CoA binding domain is exposed to the cytosolic face of the membrane. Our data showing that treatment of the intact mitochondria with trypsin causes release of adenylate kinase which indicates that trypsin has damaged the mitochondrial outer membrane, possibly allowing trypsin to enter the intermembrane space and act on CPT from within the outer membrane. Since trypsin's action is limited to arginine and lysine residues, an alternative explanation could be that the portion of the protein domain responsible for malonyl-CoA inhibition may not contain these residues. The latter explanation is plausible, since malonyl-CoA was able to protect against loss of activity and sensitivity to inhibition, but did not protect against loss of adenylate kinase, suggesting that rupture of the outer membrane is not necessarily related to loss of CPT activity. These results suggest that some protein domain that is necessary for CPT activity is exposed on the outer surface of the outer membranes. Therefore, it seems likely that trypsin would have to be able to hydrolyse protein domains of CPT that are inaccessible to Nagarse and papain.

Fetal hemoglobin levels are related to metabolic control in diabetic subjects

We have investigated the relationship between fetal hemoglobin (HbF) levels and metabolic control in subjects with insulin-dependent (N = 79) and non-insulin-dependent diabetes mellitus (N = 242). HbF and hemoglobin A1c (HbA1c) levels were increased in subjects with type 1 and type 2 diabetes as compared to levels in nondiabetic individuals (P < 0.0001), and were significantly higher in type 1 than in type 2 diabetes subjects. Lower levels of HbA1c and HbF were observed in type 2 diabetes subjects treated by diet, intermediate levels in those treated with oral hypoglycemic agents, and higher levels in those treated with insulin. HbF and HbA1c levels were correlated in type 1 diabetes (R2 = 0.57, P < 0.0001) and type 2 diabetes (R2 = 0.58, P < 0.0001) subjects. Following intense treatment, twelve diabetic patients showed significant improvement both in HbA1c and HbF values. We conclude that increased HbF levels reflect poor metabolic control in subjects with diabetes mellitus.

The sulfonylurea controversy: more questions from the heart

Myocardial ischemia and infarction are associated with substantially increased morbidity and mortality among patients with diabetes mellitus. Although many factors contribute to the increased morbidity and mortality, in patients with non-insulin-dependent (type II) diabetes mellitus, one contributor may be the use of sulfonylurea drugs, the most widely used oral hypoglycemic agents. Such a possibility, which first arose over a 25 years ago when it was observed that patients taking sulfonylurea drugs had increased cardiovascular mortality, has recently resurfaced after the discovery that sulfonylureas act by inhibiting adenosine triphosphate (ATP)-sensitive potassium channels. In the pancreas, inhibition of ATP-sensitive potassium channels induces release of insulin; but in the heart, inhibition of these channels prevents ischemic preconditioning, an endogenous cardioprotective mechanism that protects the heart from lethal injury. This review outlines the current understanding of the molecular and cellular pharmacodynamics of sulfonylurea drugs and discusses the potential clinical consequences of inhibition of ATP-sensitive potassium channels in the heart of diabetic patients with cardiac disease in whom the use of sulfonylureas may be harmful.

Intracellular targets for antidiabetic sulfonylureas and potassium channel openers

Antidiabetic sulfonylureas and potassium channel openers affect the activity of the ATP-regulated potassium channel (K(ATP) channel) present in the plasma membrane of various cells. This causes a broad spectrum of physiological responses, including the modulation of insulin release from pancreatic B-cells and the relaxation of smooth muscle. Recently, new targets for antidiabetic sulfonylureas and potassium channel openers were found in membranes of organelles, such as mitochondria and zymogen- and insulin-containing granules. By acting on these targets, the drugs modulate, independently of K(ATP) channel activity, insulin release from pancreatic B-cells, and they regulate K+ transport in mitochondria and zymogen granules. The interaction of sulfonylureas and potassium channel openers with intracellular targets gives additional basic information about their properties. Additionally, these studies could be important because of the medical applications of sulfonylureas and potassium channel openers.

