Potentiation of glucose-mediated glomerular injury by mechanical strain

Mechanical stress and glucose concentration modulate glucose transport in cultured rat podocytes

BACKGROUND

Recent studies show that mechanical stress modifies both morphology and protein expression in podocytes. Ambient glucose is another factor modulating protein synthesis in these cells. In diabetes, podocytes experience elevated glucose concentrations as well as mechanical strain generated by high intracapillary pressures. Both these factors are responsible for podocyte injury, leading to impairment of kidney glomerular function. In the present study, we examined the effects of glucose concentration and mechanical stress on glucose uptake in podocytes.

METHODS

Following a 24 h pre-incubation in low (2.5 mM, LG), normal (5.6 mM, NG) or high (30 mM, HG) glucose media, cultured rat podocytes were exposed to 4 h mechanical stress. We used the labelled glucose analogue, [3H]2-deoxy-D-glucose, to measure glucose uptake. The distribution of facilitative glucose transporters GLUT2 and GLUT4 was assessed by flow cytometry.

RESULTS

In the control (static) cells, glucose uptake was similar in the three glucose groups. In mechanically stressed podocytes, glucose uptake increased 2-fold in the LG and NG groups but increased 3-fold in the HG group. In the NG cells, mechanical load increased the membrane expression of GLUT2 and reduced the membrane-bound GLUT4. In stretched HG cells, the membrane expression of both GLUT2 and GLUT4 was decreased. High glucose decreased the plasma membrane GLUT2 content in the stretched cells, whereas both static and stretched podocytes showed an elevation in GLUT4.

CONCLUSION

Mechanical stress potentiated glucose uptake in podocytes and this effect was enhanced by high ambient glucose. The decreased expression of GLUT2 and GLUT4 on the surface of stretched cells suggests that the activity of other glucose transporters may be regulated by mechanical stress in podocytes.

C-type natriuretic peptide as a podocyte hormone and modulation of its cGMP production by glucose and mechanical stress

BACKGROUND

High glucose and mechanical strain resulting from capillary hypertension are relevant risk factors affecting glomerular cells in diabetes. Altered activity of the natriuretic peptide (NP) system acting via vasorelaxing cyclic guanosine 5' monophosphate (cGMP) has been proposed to be one of the reasons for diabetes-dependent impairment of kidney function. Podocytes possess the NP receptors (NPRs) coupled to particular guanylyl cyclase. We investigated whether mechanical stress and high ambient glucose influence cGMP generation in podocytes stimulated with NPs. Additionally, the C-type natriuretic peptide (CNP) system has been characterized in these cells.

METHODS

Conditionally immortalized mouse podocytes were stimulated with NP for 15 minutes and cGMP was determined by enzymatic immunoassay. The mRNA expression for CNP and CNP-specific NPR-B was confirmed by reverse transcriptase-polymerase chain reaction (RT-PCR). In addition, cGMP synthesis was measured in cells exposed to mechanical stress and to 30 mmol/L glucose for 3 days.

RESULTS

Production of cGMP upon stimulation of the NPR-B receptor with CNP and inhibition by an antagonist HS-142-1 was dose-dependent. RT-PCR showed that podocytes express mRNA not only for NPR-B but also for CNP. Mechanical stress reduced the cGMP response by 50%, both to atrial natriuretic peptide (ANP) and to CNP. Conversely, upon high glucose, the CNP-induced production of cGMP was elevated twofold in stretched and in control cells. Furthermore, as compared to ANP, stimulation with CNP caused a larger increase in cGMP levels in stretched as well as in nonstretched cells.

CONCLUSION

Expression of CNP together with potent NPR-B receptors suggests that in podocytes, CNP may act in an autocrine and/or paracrine manner. Furthermore, in a diabetic kidney, high glucose and mechanical stress may modulate the CNP-dependent cGMP production in podocytes in an opposite manner.

