Extinction and expression of the ribose-positive phenotype in hybrid Novikoff hepatoma cells

Systematic shut-off of the hormone receptors in intraspecific adrenal x Leydig cell hybrids

The mouse Y1 adrenal cell line was fused with mouse Leydig cells in primary culture. The selected hybrids were examined for their response to gonadotropin (hCG) and ACTH. None of them bound specifically [125I]hCG, nor did they augment their cAMP production in response to gonadotropin or ACTH stimulation, whereas their adenylate cyclase remained responsive to forskolin and cholera toxin, thus indicating a repression of hCG receptor synthesis and probably a loss of ACTH receptors, rather than a lesion of the coupling between the hormone receptor complex and the adenylate cyclase. Basal pregnenolone production in 17 hybrids was close to that of Leydig and Y1 cells and was enhanced after 8-bromo adenosine 3',5'-monophosphate (8-Br-cAMP) stimulation in 11 of them. Therefore, the negative control leading to the extinction of both parental functions acts preferentially at the first step of steroidogenesis, i.e., the gene(s) coding for the hormone receptors.

Evidence for a trans-dominant regulator of purine nucleoside phosphorylase expression in rat hepatoma cells

Purine nucleoside phosphorylase (PNP) levels are modulated during the growth cycle of rat hepatoma cells and increase two- to three-fold as cells go from early exponential growth phase to stationary growth phase. A mutant of these hepatoma cells has been isolated which is deficient in PNP activity. Quantitative immunoprecipitation tests indicate that the decrease in enzyme activity is due to a decrease in the number of PNP molecules. The low level of PNP enzyme produced by the mutant, however, is indistinguishable from the wild-type enzyme, suggesting that the mutant may be defective in the ability to modulate PNP levels. Fusion of the mutant cells to wild-type parental cells results in hybrids that express the mutant phenotype. Segregants that arise from the hybrids show chromosome loss and reexpression of the wild-type parental phenotype, the mutant parental phenotype, and a 2S wild-type phenotype. These indicate the following about the defect in modulation in the mutant PNP-100: (1) it is trans dominant to the wild-type; (2) its effect is negative; (3) some genomic element is required for its continued effect; and (4) it does not act by obliterating its functioning counterpart in hybrid cells.

Altered expression of ribokinase activity in Novikoff hepatoma variants

Ribokinase, the first enzyme in ribose catabolism, is altered in its expression in ribose-utilizing Novikoff hepatoma variants. 90% of the variants selected for their ability to use D-ribose as a sole carbon source show a change in ribokinase activity. After non-selective growth, phenotypically unstable variants lose their altered expression and regain a parental form of expression of this enzyme. In the variants, ribokinase expression in non-inducible by the carbon source and is unaffected by the growth phase of the cells. However, ribokinase expression in both parental cells and variants is cell cycle-dependent. Parental Novikoff hepatoma cells have three peaks of ribokinase activity during the S, G2 and M phases. Variants are described which have high basal levels of ribokinase and only a single peak of enzymatic activity during the S phase. In addition to changes in the level of ribokinase, changes in the subcellular localization of the enzyme have been found in some variants. While the change in the level of ribokinase seems to be a property of the variant isolated, the change in subcellular location of ribokinase can be readily achieved by culture conditions.