Modification of keratin by the chemotherapeutic drug mitoxantrone

Keratins in health and cancer: more than mere epithelial cell markers

Keratins are the intermediate filament (IF)-forming proteins of epithelial cells. Since their initial characterization almost 30 years ago, the total number of mammalian keratins has increased to 54, including 28 type I and 26 type II keratins. Keratins are obligate heteropolymers and, similarly to other IFs, they contain a dimeric central α-helical rod domain that is flanked by non-helical head and tail domains. The 10-nm keratin filaments participate in the formation of a proteinaceous structural framework within the cellular cytoplasm and, as such, serve an important role in epithelial cell protection from mechanical and non-mechanical stressors, a property extensively substantiated by the discovery of human keratin mutations predisposing to tissue-specific injury and by studies in keratin knockout and transgenic mice. More recently, keratins have also been recognized as regulators of other cellular properties and functions, including apico-basal polarization, motility, cell size, protein synthesis and membrane traffic and signaling. In cancer, keratins are extensively used as diagnostic tumor markers, as epithelial malignancies largely maintain the specific keratin patterns associated with their respective cells of origin, and, in many occasions, full-length or cleaved keratin expression (or lack there of) in tumors and/or peripheral blood carries prognostic significance for cancer patients. Quite intriguingly, several studies have provided evidence for active keratin involvement in cancer cell invasion and metastasis, as well as in treatment responsiveness, and have set the foundation for further exploration of the role of keratins as multifunctional regulators of epithelial tumorigenesis.

Multiple drug resistance and intermediate filaments

One major obstacle to the successful treatment of epithelial derived tumors, such as breast and prostate carcinoma, is the presence of a multiple drug resistance phenotype. The drug resistance which is observed in growing epithelial derived cancer cells could either be an intrinsic, selected and/or an acquired characteristic. A survey of the survival data from several laboratories suggests that epithelial derived tumor cells, which have never been challenged with damaging agents, are in some cases 10 to 2,000 times more resistant to various chemotherapeutic agents as compared to hematopoietic cell lines. An intrinsic characteristic of epithelial cells is their resistance to the lethal effects of multiple types of damaging agents. A major feature of epithelial derived tumors is the expression of the intermediate filament type proteins known as cytokeratin. The simplest cytokeratin combination, cytokeratin 8 and 18, is a major cytoplasmic element within the cells of epithelial derived tumors. Earlier work showed that cytokeratin could be modified by mitoxantrone, a chemotherapeutic agent used in the treatment of breast cancer. Increasing data indicates that the intrinsic drug resistance phenotype is due in part to the presence of continued expression of the cytokeratin 8 and 18. The cytokeratin dependent drug resistance (C-MDR) has been observed in two different cell types that were engineered to contain cytokeratin 8 and 18 expression. The cytokeratin monomers are known to self assemble into intermediate filament networks as shown by numerous basic studies. Experiments using transfected cell lines which are unable to assemble networks indicated that C-MDR does not depend upon the formation of an intermediate filament network. Selection of cytokeratin network defective tumor cells did not increase their sensitivity to chemotherapeutic agents. These data are interesting since it suggests that the C-MDR phenotype is not dependent upon the structural nature (i.e. network forming ability) of the cytokeratin. Our current working hypothesis is that the interaction of the damaging agent with cytokeratin may initiate signaling response(s) for cell survival.

Subcellular localisation of the antitumour drug mitoxantrone and the induction of DNA damage in resistant and sensitive human colon carcinoma cells

Cellular uptake and subcellular localisation of the antitumour agent mitoxantrone were studied in a human colon-carcinoma cell line and a mitoxantrone-resistant subline showing features consistent with an atypical multidrug-resistance phenotype involving altered topoisomerase II. Flow cytometry indicated a reduced uptake of mitoxantrone in the resistant line. Confocal microscopy indicated that mitoxantrone-associated fluorescence was primarily found within discrete cytoplasmic inclusions and around the periphery of the nucleus, with low levels being observed within the nucleus. The frequency of cytoplasmic inclusions was reduced in mitoxantrone-resistant cells as compared with parental cells. Fluorescence in cytoplasmic inclusions persisted throughout a 24-h post-treatment period in both cell lines. The results suggest that the persistence of mitoxantrone in cells is a determinant for the continuous induction of DNA damage, perhaps through chronic topoisomerase II trapping, and that modified sequestration may contribute to clinically relevant moderate levels of non-classic multidrug resistance.

