Bradykinin induces a calcium-store- dependent calcium influx in mouse mesangial cells

Glucagon-like peptide-1 receptor pathway inhibits extracellular matrix production by mesangial cells through store-operated Ca2+ channel

Glomerular mesangial cell is the major source of mesangial matrix. Our previous study demonstrated that store-operated Ca2+ channel signaling suppressed extracellular matrix protein production by mesangial cells. Recent studies demonstrated that glucagon-like peptide-1 receptor (GLP-1R) pathway had renoprotective effects. However, the underlying mechanism(s) remains unclear. The present study was aimed to determine if activation of GLP-1R decreased extracellular matrix protein production by mesangial cells through upregulation of store-operated Ca2+ function. Experiments were conducted in cultured human mesangial cells. Liraglutide and exendin 9–39 were used to activate and inhibit GLP-1R, respectively. Store-operated Ca2+ function was estimated by evaluating the SOC-mediated Ca2+ entry (SOCE). We found that liraglutide treatment reduced high glucose-stimulated production of fibronectin and collagen IV. The inhibitory effects of liraglutide were not observed in the presence of exendin 9–39. Exendin-4, another GLP-1R agonist also blunted high glucose-stimulated fibronectin and collagen IV production. Treatment of human mesangial cells with liraglutide for 24 h significantly attenuated the high glucose-induced reduction of Orai1 protein. Consistently, Ca2+ imaging experiments showed that the inhibition of high glucose on SOCE was significantly attenuated by liraglutide. However, in the presence of exendin 9–39, liraglutide failed to reverse the high glucose effect. Furthermore, liraglutide effects on fibronectin and collagen IV protein abundance were significantly attenuated by GSK-7975A, a selective blocker of store-operated Ca2+. Taken together, our findings suggest that GLP-1R signaling inhibited high glucose-induced extracellular matrix protein production in mesangial cells by restoring store-operated Ca2+ function.

Impact statement

Diabetic kidney disease continues to be a major challenge to health care system in the world. There are no known therapies currently available that can cure the disease. The present study provided compelling evidence that activation of GLP-1R inhibited extracellular matrix protein production by glomerular mesangial cells. We further showed that the beneficial effect of GLP-1R was attributed to upregulation of store-operated Ca2+ channel function. Therefore, we identified a novel mechanism contributing to the renal protective effects of GLP-1R pathway. Activation of GLP-1R pathway and/or store-operated Ca2+ channel signaling in MCs could be an option for patients with diabetic kidney disease.

Short-term high-glucose treatment decreased abundance of Orai1 protein through posttranslational mechanisms in rat mesangial cells

The short-term effect of high-glucose (HG) treatment on store-operated Ca²⁺ entry in mesangial cells (MCs) is not well-known. The aim of the present study was to determine whether and how HG treatment for a short period altered protein abundance of Orai1, the channel mediating store-operated Ca²⁺ entry in MCs. Rat and human MCs were exposed to HG (25 mM) for 2, 4, 8, and 24 h, and the abundance of Orai1 protein was significantly decreased at the time points of 8 and 16 h. Consistently, HG treatment for 8 h significantly reduced store-operated Ca²⁺ entry in rat MCs. However, HG treatment for the same time periods did not alter the levels of Orai1 transcript. Cycloheximide, a protein synthesis inhibitor, did not affect the HG-induced decrease of Orai1 protein, suggesting a posttranslational mechanism was involved. However, the HG effect on Orai1 protein was significantly attenuated by MG132 (a ubiquitin-proteasome inhibitor) and NH₄Cl (a lysosomal pathway inhibitor). Furthermore, HG treatment for 8 h stimulated ubiquitination of Orai1 protein. We further found that polyethylene glycol-catalase, an antioxidant, significantly blunted the HG-induced reduction of Orai1 protein. In support of involvement of reactive oxygen species in the HG effects, hydrogen peroxide (H₂O₂) itself significantly decreased abundance of Orai1 protein and increased the level of ubiquitinated Orai1. Taken together, these results suggest that a short-term HG treatment decreased abundance of Orai1 protein in MCs by promoting the protein degradation through the ubiquitination-proteasome and -lysosome mechanisms. This HG-stimulated posttranslational mechanism was mediated by H₂O₂.

