A comparison of adrenergic receptors of rat ascites hepatoma AH130 cells with those of normal rat hepatocytes

Regulation of alpha 1b-adrenergic receptor gene expression in rat liver cell lines

alpha 1b-Adrenergic receptor gene expression was investigated in two rat hepatic cell lines, Clone 9 and McA-RH7777 cells. By Northern blot analysis, Clone 9 cells expressed a 2.7 kb alpha 1b-adrenergic receptor gene transcript whereas two transcripts, 3.3 kb and 2.7 kb, were observed in total cellular RNA isolated from rat liver. A binding site for the alpha 1-adrenergic antagonist [3H]prazosin was observed in Clone 9 cell membrane preparations (Bmax = 47 +/- 7 fmol/mg protein and Kd = 0.11 +/- 0.02 nM, n = 5). In contrast, alpha 1b-adrenergic receptor gene transcripts could not be detected in total cellular RNA prepared from McA-RH7777 cells by either Northern blot analysis or ribonuclease protection assays. However, results from nuclear run-off assays indicated that the alpha 1b-adrenergic receptor gene was transcribed in McA-RH7777 cells and alpha 1b-adrenergic receptor gene transcripts were observed in McA-RH7777 cell nuclear RNA. These results suggest that alpha 1b-adrenergic receptor gene expression in liver may be regulated in part post-transcriptionally and that this level of regulation may be altered or disrupted in the Clone 9 and McA-RH7777 cell lines.

Effects of isoquinolinesulphonamide compounds on multidrug-resistant P388 cells

The effects of eight isoquinolinesulphonamide compounds on resistance to vinblastine in adriamycin-resistant mouse leukaemia cells (P388/ADR) which overexpress the relative molecular weight (M(r)) 140 kDa P-glycoprotein in the plasma membrane were investigated. N-[2-(Methylamino)ethyl]-5-isoquinolinesulphonamide (H-8) and N-(2-aminoethyl)-5-isoquinolinesulphonamide (H-9) did not reverse vinblastine resistance. N-[2-[N-[3-(4-Chlorophenyl)-2-propenyl]amino] ethyl]-5-isoquinolinesulphonamide (H-86) and N-[2-[N-[3-(4-chlorophenyl)-1-methyl-2-propenyl] amino]ethyl]-5-isoquinolinesulphonamide (H-87) caused accumulation of intracellular vinblastine and inhibition of vinblastine efflux from the cells and reversed the resistance. Addition of an aminoethyl group to the nitrogen atom of the sulphonamide group (W-66) or a formyl group at the terminal amino group (H-85) of H-86 reduced those activities. Conversion of the chlorophenyl group of H-87 to pyridinyl (H-31) or furanyl (H-34) markedly decreased activities against the drug resistance. The activity against vinblastine accumulation closely correlated with the apparent partition coefficient of compounds. These compounds dose-dependently inhibited photoaffinity labelling of a photosensitive analogue of vinblastine, N-(p-azido-(3-[125I)salicyl)-N'-beta-aminoethyl-vindesine ([125I]NASV), and there was a good correlation between inhibition of [125I]NASV-photolabelling and hydrophobicity. Although these isoquinolinesulphonamides inhibited protein kinase A with different magnitudes, this activity did not correlate with the effect on the drug resistance. These results indicate that isoquinolinesulphonamide compounds with a hydrophobic group interact with antitumour drugs on P-glycoprotein and reverse multidrug resistance without involvement of their activity on protein kinase A.

Changes in number of alpha-adrenergic receptor subtypes in hepatocytes from rats fed 3'-methyl-4-dimethylaminoazobenzene

Changes in numbers of alpha 1- and alpha 2-adrenergic receptors in the plasma membranes of hepatocytes from female Donryu rats given feed containing 0.06% of the carcinogen 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB), were examined. alpha 1-Adrenergic receptors, measured in terms of [3H]prazosin binding, decreased to half of the control 2 weeks after the start of this diet, then gradually decreased for the next 22 weeks. alpha 2-Adrenergic receptors, measured in terms of [3H]clonidine binding, transiently increased 3-fold over the control at 2 weeks. These changes in the early period of the 3'-MeDAB diet intake may be related to hepatocarcinogenesis.

Forskolin inhibits the Gs-stimulated adenylate cyclase in rat ascites hepatoma AH66F cells

Forskolin increased intracellular cyclic AMP and augmented cyclic AMP formation by prostaglandin E1 (PGE1) in normal rat hepatocytes and ascites hepatoma AH66 cells. However, in AH66F cells which were derived from the AH66 cell line, the diterpene only slightly increased the cyclic AMP level, and dose-dependently inhibited the accumulation caused by PGE1. Forskolin dose-dependently activated adenylate cyclase in these membranes, and the magnitude of activation by forskolin was largest in the following order: hepatocytes, AH66 cells, and AH66F cells. This difference may be based on the number of forskolin-binding sites. The binding affinity of forskolin for each cell membrane was similar. The number and affinity of forskolin-binding sites in these cells were not influenced by 5'-guanylylimidodiphosphate [Gpp(NH)p]. In hepatocytes and AH66 cells, forskolin and other adenylate cyclase activators such as PGE1, GTP, Gpp(NH)p, F-, and Mn2+ synergistically increased the enzyme activity. In AH66F cells, the forskolin-stimulated activity was hardly influenced by the GTP analog, and forskolin diminished the activities induced by the GTP analog in a manner similar to that of diterpene alone. Forskolin (10 microM) also significantly inhibited the activities induced by PGE1, GTP, and F-. The effect of forskolin with Mn2+ was additive in AH66F cells. The data suggest that forskolin promotes the interaction between the stimulatory guanine nucleotide-binding protein and the catalytic unit in the membrane of normal hepatocytes and AH66 cells, but it interferes with the coupling in AH66F cells.