Histochem Cell Biol. 2015 Mar;143(3):313-24.

Over-expression of muscle glycogen synthase in human diabetic nephropathy

Rodrigo Gatica, Romina Bertinat, Pamela Silva, Pamela Kairath, Felipe Slebe, Fabián Pardo, María J. Ramírez, Juan C. Slebe, José M. Campistol, Francisco Nualart, Carme Caelles, Alejandro J. Yanez.

Facultad de Medicina, Universidad San Sebastián, Puerto Montt, Chile.

 

Abstract

Diabetic nephropathy (DN) is a major complication of diabetic patients and the leading cause of end-stage renal disease. Glomerular dysfunction plays a critical role in DN, but deterioration of renal function also correlates with tubular alterations. Human DN is characterized by glycogen accumulation in tubules. Although this pathological feature has long been recognized, little information exists about the triggering mechanism. In this study, we detected over-expression of muscle glycogen synthase (MGS) in diabetic human kidney. This enhanced expression suggests the participation of MGS in renal metabolic changes associated with diabetes. HK2 human renal cell line exhibited an intrinsic ability to synthesize glycogen, which was enhanced after over-expression of protein targeting to glycogen. A correlation between increased glycogen amount and cell death was observed. Based on a previous transcriptome study on human diabetic kidney disease, significant differences in the expression of genes involved in glycogen metabolism were analyzed. We propose that glucose, but not insulin, is the main modulator of MGS activity in HK2 cells, suggesting that blood glucose control is the best approach to modulate renal glycogen induced damage during long-term diabetes.

PMID: 25371328

 

SUPPLEMENT:

Diabetic nephropathy (DN) is one of the major complications of diabetic patients and is the leading cause of end-stage renal disease, requiring dialysis or kidney transplant for patient survival. DN affects one-third of patients with either type 1 (insulin deficient) or type 2 (insulin resistant) diabetes mellitus, which together they account for 382 million people worldwide. Chronic elevation of blood glucose levels is the major consequence of diabetes, and also the main cause of long-term diabetic problems, such as DN. At this regard, tight glycemic control is widely recognized as the best method to delay diabetic progression and complications; but diabetes mellitus still remains a major health concern for humans despite lots of different anti-diabetic drugs are commercially available. Furthermore, several anti-diabetic drugs cannot be used in patients with renal problems, reducing the number of effective agents to treat DN. Hence, understanding DN at the molecular level will allow the development of new anti-diabetic drugs.

AY-Fig1

 

Glucose is a vital source for energy in mammals, and its uptake can be insulin dependent or independent. Glycogen is a polymerized version of glucose, which serves as an energy store. Liver and muscle express insulin-dependent glucose transporters, and hence glucose uptake increases in conditions of insulin increase, such as feeding. Glycogen synthesis increases as well (Figure 1). On the contrary, glucose uptake and glycogen synthesis decrease during insulinopenic states, such as fasting and diabetes (Figure 1). However, this is not the case for the kidney, where glucose uptake is mainly insulin-independent and glycogen synthesis behaves oppositely to that of liver and muscle (Figure 1). Even more, renal glycogen accumulation has long been recognized as the first pathological alteration related to the effects of diabetes on human kidney, but the triggering mechanisms are not yet understood.

AY-Fig2

 

On this basis, we decided to start the study of the mechanisms of renal glycogen synthesis and we aimed to identify the isoform of glycogen synthase (GS), i.e. the rate-limiting enzyme in glycogen synthesis, which is responsible for glycogen metabolism in human kidney. We demonstrated that, similarly to murine models, the muscle isoform of GS (MGS) is expressed in human kidney, but more importantly, we demonstrated over-expression of this enzyme (Figure 2) that could explain the enhanced glycogen accumulation in the human diabetic kidney [1]. Moreover, significant differences in the expression of genes involved in glycogen metabolism were also detected in human diabetic kidney, such as down-regulation of glycogen phosphorylase (involved in glycogen degradation), and up-regulation of glycogenin (precursor molecule for glycogen synthesis) and PTG (involved in improvement of GS activity), supporting overall increase in glycogen synthesis [1] [2]. A parallel study by Stapleton et al. [3] showed that enhanced activity of GS, reduced activity of glycogen phosphorylase and over-expression of glycogenin are also involved in enhanced renal glycogen production by diabetic rats, suggesting that similar pathological mechanisms are operating in the diabetic kidney from human and murine animal models.

To date, the study of abnormal glycogen accumulation in the diabetic kidney has been almost exclusively addressed in murine models. Hence, the importance of this study is that we have identified a piece of the mechanism that might be involved in abnormal glycogen accumulation in the human diabetic kidney, which is the over-expression of the enzyme that synthesizes glycogen. We have previously reported differences in insulin signaling between human and rat kidney [4], so this study was a logical extension to understand any further differences (or similarities) in glycogen metabolism between species. As we have mentioned, the accumulation of glycogen in the human diabetic kidney has long been associated to a pathological state, but since we don’t have any idea about the triggering mechanisms, we are not able to correct it yet. The general hypothesis is that glycogen accumulation in the diabetic kidney is insulin-independent, and that it is secondary to increased blood and urine glucose levels. However, the fact that glycogen synthesis is activated in the kidney, whereas at the same time is inactivated in liver and muscle during diabetes, strongly suggests that glycogen metabolism is differently regulated between these tissues and, more importantly, it fulfills different functions.

 

References:

[1] Gatica R, Bertinat R, Silva P, Kairath P, Slebe F, Pardo F, Ramírez MJ, Slebe JC, Campistol JM, Nualart F, Caelles C, Yáñez AJ. Over-expression of muscle glycogen synthase in human diabetic nephropathy. Histochem Cell Biol. 2015; 143(3):313-24. doi: 10.1007/s00418-014-1290-2.

[2] Woroniecka KI, Park AS, Mohtat D, Thomas DB, Pullman JM, Susztak K. Transcriptome analysis of human diabetic kidney disease. Diabetes 2011; 60(9):2354-2369. doi: 10.2337/db10-1181

[3] Lau X, Zhang Y, Kelly DJ, Stapleton DI. Attenuation of Armanni-Ebstein lesions in a rat model of diabetes by new anti-fibrotic, anti-inflammatory agent, FT011. Diabetologia. 2013; 56(3):675-9. doi: 10.1007/s00125-012-2805-9.

[4] Gatica R, Bertinat R, Silva P, Carpio D, Ramírez MJ, Slebe JC, San Martín R, Nualart F, Campistol JM, Caelles C, Yáñez AJ. Altered expression and localization of insulin receptor in proximal tubule cells from human and rat diabetic kidney. J Cell Biochem. 2013; 114(3):639-49. doi: 10.1002/jcb.24406.

 

 

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