Negative regulation of Smad1 pathway and collagen IV expression by store-operated Ca2+ entry in glomerular mesangial cells

Collagen IV (Col IV) is a major component of expanded glomerular extracellular matrix in diabetic nephropathy and Smad1 is a key molecule regulating Col IV expression in mesangial cells (MCs). The present study was conducted to determine if Smad1 pathway and Col IV protein abundance were regulated by store-operated Ca²⁺ entry (SOCE). In cultured human MCs, pharmacological inhibition of SOCE significantly increased the total amount of Smad1 protein. Activation of SOCE blunted high-glucose-increased Smad1 protein content. Treatment of human MCs with ANG II at 1 µM for 15 min, high glucose for 3 days, or TGF-β1 at 5 ng/ml for 30 min increased the level of phosphorylated Smad1. However, the phosphorylation of Smad1 by those stimuli was significantly attenuated by activation of SOCE. Knocking down Smad1 reduced, but expressing Smad1 increased, the amount of Col IV protein. Furthermore, activation of SOCE significantly attenuated high-glucose-induced Col IV protein production, and blockade of SOCE substantially increased the abundance of Col IV. To further verify those in vitro findings, we downregulated SOCE specifically in MCs in mice using small-interfering RNA (siRNA) against Orai1 (the channel protein mediating SOCE) delivered by the targeted nanoparticle delivery system. Immunohistochemical examinations showed that expression of both Smad1 and Col IV proteins was significantly greater in the glomeruli with positively transfected Orai1 siRNA compared with the glomeruli from the mice without Orai1 siRNA treatment. Taken together, our results indicate that SOCE negatively regulates the Smad1 signaling pathway and inhibits Col IV protein production in MCs.

Store-Operated Ca2+ Channels in Mesangial Cells Inhibit Matrix Protein Expression

Accumulation of extracellular matrix derived from glomerular mesangial cells is an early feature of diabetic nephropathy. Ca(2+) signals mediated by store-operated Ca(2+) channels regulate protein production in a variety of cell types. The aim of this study was to determine the effect of store-operated Ca(2+) channels in mesangial cells on extracellular matrix protein expression. In cultured human mesangial cells, activation of store-operated Ca(2+) channels by thapsigargin significantly decreased fibronectin protein expression and collagen IV mRNA expression in a dose-dependent manner. Conversely, inhibition of the channels by 2-aminoethyl diphenylborinate significantly increased the expression of fibronectin and collagen IV. Similarly, overexpression of stromal interacting molecule 1 reduced, but knockdown of calcium release-activated calcium channel protein 1 (Orai1) increased fibronectin protein expression. Furthermore, 2-aminoethyl diphenylborinate significantly augmented angiotensin II-induced fibronectin protein expression, whereas thapsigargin abrogated high glucose- and TGF-β1-stimulated matrix protein expression. In vivo knockdown of Orai1 in mesangial cells of mice using a targeted nanoparticle siRNA delivery system resulted in increased expression of glomerular fibronectin and collagen IV, and mice showed significant mesangial expansion compared with controls. Similarly, in vivo knockdown of stromal interacting molecule 1 in mesangial cells by recombinant adeno-associated virus-encoded shRNA markedly increased collagen IV protein expression in renal cortex and caused mesangial expansion in rats. These results suggest that store-operated Ca(2+) channels in mesangial cells negatively regulate extracellular matrix protein expression in the kidney, which may serve as an endogenous renoprotective mechanism in diabetes.

High glucose and diabetes enhanced store-operated Ca(2+) entry and increased expression of its signaling proteins in mesangial cells

The present study was conducted to determine whether and how store-operated Ca(2+) entry (SOCE) in glomerular mesangial cells (MCs) was altered by high glucose (HG) and diabetes. Human MCs were treated with either normal glucose or HG for different time periods. Cyclopiazonic acid-induced SOCE was significantly greater in the MCs with 7-day HG treatment and the response was completely abolished by GSK-7975A, a selective inhibitor of store-operated Ca(2+) channels. Similarly, the inositol 1,4,5-trisphosphate-induced store-operated Ca(2+) currents were significantly enhanced in the MCs treated with HG for 7 days, and the enhanced response was abolished by both GSK-7975A and La(3+). In contrast, receptor-operated Ca(2+) entry in MCs was significantly reduced by HG treatment. Western blotting showed that HG increased the expression levels of STIM1 and Orai1 in cultured MCs. A significant HG effect occurred at a concentration as low as 10 mM, but required a minimum of 7 days. The HG effect in cultured MCs was recapitulated in renal glomeruli/cortex of both type I and II diabetic rats. Furthermore, quantitative real-time RT-PCR revealed that a 6-day HG treatment significantly increased the mRNA expression level of STIM1. However, the expressions of STIM2 and Orai1 transcripts were not affected by HG. Taken together, these results suggest that HG/diabetes enhanced SOCE in MCs by increasing STIM1/Orai1 protein expressions. HG upregulates STIM1 by promoting its transcription but increases Orai1 protein through a posttranscriptional mechanism.

