Blockage of cyclin cdk's, PKC and phosphoinositol 3-kinase pathways leads to augmentation of apoptosis in drug-resistant leukemia cells: evidence for interactive effects of flavopiridol, LY 294002, roscovitine,wortmannin and UCN-01.

A mouse leukemia L1210 cell line (Y8), selected for resistance to deoxyadenosine, has a markedly altered phenotypic expression that includes loss of sensitivity to dATP as an allosteric inhibitor of ribonucleotide reductase, increased expression of c-myc, c-fos and WAF1/p21, but decreased expression of p53. In addition, the Y8 cells have a Very strong apoptotic response to a variety of agents under conditions in which the parental wild-type cells do not apoptose. In these studies, we show that flavopiridol (a cdk inhibitor) causes the Y8 cells to undergo apoptosis via a caspase-3 activation process. The apoptotic response to flavopiridol is markedly enhanced by LY294002. Data also show that the apoptotic response of the Y8 cells to roscovitine (a cdk inhibitor) is enhanced by UCN-01 (a PKC inhibitor). These data show that simultaneous blockage of specific pathways leads to increased apoptosis in the Y8 cells with essentially no effects on the parental wild-type L1210 cells.

Structure and dynamics of thioguanine-modified duplex DNA.

Mercaptopurine and thioguanine, two of the most widely used antileukemic agents, exert their cytotoxic, therapeutic effects by being incorporated into DNA as deoxy-6-thioguanosine. However, the molecular mechanism(s) by which incorporation of these thiopurines into DNA translates into cytotoxicity is unknown. The solution structure of thioguanine-modified duplex DNA presented here shows that the effects of the modification on DNA structure were subtle and localized to the modified base pair. Specifically, thioguanine existed in the keto form, formed weakened Watson-Crick hydrogen bonds with cytosine and caused a modest approximately 10 degrees opening of the modified base pair toward the major groove. In contrast, thioguanine significantly altered base pair dynamics, causing an approximately 80-fold decrease in the base pair lifetime with cytosine compared with normal guanine. This perturbation was consistent with the approximately 6 degrees C decrease in DNA melting temperature of the modified oligonucleotide, the 1.13 ppm upfield shift of the thioguanine imino proton resonance, and the large increase in the exchange rate of the thioguanine imino proton with water. Our studies provide new mechanistic insight into the effects of thioguanine incorporation into DNA at the level of DNA structure and dynamics, provide explanations for the effects of thioguanine incorporation on the activity of DNA-processing enzymes, and provide a molecular basis for the specific recognition of thioguanine-substituted sites by proteins. These combined effects likely cooperate to produce the cellular responses that underlie the therapeutic effects of thiopurines.

In vivo growth of mouse leukemia L1210 cells with metabolic alterations in the subunits of ribonucleotide reductase.

Mouse leukemia L1210 cells selected for resistance to drugs targeted specifically at each of the protein subunits of ribonucleotide reductase were studied for their ability to grow in vivo. The life-span of the mice injected with hydroxyurea-resistant L1210 cells, which have elevated levels of the mRNA and protein for the non-heme iron (NHI, R2) subunit of ribonucleotide reductase, was approximately twice that of the mice injected with equal numbers of the parental wild-type L1210 leukemia cells. The life-span of mice injected with the L1210 cells that had alterations in the effector-binding subunit (EB, R1) was considerably shorter than the mice injected with the parental wild-type L1210 cells. These results provide direct evidence that tumor cells with alterations in the properties of ribonucleotide reductase grow differently in vivo, with defined effects on the host mouse that cause either an increased survival time or a decreased survival time compared to the effects of wild-type L1210 leukemia cells on tumor-bearing mice.