Modulation of cytokeratin and actin gene expression and fibrillar organization in cultured rat hepatocytes.

Intermediate filaments of rat hepatocytes are composed of cytokeratins 8 and 18 (CK8 and CK18, respectively). Recent work from our laboratory has indicated a close relationship between the synthesis of these cytokeratins, their organization into intermediate filaments, and the promotion of growth and differentiation of cultured rat hepatocytes by insulin, epidermal growth factor, and dexamethasone. In the present study, we examined the mRNA expression, level of protein synthesis, and fibrillar distribution of cytokeratins 8 and 18 and actin in hepatocytes, isolated from normal and dexamethasone-injected rats and cultured as monolayers or spheroids in the presence of insulin, or from normal rat hepatocytes, cultured as monolayers in the presence of dexamethasone, insulin, and dimethyl sulfoxide. The CK8 mRNA level was lower in hepatocytes isolated from noninjected rats and cultured as either monolayers or spheroids, than in those from dexamethasone-injected rats. However, the CK18 mRNA level varied in a manner that was different from that of CK8 mRNA, showing that the modes of expression of the two genes were independent. The various changes in hepatocyte culture conditions led to variations in albumin mRNA levels that largely followed those observed in CK8 mRNA levels. In the case of actin, the amount of mRNAs varied from relatively high levels in hepatocyte monolayers to extremely low levels in hepatocyte spheroids, even though in both cases the cells were isolated from dexamethasone-injected rats. These changes in mRNA levels did not necessarily correlate with changes in the synthesis of cytokeratins 8 and 18, and actin. Changes in culture conditions induced a major reorganization in the distribution of cytokeratin intermediate filaments and actin filament between the region near the surface membrane and the cytoplasm. The most divergent patterns in cytokeratin intermediate filaments and actin filament distributions were observed between hepatocytes cultured as spheroidal aggregates and as monolayers in the presence of dimethyl sulfoxide. The former condition resulted in patterns of cytokeratin and actin gene expression and fibrillar organization that best matched those in situ. In the latter condition, inappropriate patterns were obtained, in spite of the fact that dimethyl sulfoxide treated hepatocytes are known to exhibit survival and functional activities equivalent to that of hepatocyte spheroids. These results demonstrate for the first time that the survival and functional activity (i.e., albumin production) of rat hepatocytes in vitro is not necessarily correlated with a particular pattern of cytokeratin and actin gene expression and fibrillar arrangement.     

Arachidonic acid suppression of fatty acid synthase gene expression in cultured rat hepatocytes

Rat hepatocytes were maintained in a serum-free, hormonally defined medium supplemented with 50-500 microM albumin-bound 20:1 (n-9) vs 20:4 (n-6). The induction of fatty acid synthase mRNA by a mix of insulin/dexamethasone/T3 was inhibited in a dose dependent fashion by 20:4 (n-6). The abundance of beta-actin mRNA was not suppressed by 20:4 (n-6). The expression of fatty acid synthase was actually stimulated 2-fold by 20:1 (n-9). It would appear that the in vivo inhibition of fatty acid synthase gene expression by dietary polyunsaturated fatty acids is a specific hepatocelluar event.     

Regulation of gene expression in adult rat hepatocytes cultured on a basement membrane matrix

Freshly isolated adult rat hepatocytes, when cultured on type I collagen (commercially available as Vitrogen), assume a polygonal shape, form a stable monolayer within 24 hours, but lose the capacity to express some liver-specific functions over time in culture. We incubated hepatocytes in a serum-free medium on a reconstituted basement membrane gel, "matrigel" (prepared from an extract of extracellular matrix of the murine Engelbreth-Holm-Swarm sarcoma), and observed that the cells adhered firmly, remained rounded as single cells or clusters, and maintained liver-specific gene expression for more than 1 week in vitro. Hepatocytes on matrigel secreted substantially higher amounts of albumin, transferrin, haptoglobin, and hemopexin, Northern blot analyses of extracted cellular RNA, expressed increased amounts of mRNA for the liver-specific protein albumin (as compared with cells on vitrogen). In cultures treated with phenobarbital, cytochrome P-450b, and cytochrome P-450e, mRNAs and proteins were barely detectable in cells on Vitrogen but were induced to levels similar to those in the liver in vivo in matrigel cultures. Likewise, the use of matrigel greatly enhanced the induction of mRNA and protein for P-450c by 3-methylcholanthrene and for P-450p by steroidal and nonsteroidal inducers. However, neither substratum permitted induction of P-450d by 3-methylcholanthrene, suggesting that the effects of matrigel are selective even for expression in liver of members of the superfamily of cytochrome P-450 genes. Within 5 days in cultures on Vitrogen, hepatocytes expressed detectable amounts of fetal liver aldolase activity and also mRNA for vimentin and type I collagen, each considered a phenotypic change reflecting hepatocyte "dedifferentiation." None of these was present in cells on matrigel. Responsiveness to mitogenic stimuli, as judged by incorporation of 3H-thymidine into DNA, was also decreased in hepatocytes cultured on matrigel. Finally, there was a remarkable increase in the levels of both matrices during the first 2 days in culture. However, the continuously cytoskeleton mRNA over time in culture than did the rounded cells on matrigel. We conclude that hepatocytes cultured on matrigel, as opposed to the standard collagen, exhibit remarkably enhanced expression of many liver-specific functions.     