Carnitine palmitoyl-transferase enzyme inhibition protects proximal tubules during hypoxia

The role of inhibition of the CPT enzymes responsible for accumulation of long chain acylcarnitines (LCAC) during hypoxia in the proximal tubule has not been previously examined. We have characterized CPT enzyme activities in mitochondrial fractions of rabbit proximal tubules. Malonyl CoA-sensitive CPT I activity (1.1 +/- 0.3 nmol/min/mg protein), and detergent-solubilized, malonyl CoA-insensitive CPT II activity (2.3 +/- 0.4 nmol/min/mg protein) were readily detected in proximal tubule mitochondrial fractions. Subjecting rabbit proximal tubules to various periods of hypoxia did not significantly change mitochondrial CPT I or CPT II activities. Thirty minutes of hypoxia resulted in an increase in lysophospholipid mass from 440 +/- 105 to 720 +/- 93 pmol/mg protein, N = 5, LCAC mass from 79 +/- 11 to 618 +/- 34 pmol/mg protein, N = 5, and lactate dehydrogenase (LDH) release from 9 +/- 1% to 46 +/- 3%, N = 8. Pretreatment of proximal tubules with two different CPT inhibitors, glybenclamide (Glyb) 400 microM and oxfenicine (Oxfe) 1 mM, resulted in reduction in the magnitude of hypoxia-induced lysophospholipid formation 490 +/- 160 (Glyb), 342 +/- 150 pmol/mg protein (Oxfe), N = 4, hypoxia-induced LCAC formation 295 +/- 27 (Glyb), 128 +/- 16 pmol/mg protein (Oxfe). N = 5, and LDH release 25 +/- 1% (Glyb) and 19 +/- 2% (Oxfe), N = 8. The protective effect of CPT inhibition was also associated with increased production of lactate suggesting the modulation of a substrate-mediated metabolic switch. Immunoblots demonstrated that hypoxia caused a time dependent hydrolysis of fodrin-alpha subunit and that CPT inhibition protected against hypoxia-induced fodrin proteolysis. These data suggest a unifying hypothesis that links phospholipase A2 (PLA2) activation, and hypoxia-mediated fodrin proteolysis to the proximal tubule mitochondrial CPT system. I propose that CPT inhibition may represent a novel mechanism to ameliorate proximal tubule cell death during hypoxia.

Which cardiac potassium channel subtype is the preferable target for suppression of ventricular arrhythmias?

Prolongation of the cardiac action potential duration is the hallmark of Class III antiarrhythmic activity. Action potential duration prolongation may be achieved by several means: enhancement of inward current and, more commonly, blockade of one or more of the many outward currents that are carried by K+. However, it is far from clear whether blockade of one particular K+ channel is more efficacious than blockade of another. The objective of this review is to consider this question with particular reference to ischaemic heart disease, a condition for which effective prevention of ventricular arrhythmias continues to be sought.

Inhibition of liver microsomal carnitine acyltransferases by sulphonylurea drugs

The sulphonylureas glibenclamide and tolbutamide inhibited carnitine acyltransferase activities in rat liver microsomes. Glibenclamide was a more potent inhibitor than tolbutamide. The effect of tolbutamide on the malonyl-CoA-inhibitable transferase was influenced by the phospholipid/detergent environment whereas the effect of glibenclamide was not. Glibenclamide was a more potent inhibitor of the malonyl-CoA-inhibitable transferase than of the malonyl-CoA-insensitive enzyme. The extent of inhibition of the malonyl-CoA-inhibitable transferase by tolbutamide was similar to its effect on VLDL triacylglycerol secretion as reported by Wiggins and Gibbons [Biochem. J. 284 (1992) 457-462] possibly supporting the suggestion that microsomal carnitine acyltransferases are involved in VLDL triacylglycerol assembly/secretion.

