Hepatic expression of regeneration marker genes following partial hepatectomy in the rat. Influence of 1,25-dihydroxyvitamin D3 in hypocalcemia

Interaction of vitamin D with membrane-based signaling pathways

Many studies in different biological systems have revealed that 1α,25-dihydroxyvitamin D₃ (1α,25(OH)₂D₃) modulates signaling pathways triggered at the plasma membrane by agents such as Wnt, transforming growth factor (TGF)-β, epidermal growth factor (EGF), and others. In addition, 1α,25(OH)₂D₃ may affect gene expression by paracrine mechanisms that involve the regulation of cytokine or growth factor secretion by neighboring cells. Moreover, post-transcriptional and post-translational effects of 1α,25(OH)₂D₃ add to or overlap with its classical modulation of gene transcription rate. Together, these findings show that vitamin D receptor (VDR) cannot be considered only as a nuclear-acting, ligand-modulated transcription factor that binds to and controls the transcription of target genes. Instead, available data support the view that much of the complex biological activity of 1α,25(OH)₂D₃ resides in its capacity to interact with membrane-based signaling pathways and to modulate the expression and secretion of paracrine factors. Therefore, we propose that future research in the vitamin D field should focus on the interplay between 1α,25(OH)₂D₃ and agents that act at the plasma membrane, and on the analysis of intercellular communication. Global analyses such as RNA-Seq, transcriptomic arrays, and genome-wide ChIP are expected to dissect the interactions at the gene and molecular levels.

Healing of the abdominal wall after parcial hepatectomy

OBJECTIVE

To evaluate the wound healing of the abdominal wall incision in hepatectomized rats as for the concentration of collagen, inflammatory reaction and angiogenesis.

METHODS

We used 48 rats randomly assigned to laparotomy with or without hepatectomy. The scars were studied in the 3rd, 7th and 14th postoperative days. We analyzed the density of collagen by the histochemical method and angiogenesis, by immunohistochemistry.

RESULTS

The analysis showed a lower total collagen concentration in skin and subcutaneous tissue in the abdominal scars of the experiment group (p3 = 0.011, p7 = 0.004 and p14 = 0.008). The density of collagen I was lower in the hepatectomy group, especially in the third day, in the skin, subcutaneous tissue (p = 0.038) and in the aponeurotic plane (p = 0.026). There was a lower concentration of collagen III in the two abdominal wall layers studied, although not statistically significant. The inflammatory response was similar at all times in both groups. It was found that angiogenesis was developed earlier in the control group (p3 = 0.005 and p7 = 0.012) and later in the experimental group (p14 = 0.048).

CONCLUSION

Hepatectomy leads to a delay in the healing process, interfering with collagen synthesis and angiogenesis.

Gc-globulin (vitamin D binding protein) is synthesized and secreted by hepatocytes and internalized by hepatic stellate cells through Ca(2+)-dependent interaction with the megalin/gp330 receptor

BACKGROUND

Gc-globulin or vitamin D binding protein is a highly expressed, multifunctional and polymorphic serum protein, which also serves as the major transporter for vitamin D metabolites in the circulation. The present study was performed to analyze the interaction between gc-globulin of hepatocytes and hepatic stellate cells, the most important fat-/retinol-storing cell type in the liver, which spontaneously transdifferentiates to myofibroblasts in culture.

METHODS

Hepatic stellate cells and hepatocytes were isolated by the pronase/collagenase reperfusion method, hepatocytes by collagenase reperfusion of the organ. Gc-globulin expression was monitored by immunocytochemistry, immunoblotting, RT-PCR, metabolic labelling with [(35)S]-methionine, and its intracellular binding to alpha-smooth-muscle actin was investigated by co-immunoprecipitation. Cytoskeletal stainings of gc-globulin and alpha-smooth-muscle actin in hepatic stellate cells and the identification of the receptors megalin/gp330, HCAM/CD44, cubilin and annexin A2 were performed with confocal immunocytochemistry, immunoblotting and/or FACS-analysis.

RESULTS

Hepatocytes synthesize and secrete gc-globulin as shown by RT-PCR and [(35)S]-methionine labelling, which could be suppressed by cycloheximide. Also, a strong signal for gc-globulin was detected in the immunoblot of native hepatic stellate cell lysates. However, no mRNA for gc-globulin was found in this cell type, which suggests no active synthesis by hepatic stellate cells. Hepatic stellate cells were tested positively for the presence of known gc-globulin interacting receptors megalin/gp330, HCAM/CD44, cubilin and annexin A2. Inhibition of the megalin/gp330 receptor by a competitive, neutralizing antibody resulted in decreased intracellular availability of gc-globulin in hepatic stellate cells. The latter effect was enhanced by additional incubation of hepatic stellate cells with EDTA for complexing Ca(2+), suggesting a Ca(2+)-dependent internalization of gc-globulin into hepatic stellate cells via the megalin/gp300 receptor. This was supported by confocal microscopy which showed a co-localization of gc-globulin with the multifunctional megalin/gp330 receptor on this cell type. Inside hepatic stellate cells, a linkage between gc-globulin and alpha-smooth muscle actin filaments of hepatic stellate cells was detected by immunocytochemistry. Intracellular binding of gc-globulin to alpha-smooth-muscle actin filaments was confirmed by co-immunoprecipitation.

