Regulator of Calcineurin 1 Isoform 4 (RCAN1.4) Is Overexpressed in the Glomeruli of Diabetic Mice

Calcineurin (CaN) is activated in diabetes and plays a role in glomerular hypertrophy and extracellular matrix (ECM) accumulation. Here, kidneys from diabetic model mice were investigated for the expression of the regulator of CaN 1 (RCAN1) isoform 4 (RCAN1.4) which had been shown to be transcriptionally upregulated by CaN activation. We found the increased immunoreactivity for RCAN1 in the glomerular cells of db/db mice and streptozotocin-induced diabetic mice. In concordance, the expression of RCAN1 protein and RCAN1.4 mRNA were elevated in the whole kidney sample from db/db mice. Interleukin-1beta (IL-1beta), tumor necrosis factor-alpha, and glycated albumin (AGE-BSA) were identified as inducers of RCAN1.4 in mesangial cells. Pretreatment of cyclosporine A blocked the increases of RCAN1.4 stimulated by IL-1beta or AGE-BSA, suggesting that activation of CaN is required for the RCAN1.4 induction. Stable transfection of RCAN1.4 in Mes-13 mesangial cells upregulated several factors relevant to ECM production and degradation. These results suggested that RCAN1.4 might act as a link between CaN activation and ECM turnover in diabetic nephropathy.

Knockdown of RCAN1.4 Increases Susceptibility to FAS-mediated and DNA-damage-induced Apoptosis by Upregulation of p53 Expression

Despite the potential importance of the human regulator of calcineurin 1 (RCAN-1) gene in the modulation of cell survival under stress, little is known about its role in death-inducing signal pathways. In this study, we addressed the effects of RCAN1.4 knockdown on cellular susceptibility to apoptosis and the activation of death pathway proteins. Transfection of siRNAs against RCAN1.4 resulted in enhanced Fas- and etoposide-induced apoptosis, which was associated with increased expression and translocation of Bax to mitochondria. Our results suggest that enhanced expression and activation of p53 was responsible for the upregulation of Bax and the increased sensitivity to apoptosis, which could be reversed by p53 knockdown. To explain the observed upregulation of p53, we propose a downregulation of the ubiquitin ligase HDM2, probably translationally. These findings show the importance of appropriate RCAN1.4 expression in the modulation of cell survival and reveal a link between RCAN1.4 and p53.