Shedding of growth-suppressive gangliosides from glomerular mesangial cells undergoing apoptosis

Glucose fluctuation increased mesangial cell apoptosis related to AKT signal pathway

INTRODUCTION

Blood glucose fluctuation is an important factor for the development of diabetic complications. Glucose fluctuation aggravated the renal injury in diabetic nephropathy. In the present study, our aim was to investigate the effects of blood glucose fluctuation on the glomerular mesangal cells and its related mechanism.

MATERIAL AND METHODS

Mesangial cells were divided into four groups: the normal glucose group (NG) cells were incubated in normal glucose conditions (5.6 mmol/l); the high glucose group (HG) cells were treated with 25 mmol/l; the glucose fluctuation (FG) group received 5.6 mmol/l and 25 mmol/l glucose repeated 3 times; the mannitol group (MG) received 5.6 mmol/l glucose plus 24.4 mmol/l mannitol as a control. Cell viability and apoptosis were detected, reactive oxygen species (ROS) level, superoxide dismutase (SOD) activity and malonaldehyde (MDA) levels were measured. Phosphorylated ser/thr protein kinase (P-AKT, phosphor-Ser473), phosphorylated glycogen synthase kinase-3β (P-GSK-3β, phosphor-Ser9) and cleaved cysteinyl aspartate-specific proteinase-3 (cleaved caspase-3) levels were assessed using western blot.

RESULTS

Data suggested that mesangial cells in the FG group show higher cell viability in 12 h, and lower cell viability from 48 h. The FG group showed cell apoptosis accompanied by a significant MDA level increase and SOD activity decrease in 48 h. More importantly, glucose fluctuation could aggravate oxidative stress in glomerular mesangial cells. Furthermore, the P-AKT level was lower, and increased P-GSK-3β and cleaved caspase-3 levels were higher in the FG group than in the HG group.

CONCLUSIONS

Glucose fluctuation aggravates mesangial cell apoptosis, which may be partly induced by activating oxidative stress and inhibiting the AKT signaling pathway.

Renal gangliosides are involved in lead intoxication

The biological effects of lead are well defined; however, neither the risk exposure level nor the subcellular mechanism of its action is completely clear. The present work was undertaken to investigate the effects of low level and long term lead exposure on the composition and expression of rat renal gangliosides. In order to identify ganglioside expression, frozen sections of kidneys were stained with monoclonal antibodies GMB16 (GM1 specific), GM28 (GM2 specific), AMR-10 (GM4 specific) and CDW 60 (9-O-Ac-GD3 specific). Strong reactivity was observed for GMB28, AMR-10 and CDW 60, while GMB16 developed only weak labelling in treated kidney compared with the control. The alterations in the expression of renal gangliosides observed by immunohistochemistry were accompanied by quantitative and qualitative changes in the thin layer chromatography of total gangliosides isolated from kidney tissues. Lead treatment produced a significant increase in 9-O-Ac GD3, a ganglioside involved in apoptotic processes. In agreement with this result, a significant decrease in the number of apoptotic glomerular cells was observed with the TUNEL assay. These findings lead us to suggest that alterations in renal gangliosides produced by low level lead exposure are associated with the apoptotic processes that take place in the kidney. These findings provide evidence that low level and long term lead exposure produces renal ganglioside alterations with urinary microalbumin excretion. The results suggest that lead levels within the limits of biological tolerance already cause molecular renal damage without clinical signs of toxicity.

Gangliosides as apoptotic signals in ER stress response

Renewed attention has been given lately to gangliosides and to their function as intracellular messengers of the adaptive responses to stress. Gangliosides are vital components of cell membranes; therefore, deleterious consequences can result from changes in their chemical composition and concentration, that is, membrane dynamics and structure can be altered as can the behavior of other membrane proteins. The importance of gangliosides in human health is evident in neurodegenerative diseases associated with defects in their degradation. As key modulators of intracellular calcium flux, gangliosides are involved in cellular processes downstream of calcium signaling. In this review, we focus on the effect of ganglioside accumulation on the endoplasmic reticulum calcium homeostasis and on the integrity of the mitochondrial membranes. We discuss how these events elicit an apoptotic program that ultimately leads to cell death. Owing to interorganelle crosstalk, these events are not necessarily self-contained, and gangliosides may serve as the common factor.

