Enhancement of nuclear Ca(2+)-ATPase activity in regenerating rat liver: involvement of nuclear DNA increase

Suppressive role of regucalcin in liver cell proliferation: involvement in carcinogenesis

Regucalcin (RGN/SMP30) was discovered in 1978 and is a unique calcium-binding protein contains no EF-hand motif calcium-binding domain. Its name, regucalcin, was proposed as it suppresses activation of enzymes related to calcium signalling. The regucalcin gene (rgn) is localized on the X chromosome. Regucalcin plays its role of suppressor protein in intracellular signalling pathways, including of protein kinases and protein phosphatase activities, protein synthesis, and DNA and RNA synthesis in liver cells. Overexpression of endogenous regucalcin has a suppressive effect on cell proliferation in modelled rat hepatoma H4-II-E cells, which are induced by various signalling stimulations in vitro. This suppressive effect is independent of apoptosis. Endogenous regucalcin plays a suppressive role on overproduction of proliferating cells in regenerating rat liver in vivo. Regucalcin mRNA expression is uniquely down-regulated in development of carcinogenesis in liver of rats in vivo. Regucalcin mRNA and protein expressions are also depressed in human hepatoma HepG2 cells, MCF-7 breast cancer cells, and prostate cancer LNCaP cells. Depression of regucalcin expression may be associated with activity progression of carcinogens. Regucalcin may be a key molecule suppressor protein in cell proliferation and carcinogenesis.

The role of calcium and nitric oxide during liver enzyme release induced by increased physical forces as evidenced in partially hepatectomized rats

Although increased plasma enzyme activities could be diagnostic for tissue damage, the mechanisms controlling cellular enzyme release remain poorly understood. We found a selective and drastic elevation of serum enzyme activities accompanying rat liver regeneration after partial hepatectomy (PH), apparently controlled by a mechanism dependent on flow-bearing physical forces. In fact, this study assesses a putative role of calcium mobilization and nitric oxide (NO) production underlying rat liver enzyme release. The role of increased shear stress (by enhancing viscosity during perfusion) and the participation of cell calcium and NO were tested in isolated livers subjected to increasing flow rate. After PH, there was a drastic elevation of serum activities for liver enzyme markers, clearly predominating those of mitochondrial localization. Liver enzyme release largely depended on extracellular calcium entry, probably mediated by stretch-sensitive calcium channels, as well as by increasing NO production. However, these effects were differentially observed when comparing liver enzymes from cytoplasmic or mitochondrial compartments. Moreover, a possible role for cell-mediated mechanotransduction in liver enzyme release was suggested by increasing shear stress (high viscosity), which also selectively affected the release of the enzymes tested. Therefore, we show, for the first time, that flow-induced shear stress can control the amount of hepatic enzymes released into the bloodstream, which is largely regulated through modifications in cell calcium mobilization and production of liver NO, events markedly elevated in the proliferating rat liver.

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.

Role of calcium-binding protein regucalcin in regenerating rat liver

Regucalcin is a novel calcium-binding protein which does not contain EF-hand motif as a Ca²⁺ -binding domain. The organization of the rat regucalcin gene consists of seven exons and six introns. Its mRNA is mainly present in liver but slightly in kidney with a size of 1.8 kb. Hepatic regucalcin mRNA expression is stimulated by various factors including calcium, calcitonin, insulin, and oestrogen in rats. The mRNA is also expressed in hepatoma cells (Morris hepatoma and HepG2). Regucalcin plays a role in the maintenance of cytosolic Ca²⁺ homeostasis in liver cells. Moreover, regucalcin has an inhibitory effect on Ca²⁺ /calmodulin-dependent enzyme activation, protein kinase C activation, and many Ca²⁺ -activated enzymes, indicating a role in the regulation of the Ca²⁺ -signalling system. Recently, regucalcin has been demonstrated to regulate nuclear function in liver cells. Regucalcin can inhibit Ca²⁺ -activated nuclear DNA fragmentation in rat isolated liver nuclei. Furthermore, the liver nuclear DNA and RNA syntheses are inhibited by regucalcin. Such an effect of regucalcin is also seen in the nuclei of regenerating rat liver. The regucalcin mRNA level is increased in regenerating liver. These findings suggest that regucalcin plays a regulatory role in the suppression for overexpression of proliferative cells.

