Effects of branched-chain amino acids on DNA synthesis and proliferation in primary cultures of adult rat hepatocytes

Role of Hepatocyte Growth Regulators in Liver Regeneration

We have studied whether growth factors, cytokines, hormones, neurotransmitters, and local hormones (autacoids) promote the proliferation of hepatic parenchymal cells (i.e., hepatocytes) using in vitro primary cultured hepatocytes. The indicators used for this purpose include changes in DNA synthesis activity, nuclear number, cell number, cell cycle, and gene expression. In addition, the intracellular signaling pathways from the plasma membrane receptors to the nucleus have been examined in detail for representative growth-promoting factors that have been found to promote DNA synthesis and cell proliferation of hepatocytes. In examining intracellular signaling pathways, the effects of specific inhibitors of presumed signaling factors involved have been pharmacologically confirmed, and the phosphorylation activities of the signaling factors (e.g., RTK, ERK, mTOR, and p70 S6K) have been evaluated. As a result, it has been found that there are many factors that promote the proliferation of hepatocytes (e.g., HGF, EGF, TGF-α, IL-1β, TNF-α, insulin, growth hormone (GH), prostaglandin (PG)), and serotonin (5-HT)), while there are very few factors (e.g., TGF-β1 and glucocorticoids) that inhibit the effects of growth-promoting factors. We have also found that 5-HT and GH promote the proliferation of hepatocytes via different autocrine factors (e.g., TGF-α and IGF-I, respectively). Using primary cultured hepatocytes, it will be possible to further study the molecular and cellular aspects of liver regeneration.

Effects of dietary branched-chain amino acid supplementation on serum and milk metabolome profiles in dairy cows during early lactation

The 3 branched-chain AA (BCAA), Val, Leu, and Ile, are essential AA used by tissues as substrates for protein synthesis and energy generation. In addition, BCAA are also involved in modulating cell signaling pathways, such as nutrient sensing and insulin signaling. In our previous study, dietary BCAA supplementation was shown to improve protein synthesis and glucose homeostasis in transition cows. However, a more detailed understanding of the changes in metabolic pathways associated with an increased BCAA availability is desired to fine-tune nutritional supplementation strategies. Multiparous Holstein cows (n = 20) were enrolled 28 d before expected calving and assigned to either the BCAA treatment (n = 10) or the control group (n = 10). Cows assigned to BCAA were fed 550 g/d of rumen-protected BCAA mixed with 200 g/d of dry molasses from calving until 35 DIM, whereas the cows assigned to the control were fed only 200 g/d of dry molasses. Serum samples were collected on d 10 before expected calving, as well as on d 4 and d 21 postpartum. Milk samples were collected on d 14 postpartum. From a larger cohort, we selected 20 BCAA-supplemented cows with the greatest plasma urea nitrogen concentration, as an indicator for greater BCAA availability, for the metabolomics analysis herein. Serum and milk samples were subjected to a liquid chromatography-mass spectrometry-based assay, detecting and measuring the abundance of 241 serum and 211 milk metabolic features, respectively. Multivariable statistical analyses revealed that BCAA supplementation altered the metabolome profiles of both serum and milk samples. Increased abundance of serum phosphocholine and glutathione and of milk Val, Ile, and Leu, and decreased abundance of milk acyl-carnitines were associated with BCAA supplementation. Altered phosphocholine and glutathione abundances point to altered hepatic choline metabolism and antioxidant balance, respectively. Altered milk acyl-carnitine abundances suggest changes in mammary fatty acid metabolism. Dietary BCAA supplementation was associated with a range of alterations in serum and milk metabolome profiles, adding to our understanding of the role of BCAA availability in modulating dairy cow protein, lipid, and energy metabolism on a whole-body level and how it affects milk composition.

