Metformin May Alter the Metabolic Reprogramming in Cancer Cells by Disrupting the L-Arginine Metabolism: A Preliminary Computational Study.

Metabolic reprogramming in cancer is considered to be one of the most important hallmarks to drive proliferation, angiogenesis, and invasion. AMP-activated protein kinase activation is one of the established mechanisms for metformin's anti-cancer actions. However, it has been suggested that metformin may exert antitumoral effects by the modulation of other master regulators of cellular energy. Here, based on structural and physicochemical criteria, we tested the hypothesis that metformin may act as an antagonist of L-arginine metabolism and other related metabolic pathways. First, we created a database containing different L-arginine-related metabolites and biguanides. After that, comparisons of structural and physicochemical properties were performed employing different cheminformatic tools. Finally, we performed molecular docking simulations using AutoDock 4.2 to compare the affinities and binding modes of biguanides and L-arginine-related metabolites against their corresponding targets. Our results showed that biguanides, especially metformin and buformin, exhibited a moderate-to-high similarity to the metabolites belonging to the urea cycle, polyamine metabolism, and creatine biosynthesis. The predicted affinities and binding modes for biguanides displayed good concordance with those obtained for some L-arginine-related metabolites, including L-arginine and creatine. In conclusion, metabolic reprogramming in cancer cells by metformin and biguanides may be also driven by metabolic disruption of L-arginine and structurally related compounds.

Buformin alleviates sepsis-induced acute lung injury via inhibiting NLRP3-mediated pyroptosis through an AMPK-dependent pathway.

BACKGROUND: NOD-like receptor family pyrin domain containing 3 (NLRP3)-mediated macrophage pyroptosis plays an important role in sepsis-induced acute lung injury (ALI). Inhibition of pyroptosis may be a way to alleviate inflammation as well as tissue damage triggered after lipopolysaccharide (LPS) stimulation. The aim of the present study was to explore whether buformin (BF), a hypoglycemic agent, could alleviate sepsis-induced ALI by inhibiting pyroptosis. METHODS: Wildtype C57BL/6 mice were randomly divided into control group, BF group, LPS group and LPS+BF group. BF group and LPS+BF group were pretreated with BF at a dose of 25 mg/kg, and the changes were observed. In addition, BF was used to interfere with THP-1 cells. The therapeutic effect of BF has been verified by intraperitoneal injection of BF in vivo after LPS stimulation. RESULTS: Inflammation and injury was significantly reduced in BF pretreated mice, and the indexes related to pyroptosis were suppressed. The phosphorylation of AMP-activated protein kinase (AMPK) in lung tissues of mice in the BF and LPS+BF groups was significantly higher. In THP-1 cells, the AMPK inhibitor, Compound C was added to demonstrate that BF worked via AMPK to inhibit NLRP3 inflammasome. It was further demonstrated that BF up-regulated autophagy, which in turn promoted NLRP3 inflammasome degradation. On the other hand, BF decreased NLRP3 mRNA level by increasing nuclear factor-erythroid 2 related factor 2 (Nrf2). And BF showed a therapeutic effect after LPS challenge. CONCLUSION: Our study confirmed that BF inhibited NLRP3-mediated pyroptosis in sepsis-induced ALI by up-regulating autophagy and Nrf2 protein level through an AMPK-dependent pathway. This provides a new strategy for clinical mitigation of sepsis-induced ALI.

Effect of Dietary Fiber on the Level of Free Hypoglycemic Agents in Vitro.

We studied the effect of dietary fibers (DFs) on the levels of free hypoglycemic agents in vitro, i.e., glimepiride and the biguanides buformin and metformin. The levels of free buformin and free metformin were not affected by mixtures of DFs, i.e., cellulose, chitosan, pectin (PE), and glucomannan (GM), in fluids of pH 1.2 and 6.8 (similar to the pH of the stomach and intestines, respectively). However, the free biguanide level was significantly reduced by mixing with PE or sodium alginate (AL), in water. The free glimepiride level was reduced in the mixture of AL, PE, and GM (in a solution with a pH of 6.8). The changes in aqueous AL solution pH seemed to reflect the free metformin levels. Therefore, the effects of DFs on free drug levels were dependent on drug type, hypoglycemic agent, and mixing solution. In this study, the oral regimen concentrations of the drug and DFs were used. Based on these results, it is important to consider the interactions between hypoglycemic agents and DFs.