Hepatic carnitine palmitoyltransferase-I has two independent inhibitory binding sites for regulation of fatty acid oxidation

Partial proteolysis of carnitine palmitoyltransferase (CPT-I) in intact mitochondria results in greatly diminished sensitivity to inhibition by its physiological inhibitor, malonyl-CoA, but inhibition by succinyl-CoA and methylmalonyl-CoA was affected to a lesser extent, whereas inhibition by coenzyme A, acetyl-CoA, and propionyl-CoA was not affected at all by proteinase treatment. These data suggested that inhibitors that are coenzyme A esters of short-chain dicarboxylic acids bind to a regulatory malonyl-CoA binding site located on the cytoplasmic face of the mitochondrial outer membrane while coenzyme A esters of monocarboxylic acids and free coenzyme A act at the active site in the mitochondrial intermembrane space. All inhibitors whose potency was altered by proteinase action provided protection against proteinases, whereas other inhibitors did not. Preincubation with the substrates carnitine, palmitoyl-CoA, or coenzyme A prior to proteolysis showed no protective effects against the loss of inhibition or loss of activity; however, preincubation with these substrates enhanced proteinase effects to more seriously diminish activity and inhibition by malonyl-CoA. Proteinases were also found to act on purified mitochondrial outer membranes to reduce inhibition by malonyl-CoA with little effect on activity. Using these outer membrane preparations it was found that the very potent inhibition of CPT-I by the active-site-directed substrate analog (+)-hemipalmitoylcarnitinium was not altered by proteinase treatment; however, inhibition by the malonyl-CoA analog Ro 25-0187, which is a more potent inhibitor than malonyl-CoA, was drastically reduced by proteinase treatment of mitochondrial outer membranes, confirming the different locations for the malonyl-CoA site and the active site. Coenzyme A and malonyl-CoA both act as competitive inhibitors with respect to the acyl-CoA substrate, but coenzyme A lacks cooperative effects seen with malonyl-CoA. For ligand binding to the malonyl-CoA regulatory site, there appears to be a requirement for two carbonyl groups in close juxtaposition, but there is apparently no requirement for the coenzyme A moiety per se. Current evidence, including the recently deduced primary structure for CPT-I, favors the hypothesis that (a) inhibitors of CPT-I may act at two distinct sites, (b) malonyl-CoA binds primarily to a regulatory site that is distinct from the active site of carnitine palmitoyltransferase-I, and (c) the two inhibitory sites are located on opposite sides of the mitochondrial outer membrane.

Recent insights pertaining to sarcolemmal phospholipid alterations underlying arrhythmogenesis in the ischemic heart

Myocardial ischemia in vivo is associated with dramatic electrophysiologic alterations that occur within minutes of cessation of coronary flow and are rapidly reversible with reperfusion. This suggests that subtle and reversible biochemical alterations within or near the sarcolemma may contribute to the electrophysiologic derangements. Our studies have concentrated on two amphipathic metabolites, long-chain acylcarnitines and lysophosphatidylcholine (LPC), which have been shown to increase rapidly in ischemic tissue in vivo and to elicit electrophysiologic derangements in normoxic tissue in vitro. Incorporation of these amphiphiles into the sarcolemma at concentrations of 1 to 2 mole%, elicits profound electrophysiologic derangements analogous to those observed in ischemic myocardium in vivo. The pathophysiological effects of the accumulation of these amphiphiles are thought to be mediated by alterations in the biophysical properties of the sarcolemmal membrane, although there is a possibility of a direct effect upon ion channels. Inhibition of carnitine acyltransferase I (CAT-I) in the ischemic cat heart was found to prevent the increase in long-chain acylcarnitines and LPC and to significantly reduce the incidence of malignant arrhythmias including ventricular tachycardia and fibrillation. This review focuses on the electrophysiologic derangements that are observed during early ischemia and presents data supporting the concept that accumulation of these amphiphiles within the sarcolemma contributes to these changes. The potential contribution of these amphiphiles to the increases in extracellular potassium and intracellular calcium are examined. Finally, recent data pertaining to the accumulation of long-chain acylcarnitines on cell-to-cell uncoupling are presented. In addition to the events reviewed here, there are many other alterations that occur during early myocardial ischemia, but the results from multiple studies over the past two decades indicate that the accumulation of these amphiphiles contributes importantly to arrhythmogenesis and that development of specific inhibitors of CAT-I or phospholipase A2 may be a promising therapeutic strategy to attenuate the incidence of lethal arrhythmias associated with ischemic heart disease in man.

