No abstract

The activation of protein kinase C (PKC) found in diabetic glomeruli and glomerular mesangial cells cultured under high glucose conditions has been proposed to contribute to the development of diabetic nephropathy. However, the abnormalities distal to PKC have not been fully elucidated yet. Herein, we provide the evidence that mitogen-activated protein kinase (MAPK) cascade, an important kinase cascade downstream to PKC and an activator of cytosolic phospholipase A2 (cPLA2) by direct phosphorylation, is activated in glomeruli isolated from streptozotocin-induced diabetic rats. MAPK cascade was also activated in glomerular mesangial cells cultured under high glucose (27.8 mmol/l) conditions for 5 days, and the activation of MAPK cascade was inhibited by treating the cells with calphostin C, an inhibitor of PKC. Furthermore, the activities of cPLA2 also increased in cells cultured under the same conditions and this activation was inhibited by both calphostin C and PD 098059, an inhibitor of MEK (MAPK or extracellular signal-regulated kinase [ERK] kinase). These results indicate that MAPK cascade is activated in glomeruli and mesangial cells under the diabetic state possibly through the activation of PKC. Activated MAPK, in turn, may induce various functional changes of mesangial cells at least through the activation of cPLA2 and contribute to the development of diabetic nephropathy.

No abstract

To evaluate an interaction between vasoconstrictive (Ang II) and vasodilating (ANP) peptides, we examined the effect of Ang II on ANP-induced accumulation of cGMP in cultured glomerular mesangial cells. ANP rapidly increased intracellular cGMP levels, with a peak stimulation at one minute in the absence of IBMX and at ten minutes in the presence of IBMX. The ANP-induced cGMP accumulation was significantly inhibited when the cells were treated with Ang II simultaneously with ANP for one minute in the absence of IBMX. This inhibitory effect of Ang II was completely abolished by IBMX and significantly reduced in calcium-free media or by W7, but not affected by H7. Similar inhibitory effect was observed when cells were treated with A23187 but not with TPA for one minute. In the presence of IBMX, Ang II inhibited ANP-induced cGMP accumulation when cells were treated with Ang II for 15 minutes prior to the stimulation by ANP. This inhibition by Ang II was blocked by H7. ANP-induced increase in particulate guanylate cyclase activity was significantly reduced in the cells treated with Ang II or TPA. This reduction of enzyme activity was also prevented by H7. These results indicate that Ang II inhibits ANP-induced cGMP accumulation in cultured glomerular mesangial cells through at least two mechanisms; one is the activation of calcium-dependent, calmodulin-stimulated cyclic nucleotide phosphodiesterase in the initial phase, and the other is the inhibition of guanylate cyclase resulting from protein kinase C activation in the maintenance phase.

Summary. Type IV collagen production by cultured glomerular mesangial cells and the effect of glucose on it were evaluated in order to explore the possible contribution of mesangial cells to the accumulation of type IV collagen in mesangial matrix typically seen in diabetes. Type IV collagen was measured quantitatively by enzyme-linked immunosorbent assay. The majority of type IV collagen was secreted into culture media and secreted-type IV collagen increased with cell growth in early log phase and decreased in late log phase and after confluency. By exposing the cells to high concentrations of glucose (27.8 retool/l), both secreted-and cellassociated-type IV collagens increased significantly compared with the cells cultured under normal glucose concentrations (5.6 mmol/1) or under equivalent concentrations of mannitol, resulting in a significant increase in total type IV collagen accumulation from 32.1+6.4 (under 5.6 retool/1 glucose) to 51.0 _+ 4.6 gg/dish (mean + SD, n = 4) on day4, from 113.6+6.6 to 156.8+7.1 on day6, from 248.5 + 15.2 to 310.0 + 12.6 on day 8 and from 372.4 + 14.8 to 507.9 + 17.2 on day 12. These results indicate the importance of glucose-induced alteration of mesangial cell function in the development of diabetic mesangial expansion.