Expression of cytokeratin confers multiple drug resistance

The cytokeratin network is an extensive filamentous structure in the cytoplasm whose biological function(s) is unknown. Based upon previous data showing the modification of cytokeratin by mitoxantrone, we investigated the ability of cytokeratin networks to influence the survival response of cells to chemotherapeutic agents. We have compared the survival of mouse L fibroblasts lacking cytokeratins with that of L cells transfected with cytokeratins 8 and 18 in the presence of chemotherapeutic drugs. The expression of cytokeratins 8 and 18 conferred a multiple drug resistance phenotype on cells exposed to mitoxantrone, doxorubicin, methotrexate, melphalan, Colcemid, and vincristine. The degree of drug resistance was 5-454 times that of parental cells, depending upon the agent used. Drug resistance could not be attributed to altered growth characteristics, altered drug accumulation, or an altered drug efflux in the transfected cells. Cytokeratin does not confer resistance to ionizing radiation, which damages DNA independently of intracellular transport mechanisms. These data suggest a role for cytokeratin networks in conferring a drug resistance phenotype.

Release of specific proteins from nuclei of HL-60 and MOLT-4 cells by antitumor drugs having affinity to nucleic acids

The binding sites for mitoxantrone (MIT), Ametantrone (AMT), doxorubicin (DOX), actinomycin D (AMD) and ethidium bromide (EB) in nuclei from exponentially growing and differentiating human promyelocytic HL-60 and lymphocytic leukemic MOLT-4 cells were studied by gel electrophoresis of proteins selectively released during titration of these nuclei with the drugs. Each drug at different drug: DNA binding ratios resulted in a characteristic pattern of protein elution and/or retention. For example, in nuclei from exponentially growing HL-60 cells, MIT affected 44 nuclear proteins that were different from those affected by EB; of these 29 were progressively released at increasing MIT:DNA ratios, 11 were transiently released (i.e. only at a low MIT:DNA ratio) and 4 entrapped. Patterns of proteins displaced from nuclei of exponentially growing HL-60 cells differed from those of cells undergoing myeloid differentiation as well as from those of exponentially growing MOLT-4 cells. The first effects were seen at a binding density of approximately one drug molecule per 10-50 base pairs of DNA. The observed selective displacement of proteins may reflect drug-altered affinity of the binding sites for those proteins, for example due to a change of nucleic acid or protein conformation upon binding the ligand. The data show that the binding site(s) for each of the ligands studied is different and the differences correlate with variability in chemical structure between the ligands. The nature of the drug-affected proteins may provide clues regarding antitumor or cytotoxic mechanisms of drug action.

Persistent intracellular binding of mitoxantrone in a human colon carcinoma cell line

Incubation of human carcinoma cells with mitoxantrone resulted in an intracellular distribution of the drug into cytoplasmic, nuclear and cytoskeletal compartments occurring within 1 min of drug treatment. Incubation of the cells in drug-free medium resulted in an efflux of the drug such that 80% of the intracellular drug was eliminated from the cells by 72 hr. Approximately 20% of the initial intracellular drug concentration remained in the cells after the drug had been removed from the medium. The majority of the persistent intracellular drug was associated with soluble cytoplasmic proteins and fractions enriched in nucleic acid. Approximately 10% of the persistent drug binding was associated with cellular structures that had been depleted of soluble cytoplasmic protein and nucleic acid. During the persistent drug binding, the cells enlarged at least 2-fold as determined by microscopic examination. An increasing percentage of the cells was also observed to contain a DNA content consistent with a G2 cell cycle arrest. Taken together, these data suggest that the persistent intracellular binding of mitoxantrone results in a G2 cell cycle arrest and cellular damage.