A dual-functional buformin-mimicking poly(amido amine) for efficient and safe gene delivery.

Biguanides (i.e. metformin, phenformin and buformin) are antidiabetic drugs with potential antitumor effects. Herein, a polycationic polymer, N,N'-bis(cystamine)acrylamide-buformin (CBA-Bu), containing multiple biodegradable disulphide bonds and buformin-mimicking side chains was synthesised. CBA-Bu was equipped with high efficiency and safety profile for gene delivery, meanwhile exhibiting potential antitumor efficacy. As a gene vector, CBA-Bu was able to condense plasmid DNA (pDNA) into nano-sized (<200 nm), positively-charged (>30 mV) uniform polyplexes that were well resistant to heparin and DNase I. Due to the reduction responsiveness of the disulphide bonds, CBA-Bu/pDNA polyplexes could release the loaded pDNA in the presence of dithiothreitol, and induce extremely low cytotoxicity in NIH/3T3 and U87 MG cells. The transfection results showed that CBA-Bu had a cellular uptake efficiency comparable to 25 kDa PEI, while a significantly higher gene expression level. Additionally, CBA-Bu had a lower IC50 value than its non-biguanide counterpart in two cancer cell lines. Furthermore, CBA-Bu could activate AMPK and inhibit mTOR pathways in U87 MG cells, a mechanism involved in the antitumor effect of biguanides. Taken together, CBA-Bu represented an advanced gene vector combining desirable gene delivery capability with potential antitumor activity, which was promising to achieve enhanced therapeutic efficacy in antitumor gene therapy.

Postmortem distribution/redistribution of buformin in body fluids and solid tissues in an autopsy case using liquid chromatography-tandem mass spectrometry with QuEChERS extraction method.

An autopsy for a suicidal case of a male in his 40s, who had died of poisoning due to ingestion of a large amount of buformin, was performed at our department. Buformin is biganide class agent used for patients of diabetes mellitus, which can occasionally cause severe lactic acidosis. The autopsy was performed about 10 days after his death, and the direct cause of his death was judged as asphyxia due to the aspiration of stomach contents into the airway. The nine body fluids and eight solid tissues specimens were dealt with for investigating postmortem distribution/redistribution of buformin in a whole body; femoral vein blood, right and left heart blood, pericardial fluid, urine, bile, stomach contents, small intestine contents, cerebrospinal fluid, the brain, lung, heart muscle, liver, spleen, kidney and skeletal muscle were examined. For extracting buformin from specimens, a modified QuEChERS method including dispersive solid-phase extraction was employed, followed by the analysis by liquid chromatography tandem mass spectrometry (LC-MS/MS). Buformin in various kinds of human matrices were quantified by the standard addition method in this study, which can overcome the matrix effects and recovery rates without use of blank human matrices. All concentrations of buformin in specimens examined in this case were extremely higher than those of previously reported poisoning cases. The concentrations of buformin in left and right heart blood and femoral vein blood specimens of this case were 399, 216 and 261µg/mL, respectively; although the direct cause of his death was judged as asphyxia due to occlusion of airway with stomach contents, the vomiting was thought to be provoked by buformin poisoning. In this study, marked differences of buformin concentrations between brain tissue and cerebral spiral fluids, and other specimens were observed, which suggested that its distribution was influenced also by blood-brain-barrier. Although a number of buformin poisoning cases were published so far, they gave sporadic data on its concentrations and/or distribution in some limited human specimens. This study is the first to describe detailed distribution/redistribution of buformin in a whole human body quantified by using LC-MS/MS.

Biguanide-induced mitochondrial dysfunction yields increased lactate production and cytotoxicity of aerobically-poised HepG2 cells and human hepatocytes in vitro.