Dissociation between cellular K+ loss, reduction in repolarization time, and tissue ATP levels during myocardial hypoxia and ischemia

The mechanisms underlying the marked increase in [K+]o in response to ischemia are not fully understood. Accordingly, the present study was performed to assess the contribution of ATP-regulated K+ channels by using simultaneous measurements of cellular K+ efflux, [K+]o, transmembrane action potentials, and tissue ATP, ADP, phosphocreatine, and creatine content in a unique isolated, blood-perfused papillary muscle preparation during hypoxia compared with ischemia. During 15 minutes of hypoxic perfusion (PO2, 6.1 +/- 0.9 mm Hg) with normal [K+]o of 4.1 +/- 0.1 mM, action potential duration (APD) was not altered even though tissue ATP levels decreased markedly from 33.5 +/- 1.8 to 14.7 +/- 2.0 nmol.mg protein-1 (p < 0.01). Net cellular K+ efflux, based on measured differences of [K+] between the venous effluent and the perfusate, was 13.23 +/- 0.79 mumol.g wet wt-1 during hypoxia. In contrast, after 15 minutes of zero-flow ischemia, APD at 80% of repolarization (APD80) decreased by 47% from 171 +/- 5 to 92 +/- 5 msec (p < 0.01), but integrated net cellular K+ efflux over 15 minutes of ischemia was 8.4-fold less (1.57 +/- 0.13 mumol.g wet wt-1) than during hypoxia. Tissue ATP levels, however, decreased by only 35.2% to 21.7 +/- 2.1 nmol.mg protein-1, which was significantly less than that induced by 15 minutes of hypoxia. Perfusion with hypoxic blood containing high [K+]o of 10.3 +/- 0.3 mM resulted in APD shortening similar to that observed during ischemia. Cellular K+ loss, however, was inhibited markedly by high [K+]o perfusion (only 4.51 +/- 0.28 mumol.g wet wt-1). Pretreatment with glibenclamide (5 microM), a drug that has been reported to inhibit ATP-regulated K+ channels and accelerate glycolysis in normoxic tissue, partially inhibited cellular K+ efflux during hypoxic perfusion with normal [K+]o (7.35 +/- 0.71 versus 13.23 +/- 0.79 mumol.g wet wt-1, p < 0.01) but had no significant influence on repolarization time or tissue ATP levels. Although glibenclamide partially prevented action potential shortening induced by hypoxic perfusion in the presence of elevated [K+]o, the proportion of cellular K+ efflux reduced by glibenclamide was less (23%) than that observed with glibenclamide in hypoxic perfusion with normal [K+]o (44%).(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of tolbutamide on fructose-2,6-bisphosphate formation and ketogenesis in hepatocytes from diabetic rats

To assess the extrapancreatic action of sulfonylurea directly in the diabetic, effects of tolbutamide on hepatocyte fructose-2,6-bisphosphate (F-2,6-P2) formation and ketone production were investigated using isolated hepatocytes from streptozotocin (STZ)-induced diabetic rats. The basal level of hepatocyte F-2,6-P2 was significantly higher in diabetic rats within 2 weeks after STZ (40 mg/kg body weight) injection compared with that in the nondiabetic control group. Ultimately, a marked decrease in the F-2,6-P2 level was observed at 4 weeks after STZ administration (10% of the control). Although the addition of tolbutamide further increased the hepatocyte F-2,6-P2 level during the first week after STZ injection, no significant effect was observed after the second week and on from the initial STZ. Treatment of diabetes with insulin restored the stimulatory effect of tolbutamide on the hepatocyte F-2,6-P2 formation. Tolbutamide, independently of insulin treatment, lowered the ketone production of hepatocytes from diabetic rats. The present results indicate that insulin is necessary, in advance, for sulfonylurea to stimulate the liver F-2,6-P2 formation, while tolbutamide inhibition of hepatocyte ketone production is independent of insulin. These results provide further support for the role of sulfonylurea in regulating hepatic energy metabolism in the diabetic.

