DNA strand scission and ADP-ribosyltransferase activity during murine erythroleukemia cell differentiation

The accumulation of DNA strand breaks and activation of ADP-ribosyltransferase (ADPRT) have recently been associated with cellular differentiation. Murine erythroleukemia (MEL) cells undergo erythropoietic differentiation when exposed to dimethyl sulfoxide (Me2SO) and several studies have suggested that DNA strand scission induced by this agent is a prerequisite for expression of the differentiated phenotype. Me2SO induction of MEL cells has also been associated with increases in ADPRT activity in one study, but not in another. We have monitored the effects of Me2SO on DNA strand breaks in preformed and replicating MEL cell DNA. The results clearly demonstrate that DNA fragmentation is not detectable during Me2SO induction of MEL differentiation, even in the presence of 3-aminobenzamide, an inhibitor of ADPRT. Further, these results are consistent with an absence of detectable changes in both endogenous and total potential ADPRT activity during Me2SO-induced MEL differentiation. These findings would argue against Me2SO induction of DNA strand scission and ADPRT in MEL cells undergoing differentiation.     

Increased ornithine decarboxylase activity and polyamine biosynthesis are required for optimal cytolytic T lymphocyte induction

The objective of the present investigation was to evaluate the requirement for increased ornithine decarboxylase (ODC) activity and polyamine biosynthesis in the induction of cytolytic T lymphocytes (CTL). In this regard, we have utilized alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ODC. DFMO treatment completely abrogated Con A-induced NW T-cell ODC activity. Similarly, DFMO treatment reduced putrescine and spermidine biosynthesis 100 and 87% respectively by the end of a 48-hr incubation period. Polyamine depletion reduced the Con A-mediated polyclonal induction of CTL by 52 and 81% at 24 and 48 hr of culture, respectively. The effect of DFMO on CTL induction could be reversed by the addition of exogenous putrescine. These data indicate that the observed effects of DFMO on CTL induction were mediated through inhibition of polyamine biosynthesis. Therefore, increased ODC activity and polyamine biosynthesis are required for optimal CTL induction. Furthermore, polyamine depletion did not impair IL-2 production; however, IL-2-dependent proliferation was reduced. These data are the first to discriminate between the requirement for polyamines with regard to IL-2 responsiveness, rather than IL-2 production, during a primary T-cell mitogenic response.     

Effect of c-myc gene expression on early inducible reactions required for erythroid differentiation in vitro.

By employing cell fusion between two genetically marked mouse erythroleukemia (MEL) cells in which an artificially introduced c-myc gene had been placed under the control of human metallothionein promoter, we investigated the mechanism of the suppressive action of c-myc gene expression in erythroid differentiation. The results indicated that the expression of the c-myc gene blocked the induction of dimethyl sulfoxide-inducible activity, one of the two early activities required for triggering the differentiation.     

Regulation of ornithine decarboxylase mRNA translation by polyamines. Studies using a cell-free system and a cell line with an amplified ornithine decarboxylase gene

The translational control of ornithine decarboxylase (ODCase) by polyamines has been studied using a cellular as well as a cell-free system. A mutant L1210 cell line, in which ODCase represents 4-5% of all soluble protein synthesized, was isolated by stepwise selection for resistance to the ODCase inhibitor 2-difluoromethylornithine (DFMO). The exceptionally high expression of ODCase in these cells was due to amplification of the ODCase gene. When the cells were grown in the absence of DFMO, dramatic increases in cellular putrescine and spermidine levels occurred. These increases were accompanied by a rapid decrease in ODCase synthesis. The change in ODCase synthesis was not associated with an alteration in the amount of ODCase mRNA, demonstrating a translational control in these cells. The effects of polyamines on ODCase mRNA translation were also studied in rabbit reticulocyte lysates using mRNA isolated from the DFMO-resistant cells. Low concentrations of spermidine stimulated synthesis of ODCase and that of total protein, when added to gel-filtered lysates. Notably, optimal stimulation of ODCase synthesis was achieved at a spermidine concentration lower than that required for an optimal rate of total protein synthesis. Higher concentrations of spermidine were inhibitory, and their effects of ODCase synthesis were stronger than on protein synthesis in general, resulting in a decrease in the fraction of protein synthesis accounted for by ODCase. The present results demonstrate that at least part of the feedback regulation of ODCase exerted by the polyamines is due to direct inhibition of ODCase mRNA translation.