Stimulatory effect of regucalcin on proteolytic activity in rat renal cortex cytosol: involvement of thiol proteases

Regucalcin confers resistance to amyloid-β toxicity in neuronally differentiated PC12 cells

Amyloid‐β (Aβ), a primary component of amyloid plaques, has been widely associated with the pathogenesis of Alzheimer's disease. The Ca²⁺‐binding protein regucalcin (RGN) plays multiple roles in maintaining cell functions by regulating intracellular calcium homeostasis, various signaling pathways, and gene expression systems. Here, we investigated the functional role of RGN against Aβ‐induced cytotoxicity in neuronally differentiated PC12 cells. Overexpression of RGN reduced Aβ‐induced apoptosis by reducing mitochondrial dysfunction and caspase activation. It also attenuated Aβ‐induced reactive oxygen species production and oxidative damage and decreased Aβ‐induced nitric oxide (NO) overproduction, upregulation of inducible NO synthase by nuclear factor‐κB, and nitrosative damage. Interestingly, the genetic disruption of RGN increased the susceptibility of neuronally differentiated PC12 cells to Aβ toxicity. Thus, RGN possesses antioxidant activity against Aβ‐induced oxidative and nitrosative stress and may play protective roles against Aβ‐induced neurotoxicity in Alzheimer's disease.

The diverse roles of calcium-binding protein regucalcin in cell biology: from tissue expression and signalling to disease

Regucalcin (RGN) is a calcium (Ca(2+))-binding protein widely expressed in vertebrate and invertebrate species, which is also known as senescence marker protein 30, due to its molecular weight (33 kDa) and a characteristically diminished expression with the aging process. RGN regulates intracellular Ca(2+) homeostasis and the activity of several proteins involved in intracellular signalling pathways, namely, kinases, phosphatases, phosphodiesterase, nitric oxide synthase and proteases, which highlights its importance in cell biology. In addition, RGN has cytoprotective effects reducing intracellular levels of oxidative stress, also playing a role in the control of cell survival and apoptosis. Multiple factors have been identified regulating the cell levels of RGN transcripts and protein, and an altered expression pattern of this interesting protein has been found in cases of reproductive disorders, neurodegenerative diseases and cancer. Moreover, RGN is a serum-secreted protein, and its levels have been correlated with the stage of disease, which strongly suggests the usefulness of this protein as a potential biomarker for monitoring disease onset and progression. The present review aims to discuss the available information concerning RGN expression and function in distinct cell types and tissues, integrating cellular and molecular mechanisms in the context of normal and pathological conditions. Insight into the cellular actions of RGN will be a key step towards deepening the knowledge of the biology of several human diseases.

The role of regucalcin in nuclear regulation of regenerating liver

Regucalcin was discovered in 1978 as a Ca(2+)-binding protein that does not contain EF-hand motif of Ca(2+)-binding domain [Yamaguchi, M., and Yamamoto T., Chem. Pharm. Bull. 26, 1915-1918, 1978]. The name regucalcin was proposed for this Ca(2+)-binding protein, which can regulate liver cell functions related to Ca(2+). Regucalcin has been demonstrated to play a multifunctional role in liver and kidney cells, for which regucalcin mRNA expression and its protein content are pronounced. Hepatic regucalcin mRNA expression has been shown to be mediated through signaling pathway of Ca(2+)/calmodulin-dependent protein kinase, protein kinase C, and tyrosine kinase. AP-1- and NF-1-like factors can bind to the promotor region of the rat regucalcin gene to mediate the Ca(2+) response for transcriptional activation. Growing evidence supports the view, moreover, that regucalcin plays an important role in the regulation of Ca(2+) signaling from the cytoplasm to nuclei in the proliferative cells of regenerating rat liver. Also, regucalcin has been demonstrated to be transported to liver nucleus, and it can inhibit nuclear protein kinase, protein phosphatase, and DNA and RNA synthesis in regenerating liver. Regucalcin plays a physiologic role in the control for overexpression of proliferative cells. Regucalcin has been proposed to be an important regulatory protein in nuclear signaling system.

Role of regucalcin in calcium signaling

Regucalcin was discovered in 1978 as a calcium-binding protein that does not contain EF-hand motif of Ca(2+)-binding domain [M. Yamaguchi and T. Yamamoto, Chem. Pharm. Bull. 26 1915-1918 (1978)]. In recent years, regucalcin has been demonstrated to play an important role as a regulatory protein in Ca2+ signaling in rat liver and kidney cells. The organization of the rat regucalcin gene consists of seven exons and six introns. The mRNA is mainly present in liver and kidney with a size of 1.8 kb. Hepatic regucalcin mRNA expression has been shown to be stimulated by various factors including calcium, calcitonin, insulin, and estrogen in rats. The mRNA is also expressed in hepatoma cells (Morris hepatoma, HepG2, and rat hepatoma H4-II-E cells). Regucalcin plays a role in the maintenance of intracellular Ca2+ homeostasis due to activating Ca2+ pump enzymes in the plasma membrane (basolateral membrane) and microsomes of liver and renal cortex cells. Moreover, regucalcin has an inhibitory effect on the activation of Ca2+/calmodulin-dependent enzymes and protein kinase C. Also, regucalcin has been demonstrated to regulate nuclear function in liver cells; it can inhibit Ca(2+)-activated DNA fragmentation, DNA and RNA synthesis, protein kinase and protein phosphatase activities in the nuclei. Such an effect is also seen in the nuclei of regenerating rat liver. Regucalcin may play a physiological role in the control for overexpression of proliferative cells. Regucalcin has been proposed to be an important regulatory protein in Ca2+ signaling system, and it plays a multifunctional role in liver and kidney cells.

Stimulatory effect of regucalcin on proteolytic activity is impaired in the kidney cortex cytosol of rats with saline ingestion

The effect of regucalcin (RC) on neutral proteolytic activity in the cytosol of rat kidney cortex was investigated. Proteolytic activity was significantly increased by the presence of RC (0.01-0.10 microM) in the enzyme reaction mixture. This increase was completely abolished by the addition of anti-RC monoclonal antibody (150 ng/ml). When the renal cortex cytosol was incubated without RC addition, the degradation of globin of substrate was demonstrated by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis. This degradation was clearly inhibited by the addition of anti-RC antibody (150 ng/ml), indicating that protein degradation results partly from the cytosolic endogenous RC. Meanwhile, proteolytic activity was significantly decreased in the renal cortex cytosol of rats with saline ingestion for 2, 7, and 14 days. The effect of RC (0.1 microM) in increasing proteolytic activity was weakened in the kidney cortex cytosol of saline-ingested rats. The present study suggests that endogenous RC plays a role in the activation of proteases in the renal cortex cytosol, and that the RC effect is impaired in saline-ingested rats.