ABSTRACT
MLH1 is an integral part of the mismatch repair complex, and the loss of this protein is associated with the acquisition of a mutator phenotype, microsatellite instability, and a predisposition to cancer. Deficiencies in the mismatch repair complex, including the loss of MLH1, result in elevated resistance to specific inducers of DNA damage, yet the mechanisms involved in this DNA-damage resistance are largely unknown. Abnormal cellular responses to DNA damage can lead to the selection of cells with a greater propensity for neoplastic transformation and might also reduce the effectiveness of certain chemotherapeutic drugs. It is therefore important to identify agents that provide selective pressure for growth of MLH1-deficient cells and to characterize further the pathways involved. In this study, we show that both human epithelial and mouse embryo fibroblast cell lines lacking the MLH1 protein are more resistant to two inducers of oxidative stress, hydrogen peroxide and tert-butyl hydroperoxide. Our analyses suggest that the observed differences in cellular viability are mediated primarily through apoptotic pathways and not through deficiencies in cell cycle checkpoint controls. Additional characterization of the signaling pathways for hydrogen peroxide-induced apoptosis in MLH1-proficient cells demonstrates the involvement of increased mitochondrial permeability, the release of cytochrome c, and caspase 3 activation. Together, our data indicate that cells lacking MLH1 may possess a selective growth advantage under oxidatively stressed conditions via the disregulation of apoptosis, possibly involving the mitochondria.
Subject(s)
Apoptosis/physiology , Neoplasm Proteins/physiology , Oxidative Stress/physiology , Peroxides/toxicity , Adaptor Proteins, Signal Transducing , Adenocarcinoma/metabolism , Adenocarcinoma/pathology , Animals , Apoptosis/drug effects , Base Pair Mismatch , Carrier Proteins , Cell Cycle/drug effects , Cell Line , Colorectal Neoplasms/metabolism , Colorectal Neoplasms/pathology , DNA Repair , Fibroblasts/cytology , Fibroblasts/drug effects , Fibroblasts/metabolism , Humans , Hydrogen Peroxide/toxicity , Mice , MutL Protein Homolog 1 , Nuclear Proteins , Oxidative Stress/drug effects , Reactive Oxygen Species/metabolism , Signal Transduction/physiology , Tumor Cells, Cultured , tert-Butylhydroperoxide/toxicityABSTRACT
In this study, we tested the hypothesis that insulin-like growth factor-1 (IGF-1) modulates apoptosis in human breast cancer cells, HBL100, induced by diverse chemotherapeutic drugs. IGF-1 increased cell survival of HBL100 cells treated with 5-fluorouracil (antimetabolite), methotrexate (antimetabolite), tamoxifen (antiestrogen/antiproliferative), or camptothecin (topoisomerase 1 inhibitor) and after serum withdrawal. Elevated cell survival was not due to an increase in cell proliferation by IGF-1, but rather to an inhibition of apoptosis. Evidence for death by apoptosis was supported by cellular morphology and DNA fragmentation. There were no changes observed in Bcl-2 protein or bax mRNA levels. Extracellular matrix (ECM) is known to influence the apoptotic response of cells; therefore, the antiapoptotic effect of IGF-1 on breast cancer cells was examined using different ECMs: laminin, collagen IV, or Matrigel. IGF-1 protected cells from apoptosis induced by methotrexate on all ECMs tested, providing the first evidence that IGF-1 protects against apoptosis in three-dimensional culture systems. These data provide the rationale to search for drugs that lower serum IGF-1 in an effort to improve the efficacy of chemotherapeutic drugs for the treatment of breast cancer.