As a class, the biguanides induce lactic acidosis, a hallmark of mitochondrial impairment. To assess potential mitochondrial impairment, we evaluated the effects of metformin, buformin and phenformin on: 1) viability of HepG2 cells grown in galactose, 2) respiration by isolated mitochondria, 3) metabolic poise of HepG2 and primary human hepatocytes, 4) activities of immunocaptured respiratory complexes, and 5) mitochondrial membrane potential and redox status in primary human hepatocytes. Phenformin was the most cytotoxic of the three with buformin showing moderate toxicity, and metformin toxicity only at mM concentrations. Importantly, HepG2 cells grown in galactose are markedly more susceptible to biguanide toxicity compared to cells grown in glucose, indicating mitochondrial toxicity as a primary mode of action. The same rank order of potency was observed for isolated mitochondrial respiration where preincubation (40 min) exacerbated respiratory impairment, and was required to reveal inhibition by metformin, suggesting intramitochondrial bio-accumulation. Metabolic profiling of intact cells corroborated respiratory inhibition, but also revealed compensatory increases in lactate production from accelerated glycolysis. High (mM) concentrations of the drugs were needed to inhibit immunocaptured respiratory complexes, supporting the contention that bioaccumulation is involved. The same rank order was found when monitoring mitochondrial membrane potential, ROS production, and glutathione levels in primary human hepatocytes. In toto, these data indicate that biguanide-induced lactic acidosis can be attributed to acceleration of glycolysis in response to mitochondrial impairment. Indeed, the desired clinical outcome, viz., decreased blood glucose, could be due to increased glucose uptake and glycolytic flux in response to drug-induced mitochondrial dysfunction.

Buformin suppresses proliferation and invasion via AMPK/S6 pathway in cervical cancer and synergizes with paclitaxel.

Buformin is an old anti-diabetic agent and manifests potent anti-tumor activities in several malignancies. In the present study, we aimed to explore the functions of buformin in human cervical cancer. As our data shown, buformin exhibited significant anti-proliferative effects in a dose-dependent manner in 4 cervical cancer cell lines. Compared to the control, buformin notably suppressed colony formation and increased ROS production in C33A, Hcc94 and SiHa cells. Flow cytometric analysis showed that buformin induced marked cell cycle arrest but only resulted in mild apoptosis. The invasion of C33A and SiHa cells sharply declined with buformin treatment. Consistently, western blotting showed that buformin activated AMPK and suppressed S6, cyclin D1, CDK4, and MMP9. Moreover, we found that buformin enhanced glucose uptake and LDH activity, increased lactate level, while decreased ATP production in cervical cancer cells. In addition, low doses of buformin synergized with routine chemotherapeutic drugs (such as paclitaxel, cisplatin, and 5-FU) to achieve more significant anti-tumor effects. In vivo, a single use of buformin exerted moderate anti-tumor effects, and the combination with buformin and paclitaxel exhibited even greater suppressive effects. Buformin also consistently showed synergistic effects with paclitaxel in treating primary cultures of cervical cancer cells. Take together, we are the first to demonstrate that buformin suppresses cellular proliferation and invasion through the AMPK/S6 signaling pathway, which arrests cell cycle and inhibits cellular invasion. Buformin also could synergize with routine chemotherapies, producing much more powerful anti-tumor effects. With these findings, we strongly support buformin as a potent choice for treating cervical cancer, especially in combination with routine chemotherapy.

Effects of Biguanides on Growth and Glycolysis of Bladder and Colon Cancer Cells.

Background/ Aim: There is evidence that inhibitory effects of biguanides on oxidative phosphorylation require uptake of biguanides into the mitochondria. In this study the action of two biguanides that enter the mitochondria (buformin and phenformin) were compared with the action of two biguanides with poor uptake (phenyl biguanide and proguanil). MATERIALS AND METHODS: Effects on growth, glucose uptake and medium acidification were studied with two human colon cancer cells and seven bladder cancer cell lines. RESULTS: Growth inhibition was greatest with proguanil followed by phenformin, buformin and phenylbiguanide. In contrast, increased glucose uptake and acidification of the medium was observed with buformin and phenformin, with no change or less acidification of the medium with phenyl biguanide and proguanil. CONCLUSION: The effect of biguanides on glucose metabolism requires mitochondrial uptake while the mechanism for growth inhibition by biguanides remains to be defined.