Regulation of protein synthesis in porcine mammary epithelial cells by L-valine

This study was conducted to determine the catabolism of L-valine in porcine mammary epithelial cells (PMECs) and its role in stimulating protein synthesis in these cells. PMECs were incubated with 0.05-, 0.10-, 0.25-, 0.5-, and 1.0-mM L-valine at 37 ^oC for 2 h. Cell viability and expressions of α-lactalbumin and β-casein were measured after culture with L-valine for 3 days. L-[1-¹⁴C]valine was used to study valine catabolism, whereas [³H]phenylalanine was employed as a tracer to determine protein synthesis and degradation in PMECs. The abundances of proteins involved in the mTOR signaling pathway and the mRNA levels for the related key genes were determined using the western blot and RT-PCR techniques, respectively. Cell numbers and the synthesis of proteins (including α-lactalbumin and β-casein) were greater (P < 0.05) in the presence of 0.5-mM L-valine, compared with 0.05- or 0.1-mM L-valine. L-Valine at 0.5 mM also enhanced (P < 0.05) the production of α-lactalbumin by PMECs, in comparison with 0.25 mM L-valine. Increasing the extracellular concentration of L-valine from 0.05 to 0.5 mM stimulated protein synthesis in a concentration-dependent manner without affecting proteolysis. Although L-valine was actively transaminated in PMECs, its α-ketoacid product (α-ketoisovalerate) at 0.05-0.2 mM did not affect protein synthesis or degradation in the cells. Thus, the effect of L-valine on protein synthesis was independent of its metabolism to yield α-ketoisovalerate. At the molecular level, 0.5-mM L-valine increased (P < 0.05) the mRNA levels for Ras, ERK1/2, and p70S6K, and the abundances of mTOR, p-4EBP1, total 4EBP1, p-ERK1/2, and total ERK1/2 proteins. These findings establish the critical role of L-valine in enhancing PMEC growth and milk protein synthesis possibly by regulating the mTOR and Ras/ERK signaling pathways. Further studies are warranted to understand how L-valine regulates gene expression and mTOR activation in PMECs.

Fabrication and in vitro evaluation of a packed-bed bioreactor based on an optimum two-stage culture strategy

A packed-bed (PB) bioreactor for bioartificial liver (BAL) was fabricated based on an optimum two-stage culture strategy and evaluated in vitro in this research. Human induced hepatocytes (hiHeps) were first expanded using Cytodex 3 microcarriers and the choice of microcarrier concentration and fetal bovine serum (FBS) content was optimized. Then, the cells expanded under the optimum expansion condition were perfused into a perfusion system containing Fibra-Cel (FC) disks to fabricate a PB bioreactor. Operating parameters including flow rate and seeding density for perfusion culture were optimized, respectively. Results indicated that during suspension culture, rapid cell proliferation and favorable amino acid metabolism were achieved at 3 mg/mL microcarriers combined with 1% FBS. While for the perfusion culture, the most effective flow rate and seeding density were 2 mL/min and 1 × 10⁶ cells/mL, respectively. Under this optimum perfusion condition, hiHeps showed good proliferation ability, high viability, homogeneous distribution, high metabolism activities and efficient albumin secretion as well as high liver-specific genes expression. Therefore, the two-stage culture strategy based on operating parameters optimization provides a new method for the development of PB bioreactors.

Effect of amino acids on expression of signal molecules in organotypic culture of the spleen

Opposite effects of amino acids on proliferation and differentiation of immune cells in organotypic culture of the spleen were demonstrated. It was found that methionine stimulates differentiation of CD5(+) lymphocyte precursors into T-helpers, cytotoxic T lymphocytes, and B cells and induces proliferation of CD68(+)-macrophages. Glutamic acid and arginine induce proliferation of B cells, while histidine and leucine promote differentiation of precursors towards cytotoxic T cells. The opposite effects of amino acids on immune cells of the spleen are related to different hydrophobicity of their side chains determining the type of intermolecular interaction.

Isolation and culture of adult mouse hepatocytes

The liver performs a multitude of functions including the regulation of carbohydrate, fat, and protein metabolism, the detoxification of endo- and xenobiotics, and the synthesis and secretion of plasma proteins and bile. Isolated hepatocytes constitute a useful technique for studying liver function in an in vitro setting. Here we describe a method for the isolation of hepatocytes from adult mouse liver. The principle of the method is the two-step collagenase perfusion technique which involves sequential perfusion of the liver with ethylenediaminetetraacetic acid and collagenase. Following isolation, the cells can either be used for short-term studies or, alternatively, maintained in culture for prolonged periods to study long-term changes in gene expression. The protocol for mouse hepatocyte isolation may be applied to both normal and transgenic mice.

