Genomic hypomethylation and far-5'' sequence alterations are associated with carcinogen-induced activation of the hamster thymidine kinase gene.

We have investigated the mechanism of activation of an inactive but functionally intact hamster thymidine kinase (TK) gene by the chemical carcinogen N-methyl-N'-nitro-N-nitrosoguanidine. Following carcinogen treatment of TK- RJK92 Chinese hamster cells, aminopterin-resistant (HATr) colonies appeared at a frequency 50-fold higher than in untreated controls. More than 80% of these HATr variants expressed TK enzymatic activity and were divided into high- and low-activity classes. In all TK+ variants, TK expression was correlated with demethylation in the 5' region of the TK gene and the appearance a 1,400-nucleotide TK mRNA. Using high-performance liquid chromatography to measure the level of genomic methylation, we found that four of five high-activity lines demonstrated extensive genomic hypomethylation (approximately 25% of normal level) that was associated with demethylation of all TK gene copies. Restriction endonuclease analysis of 15 low-activity lines revealed four instances of sequence alterations in the far-5' region of the TK gene and one instance of a tandem low-copy amplification. In these lines, the structurally altered gene copy was demethylated. Thus, we propose that a chemical carcinogen can activate TK expression by several different mechanisms. Focal demethylation with or without gene rearrangement was associated with low TK activity, whereas demethylation throughout the genome was associated with high TK activity.     

Classical conditioning, decay and extinction of cocaine-induced hyperactivity and stereotypy

Following 10 daily pairings of multiple conditioned stimuli with injection of cocaine (15 mg/kg), the presentation of the stimuli alone elicited behaviors in rats similar to those induced by cocaine. The behaviors included increased duration or frequency of rearing, sniffing, head bobbing, and horizontal locomotor activity (crossing). The level of the conditioned response for several of these behaviors approximated that induced by the drug itself. The conditioned drug effect showed decay over 15 days but little extinction during 4 daily trials. Brain concentrations of the dopamine metabolites, homovanillic acid and dihydroxyphenylacetic acid, were similar in the conditioned and pseudoconditioned control groups in both the caudate and mesolimbic areas. The behavioral results demonstrate that, in a classical conditioning paradigm, previously neutral stimuli can elicit behaviors similar to those induced by cocaine and that certain conditioned responses show time related decline. This agrees with the reported conditioning of amphetamine's behavioral effects but differs in terms of the action on brain dopamine turnover.     

A Calcium-Selective Site in Photosystem II of Spinach Chloroplasts

After acid-treatment of spinach (Spinacia oleracea) chloroplasts, various partial electron transport reactions are inactivated from 25 to 75%. Divalent cations in concentrations from 10 to 50 millimolar can partially restore electron transport rates. Two cation-specific sites have been found in photosystem II: one on the 3-(3,4-dichlorophenyl)-1, 1-dimethylurea-insensitive silicomolybdate pathway, which responds better to restoration by Mg²⁺ than by Ca²⁺ ions, the other on the forward pathway to photosystem I, located on the 2,5-dimethylbenzoquinone pathway. This site is selectively restored by Ca²⁺ ions. When protonated chloroplasts are treated with N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)aziridine, a carboxyl group modifying reagent, presumed to react with glutamic and aspartic acid residues of proteins, restoration of electron transport at the Ca²⁺-selective site on the 2,5-dimethylbenzoquinone pathway is impaired, while no difference in restoration is seen at the Mg²⁺ site on the 3-(3,4-dichlorophenyl)-1,1-dimethylurea-insensitive silicomolybdate pathway.

Trypsin treatment of chloroplasts modifies the light-harvesting pigment-protein complex, destroys the dibromothymoquinone-insensitive 2,5-dimethyl-benzoquinone reduction, but does not interfere with the partial restoration of activity of this pathway by Ca²⁺ ions, implying that the selective Ca²⁺ effect on photosystem II (selective Ca²⁺ site) is different from its effects as a divalent cation on the light-harvesting pigment-protein complex involved in the excitation energy distribution between the two photosystems.

     

Karyological and biochemical evidence for chromosomal dedifferentiation in neoplasia

Summary Quantitative analysis of the X-linked enzyme, glucose 6-phosphate dehydrogenase (G-6-PD), was performed on tumor cells lines from two human females. Both tumor cells were hyperdiploid, having complete or redundant C+X groups. One, no. 930, lacked the X chromatin body and exhibited twice the level of G-6-PD as in the X chromatin-positive tumor cells, ME-180. Hence, in the no. 930 cell, reversal of X chromosomal condensation was associated with loss of the X chromatin body and doubling of genetic activity. Cells of no. 930 were subsequently placed in culture where after three passages they developed an X chromatin body (or bodies). G-6-PD determinations made at that time showed enzyme levels comparable to the X chromatin-positive tumor cells (ME-180). This research was supported by United States Public Health Service Grant CA 08791-03.     

The Occurence of δ-Tocopherylquinone in Higher Plants and Its Relation to Senescence

δ-Tocopherylquinone (δTQ) content was determined in tobacco and yellow maple leaves, green ivy leaves and cactus tissues. It was found that the concentration of δ-TQ was highest in mature or senescent tissues, such as white tobacco leaves (0.02 μmole/g dry wt) while its detection was uncertain in young, green leaves from the apex of tobacco plants. Fractionation by centrifugation of senescent tobacco leaves showed that the osmiophilic globule fraction was enriched in δ-TQ. Electron microscope studies of young, mature and senescent tobacco tissues showed progressive changes in the size and number of osmiophilic globules. After chloroplast breakdown in senescent tobacco leaves, these globules became the predominant constituents of the organelle. δ-TQ which is associated with osmiophilic globules may play a role in the development of plants, particularly during senescence.     

The Occurence of delta-Tocopherylquinone in Higher Plants and Its Relation to Senescence

delta-Tocopherylquinone (deltaTQ) content was determined in tobacco and yellow maple leaves, green ivy leaves and cactus tissues. It was found that the concentration of delta-TQ was highest in mature or senescent tissues, such as white tobacco leaves (0.02 mumole/g dry wt) while its detection was uncertain in young, green leaves from the apex of tobacco plants. Fractionation by centrifugation of senescent tobacco leaves showed that the osmiophilic globule fraction was enriched in delta-TQ. Electron microscope studies of young, mature and senescent tobacco tissues showed progressive changes in the size and number of osmiophilic globules. After chloroplast breakdown in senescent tobacco leaves, these globules became the predominant constituents of the organelle. delta-TQ which is associated with osmiophilic globules may play a role in the development of plants, particularly during senescence.