Connective tissue growth factor [CTGF]/CCN2 stimulates mesangial cell migration through integrated dissolution of focal adhesion complexes and activation of cell polarization

Connective tissue growth factor [CTGF]/CCN2 is a prototypic member of the CCN family of regulatory proteins. CTGF expression is up-regulated in a number of fibrotic diseases, including diabetic nephropathy, where it is believed to act as a downstream mediator of TGF-beta function; however, the exact mechanisms whereby CTGF mediates its effects remain unclear. Here, we describe the role of CTGF in cell migration and actin disassembly in human mesangial cells, a primary target in the development of renal glomerulosclerosis. The addition of CTGF to primary mesangial cells induced cell migration and cytoskeletal rearrangement but had no effect on cell proliferation. Cytoskeletal rearrangement was associated with a loss of focal adhesions, involving tyrosine dephosphorylation of focal adhesion kinase and paxillin, increased activity of the protein tyrosine phosphatase SHP-2, with a concomitant decrease in RhoA and Rac1 activity. Conversely, Cdc42 activity was increased by CTGF. These functional responses were associated with the phosphorylation and translocation of protein kinase C-zeta to the leading edge of migrating cells. Inhibition of CTGF-induced protein kinase C-zeta activity with a myristolated PKC-zeta inhibitor prevented cell migration. Moreover, transient transfection of human mesangial cells with a PKC-zeta kinase inactive mutant (dominant negative) expression vector also led to a decrease in CTGF-induced migration compared with wild-type. Furthermore, CTGF stimulated phosphorylation and activation of GSK-3beta. These data highlight for the first time an integrated mechanism whereby CTGF regulates cell migration through facilitative actin cytoskeleton disassembly, which is mediated by dephosphorylation of focal adhesion kinase and paxillin, loss of RhoA activity, activation of Cdc42, and phosphorylation of PKC-zeta and GSK-3beta. These changes indicate that the initial stages of CTGF mediated mesangial cell migration are similar to those involved in the process of cell polarization. These findings begin to shed mechanistic light on the renal diabetic milieu, where increased CTGF expression in the glomerulus contributes to cellular dysfunction.

Rat mesangial α-endosulfine

BACKGROUND

Sulfonylurea agents exert their physiologic effects via binding to specific sulfonylurea receptors (SUR) in adenosine triphosphate-sensitive potassium (K(ATP)) channels. Mesangial cells express K(ATP) and respond to sulfonylureas by altering glucose metabolism, elevating intracellular calcium and contracting. A putative endogenous sulfonylurea, alpha-endosulfine, has been demonstrated in diverse tissues and is a member of the cyclic adenosine monophosphate (cAMP)-regulated family of phosphoproteins. This study investigates mesangial cell expression of ENSA, the gene encoding alpha-endosulfine, and its regulation by glucose.

METHODS

Expression of rat glomerular and mesangial ENSA was examined by reverse transcription-polymerase chain reaction (RT-PCR) and Northern analysis. In situ hybridization studies were carried out to investigate the presence and distribution of ENSA in kidney cortex. Expression of mesangial cell alpha-endosulfine was studied by immunoblotting, immunofluorescence, and confocal microscopy.

RESULTS

RT-PCR with gene-specific primers and Northern blotting disclosed abundant expression of two major ENSA transcripts at 2.4 kb and 1.2 kb in whole rat kidney, kidney cortex, and mesangial cells. In situ hybridization of rat kidney demonstrated renal ENSA expression, particularly within glomeruli. Confocal microscopy revealed a diffusely granular, cytosolic distribution of alpha-endosulfine. High glucose concentrations increased ENSA expression by 24 hours, an effect that persisted for at least 10 days. Protein expression paralleled gene expression.

CONCLUSION

ENSA and alpha-endosulfine are expressed in rat glomeruli and mesangial cell and gene and protein expression are up-regulated by a high glucose environment. alpha-Endosulfine has potential roles as a regulator of metabolism and contractility via mesangial cell K(ATP).