Enhanced lipolysis in 3T3-L1 adipocytes following prolonged exposure to tolbutamide

The effect of tolbutamide on lipolysis was examined in 3T3-L1 adipocytes. Whereas lipolysis was reversibly inhibited by tolbutamide, prolonged treatment with this agent dose-dependently increased both basal and isoproterenol-stimulated lipolysis in washed adipocytes. The latter effect of tolbutamide was not accompanied with altered cAMP levels in the cells and was abolished in the presence of cycloheximide. Moreover, the lipolytic responses induced by isobutylmethylxanthine, forskolin and dibutyryl cAMP were also augmented by prolonged treatment of adipocytes with tolbutamide. Thus, it appears that development of enhanced lipolysis in 3T3-L1 adipocytes following prolonged exposure to tolbutamide requires continuous protein synthesis and probably involves a step distal to cAMP production.

The metabolic syndrome and signal transduction of gene expression

Epidemiological studies have clearly shown that the so-called metabolic syndrome which is linked to insulin resistance and a reduced glucose utilization of muscle represents an important risk factor for cardiovascular disease. However, only little is known of the intracellular consequences of insulin resistance. An important feature of an altered substrate utilization is related to signal transduction of gene expression. For the example of myosin heavy chain expression, it is shown that metabolic signals exist which reflect the fuel flux and substrate utilization of the heart muscle cell. The signals were characterized in functional states of the heart associated with altered metabolic influences (fasting, diabetes, sucrose feeding, increased calorie intake, carnitine palmitoyltransferase inhibition). In the pressure-overloaded heart, metabolic interventions which are expected to increase glucose utilization (sucrose feeding, captopril treatment) have a pronounced effect. Although a link with gene expression remains to be established, it should be noted that the sarcoplasmic reticulum Ca(2+)-pump activity seems to be affected in a functionally comparable manner. It is concluded that metabolic signals alter the protein phenotype of heart muscle and it is expected that a deranged signal transduction affects, not only the heart, but also vascular muscle.

Inhibition of mitochondrial carnitine palmitoyltransferases by adriamycin and adriamycin analogues

Adriamycin (ADR; doxorubicin) and its highly lipophilic, less toxic analogue N-benzyl-adriamycin-14-valerate (AD 198) were found to inhibit rat heart and liver carnitine palmitoyltransferases of both mitochondrial outer and inner membranes. The outer membrane enzyme was more sensitive to inhibition by these drugs than the inner membrane enzyme, and AD 198 was a more potent inhibitor of these enzymes than ADR. Other analogues of ADR, N-trifluoroacetyladriamycin-14-valerate (AD 32) and N-trifluoroacetyladriamycin-14-O-hemiadipate (AD 143), which are documented as being noncardiotoxic, were also more potent inhibitors of the mitochondrial carnitine palmitoyltransferases than ADR. Overall, the cardiac mitochondrial carnitine palmitoyltransferases seemed to be slightly more sensitive to the inhibitory effects of ADR and its analogues than the liver enzyme. ADR was an uncompetitive inhibitor with respect to palmitoyl-CoA and a noncompetitive inhibitor with respect to carnitine for both mitochondrial outer and inner membrane enzymes. Our data suggest that mitochondria can take up ADR and concentrate it within the matrix, as is known to happen with other positively-charged compounds. More ADR was found associated with the mitochondrial inner membrane than with the outer membrane; this could be due to the greater protein content of the inner membrane rather than drug binding to cardiolipin. Although inhibition of cardiac inner membrane carnitine palmitoyltransferase has been implicated previously as part of the cardiotoxicity mechanism of ADR, the present findings with ADR and its noncardiotoxic analogues do not support this view.

