Effect of benzamide on cell growth, NAD and ATP levels in cultured chick limb bud cells

3-aminobenzamide protects the mouse thymocytes in vitro from dexamethasone-mediated apoptotic cell death and cytolysis without changing DNA strand breakage

Exposure of mouse thymocytes to 1 microM dexamethasone (Dex) resulted in a dramatic increase in the degree of DNA strand breakage up to 80% between 4 to 6 h postincubation. During incubation a marked decrease in the number of total and viable cells as well as an increase in the release of lactate dehydrogenase into medium were detectable, indicating a strong cytotoxicity of Dex on the mouse thymocytes. Agarose gel electrophoresis of DNA from cells exposed to Dex for 6 h clearly demonstrated an increased laddering of DNA fragments multiple of approx. 200 base pairs as a characteristic feature of an apoptosis or programmed cell death. The cytotoxicity of Dex, as judged by the decrease in the viability and increase in the cell lysis, was effectively prevented by 3-aminobenzamide, a potent inhibitor of poly(ADP-ribose) synthesis. Furthermore, the lowering of intracellular NAD levels, which was observable in the present study most probably as a result of activation of poly(ADP-ribose) synthesis due to Dex-mediated DNA strand breakage, was also specifically prevented by the inhibitor, although the DNA strand breakage itself was not affected under these conditions. Our present results indicate that the Dex-mediated thymocyte death and cytolysis and probably intrathymic apoptotic thymocytolysis could be attributable primarily to the loss of intracellular NAD.

Chromium effects on chondrocytic differentiation in vitro

A tissue culture study was conducted on the effects of chromium on chondrocytic differentiation. Mesenchymal cells from stage 22-24 chick limb buds were dispersed and cultured as micromasses, where they differentiated into chondrocytes. Addition of chromium(VI) to the cultures indicated that the production of proteoglycans (as detected by Alcian blue staining) was more sensitive to chromium's effects than was cell proliferation. Whereas Alcian blue nodule formation was inhibited by 1 microM Cr(VI), cell proliferation (as detected by cell counts) was not. Chromium (VI) was added to cultures at daily intervals, and these studies indicated that the interval of d 1-2 was the most sensitive period. ADP-ribose transferase activity in these cultures was measured; the pattern of enzyme activity in control cultures was high 1 and 24 h after the start of culture, decreased abruptly between 24 and 48 h, and then decreased more gradually. In the presence of Cr(VI), elevated ADP-ribose transferase levels were maintained throughout the culture period. We suggest that, in presence of 1.0 microM chromium(VI) or higher concentrations, the balance of events favors nucleolytic action rather than repair of damage.

DNA repair: relationship to drug and radiation resistance, metastasis and growth factors

DNA repair mechanisms are important for the recovery of both normal and malignant tissues from radiation and chemotherapy. Drug 'resistance' may merely reflect the similarity of cancer to normal tissues. Investigating the normal repair mechanisms by cloning human DNA repair genes will permit a much better comparison. Therapeutic inhibition of DNA repair may be possible with poly-ADP-ribose polymerase inhibitors. A differential effect may be obtained since less-differentiated cells have a higher poly-ADP-ribose polymerase activity. Clinical application of repair inhibitors can be achieved by using antimetabolites such as high-dose hydroxyurea which produces levels of 1-3 mmol litre -1/24 hours. The whole cell and tissue response to DNA damage is more complex than removal of adducts and joining strand breaks. DNA damage can result in an increase in growth-factor receptors, the release of soluble mediators that affect undamaged cells and stimulation of plasminogen activator. These changes may enhance growth and recovery as well as bypass or repair the damage. The generation of heterogeneity in a tumour population may be mediated by DNA rearrangements. Genetic instability is much higher in metastatic clones and a comparison of DNA strand-break repair in a metastatic and a non-metastatic line showed more rapid repair in the former. Aberrant use of DNA repair stimulated by growth factors may mediate tumour progression and heterogeneity as well as drug resistance.