Diabetes & Metabolism Journal

Original Articles

Changes in the Amount and Function of Gi Protein in the Liver Cells of Streptozotocin-Induced Diabetic Rats.: Sun Myeong Ock, Hyun Shik Son, Oak Kee Hong, Jung Min Lee, Sung Rae Kim, Sang Ah Chang, Kun Ho Yoon, Moo Il Kang, Bong Yun Cha, Kwang Woo Lee, Ho Young Son, Sung Koo Kang; Korean Diabetes J. 2000;24(6):666-677. Published online January 1, 2001

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Abstract PDF: BACKGROUND
The functional and expressional changes of Gi proteins in diabetes have been investigated extensively, no agreement has been reached in the results. Moreover, studies using rats with different diabetic duration, and using subunits (Gialpha) of Gi proteins are lacking in literatures. Thus, we assessed the changes according to the duration of diabetes and examined the expressional changes of Gialphaand functional changes of Gi proteins in hepatocytes from streptozotocin-induced diabetic rats. METHODS: Male Sprague-Dawley rats were injected with streptozotocin to induce diabetes ; 1, 2, 3 and 5 weeks after the onset of diabetes, livers from the control and diabetic rats were fractionated into homogenate, interface, and plasma membrane. The levels of Gialpha1&2, Gialpha3 were quantified with western blots in each fraction. The functional changes of Gi proteins were evaluated by performing pertussis toxin-catalyzed ADP-ribosylation and measuring GTP S binding activity. RESULTS: 1) Gialpha2 and Gialpha3 were present mainly in the plasma membrane of hepatocytes in the diabetic and control rats, but the levels of these subunits were significantly higher in the diabetic rates than in the control rats (p<0.01). The levels of these subunits were not affected by the duration of diabetes. 2) In streptozotocin-induced diabetic rats, the levels of ADP-ribosylation of Gi proteins in liver plasma membranes decreased when pertussis toxin-catalyzed ADP-ribosylation was performed with liver tissues. However, the levels of these proteins were not affected by the duration of diabetes. 3) For the GTP S binding activity of Gi proteins in liver plasma membranes, the diabetic rats showed significantly less activity than the control rats (p<0.01). However, the activity was not affected by the duration of diabetes. The activity was somewhat restored by the insulin treatment of liver plasma membranes in diabetic rats. CONCLUSION: These results suggest that the insulin-deficient diabetic state induces the quantitative and functional changes in Gi proteins of hepatocytes regardless of the duration of diabetes. Therefore, these changes in Gi proteins may be the important compensatory reactions for the insulin resistance occurring in the insulin deficient state.

Effect of Overexpression of Gi Proteins on Insulin Actions in 3T3-L1 Adipocytes.: Hyun Shik Son, Bong Yun Cha, Sung Dae Moon, Jung Min Lee, Ok Ki Hong, Sang Ah Chang, Yu Bae Ahn, Kun Ho Yoon, Kwang Woo Lee, Ho Young Son, Sung Koo Kang; Korean Diabetes J. 2000;24(4):404-412. Published online January 1, 2001

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Abstract PDF: BACKGROUND
It has been reported that G proteins are involved in biological actions of insulin. Especially, Gi protein is more associated with insulin actions than Gs proteins. Gi protein has at least three different subtypes of Gi 1, Gi 2 and Gi 3 protein. However, it is not certain which subtypes of Gi proteins are associated with biological actions of insulin. METHODS: To investigate which subtypes of Gi proteins are associated with insulin action, we overexpressed three different kinds of Gi protein, Gi 1, Gi 2 and Gi 3 protein, in 3T3-L1 adipocytes using DNA-polylysine-adenovirus complex transfection method. After incubating for 2 hours, 3T3-L1 adipocytes were treated with 100 nM insulin for the evaluation of biological actions of insulin. Moreover, to elucidate insulin stimulated insulin receptor autophosphorylation and IRS-1 phosphorylation, 3T3-L1 adipocytes were stimulated with 100 nM insulin for 10 minutes, homogenized and immunoprecipitated with anti-phosphotyrosine antibody. RESULTS: Transfection with Gi 2 gene resulted in increment in insulin-stimulated [3H]2-deoxyglucose (DOG) uptake without affecting basal 2-DOG uptake, but not with Gi 1 and Gi 3 gene transfection. There was unchanged glycogen synthesis rate in all three Gialphasubtypes. Insulin-induced increments of insulin receptor autophos phorylation and IRS-1 phosphorylation were found in Gi 2 protein overexpressed group, only. CONCLUSION: These results suggest that Gi 2 protein may be associated with regulation of biological actions of insulin.

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