Buformin inhibits the stemness of erbB-2-overexpressing breast cancer cells and premalignant mammary tissues of MMTV-erbB-2 transgenic mice.

BACKGROUND: Metformin, an FDA-approved drug for the treatment of Type II diabetes, has emerged as a promising anti-cancer agent. Other biguanide analogs, including buformin and phenformin, are suggested to have similar properties. Although buformin was shown to reduce mammary tumor burden in carcinogen models, the anti-cancer effects of buformin on different breast cancer subtypes and the underlying mechanisms remain unclear. Therefore, we aimed to investigate the effects of buformin on erbB-2-overexpressing breast cancer with in vitro and in vivo models. METHODS: MTT, cell cycle, clonogenic/CFC, ALDEFLUOR, tumorsphere, and Western blot analyses were used to determine the effects of buformin on cell growth, stem cell populations, stem cell-like properties, and signaling pathways in SKBR3 and BT474 erbB-2-overexpressing breast cancer cell lines. A syngeneic tumor cell transplantation model inoculating MMTV-erbB-2 mice with 78617 mouse mammary tumor cells was used to study the effects of buformin (1.2 g buformin/kg chow) on tumor growth in vivo. MMTV-erbB-2 mice were also fed buformin for 10 weeks, followed by analysis of premalignant mammary tissues for changes in morphological development, mammary epithelial cell (MEC) populations, and signaling pathways. RESULTS: Buformin significantly inhibited SKBR3 and BT474 cell growth, and in vivo activity was demonstrated by considerable growth inhibition of syngeneic tumors derived from MMTV-erbB-2 mice. In particular, buformin suppressed stem cell populations and self-renewal in vitro, which was associated with inhibited receptor tyrosine kinase (RTK) and mTOR signaling. Consistent with in vitro data, buformin suppressed mammary morphogenesis and reduced cell proliferation in MMTV-erbB-2 mice. Importantly, buformin decreased MEC populations enriched with mammary reconstitution units (MRUs) and tumor-initiating cells (TICs) from MMTV-erbB-2 mice, as supported by impaired clonogenic and mammosphere formation in primary MECs. We further demonstrated that buformin-mediated in vivo inhibition of MEC stemness is associated with suppressed activation of mTOR, RTK, ER, and ß-catenin signaling pathways. CONCLUSIONS: Overall, our results provide evidence for buformin as an effective anti-cancer drug that selectively targets TICs, and present a novel prevention and/or treatment strategy for patients who are genetically predisposed to erbB-2-overexpressing breast cancer.

The enhancement of oxidative DNA damage by anti-diabetic metformin, buformin, and phenformin, via nitrogen-centered radicals.

Metformin (N,N-dimethylbiguanide), buformin (1-butylbiguanide), and phenformin (1-phenethylbiguanide) are anti-diabetic biguanide drugs, expected to having anti-cancer effect. The mechanism of anti-cancer effect by these drugs is not completely understood. In this study, we demonstrated that these drugs dramatically enhanced oxidative DNA damage under oxidative condition. Metformin, buformin, and phenformin enhanced generation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) in isolated DNA reacted with hydrogen peroxide (H2O2) and Cu(II), although these drugs did not form 8-oxodG in the absence of H2O2 or Cu(II). An electron paramagnetic resonance (EPR) study, utilizing alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone and 3,3,5,5-tetramethyl-1-pyrroline-N-oxide as spin trapping agents, showed that nitrogen-centered radicals were generated from biguanides in the presence of Cu(II) and H2O2, and that these radicals were decreased by the addition of DNA. These results suggest that biguanides enhance Cu(II)/H2O2-mediated 8-oxodG generation via nitrogen-centered radical formation. The enhancing effect on oxidative DNA damage may play a role on anti-cancer activity.

