Cadmium increases GLUT1 substrate binding affinity in vitro while reducing its cytochalasin B binding affinity

Negatively interactive effect of chromium and cadmium on obesity: Evidence from adults living near ferrochromium factory

BACKGROUND

Researchers have reported that chromium (Cr) exposure may be associated with metabolism of glucose and lipids in residents living in a long-term Cr polluted area. Previous statistical analysis is mainly focused on individual chromium exposure. Furtherly, we aim to investigated the independent, combined, and interaction effects of the co-exposure of urine Cr (UCr) with cadmium (UCd), lead (UPb) and manganese (UMn) on body mass index (BMI), waist circumference, and the risk of overweight and abdominal obesity.

METHOD

We enrolled 1187 participants from annual surveys between 2017 and 2019. Heavy metal concentrations in urine were standardized using covariate-adjusted urine creatinine levels. Multiple linear/logistic regression models were applied to measure the single effect of urine heavy metal concentration on the outcomes. The quantile-based g-computation (g-comp) model was used to evaluate the combined effect of metal mixture on the outcomes and to compare the contribution of each metal. Both additive and multiplicative interactions were measured for UCr with UCd, UPb, UMn on the outcomes. Analysis was performed on the overall population and stratified by smoking habit.

RESULTS

For the overall study population, UCr was positively associated with BMI (p trend = 0.023) and waist circumference (p trend = 0.018). For smoking participants, the g-comp model demonstrated that the metal mixture was negatively associated with BMI, with UCr and UCd contributing the most in the positive and negative direction. A negative additive interaction was observed between UCr and UCd on BMI and abdominal obesity. We did not observe a significant interaction effect of UCr with UPb or UMn.

CONCLUSION

Our study indicated that Cr and Cd exposure may be associated with BMI and waist circumference, with combined and interaction effects of the heavy metals noted. Further epidemiological and experimental researches could simultaneously consider single and complex mixed exposure to verify the findings and biological mechanisms.

The endocrine disruptor cadmium: a new player in the pathophysiology of metabolic diseases

Cadmium (Cd), a highly toxic heavy metal, is found in soil, environment and contaminated water and food. Moreover, Cd is used in various industrial activities, such as electroplating, batteries production, fertilizers, while an important non-occupational source is represented by cigarette smoking, as Cd deposits in tobacco leaves. Since many years it is clear a strong correlation between Cd body accumulation and incidence of many diseases. Indeed, acute exposure to Cd can cause inflammation and affect many organs such as kidneys and liver. Furthermore, the attention has focused on its activity as environmental pollutant and endocrine disruptor able to interfere with metabolic and energy balance of living beings. Both in vitro and in vivo experiments have demonstrated that the Cd-exposure is related to metabolic diseases such as obesity, diabetes and osteoporosis even if human studies are still controversial. Recent data show that Cd-exposure is associated with atherosclerosis, hypertension and endothelial damage that are responsible for cardiovascular diseases. Due to the large environmental diffusion of Cd, in this review, we summarize the current knowledge concerning the role of Cd in the incidence of metabolic and cardiovascular diseases.

The role of cadmium in obesity and diabetes

Multiple studies have shown an association between environmental exposure to hazardous chemicals including toxic metals and obesity, diabetes, and metabolic syndrome. At the same time, the existing data on the impact of cadmium exposure on obesity and diabetes are contradictory. Therefore, the aim of the present work was to review the impact of cadmium exposure and status on the risk and potential etiologic mechanisms of obesity and diabetes. In addition, since an effect of cadmium exposure on incidence of diabetes mellitus and insulin resistance was suggested by several epidemiologic studies, we carried out a meta-analysis of all studies assessing risk of prevalence and incidence of diabetes. By comparing the highest versus the lowest cadmium exposure category, we found a high risk of diabetes incidence (odds ratio=1.38, 95% confidence interval 1.12-1.71), which was higher for studies using urine as exposure assessment. On the converse, results of epidemiologic studies linking cadmium exposure and overweight or obesity are far less consistent and even conflicting, also depending on differences in exposure levels and the specific marker of exposure (blood, urine, hair, nails). In turn, laboratory studies demonstrated that cadmium adversely affects adipose tissue physiopathology through several mechanisms, thus contributing to increased insulin resistance and enhancing diabetes. However, intimate biological mechanisms linking Cd exposure with obesity and diabetes are still to be adequately investigated.

Cadmium induces impaired glucose tolerance in rat by down-regulating GLUT4 expression in adipocytes

Cadmium (Cd) has been known to cause hyperglycemia with diabetes-related complications in experimental animals; however, the molecular basis underlying this Cd-induced hyperglycemia is not known. Here, we report the novel finding that the impaired glucose tolerance (IGT) in rats induced by CdCl(2) is accompanied by a drastic (by as much as 90%) and dose-dependent reduction in GLUT4 protein and GLUT4 mRNA levels in adipocytes. The effect was specific to GLUT4; neither GLUT1 nor insulin-responsive aminopeptidase in adipocytes was affected. GLUT2 in hepatocytes was also not affected. Interestingly, the effect on GLUT4 was also specific to adipocytes; the muscle tissues of the Cd-treated rats showed only a slight (<25%) reduction in GLUT4 protein level with no change in GLUT4 message level, and again with no change in GLUT1 protein and its message levels. Although the insulin-induced GLUT4 translocation in adipocytes was not affected by the Cd treatment, the 3-O-methy-D-glucose flux in insulin-stimulated adipocytes of Cd-treated rat was drastically reduced. Together these findings clearly demonstrate that Cd induces IGT in rats by selectively down-regulating GLUT4 expression in adipocytes.