Tumor necrosis factor (TNF) receptor-2-mediated DNA synthesis and proliferation in primary cultures of adult rat hepatocytes: The involvement of endogenous transforming growth factor-alpha

We investigated the effects of tumor necrosis factor (TNF)-alpha on DNA synthesis and proliferation, and its signal transduction pathways in primary cultures of adult rat hepatocytes. TNF-alpha induced time- and dose-dependent increases in hepatocyte DNA synthesis and proliferation. The hepatocyte proliferation stimulated by 30 ng/ml TNF-alpha was significantly inhibited by anti-TNF receptor 2 antibody, but not by anti-TNF receptor 1 antibody. TNF-alpha-induced hepatocyte DNA synthesis and proliferation were blocked by AG1478 (10(-7) M), PD98059 (10(-6) M), LY 294002 (10(-7) M), and rapamycin (100 ng/ml). TNF-alpha at 30 ng/ml significantly increased phosphorylation of receptor tyrosine kinase (175 kDa) and p42 mitogen-activated protein (MAP) kinase. This data suggests that the proliferative signal for primary cultured hepatocytes induced by TNF-alpha is mediated by TNF receptor 2 and the receptor tyrosine kinase/MAP kinase pathway. In addition, TNF-alpha-induced hepatocyte mitogenesis was significantly blocked by somatostatin (10(-6) M), adenylate cyclase inhibitor dideoxyadenosine (10(-7) M), protein kinase A inhibitor H-89 (10(-7) M), and neutralizing antibody to transforming growth factor (TGF)-alpha in culture. Indeed, 30 ng/ml TNF-alpha was found to rapidly stimulate secretion of TGF-alpha, and this secretion was also blocked by anti-TNF receptor 2 antibody. Moreover, TGF-alpha secretion induced by TNF-alpha was suppressed by dideoxyadenosine, H-89, and somatostatin. Together, these results indicate that stimulation of TNF receptor 2 by 30 ng/ml TNF-alpha induces autocrine secretion of TGF-alpha via the adenylate cyclase/protein kinase A pathway, after which TGF-alpha induces hepatocyte DNA synthesis and proliferation through the TGF-alpha receptor-linked tyrosine kinase (175 kDa)/MAP kinase signaling system.

Regulation of DNA synthesis in mouse embryonic stem cells by transforming growth factor-alpha: involvement of the PI3-K/Akt and Notch/Wnt signaling pathways

This study examined the mechanisms by which transforming growth factor (TGF)-alpha regulates proliferation of mouse embryonic stem (ES) cells. TGF-alpha increased [3H] thymidine and BrdU incorporation in a time- (0-72 h) and dose-dependent (0-10 ng/ml) manner. TGF-alpha stimulated the phosphorylation of Akt, mammalian target of rapamycin (mTOR), p70S6K1 and p44/42 mitogen-activated protein kinases (MAPKs). TGF-alpha also increased the protein levels of Notch, Notch intracellular domain, Hes-1 and Wnt1. However, TGF-alpha-induced DNA synthesis was blocked by inhibition of Akt, mTOR, p44/42 MAPKs and Notch. TGF-alpha increased the gene expression of c-jun, c-myc and c-fos. Moreover, TGF-alpha increased cyclin D/CDK 4 and cyclin E/CDK 2 levels, while decreasing p21cip1/waf1 and p27kip1, which were blocked by the inhibition of Akt, mTOR and Notch. In conclusion, TGF-alpha regulated DNA synthesis of mouse ES cells via PI3-K/Akt, p44/42 MAPKs and Notch/Wnt pathways.

Isoleucine, an essential amino acid, prevents liver metastases of colon cancer by antiangiogenesis

In spite of recent advances in the treatment of colon cancer, multiple liver metastases of colon cancer are still difficult to treat. Some chemotherapeutic regimens have been reported to be efficient, but there is a high risk of side effects associated with these. Here, we show that isoleucine, an essential amino acid, prevents liver metastases in a mouse colon cancer metastatic model. Because isoleucine is a strong inducer of beta-defensin, we first hypothesized that it prevented liver metastases via the accumulation of dendritic cells or memory T cells through up-regulation of beta-defensin. However, neither beta-defensin nor immunologic responses were induced by isoleucine in both mouse livers and spleens. Furthermore, isoleucine prevented liver metastasis in nude mice, which lack T cells and natural killer T cells. Finally, we discovered a novel mechanism of isoleucine: down-regulation of angiogenesis via inhibition of vascular endothelial growth factor, partially through the mammalian target of the rapamycin pathway, independent of hypoxia-inducible factor 1-alpha. Importantly, isoleucine is safe for administration to humans because it does not affect cell viability. Isoleucine could be a novel prophylactic drug for the prevention of liver metastases of colon cancer.