Modifications of intracellular calcium release channels and calcium mobilization following 70% hepatectomy

The aim of this study was to investigate the properties of ryanodine and IP3 receptors in regenerating liver following 70% hepatectomy, and to evaluate the hepatic Ca2+ distribution and mobilization during this process. Specific [3H]ryanodine and [3H]IP3 binding to hepatic smooth endoplasmic reticulum membranes, as well as subcellular Ca2+ determination by atomic absorption flame photometry and Ca2+ mobilization by INDO-1 AM spectrofluorescence in hepatocytes, was performed in regenerating livers after surgical 70% hepatectomy. Incorporation of 14C amino acids into proteins and of 32P into phospholipids was done in subcellular fractions. Ryanodine receptor Kd presented a dramatic increase after 12 h of surgery and remained high up to 2 days of treatment. IP3 receptor Bmax showed a significant augmentation starting at 6 h after hepatectomy and returning to normal values after 1 week. Cytosolic total calcium content decreased from 12 h until 4 days after hepatectomy whereas the microsomal and mitochondrial total calcium increased at 1 and 2-4 days of liver regeneration, which coincided with the differential turnover of proteins and phospholipids in these fractions. ATP-induced Ca2+ transients in hepatocytes of 24-h-hepatectomized rats confirmed the altered sensitivity of the ryanodine receptor toward its ligand, since 10 times more ryanodine was necessary to alter the ATP-induced Ca2+ transient. The data support the notion that the calcium release channels are targets of mechanisms of metabolic control during the proliferative response following 70% hepatectomy and might be part of the modified intracellular Ca2+ dynamics during liver regeneration.

Inhibitory effect of calcium-binding protein regucalcin on ribonucleic acid synthesis in isolated rat liver nuclei

The effect of regucalcin, a Ca2+-binding protein isolated from rat liver cytosol, on ribonucleic acid (RNA) synthesis in the nuclei of normal rat liver and of regenerating rat liver was investigated. The liver weight at 1 day after partial hepatectomy was increased about 50% of that of sham-operated (control) rats. Calcium chloride (1.0-20 microM Ca2+ as final concentration) was added into the reaction mixture of nuclear RNA synthesis. RNA synthesis was established by incorporation of [3H]-uridine 5'-triphosphate (UTP) into the nuclear RNA. Addition of Ca2+ (5 and 10 microM) caused a significant increase of RNA synthesis in the nuclei from control rat liver. Such effect of Ca2+ was potentiated in the nuclei of regenerating liver; nuclear RNA synthesis was increased about 2 fold by the 1.0 and 2.5 microM Ca2+ addition. The stimulatory effect of Ca2+ was significantly inhibited by the presence of alpha-amanitin (10(-8) M), an inhibitor of RNA polymerase II. The presence of regucalcin (0.25 and 0.5 microM) significantly inhibited RNA synthesis in the nuclei from control rat liver and from regenerating rat liver. The inhibitory effect of regucalcin was remarkable in the presence of EGTA (0.5 mM), and it was weakened by the addition of Ca2+ (5 microM). Such regucalcin effect was not seen in the presence of alpha-amanitin. The presence of anti-regucalcin IgG in the reaction mixture significantly increased RNA synthesis in the nuclei from control rat liver, indicating that the endogenous regucalcin may be involved in nuclear RNA synthesis. The present results demonstrate that regucalcin can inhibit nuclear RNA synthesis in rat liver. Regucalcin may have an inhibitory role in the regulation of liver nuclear RNA synthesis.