Adenosine and adenosine triphosphate modulate the substrate binding affinity of glucose transporter GLUT1 in vitro

Evidence indicates that a large portion of the facilitative glucose transporter isoform GLUT1 in certain animal cells is kept inactive and activated in response to acute metabolic stresses. A reversible interaction of a certain inhibitor molecule with GLUT1 protein has been implicated in this process. In an effort to identify this putative GLUT1 inhibitor molecule, we studied here the effects of adenosine and adenosine triphosphate (ATP) on the binding of D-glucose to GLUT1 by assessing their abilities to displace cytochalasin B (CB), using purified GLUT1 in vesicles. At pH 7.4, adenosine competitively inhibited CB binding to GLUT1 and also reduced the substrate binding affinity by more than an order of magnitude, both with an apparent dissociation constant (K(D)) of 3.0 mM. ATP had no effect on CB and D-glucose binding to GLUT1, but reduced adenosine binding affinity to GLUT1 by 2-fold with a K(D) of 30 mM. At pH 3.6, however, ATP inhibited the CB binding nearly competitively, and increased the substrate binding affinity by 4--5-fold, both with an apparent K(D) of 1.22 mM. These findings clearly demonstrate that adenosine and ATP interact with GLUT1 in vitro and modulate its substrate binding affinity. They also suggest that adenosine and ATP may regulate GLUT1 intrinsic activity in certain cells where adenosine reduces the substrate-binding affinity while ATP increases the substrate-binding affinity by interfering with the adenosine effect and/or by enhancing the substrate-binding affinity at an acidic compartment.

The role of the GLUT 4 transporter in regulating rat myoblast glucose transport processes

Previous studies revealed an inverse relationship between GLUT 1 and GLUT 4 expression in rat myoblasts [L. Xia, Z. Lu, T.C.Y. Lo, J. Biol. Chem., 268 (1993) 23258-23266]. It was not clear whether these were coincidental or causal occurrences. To examine the regulatory roles of the GLUT 4 isoform, rat L6 myoblasts were transfected with full length GLUT 4 cDNAs (2.5 kb) in the sense or antisense orientation. L6 myoblasts transfected with the GLUT 4 sense cDNA (L6/G4S transfectants) possessed much elevated levels of both endogenous GLUT 4 transcripts (1.4 kb and 2.8 kb). Transport and immunofluorescence studies showed that this GLUT 4 sense cDNA was responsible for a functional GLUT 4 transporter. L6 cells transfected with the GLUT 4 antisense cDNA (L6/G4A transfectants) possessed only 6% of the L6 level in day 6 cultures. These antisense transfectants were essentially devoid of any functional GLUT 4 transporter. The activation of transcription of the endogenous GLUT 4 gene in L6/G4S myoblasts suggested auto-regulation of GLUT 4 expression. GLUT 3 expression and activity were not altered in both sense and antisense GLUT 4 transfectants. More interestingly, GLUT 1 expression was reduced in L6/G4S myoblasts, whereas it was elevated in L6/G4A myoblasts. This was the first direct evidence indicating GLUT 4 might play an important role in suppressing GLUT 1 expression.

Effects of noradrenaline on the cell-surface glucose transporters in cultured brown adipocytes: novel mechanism for selective activation of GLUT1 glucose transporters

Glucose transport into rat brown adipocytes has been shown to be stimulated directly by the sympathetic neurotransmitter, noradrenaline, without a significant increase in the protein content of either GLUT1 or GLUT4 glucose transporter in the plasma membrane [Shimizu, Kielar, Minokoshi and Shimazu (1996) Biochem. J. 314, 485-490]. In the present study, we labelled the exofacial glucose-binding sites of GLUT1 and GLUT4 with a membrane-impermeant photoaffinity reagent, 2-N-[4-(1-azitrifluoroethyl)benzoyl]-[2-3H]1,3-bis- (D-mannos-4-yloxy)-2-propylamine (ATB-[3H]BMPA), to determine which isoform is responsible for the noradrenaline-induced increase in glucose transport into intact brown adipocytes in culture. Insulin stimulated the rate of hexose transport by increasing ATB-[3H]BMPA-labelled cell-surface GLUT4. In contrast, the noradrenaline-induced increase in glucose transport was not accompanied by an increased ATB-[3H]BMPA labelling of GLUT4, nor with an increased amount of GLUT4 in the plasma membrane fraction as assessed by Western blotting, indicating that noradrenaline does not promote the translocation of GLUT4. However, noradrenaline induced an increase in photoaffinity labelling of cell-surface GLUT1 without an apparent increase in the immunoreactive GLUT1 protein in the plasma membrane. This is suggestive of an increased affinity of GLUT1 for the ligand. In fact, the Ki value of non-radioactive ATB-BMPA for 2-deoxy-D-glucose uptake was significantly decreased after treatment of the cells with noradrenaline. The increased photoaffinity labelling of GLUT1 and increased glucose transport caused by noradrenaline were inhibited by a cAMP antagonist, cAMP-S Rp-isomer. These results demonstrate that noradrenaline stimulates glucose transport in brown adipocytes by enhancing the functional activity of GLUT1 through a cAMP-dependent mechanism.