Insulin-like Growth Factor-I Induces Renal Cell Hypertrophy via a Calcineurin-dependent Mechanism

Abstract

Insulin-like growth factor-I (IGF-I) may play an important role in the development of renal hypertrophy. In this study we determined the effect of IGF-I on cultured mesangial cells (MCs) and examined activation of key signaling pathways. IGF-I induced hypertrophy as determined by an increase in cell size and an increase in protein to DNA ratio and increased accumulation of extracellular matrix (ECM) proteins. IGF-I also activated both Erk1/Erk2 MAPK and phosphatidylinositol 3-kinase (PI3K) in MCs. Inhibition of either MAPK or PI3K, however, had no effect on IGF-I-induced hypertrophy or ECM production. Next, we examined the effect of IGF-I on activation of the calcium-dependent phosphatase calcineurin. IGF-I treatment stimulated calcineurin activity and increased the protein levels of calcineurin and the calcineurin binding protein, calmodulin. Cyclosporin A, an inhibitor of calcineurin, blocked both IGF-I-mediated hypertrophy and up-regulation of ECM. In addition, calcineurin resulted in sustained Akt activation, indicating possible cross-talk with other signaling pathways. Finally, IGF-I treatment resulted in the calcineurindependent nuclear localization of NFATc1. Therefore, IGF-I induces hypertrophy and increases ECM accumulation in MCs. IGF-I-mediated hypertrophy is associated with activation of Erk1/Erk2 MAPK and PI3K but does not require either of these pathways. Instead, IGF-I mediates hypertrophy via a calcineurin-dependent pathway.In response to stress or injury, kidney tissue undergoes hypertrophy, and to a lesser extent hyperplasia, resulting in a net gain in the size of the kidney. Glomeruli, the filtering microvascular structures, are particularly susceptible to hypertrophy, which eventuates in fibrosis. At the cellular level, hypertrophy is characterized by cessation of the cell cycle at G 1 , a halt in DNA synthesis, and continued production and/or decreased degradation of cellular proteins (1). The net result is an increase in protein concentration disproportionate to DNA and an increase in the overall size of the cell. In addition to the increase in cell size, expansion of the extracellular matrix (ECM), 1 including fibronectin and collagen type IV, contributes to tissue hypertrophy. Studies in humans and in animal models of renal hypertrophy indicate that early hypertrophy and ECM accumulation are potentially reversible (2). Therefore, understanding the mechanisms that are required for the induction and maintenance of hypertrophy and ECM accumulation by growth factors, hormones, and cytokines may be critical for developing therapies that prevent or reverse renal hypertrophy. The insulin-like growth factor (IGF) system has been implicated in glomerular hypertrophy. In patients with type I diabetes, elevated amounts of IGF-I in the urine are associated with hypertrophy and progression of kidney disease (3). Moreover, endogenous kidney IGF-I levels are elevated within 2-3 days of streptozotocin-induced type I diabetes in rats (4, 5) and IGF-I receptor is up-regulated after prolonged hyperglycemia ...

NFATc is required for TGFβ-mediated transcriptional regulation of fibronectin

“…But in glomeruli, inhibition of calcineurin actually reduced fibronectin and collagen IV expression in the glomerulus [7]. Similarly, we found that both IGF-I and TGFβ signaling pathways involve activation of calcineurin in cultured mesangial cells [8,9]. Inhibition of calcineurin in this cell type blocks TGFβ-mediated induction of matrix proteins including fibronectin.…”

“…NFATc proteins are known to be expressed in the kidney, and are regulated in mesangial cells in response to IGF-I [9], TGFβ [8], and endothelin [10,11]. In vivo, NFATc has also been found to be regulated by calcineurin activation in the glomerulus [7].…”

Pharmacologic profiling of transcriptional targets deciphers promoter logic

“…Recent findings that calcineurin stabilizes the active phosphorylation state of Akt suggest a plausible mechanism for CSA repression. 39 This idea is confirmed in Figure 7c, which shows that the early induction of Akt by aCD3 and aCD3/aCD28 is antagonized by CSA, thus providing a mechanism for the repression of NF-kB transcriptional pathways by CSA. Furthermore, the aCD28-costimulated increase in AKT phosphorylation partly explains the relative resistance of IkBa phosphorylation to CSA under these conditions ( Figure 7a and b).…”

“…This suppression of Akt phosphorylation by CSA represents a novel finding in human T cells, although it has been previously suggested in cells of renal origin. 39 How wortmannin and CSA are able to stimulate AP1 is puzzling. Analysis of the activating phosphorylation state of c-Jun following wortmannin treatment shows that c-Jun phosphorylation is upregulated in the presence of wortmannin, providing a plausible reason for the increase in transcriptional activity (Figure 7d).…”

Expression of Myosin Light Chain Kinase in Kidney of Streptozotocin-Induced Diabetic Rats

“…Diabetic nephropathy is characterized by thickening of basement membranes and mesangial expansion with progression into glomerulosclerosis, tubular atrophy and interstitial fibrosis, ultimately resulting in renal failure [1][2][3][4]. A wide variety of mechanisms in the pathogenesis of diabetes have been proposed, including accumulation of nonenzymatic glycated end products in the kidney, oxidation of renal glycoproteins by reactive oxygen species, intracellular accumulation of sorbitol generated by the reduction of glucose by aldose reductase, involvement of mitogen-activated protein kinase and growth factors [5][6][7][8]. The diverse mechanisms indicates that a vast number of molecules and different signal transduction pathways are involved in its pathogenesis [9,10].…”