Age-related decline of inducible nitric oxide synthase gene expression in primary cultured rat hepatocytes

Hepatocytes exhibit a non-specific immune response by expressing the enzyme inducible nitric oxide (NO) synthase (iNOS, NOS2) through the stimulation of a mixture of cytokines, or a single cytokine such as interleukin-1beta. We examined the age-dependent inducibility of the iNOS gene expression and the capacity of NO production in response to lipopolysaccharide (LPS) or interleukin-1beta (IL-1beta) in primary cultured rat hepatocytes that were isolated from the livers of rats, 3 (young) and 24 (aging) months of age. NO production (NO2-), indicating iNOS activity, was much higher in the young rat hepatocytes following stimulation with LPS or IL-1beta. Likewise, in the young hepatocytes, Western blot analyses showed much higher protein levels in the iNOS expression; it was also a little higher in mRNA levels that were analyzed by RT-PCR. Furthermore, after stimulation with IL-1beta, the levels of transactivation of nuclear factor-KB (NF-kappaB) that were involved in the induction of the iNOS gene were reduced without a significant difference in the aged cells. Therefore, the decrease of NO formation in the aged hepatocytes was due to the belated and incomplete inducibility of the iNOS protein expression, together with a minor contribution of the reduced-transactivation of NF-kappaB. These results suggest that the age-related decline of the iNOS gene expression in primary rat hepatocytes may be associated with the increased incidence of many infective diseases with aging.     

Hormonal modulation of hepatic plasma membrane lactate transport in cultured rat hepatocytes

Hormonal modulation of hepatic plasma membrane lactate transport was studied in primary cultures of isolated hepatocytes from fed rats to examine the mechanism for the known enhancement of lactate transport in starvation and diabetes. Total cellular lactate entry was increased by 14% in the presence of dexamethasone; this was accounted for by an approximately 40% increase in the carrier-mediated component of entry with no effect on diffusion. A trend of similar magnitude was evident with glucagon. The effects of dexamethasone and glucagon on lactate transport constitute an additional potential mechanism for enhancement of gluconeogenesis by these hormones.     

转录后水平沉默与基因表达

基因沉默是1个非常复杂和普遍的现象。转录后水平的基因沉默是指转基因在细胞核里能稳定转录,细胞质里却无相应的稳定态mRNA存在的现象。它往往被称为共抑制、静息作用或RNA干预等。本文介绍了转录后水平的基因沉默现象的发现、分子机理和应用等方面的进展。提出了克服转录后水平基因沉默的一些对策。     

Influence of rat strain on P-glycoprotein expression in cultured hepatocytes

The amount and activity of the multi-drug transport P-glycoprotein (Pgp) have been measured in cultured hepatocytes derived from different rat strains. A marked increase in Pgp, as revealed by Western blotting, occurred 48 h after seeding in hepatocytes from Sprague-Dawley, Wistar and Fischer 344 rats, the last showing the highest value. The addition of dexamethasone (DEX) to culture medium delayed Pgp overexpression in all the strains, proportionally to the protein amount in the absence of hormone. The R-123 functional test for Pgp showed that Fischer 344 hepatocytes had the lowest ability to extrude the fluorescent dye as compared with Sprague-Dawley or Wistar rats. These results suggest that the Fischer 344 rat is more prone than other strains to culture stressing conditions, leading to an overexpression of Pgp that is not necessarily functional.Abbreviations DEX dexamethasone - FCS fetal calf serum - HRP horseradish peroxide - Pgp P-glycoprotein     