L503F variant of carnitine/organic cation transporter 1 efficiently transports metformin and other biguanides.

OBJECTIVES: Carnitine/organic cation transporter 1 (OCTN1) is involved in gastrointestinal absorption and mitochondrial toxicity of biguanides in rodents, but its pharmacokinetic roles in humans are largely unknown. The purpose of this study was to clarify the transport activities of two major OCTN1 variants, L503F and I306T, for gabapentin and three biguanide drugs, metformin, buformin and phenformin. METHODS: HEK293 cells were transfected with OCTN1 gene, its variants, or vector alone, and the uptake and cytotoxicity of each drug were examined. KEY FINDINGS: Buformin was identified to be an OCTN1 substrate. Uptake of biguanides, especially metformin, mediated by OCTN1 variant L503F, which is commonly found in Caucasians, was much higher than that by the wild-type transporter (WT-OCTN1). Cytotoxicity of metformin was also greater in HEK293 cells expressing the L503F variant, compared with WT-OCTN1. Uptake of gabapentin mediated by OCTN1 variant I306T, which is commonly found in both Asians and Caucasians, was lower than that by WT-OCTN1, although uptake of the typical OCTN1 substrate ergothioneine was similar. CONCLUSION: Organic cation transporter 1 variant L503F transports biguanides, especially metformin, more efficiently than WT-OCTN1, whereas the I306T variant transports gabapentin less efficiently than WT-OCTN1, suggesting that the common OCTN1 variants may alter pharmacokinetics of these drugs.

Therapeutic inhibition of mitochondrial function induces cell death in starvation-resistant renal cell carcinomas.

Renal cell carcinomas (RCC) have two types of cells for carbon metabolism and for cell signaling under nutrient-deprivation conditions, namely starvation-resistant and starvation-sensitive cells. Here, we evaluated the mitochondrial characteristics of these cell types and found that the resistant type possessed higher activities for both mitochondrial oxidative phosphorylation and glycolysis than the sensitive types. These higher activities were supported by the stored carbon, lipid and carbohydrate sources, and by a low level of mitochondrial reactive oxygen species (ROS) due to sustained SOD2 expression in the resistant RCC cells. In metastatic RCC cases, higher SOD2 expression was associated with a significantly shorter survival period. We found that treatment with the drugs etomoxir and buformin significantly reduced mitochondrial oxidative phosphorylation and induced cell death under glucose-deprivation conditions in starvation-resistant RCC cells. Our data suggest that inhibitory targeting of mitochondria might offer an effective therapeutic option for metastatic RCC that is resistant to current treatments.

Effects of sulfonylureas on membrane-bound low Km cyclic AMP phosphodiesterase in rat fat cells.

The effects of sulfonylureas and a biguanide on membrane-bound low Km cyclic AMP phosphodiesterase and lipolysis were examined in rat fat cells. Pharmacologically active sulfonylureas, such as tolbutamide (10 mM), acetohexamide (10 mM) and glibenclamide (200 microM) activated the phosphodiesterase when incubated with fat cells and suppressed lipolysis induced by isoproterenol. However, neither of these actions was observed in the presence of a pharmacologically inactive sulfonylurea, carboxytolbutamide (10 mM) and a biguanide, buformin (500 microM). Tolbutamide (0.5-10 mM) activated the enzyme, concentration dependently, and this manner of activation appears to coincide with that of the suppressive effect on the lipolysis. The time course of the enzyme activation was similar to that seen with insulin. Km, optimal pH and sensitivity to temperature of the enzyme from tolbutamide-treated cells were the same as those of the enzyme from control and insulin-treated cells. Direct incubation of the enzyme from control cells with tolbutamide did not affect the activity, while as little as 10 microM 3-isobutyl-1-methylxanthine markedly inhibited the enzyme. Tolbutamide continued to activate the enzyme in cells in which insulin receptor had been destroyed by trypsin-pretreatment. These results are compatible with the idea that the enzyme activated by sulfonylurea and that activated by insulin may be the same species of phosphodiesterase and that the antilipolytic action of sulfonylurea may be mediated by the activation of the enzyme which does not occur through the insulin receptor.