CONCLUSION

We give evidence to the expression of the megalin/gp330 receptor on hepatic stellate cells and that this receptor is involved in the Ca(2+)-dependent internalization of gc-globulin into hepatic stellate cells, a protein synthesized and secreted into the extracellular space and circulation by hepatocytes. Inside hepatic stellate cells, it co-localizes with and binds to alpha-smooth muscle actin filaments. Under consideration of the available literature, these findings propose a participation of gc-globulin in hepatic vitamin D metabolism as well as in hepatic stellate cell stability and apoptosis as important mechanisms of liver regeneration.

The normal liver harbors the vitamin D nuclear receptor in nonparenchymal and biliary epithelial cells

The liver is generally considered negative for the vitamin D nuclear receptor (VDR(n)), even though several studies have shown significant effects of 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) on liver cell physiology. The low abundance of VDR(n) in the liver led us to propose that hepatocytes (the largest hepatic cell population) were most likely negative for the receptor, whereas the small hepatic sinusoidal and ductular cell populations that contain cell types known to express VDR(n) in other tissues should express the receptor. Using freshly isolated cells from normal livers as well as biliary and epithelial hepatic cell lines, our data show that the human, rat, and mouse hepatocytes express very low VDR(n) messenger RNA (mRNA) and protein levels. In contrast, sinusoidal endothelial, Kupffer, and stellate cells of normal rat livers as well as the mouse biliary cell line BDC and rat hepatic neonatal epithelial SD6 cells clearly expressed both VDR(n) mRNA and protein. In addition, specimens of human hepatocarcinoma as well as intrahepatic colon adenocarcinoma metastases were also found to express the VDR(n) gene transcript. Kupffer, stellate, and endothelial cells responded to 1,25(OH)(2)D(3) by a significant increase in the CYP24, indicating that the VDR(n) is fully functional in these cells. In conclusion, selective hepatic cell populations are targets for the vitamin D endocrine/paracrine/intracrine system.

Effect of cyclosporine a on hepatic compensatory growth: role of calcium status

Cyclosporine A (CsA) has been reported to positively influence hepatic compensatory growth (HCG) in normal animals. The role of calcium in the CsA-mediated influence on HCG was studied in normal and in chronically hypocalcemic rats, a model in which HCG is perturbed. CsA (3.33 mg/kg/day for 10 days) was administered before 2/3 partial hepatectomy (PHx). CsA did not influence serum Ca(2+) but significantly increased concentrations of the vitamin D hormone calcitriol. After PHx in normal animals, CsA accelerated DNA synthesis without influencing liver weight restitution, suggesting that its main effect was to mediate an accelerated progression through the cell cycle G(0) to G(1)/S phase(s). In hypocalcemic rats, CsA did not influence DNA synthesis, but normalization of circulating calcium alone accelerated DNA synthesis but abrogated the stimulatory effect of CsA, indicating that CsA could not superimpose its stimulatory effect on the calcium effect. In vitro investigation on the CsA mechanisms of action revealed a dose-dependent increase in hepatocyte basal resting cytoplasmic Ca(2+) and an increase in inositol-1,4,5-trisphosphate-sensitive Ca(2+) pool, which was dependent on the presence of normal extracellular Ca(2+) during CsA exposure. CsA also mediated a significant increase in cellular Ca(2+) mobilization by phenylephrine, vasopressin, and epidermal growth factor (EGF) in the presence of extracellular Ca(2+) concentration. Our data, therefore, demonstrate that CsA accelerates HCG after PHx by, in part, increasing the cellular Ca(2+) pools and the response to EGF and Ca(2+)-mobilizing hormones known to be comitogens for hepatocytes.

Hypocalcemia modifies the intracellular calcium response to the alpha 1-adrenergic agent phenylephrine in rat hepatocytes

In vivo, extracellular calcium ([Ca2+]e) homeostasis is maintained within a very narrow range by the calcium regulating hormones. At the cellular level, the response to many agents is transduced by changes in cytosolic Ca2+ ([Ca2+]i) which involves both mobilization of cellular pools and entry of [Ca2+]e through plasma membrane channels. To investigate the cellular effects of chronic hypocalcemia (Ca-) on [Ca2+]i homeostasis, hepatocytes, a cell type well characterized for its [Ca2+]i response, were used. Data indicate that Ca- leads to a significant shift to the left in the basal resting cytosolic Ca2+ concentration distribution curve with half-maximum cumulative frequency of 119 versus 149 nM in Ca- and normal rats (N) respectively (P < 0.0001). The response to the alpha 1-adrenergic agonist phenylephrine (Phe) was also influenced by Ca- with a dampening of the dose-response curve, a significant decrease in the frequency of sustained responses (P < 0.001), and significant changes in the oscillation pattern. Indeed, hepatocytes obtained from Ca- exhibited a higher frequency of large amplitude, low frequency oscillations than N most particularly at the 2 and 5 microM Phe dose while N predominantly exhibited low amplitude, high frequency oscillations on sustained plateaus (P < 0.001). IP3 receptor (IP3R) binding studies and Ca2+ mobilization from IP3-sensitive pools showed that IP3R was highly sensitive to the prevailing Ca2+ with, in the range of resting [Ca2+]i, R affinity significantly lower in Ca- than in N. Upon exposure of permeabilized cells to 25 microM IP3, Ca2+ mobilization from the IP3-sensitive intracellular pool was significantly reduced by Ca- (P < 0.05) suggesting a decrease in the IP3-mobilizable Ca2+ pool in Ca-. Our results indicate that hypocalcemia significantly alters [Ca2+]i signalling by perturbing the initial response to agonist and the [Ca2+]i response pattern. In addition, the decrease in Ca2+ mobilization from IP3-sensitive pools suggests that hypocalcemia may also lead to a decrease in the Ca2+ content of intracellular pools.