NF-kappaB activates fibronectin gene expression in rat hepatocytes

Resveratrol (RSVL), an edible polyphenolic stilbene, claims a myriad of cardiovascular benefits. However, the molecular underpinnings of such actions are poorly understood. Currently, in sheep coronary arteries (SCA), RSVL markedly (threefold) enhanced cGMP formation (t(1/2): 6.5 min; EC(50): 3 microM). This response was not abrogated by the phosphodiesterase inhibitor (IBMX, 0.5 mM), but was partly sensitive (20-30%) to either removal of the endothelium, treatment with the nitric oxide synthase-inhibitor (L-NMMA, 10 microM), or with the soluble GC (sGC)-inhibitor (ODQ, 10 microM). In membrane preparations from denuded SCA, either RSVL or the pGC agonist atrial natriuretic peptide (ANP, 0.1-1 microM) activated GC in the particulate, but not in the soluble, membrane fraction. By contrast, the nitric oxide donor, sodium nitroprusside (SNP, 1-10 microM), stimulated GC only in the soluble fraction. Further, pretreatment with RSVL partly desensitized the ANP response, but was additive to that of SNP. In arterial tension studies, RSVL relaxed PGF(2alpha)-precontracted denuded rings in a concentration-dependent manner, a response that was markedly enhanced (approximately 18 fold) in the presence of IBMX. Conversely, precontraction with phorbol ester, which also desensitizes pGC, blunted relaxations to RSVL but not to forskolin or SNP. These findings demonstrate that RSVL increases cGMP in coronary arteries, mostly by activation of pGC. This pathway triggers vasorelaxant responses that remain effective in endothelium-disrupted arteries.     

Norepinephrine stimulates interleukin-6 mRNA expression in primary cultured rat hepatocytes

Non-invasive immobilization stress causes an increase in the plasma interleukin (IL)-6 level accompanied by increased IL-6 mRNA expression and IL-6 immunoactivity in the liver [Biochem. Biophys. Res. Commun. (1997) 238, 707-711]. In the present study, using rat primary cultured hepatocytes and non-parenchymal liver cells, the effect of norepinephrine (NE) on IL-6 mRNA expression was determined. IL-6 mRNA expression in hepatocytes, but not in non-parenchymal liver cells, increased when the cells were treated with NE. The stimulatory effect of NE was inhibited by the combined use of alpha- and beta-adrenergic antagonists. IL-6 mRNA expression in hepatocytes also increased on incubation with the culture medium of non-parenchymal liver cells treated with NE. The effect of the medium was blocked by an IL-1 receptor antagonist. Moreover, exogenous IL-1beta stimulated IL-6 mRNA expression in hepatocytes. IL-1beta was present in the medium of non-parenchymal liver cells and increased with NE-treatment. These results suggest that NE released from sympathetic nerve terminals during stress can directly increase IL-6 mRNA expression in hepatocytes and indirectly through IL-1beta production from non-parenchymal liver cells.     

Regulation of the expression of the phosphoenolpyruvate carboxykinase gene in cultured rat hepatocytes by glucagon and insulin

The induction of phosphoenolpyruvate carboxykinase (PEPCK) by glucagon was studied in primary rat hepatocyte cultures by determining the time course of the sequential events, increases in the enzyme's mRNA abundance, synthesis rate, amount and activity, and by investigating the antagonistic action of insulin on the induction by glucagon. 1. The mRNA of PEPCK was induced maximally 2-3 h after addition of 10 nM glucagon, as detected by Northern-blot analysis after hybridization with a biotinylated antisense RNA of PEPCK. 2. The synthesis rate of PEPCK increased maximally 2-3 h after application of glucagon as revealed by pansorbin-linked immunoprecipitation of [35S]methionine-labelled PEPCK. 3. The enzyme amount and activity was maximally induced 4 h after glucagon application. 4. The mRNA of PEPCK was half-maximally induced by 0.1 nM and maximally by 1 nM and 10 nM glucagon. The half-maximal induction by 0.1 nM glucagon was antagonized almost totally, and the maximal induction by 1 nM glucagon partially, while the maximal induction by 10 nM glucagon remained unaffected by 10 nM insulin. The results show that in cultured rat hepatocytes physiological concentrations of glucagon stimulated the induction of PEPCK by an increase in mRNA, that the glucagon-dependent increase in mRNA and enzyme-synthesis rate occurred in parallel and preceded the increase of enzyme amount and activity by 1-1.5 h, and that physiological levels of insulin antagonized the induction by glucagon in the physiological concentration range, with glucagon being the dominant hormone.