Comparison of human adult and fetal expression and identification of 535 housekeeping/maintenance genes.

Gene expression levels of about 7,000 genes were measured in 11 different human adult and fetal tissues using high-density oligonucleotide arrays to identify genes involved in cellular maintenance. The tissues share a set of 535 transcripts that are turned on early in fetal development and stay on throughout adulthood. Because our goal was to identify genes that are involved in maintaining cellular function in normal individuals, we minimized the effect of individual variation by screening mRNA pooled from many individuals. This information is useful for establishing average expression levels in normal individuals. Additionally, we identified transcripts uniquely expressed in each of the 11 tissues.

TNF-alpha modulates expression of the tissue transglutaminase gene in liver cells.

One of several postulated roles for tissue transglutaminase (tTG) is the stabilization and assembly of extracellular matrix via peptide cross-linking. We previously determined that tTG activity increased in an animal model of hepatic fibrogenesis and in human liver disease. To further study the role of tTG in liver disease, we initiated investigations into the effect of a proinflammatory mediator, tumor necrosis factor (TNF)-alpha, on tTG activity in cultured liver cells. Treatment of human Hep G2 cells with 1 ng/ml TNF-alpha increased [14C]putrescine cross-linking to cellular proteins. An increase in tTG mRNA content was observed 1 h after addition of TNF-alpha, and levels of tTG mRNA remained elevated after 24 h. Hep G2 cells, transiently transfected with a luciferase reporter containing 1.67 kb of the human tTG promoter, showed an increase in reporter activity after addition of TNF-alpha. Gel shift experiments using nuclear extracts from TNF-alpha-treated cells and oligonucleotides containing the tTG nuclear factor (NF)-kappa B motif revealed increased binding, concordant with mRNA data. Transient transfections with a truncated reporter construct lacking the tTG NF-kappa B sequence showed an attenuated response to TNF-alpha treatment. Similar responses were seen in stably transfected HeLa cells. Primary hepatocytes isolated from a transgenic mouse line containing the mouse tTG promoter driving the beta-galactosidase reporter, show similar time-dependent increases in promoter activity when treated with TNF-alpha. Furthermore, Hep G2 cells are incapable of upmodulating tTG promoter reporter activity in the presence of TNF-alpha when those cells overexpress a transdominant, negative mutant NF-kappa B subunit. Because TNF-alpha expression is upregulated in hepatic inflammation, the data suggest TNF-alpha-mediated increases in tTG expression may play an important role in the process of hepatic fibrogenesis.

The antioxidant function of the physiological content of vitamin C.

The synthesis of vitamin C is substantially reduced in Osteogenic Disorder Shionogi (ODS) rats. Hepatocytes prepared from these rats contained approximately 12% of the wild-type content of this vitamin. In culture, the ascorbate content remained low in the absence of supplementation of the medium. Independent of their vitamin C status, cultured hepatocytes become depleted of vitamin E. Supplementation of the culture medium with 10C microM ascorbate and 1.2 microM alpha-tocopherol phosphate maintained the physiological content of both vitamins C and E in ODS hepatocytes. Thus, the antioxidant function of vitamins C and E could be evaluated in the presence of both or either vitamin or in the absence of both vitamins. Hepatocytes deficient in both vitamins were the most susceptible to lipid peroxidation (as measured by thiobarbituric acid) and to cell kllling within a 90-min exposure to 125-500 microM tert-butyl hydroperoxide (TBHP). Supplementation to achieve a physiological content of both vitamins C and E reduced the evidence of lipid peroxidation and abolished the cell killing. Supplementation with either vitamin alone resulted in an intermediate degree of both lipid peroxidation and cell killing. In ODS hepatocytes treated with TBHP, the decline in vitamin E preceded the decline in vitamin C. In ODS hepatocytes depleted of vitamin C, the loss of vitamin E after exposure to TBHP was greater than that in the presence of physiological levels of ascorbate. This greater loss of vitamin E in the face of a depletion of vitamin C was readily attributable to the increased peroxidation of lipids. Thus, the physiological level of vitamin C in cells does not seem to regenerate vitamin E. In contrast, the rate and extent of the depletion of vitamin C correlate with the degree of cell killing. These data document the antioxidant function of the physiological level of cellular vitamin C and relate this function to protection against peroxidative cell injury.