Effect of buformin and metformin on formation of advanced glycation end products by methylglyoxal.

BACKGROUND: The formation and accumulation of advanced glycation end products (AGE) in various tissues are known to be involved in the aging process and complications of long-term diabetes. Aminoguanidine as AGE inhibitors was first studied, and metformin as biguanide compounds have been reported to react with reactive dicarbonyl precursors such as methylglyoxal. METHODS: We studied the effects of the biguanides of buformin and metformin on AGE formation by the methods of specific fluorescence, and enzyme-linked immunosorbent assay and a Western blot analysis using the anti-AGE antibody after incubating BSA or RNase with methylglyoxal. RESULTS: Buformin is a more potent inhibitor of AGE formation than metformin, and suggests that the amino group of buformin trap the carbonyl group of methylglyoxal to suppress formation of AGE. CONCLUSION: In addition to that of metformin, buformin may be clinically useful to prevent diabetic complications.

[A case of lactic acidosis caused by buformin in an oldest elderly diabetic patient].

A 93-year-old male was urgently admitted to our hospital with dyspnea and disturbance of consciousness. The patient had been visiting a general physician regularly for ten years, for treatment of type 2 diabetes. He had been treated with glibenclamide and voglibose, until voglibose was replaced with buformin 3 months before admission. During pre-admission treatment, his HbA1c was 10-12% and serum Cr level was around 2mg/dL, but insulin therapy had never been considered because of "being too old". The patient had started taking furosemide one year before admission, because of edema of the lower legs, and also spironolactone two months before admission. Anorexia had continued for one month before admission on May 29, 2003. On admission, his laboratory data were; blood glucose 87mg/dL, HbA1c 12.5%, BUN 75mg/dL, Cr 3.9mg/dL, lactate 253.1 mg/dL, and blood gas analysis; pH 6.97, anion gap 45.3mmol/L breathing room air, suggesting marked lactic acidosis with renal failure. Intensive care with bicarbonate and fluid therapy was successful, and his glycemic control improved markedly with insulin. On the other hand, his activity of daily living (ADL) severely deteriorated while in hospital Home follow-up was therefore not indicated, and he had to change a hospital for further follow-up. This case report gives rise to the question of how we should manage diabetes in the oldest elderly, including the use of insulin and biguanides. In addition, complications of biguanides in the elderly are reviewed.

Effects of metformin, buformin, and phenformin on the post-initiation stage of chemically induced mammary carcinogenesis in the rat.

Metformin is a widely prescribed drug for the treatment of type II diabetes. Although epidemiologic data have provided a strong rationale for investigating the potential of this biguanide for use in cancer prevention and control, uncertainty exists whether metformin should be expected to have an impact in nondiabetic patients. Furthermore, little attention has been given to the possibility that other biguanides may have anticancer activity. In this study, the effects of clinically relevant doses of metformin (9.3 mmol/kg diet), buformin (7.6 mmol/kg diet), and phenformin (5.0 mmol/kg diet) were compared with rats fed control diet (AIN93-G) during the post-initiation stage of 1-methyl-1-nitrosourea-induced (50 mg/kg body weight) mammary carcinogenesis (n = 30/group). Plasma, liver, skeletal muscle, visceral fat, mammary gland, and mammary carcinoma concentrations of the biguanides were determined. In comparison with the control group, buformin decreased cancer incidence, multiplicity, and burden, whereas metformin and phenformin had no statistically significant effect on the carcinogenic process relative to the control group. Buformin did not alter fasting plasma glucose or insulin. Within mammary carcinomas, evidence was obtained that buformin treatment perturbed signaling pathways related to energy sensing. However, further investigation is needed to determine the relative contributions of host systemic and cell autonomous mechanisms to the anticancer activity of biguanides such as buformin.