Glucose oxidation rates in fatty acid-perfused isolated working hearts from diabetic rats

The effect of fatty acids and the carnitine palmitoyltransferase I (CPT I) inhibitor, Etomoxir, on myocardial glucose oxidation in diabetes was studied. 14CO2 production from 11 mM [14C]glucose was measured in control or 6-week streptozotocin-diabetic isolated working rat hearts perfused with or without 1.2 mM palmitate (bound to 3% albumin). In control hearts, addition of palmitate to the buffer resulted in a marked reduction (13-fold) in glucose oxidation rates. Glucose oxidation in diabetic rat hearts perfused with palmitate was almost abolished. Even though glucose oxidation rates were low, exogenous palmitate oxidation rates, measured as 14CO2 production from [14C]palmitate, were not increased in diabetic versus control hearts. Addition of the CPT 1 inhibitor, Etomoxir (1.10(-6) M), resulted in a doubling of glucose oxidation rates in both control and diabetic rat hearts, in the presence or absence of palmitate. The effects of Etomoxir on glucose oxidation could not be explained by reduced exogenous palmitate oxidation or decreased levels of citrate. Cardiac function, as measured by the heart rate x peak systolic pressure product, was reduced in diabetic rat hearts. Etomoxir significantly increased heart function in palmitate-perfused hearts from both control and diabetic rats. These data suggest that fatty acids contribute to decreased glucose oxidation and cardiac function in diabetic rat hearts. These effects of fatty acids can be partially reversed with the CPT 1 inhibitor, Etomoxir.

Effects of 8-N,N-diethylamino-octyl-3,4,5-trimethoxybenzoate (TMB-8) HCl and verapamil on the metabolism of free fatty acid by hepatocytes

The influence of calcium antagonists on hepatic lipid metabolism was investigated in freshly dispersed rat hepatocytes incubated with [1-14C]oleate and verapamil or 8-N,N-diethylamino-octyl-3,4,5-trimethoxybenzoate (TMB-8). Synthesis of triglyceride was calculated from the specific radioactivity of [1-14C]oleate in extracted total lipid, after separation of each lipid class by thin-layer chromatography. Ketogenesis was measured enzymatically or as the amount of radioactivity incorporated into neutralized acid-soluble extracts. Neither verapamil nor TMB-8 affected triglyceride synthesis. In contrast, TMB-8 and verapamil exerted a concentration-dependent inhibition of ketogenesis, with TMB-8 being more potent than verapamil (inhibition by 50 microM TMB-8 was 73 +/- 9% versus 38 +/- 2% inhibition by 50 microM verapamil). Increasing the concentrations of calcium (0 to 4.2 mM) or oleate (0 to 2.0 mM) increased the rate of ketogenesis but did not alter the antiketogenic potency of TMB-8 or verapamil, indicating that inhibition of ketogenesis by these drugs was not calcium dependent. Since the calcium antagonists did not affect ketogenesis from octanoic acid, and since carnitine stimulated ketogenesis from [1-14C]oleate by 25% and reversed the antiketogenic effects of TMB-8 and verapamil, it appeared that the two calcium antagonists inhibited ketogenesis by interfering with the activity of the outer mitochondrial carnitine palmitoyltransferase. In assays of the outer carnitine palmitoyltransferase in isolated mitochondria, both TMB-8 and verapamil were inhibitory. TMB-8 was the more potent inhibitor of this enzyme, and carnitine was able to overcome inhibition by each of the inhibitors. These results suggest that verapamil and TMB-8 may inhibit ketogenesis by mechanisms independent of their well known effects on cellular calcium concentrations, and that inhibition may be competitive with respect to carnitine concentration. However, direct inhibition of carnitine palmitoyltransferase may not explain completely the inhibition of ketogenesis by these drugs, since concentrations required for enzyme inhibition were greater than those required for inhibition of ketogenesis in isolated hepatocytes.