Hepatic glucokinase promoter polymorphism is associated with hepatic insulin resistance in Asian Indians.

Abstract

) were estimated from the oral glucose tolerance test. Beta cell function was estimated using %B from the Homeostasis Model Assessment and insulingenic index (dI/dG). (p=0.089) and %B (p=0.083). Furthermore, it had no independent effect on dI/dG (p=0.135). These data suggest that the G-to-A polymorphism in the hepatic GCK promoter is associated with hepatic insulin resistance in Asian Indians.

Background

) were estimated from the oral glucose tolerance test. Beta cell function was estimated using %B from the Homeostasis Model Assessment and insulingenic index (dI/dG).

Result

(p=0.089) and %B (p=0.083). Furthermore, it had no independent effect on dI/dG (p=0.135).

Conclusions

These data suggest that the G-to-A polymorphism in the hepatic GCK promoter is associated with hepatic insulin resistance in Asian Indians.

Introduction

]. Thus, the mutated GCKs do not play a key role in the pathogenesis of most forms of diabetes.

], we hypothesized that this polymorphism could be related to insulin resistance.

Results

], only those subjects with a fasting plasma glucose concentration less than 6.1 mM, interval plasma glucose concentrations less than 11.1 mM, and a two-hour plasma glucose concentration less than 7.8 mM were enrolled in the study. By eliminating factors that contribute independently to insulin resistance, such as hypertension and abnormal glucose tolerance, any effect regarding genetic influence per se becomes more apparent.

) and higher plasma insulin concentrations at fasting and 60 minutes than the GG group (p=0.003 and p=0.008, respectively).

p=0.008.

). Demographic features and glycemic parameters by genotypes).

(p=0.089). In contrast to hepatic insulin sensitivity, this polymorphism had less impact on beta cell function (9.5% and 7.5% of the variations in %B and dI/dG, respectively). Multivariate analyses showed that this polymorphism was weakly associated with %B (p=0.083), but not dI/dG (p=0.135).

Discussion

. Since all the subjects were glucose tolerant, their beta cell function will compensate for the prevailing insulin resistance to maintain plasma glucose concentration wthin a relatively narrow physiological range. The observed differences in %B and dI/dG between the two groups are most likely due to the compensatory increase of beta cell response to the differences in insulin sensitivity. This interpretation is consistent with the nature of this polymorphism, which occurs within the hepatic GCK promoter and not in the beta cell GCK promoter. Therefore, these results indicate that the polymorphism mainly affects hepatic insulin sensitivity.

for the GA/AA subjects). Initially, glucose homeostasis is maintained by the compensatory hyperinsulinemia (as observed from the higher plasma insulin concentration for the GA/AA subjects in this study) through an increase in insulin secretion by the pancreatic beta cells, which was also observed in this study (a higher %B and dI/dG for the GA/AA subjects). However, the cause-effect relationship between this polymorphism and insulin resistance remains to be elucidated.

for rat PEPCK). Human PEPCK has two copies of insulin regulatory sequences in its promoter and the transcription start site has not been mapped yet (Chiu KC, unpublished data). The underlined A in the variant human liver glucokinase is the position where the G-to-A substitution occurs. The blocked area is conserved between human and rat and also between PEPCK and glucokinase.

In conclusion, we demonstrated that the G-to-A polymorphism at the -258 nucleotide position of hepatic GCK promoter is associated with hepatic insulin resistance in normotensive and glucose tolerant subjects. To our knowledge, this is the first study, which attempts to dissect genetic influence among hepatic and whole body insulin sensitivity and beta cell function. However, to understand the molecular basis of insulin resistance of this polymorphism requires further studies.

Studied subjects

The study was approved by the Institutional Review Board and written informed consent was obtained at the entry of the study from each participant. We confirm that the study has complied with the recommendations of the Declaration of Helsinki. Asian Indians who resided in the metropolitan Los Angeles area were recruited from local Indian temples. Only normotensive subjects, who were not taking any medications, were included. Glucose tolerance was determined by an oral glucose tolerance test (OGTT) after an overnight fast. The subjects were biologically unrelated. They were instructed to fast overnight and not to take any medication before the study. Two baseline blood samples were obtained at -10 and -5 minutes before an oral glucose challenge with 75 gm glucose. Four additional blood samples were obtained at 30, 60, 90, and 120 minutes. Blood pressure was measured three times with a mercury sphygmomanometer while the subject was in the seated position. The mean of the last two measurements was used in the analysis.

] and dI/dG [(plasma insulin concentration at 30 minutes - fasting plasma insulin concentration in μU/mL) / (plasma glucose concentration at 30 minutes - fasting plasma glucose concentration in mmol/L)].

Laboratory methods

to produce a smaller fragment (154 bp).

Statistical analysis

, %B, and dI/dG. Data were presented as means (or geometric means when appropriate) with 95% confidence intervals, unless otherwise specified. Two-sided t-tests or chi-square tests were used to evaluate the differences between the two groups. To examine the influence of multiple variables on either insulin sensitivity or beta cell function, multivariate analysis was performed with a backward stepwise option. The probability to enter or to remove was set at 0.10. A nominal P value of less than 0.05 was considered significant. SYSTAT 8.0 for Windows from SPSS, Inc. (Chicago, Illinois) was used for the statistical analyses.

Tables

Table 1: Clinical characteristics of the studied subjects

Table 2: Demographic features and glycemic parameters by genotypres

Table 3: Stepwise regression analysis of the estimated indices for insulin sensitivity snd beta cell function

Acknowledgement

Author (KCC) expresses a special acknowledgement to M. Alan Permutt, M.D. of Washington University School of Medicine, in whose laboratory the idea was conceptualized and the work was initiated. We also thank George P. Tsai, Jennifer M. Ryu, Jennifer L. McGullam and Jennifer E. McCarthy for their laboratory assistance. The work was supported in parts by grants from NIH/NIDDK RO1DK52337-01 (KCC), Diabetes Action Research and Education Foundation (KCC), and American Diabetes Association (KCC).