Regulation of STIM1, store-operated Ca2+ influx, and nitric oxide generation by retinoic acid in rat mesangial cells

It has been shown that store-operated Ca(2+) influx (SOC) plays critical roles in the activation of endothelial nitric oxide (NO) synthase (eNOS) and generation of NO in endothelial cells. Recent studies indicate stromal interaction molecule 1 (STIM1) is the molecule responsible for SOC activation following Ca(2+) depletion in the ER. Retinoic acids (RA) have beneficial effects in the treatment of renal diseases. The mechanism of the RA action is still largely unknown. In the current study, we used primary cultured rat mesangial cells to examine the effect of RA on SOC and STIM1. In these cells, BK caused concentration-dependent [Ca(2+)](i) mobilization. Treatment of the cells with RA, while it had no effect on the initial peak, reduced the plateau phase of BK-mediated [Ca(2+)](i) response, indicating the inhibition of SOC by RA. The level of STIM1 protein but not mRNA in RA-treated cells was significantly reduced. RA treatment did not affect TGF-beta-mediated gradual Ca(2+) influx which occurred by superoxide anion-mediated mechanism, indicating RA treatment specifically inhibited SOC in mesangial cells. RT-PCR and Western blot analysis demonstrated that eNOS was expressed in rat mesangial cells grown in media containing 11 and 30 but not 5.5 mM glucose. Downregulation of STIM1 protein and BK-induced SOC by RA treatment or STIM1 dsRNA were associated with abolished NO production. The 26S proteasome inhibitor lactacystin blocked the RA-mediated downregulation of BK-induced SOC, suggesting that ubiquitin-proteasome pathway may be involved in RA-mediated STIM1 protein downregulation in rat mesangial cells. Our data suggest that glucose-induced eNOS expression and NO production in mesangial cells may contribute to hyperfiltration in diabetes and RA may exert beneficial effects by downregulation of STIM1 and SOC in mesangial cells.

Ion channels in mesangial cells: function, malfunction, or fiction

Ion channels in glomerular mesangial cells from humans, rats, and mice have been studied by electrophysiological, molecular, and gene-knockout methods. Two channels, a large, Ca(2+)-activated K(+) channel (BK) and a store-operated Ca(2+) channel (SOCC), can be defined with respect to molecular structure and function. Human BK, comprised of a pore-forming alpha-subunit and an accessory beta1-subunit, operate as Ca(2+)-sensing feedback modulators of contractile tone. SOCC have also been characterized in a mouse cell line; they are comprised of molecules belonging to the transient receptor potential subfamily.

Adriamycin impairs the contraction of mesangial cells through the inhibition of protein kinase C and intracellular calcium

The effects of adriamycin on the contractile function of cultured mesangial cells were measured by the changes in planar surface area in response to treatment with agonists. Incubation of mesangial cells with adriamycin (0.2 microg/ml) for 24 h significantly decreased the contractile responses to the calcium channel activator BAY K 8644 (1 microM) and to the PKC activator PMA (1 microM). Intracellular calcium concentration ([Ca(2+)](i)), measured by changes in fura 2 levels in response to ATP (0.1 mM), was significantly inhibited in adriamycin-treated mesangial cells compared with control cells. In the absence of extracellular calcium, treatment with ionomycin (0.1 mM) or thapsigargin (10 microM) resulted in a significantly smaller increase in [Ca(2+)](i) in adriamycin-treated mesangial cells compared with control, suggesting an important role of the endoplasmic reticulum in the effects of adriamycin. Using PKC-specific antibodies, adriamycin significantly decreased the cytosolic and membranous fractions of PKC-alpha in mesangial cells to 75 +/- 6 and 70 +/- 12% of control, respectively. The PKC activity of mesangial cells was also significantly inhibited after incubation with adriamycin for 24 h. In conclusion, adriamycin induces hypocontractility of mesangial cells, which may mediate this effect by inhibiting PKC-alpha and [Ca(2+)](i).