Identification of cis-acting DNA elements involved in the regulation of angiotensinogen gene expression.

Angiotensinogen is the precursor molecule of one of the most potent vasoactive substances, angiotensin-II. Angiotensinogen is normally synthesized in the liver and secreted into the plasma where it is converted into angiotensin-II by the combined proteolytic action of renin and angiotensin converting enzyme. Angiotensinogen levels in the plasma are modulated by a number of pathological and physiological factors. In order to understand the regulation of angiotensinogen gene expression, we have constructed an expression vector in which 688 bp of the 5'-flanking region of the rat angiotensinogen gene were attached to the chloramphenicol acetyl transferase (CAT) coding sequence. We have also obtained 5'-sequential deletion mutants from the rat angiotensinogen promoter attached to the CAT gene, and have identified multiple cis-acting DNA sequences involved in the regulation of angiotensinogen gene expression by transient transfection of these recombinant DNA molecules in human hepatoma cell lines, Hep3B, and HepG2.     

Induction of mdr1b mRNA and P-glycoprotein expression by tumor necrosis factor alpha in primary rat hepatocyte cultures

Mammalian liver exhibits expression of members of the family of multidrug resistance (mdr) transporters (P-glycoproteins). P-glycoprotein isoforms encoded by mdr1 genes participate in extrusion of an array of xenobiotics into the bile. Induction of mdr1b mRNA expression has been shown to occur in rat hepatocytes in response to hepatotrophic growth factors. As the cytokine tumor necrosis factor alpha (TNF-α) is known to exert a direct mitogenic effect on hepatocytes, its influence on mdr1b expression was investigated. In primary rat hepatocytes cultured in the absence of TNF-α, a time-dependent increase in basal expression of mdr1b mRNA and in immunodetectable P-glycoprotein was observed. In cells treated with TNF-α (4,000 U/ml) for 3 days, expression of mdr1b mRNA and of immunodetectable P-glycoprotein was induced approximately twofold. Moreover, intracellular steady-state levels of the mdr1 substrate rhodamine 123 were decreased in cells pretreated with TNF-α in comparison to controls, indicating an increase in functional transporter(s) mediating dye extrusion. Treatment of hepatocytes with antioxidants (1 mM ascorbic acid and 2% dimethyl sulfoxide) for 3 days markedly suppressed mdr1b mRNA and P-glycoprotein expression both in cells cultured in the presence of TNF-α and in the absence of the cytokine, but did not fully abolish mdr1b mRNA induction by TNF-α, supporting the notion that reactive oxygen species participate in regulation of basal mdr1b gene expression during hepatocyte culture. In conclusion, the present data indicate that by inducing mdr1b expression in hepatocytes, TNF-α may affect the capacity of the liver for extrusion or detoxification of endogenous or xenobiotic mdr1 substrates. J. Cell. Physiol. 176:506–515, 1998. © 1998 Wiley-Liss, Inc.     

Puromycin selectively increases mdr1a expression in immortalized rat brain endothelial cell lines

The blood-brain barrier (BBB) plays an important role in controlling the passage of molecules from blood to brain extracellular fluid. The multidrug efflux pump P-glycoprotein (P-gp) is highly expressed in the luminal membrane of brain endothelium and contributes to the formation of a functional barrier to lipid-soluble drugs such as anticancer agents. The mdr1a P-gp-encoding gene is exclusively expressed in the rodent BBB. Primary cultures of rat brain endothelial cells and GP8.3 cells showed a dramatic decrease in mdr1a mRNA level and some expression of mdr1b mRNA. GPNT cells, derived from GP8.3 cells after transfection with a puromycin resistance gene, were chronically treated with 5 microg/mL puromycin, a P-gp substrate. Compared with rat brain endothelial cells and GP8.3 cells, GPNT cells exhibited a very high level of expression of mdr1a mRNA together with a moderate level of mdr1b mRNA expression. Accordingly, P-gp expression and activity were strongly increased. When GP8.3 and puromycin-starved GPNT cells were treated with puromycin, mdr1a expression was selectively increased. High expression of mdr1a mRNA in GPNT cells may thus be related to the chronic treatment with puromycin. We conclude that GPNT cells may be used as a valuable rat in vitro model for studying the regulation of mdr1a expression at the BBB level.     

Molecular mechanism of multidrug resistance in tumor cells

The ability of tumor cells to develop simultaneous resistance to multiple lipophilic cytotoxic compounds represents a major problem in cancer chemotherapy. This review describes recent molecular biological studies which resulted in the identification and cloning of the gene responsible for multidrug resistance in human tumor cells. This gene, designated mdr1, is overexpressed in all and amplified in many of the multidrug-resistant cell lines analyzed. Gene transfer and expression assays have indicated that the mdr1 gene is both necessary and sufficient for multidrug resistance. The product of the mdr1 gene is P-glycoprotein, a transmembrane protein which shares homology with several bacterial proteins involved in active membrane transport. P-glycoprotein appears to function as an energy-dependent efflux pump responsible for the removal of drugs from multidrug-resistant cells. The functions of the mdr system in normal cells and its potential clinical implications are discussed.