No abstract

In response to anticancer therapeutics, human colon cancer cells growing in vitro either enter into a stable arrest or die, depending on the integrity of their cell-cycle checkpoints. To test whether altered checkpoints can modulate sensitivity to treatment in vivo, xenografts were established from isogenic lines differing only in their p21 checkpoint status. Although all tumors with intact checkpoint function underwent regrowth after treatment with gamma-radiation, a significant fraction of checkpoint-deficient tumors were completely cured. This difference in sensitivity was not detected by the clonogenic survival assay, because both arrest and death preclude outgrowth of colonies. These results demonstrate that checkpoint status affects sensitivity to anticancer treatments in vivo, and these findings have important implications for identifying and testing new therapeutic compounds.

Mutations in the mitochondrial genome leading to mitochondrial dysfunction have been reported in a variety of cancers. However, the potential implication of these ®ndings in the cellular response to cancer therapeutic agents is unclear. To examine the importance of mitochondrial DNA (mitDNA) encoded functions in cancer therapeutic response, we determined the clonogenic survival of HSL2 (Rho + , HeLa subline), and its derivative cell line lacking mitDNA (Rho 0 ) after exposure to di erent anticancer agents. We found that isogenic Rho 0 cells lacking mitDNA were extremely resistant to adriamycin and photodynamic therapy (PDT) induced cell death, whereas the Rho + cell line was sensitive. However, there was no measurable di erence in the responses of these cell lines to either alkylating agent or g-radiation. We show that the development of resistance to adriamycin was not due to changes in apoptotic cell death, cell cycle response or to the uptake of adriamycin in isogenic Rho 0 cells. We also demonstrate that exposure of HeLa cells to adriamycin leads to mutations in mitDNA. These studies provide direct evidence that mitDNA plays an important role in cellular sensitivity to cancer therapeutic agents.

Small cell lung cancer (SCLC) accounts for 25% of all lung cancers, and is almost uniformly fatal. Unlike other lung cancers, ras mutations have not been reported in SCLC, suggesting that activation of ras-associated signal transduction pathways such as the raf-MEK mitogen-activated protein kinases (MAPK) are associated with biological consequences that are unique from other cancers. The biological effects of raf activation in small cell lung cancer cells was determined by transfecting NCI-H209 or NCI-H510 SCLC cells with a gene encoding a fusion protein consisting of an oncogenic form of human Raf-1 and the hormone binding domain of the estrogen receptor ( ⌬ Raf-1:ER), which can be activated with estradiol. ⌬ Raf-1:ER activation resulted in phosphorylation of MAPK. Activation of this pathway caused a dramatic loss of soft agar cloning ability, suppression of growth capacity, associated with cell accumulation in G1 and G2, and S phase depletion. Raf activation in these SCLC cells was accompanied by a marked induction of the cyclin-dependent kinase (cdk) inhibitor p27 kip1 , and a decrease in cdk2 protein kinase activities. Each of these events can be inhibited by pretreatment with the MEK inhibitor PD098059. These data demonstrate that MAPK activation by ⌬ Raf-1:ER can activate growth inhibitory pathways leading to cell cycle arrest. These data suggest that raf/MEK/ MAPK pathway activation, rather than inhibition, may be a therapeutic target in SCLC and other neuroendocrine tumors. ( J. Clin. Invest. 1998. 101:153-159.) Key words: SCLC • activated raf • MAP kinase • cell cycle • p27 kip1 • MEK inhibitor PD098059

We propose that porphyrin photodynamic therapy is mediated by mitochondrial benzodiazepine receptors.