Divergent effects of arginine vasopressin and angiotensin II on proliferation and expression of the immediate early genes c-fos, c-jun and Egr-1 in cultured rat glomerular mesangial cells


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Abstract

ObjectivesThe vasoactive peptides arginine vasopressin (AVP) and angiotensin II (Angll) induce similar second messengers in cultured glomerular mesangial cells, as shown by the rise in intracellular calcium and the activation of phospholipases C and A2. In contrast, AVP is a strong mitogen for cultured rat mesangial cells while Angll is not. To elucidate the level of signal divergence, we examined the effects of AVP and Angll on the expression of the immediate early genes c-fos, c-jun and Egr-1, which have been associated with cell growth. We also tested the effect of AVP and Ang II on the induction of the ornithine decarboxylase gene and enzyme in order to examine a process that is induced in cell activation, e.g. it has been associated with the C-j phase after mitogen-receptor binding.Design and methodsCellular effects of Angll and AVP were studied in rat mesangial cells in conventional two-dimensional culture. Proliferation of the mesangial cells was measured by cell-counting and by [3H]-thymidine uptake. The presence of receptors for Angll and AVP was assessed by measurement of prostaglandin E2 by a radioimmunoassay following stimulation with vasoactive peptides and a receptor-binding assay for Angll. Messenger (m)RNA levels of c-fos, c-jun Egr-1 and ornithine decarboxylase were determined by Northern blot analysis. Ornithine decarboxylase activity was measured by an enzyme substrate assay.ResultsAVP (10˜7mol/l) stimulated [3H]-thymidine uptake by 3.7-fold after 48h and increased mesangial cell counts by 42% (P<0.05), while Angll was not mitogenic. Stimulation of resting mesangial cells with AVP (10˜9 to 10˜6mol/I) caused maximal expression of the immediate early genes after 0.5-1 h, which disappeared after 2-4 h. The relative increases were: c-fos, 15-fold; c-jun, 12-fold; Egr-1, sixfold. Angll (10˜9 to 10˜6mol/l) did not induce these genes at any time. In contrast, ornithine decarboxylase mRNA and enzyme activity were induced by both AVP and Angll.ConclusionsThe results demonstrate that AVP, but not Angll, is a strong inducer of the immediate early genes c-fos, c-jun and Egr-1 in cultured mesangial cells. We conclude that signalling pathways activated by AVP and Angll in mesangial cells diverge within 30 min after ligand-receptor binding and proximal to the transient expression of immediate early genes. While mitogenesis of cultured mesangial cells, as effected by AVP, involves the expression of immediate early genes, Angll-induced non-mitogenic changes in the mesangial cell phenotype do not. The increase in ornithine decarboxylase mRNA and enzyme activity following stimulation with both AVP and Angll indicates that its induction can occur independently of the immediate early gene expression and mitogenesis.

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