Shedding and uptake of gangliosides and glycosylphosphatidylinositol-anchored proteins

Gangliosides and glycosylphosphatidylinositol (GPI)-anchored proteins have very different biosynthetic origin, but they have one thing in common: they are both comprised of a relatively large hydrophilic moiety tethered to a membrane by a relatively small lipid tail. Both gangliosides and GPI-anchored proteins can be actively shed from the membrane of one cell and taken up by other cells by insertion of their lipid anchors into the cell membrane. The process of shedding and uptake of gangliosides and GPI-anchored proteins has been independently discovered in several disciplines during the last few decades, but these discoveries were largely ignored by people working in other areas of science. By bringing together results from these, sometimes very distant disciplines, in this review, we give an overview of current knowledge about shedding and uptake of gangliosides and GPI-anchored proteins. Tumor cells and some pathogens apparently misuse this process for their own advantage, but its real physiological functions remain to be discovered.

Modulation of cell proliferation and hypertrophy by gangliosides in cultured human glomerular mesangial cells

Glomerular mesangial cells (GMCs) in diverse renal diseases undergo cell proliferation and/or hypertrophy, and gangliosides have been reported to play an important role in modulating cell structure and function. This study compared the effects of transforming growth factor-beta1 (TGF-beta1) and the effects of the application of exogenous gangliosides on GMCs and investigated whether the application of exogenous gangliosides regulated cellular proliferation and hypertrophy. Human GMCs were cultured with exogenous gangliosides and TGF-beta1 in a media containing 10% fetal bovine serum and in a media without the fetal bovine serum. Exogenous gangliosides biphasically changed the proliferation of human GMCs (0.1-1.0 mg/mL). A low concentration (0.1 mg/mL) of gangliosides mainly increased the number of human GMCs, whereas cellular proliferation was significantly reduced by raising the concentration of exogenous gangliosides. TGF-beta1 greatly reduced the number of human GMCs in a concentration-dependent manner (1-10 ng/mL). Serum deprivation accelerated the gangliosides- and TGF-beta1-induced inhibition of mesangial cell proliferation to a greater extent. Gangliosides (1.0 mg/ mL) and TGF-beta1 (10 ng/mL) both caused a significant increase in the incorporation of [3H]leucine per cell in the serum-deprived condition, whereas it was completely reversed in serum-supplemented condition. Similar results to the [3H]leucine incorporation were also observed in the changes in cell size measured by flow cytometric analysis. These results show that exogenous gangliosides modulate cell proliferation and hypertrophy in cultured human GMCs, and these cellular responses were regulated differently based on whether the media contained serum or not. Results from the present study raise new possibilities about the potential involvement of gangliosides in the development of mesangial cell proliferation and hypertrophy.

Mechanism for the negative regulation of cell proliferation by ganglioside GM3 in high glucose-treated glomerular mesangial cells

We recently identified ganglioside GM3 as a modulator of glomerular hypertrophy in streptozotocin-induced diabetic rats (Life Sci., 72: 1997-2006, 2003). This study examined whether alteration of ganglioside GM3 expression could modulate the high glucose-induced proliferation of glomerular mesangial cells (GMCs). GMCs isolated from rat kidneys were cultured under normal (5.6 mM) or high (25 mM) glucose condition for 24-72 hrs. Cell proliferation was predominantly stimulated when GMCs were cultured with high glucose as well as 20 microM of d-threo-PDMP, an inhibitor of ganglioside biosynthesis, for 24 hrs, whereas raising ambient glucose significantly reduced the mesangial sialic acid contents. Based upon mobility on high-performance thin-layer chromatography (HPTLC), GMCs showed a complex pattern of ganglioside expression that consisted of three major components of gangliosides, mainly GM3. High glucose induced a significant reduction of ganglioside expression with apparent changes in the composition of major ganglioside GM3, and semi-quantitative analysis by HPTLC showed that ganglioside GM3 was reduced to 62% of GMCs cultured under normal glucose condition. A prominent immunofluorescence microscopy using anti-GM3 monoclonal antibody also showed a dramatic disappearance of immunoreactivity in high glucose-treated GMCs. Moreover, high glucose significantly lowered the Km values of GM3 synthase (16 microM vs. 49 microM), but did not change the Vmax. These results provide the pathophysiological relationship between the high glucose-induced proliferation of GMCs and the decreased expression of ganglioside GM3, indicating a mechanism for the negative regulation of mesangial proliferation by ganglioside GM3. This mechanism may play an important role in the development of diabetic glomerulopathy.