Plasma BNP levels in the treatment of type 2 diabetes with pioglitazone.

We monitored the change in plasma ANP and BNP levels (as markers for left ventricular dysfunction (LVD)) in DM2 patients treated with pioglitazone (Pio) for 4 weeks. Thirty DM2 patients with no sign of heart failure were treated with Pio (15 mg/day), and their plasma ANP (normal levels

Treatment of reactive hypoglycemia with buformin.

The therapeutic effect of short-term buformin (l-butylbiguanide) treatment was investigated in 12 patients with reactive hypoglycemia. Eleven of them were classified as having idiopathic reactive hypoglycemia, nine obese and two nonobese. None of these patients had a degree of hyperglycemia during glucose tolerance tests which would indicate diabetes mellitus. In one patient reactive hypoglycemia was related to chemical diabetes. The diagnosis of reactive hypoglycemia was established on the basis of patient's hypoglycemic reaction and low blood glucose levels during 6-hour oral glucose tolerance tests. The patient's received 200 mg of buformin daily for 7 days and its therapeutic effectiveness was assessed by repeat testing. Buformin treatment resulted in significant increase of blood glucose values between 180 and 360 min after oral glucose challenge and in considerable improvement of hypoglycemia in nine obese patients with idiopathic reactive hypoglycemia and in the patient with chemical diabetes. Buformin also significantly reduced maximal insulin response and incremental insulin areas. In two nonobese patients hypoglycemic reaction was deteriorated after buformin therapy.

Stimulation of the intracellular portion of the human insulin receptor by the antidiabetic drug metformin.

Our prior work suggested that the antidiabetic metformin must enter the cell to act and that the drug stimulates tyrosine kinase activity. We now report that therapeutic concentrations (approximately 1 microg/mL) of metformin stimulated the tyrosine kinase activity of the intracellular portion of the beta-subunit of the human insulin receptor (IPbetaIRK), the intracellular portion of the epidermal growth factor receptor and pp60-src, but not cAMP-dependent protein kinase. A derivative of metformin unable to lower glucose was ineffective in stimulating IPbetaIRK. Two derivatives more effective than metformin in patients were also more effective than metformin in stimulating IPbetaIRK. Higher levels (10-100 microg/mL) of metformin or methylglyoxyl bis(guanylhydrazone) inhibited the tyrosine kinases, and this inhibition may be responsible for the ability of these two drugs to block cell proliferation.

Effect of buformin on splanchnic carbohydrate and substrate metabolism in healthy man.

The effect of buformin (100 mg b.i.d. for 5 days) on carbohydrate metabolism, both splanchnic glucose output (SGO) and net substrate exchange were studied in 6 healthy male volunteers in the basal state and following glucose ingestion (100 g). Control studies without buformin were also performed in 5 men. Splanchnic glucose and substrate exchange was determined by means of the hepatic venous catheter technique. SGO was 154 +/- 18 (SEM) mg/min in the postabsorptive state and increased 33.3 +/- 2.8 g above the basal level during the 150 min period following glucose ingestion. Buformin administration did not alter basal SGO (157 +/- 26 mg/min), nor the splanchnic exchange of pyruvate, alanine, glycerol, OH-butyrate and acetoacetate. Splanchnic lactate balance was altered by buformin and net lactate output occurred. Following glucose ingestion the rise in splanchnic lactate output was increased, whereas no change in SGO (32.9 +/- 3.5 g/150 min) and splanchnic exchange of the other substrates was observed. The increase in arterial blood glucose concentration following oral glucose loading was reduced by buformin pretreatment (p less than 0.005). The insulin production rate (basal, 16 +/- 2 mU/min; following oral glucose, 13 +/- 2 U/150 min) as calculated from C-peptide release from the splanchnic area was unchanged by buformin. Except for a marked rise in splanchnic lactate production, buformin did not alter splanchnic carbohydrate metabolism after orally ingested glucose in healthy man. The diminished increase in arterial blood glucose concentration associated with unaltered insulin production suggests that buformin facilitates glucose utilization by peripheral tissues.