Interferon-resistant Daudi Cell Line with a Stat2 Defect Is Resistant to Apoptosis Induced by Chemotherapeutic Agents

Abstract

Interferon-␣ (IFN␣) has shown promise in the treatment of various cancers. However, the development of IFN resistance is a significant drawback. Using conditions that mimic in vivo selection of IFN-resistant cells, the RST2 IFN-resistant cell line was isolated from the highly IFN-sensitive Daudi human Burkitt lymphoma cell line. The RST2 cell line was resistant to the antiviral, antiproliferative, and gene-induction actions of IFN␣. Although STAT2 mRNA was present, STAT2 protein expression was deficient in RST2 cells. A variant STAT2 mRNA, which resulted from alternative splicing within the intron between exon 19 and 20, was expressed in several human cell lines but at relatively high levels in RST2 cells. Most importantly, the RST2 line showed an intrinsic resistance to apoptosis induced by a number of chemotherapeutic agents (camptothecin, staurosporine, and doxorubicin). Expression of STAT2 in RST2 cells not only rescued their sensitivity to the biological activities of IFNs but also restored sensitivity to apoptosis induced by these chemotherapeutic agents. The intrinsic resistance of the RST2 cells to IFN as well as chemotherapeutic agents adds a new dimension to our knowledge of the role of STAT2 as it relates to not only biological actions of IFN but also resistance to chemotherapyinduced apoptosis. IFN2 ␣/␤ regulates a number of cellular responses, such as proliferation, differentiation, and development (1). Although IFN triggers the death of some tumor cells by inducing proapoptotic proteins (tumor necrosis factor-related apoptosis-inducing ligand, PKR, etc.), IFN also promotes cell survival through a nuclear factor B-dependent pathway (2-4). IFN is used to treat various human malignancies (chronic myeloid leukemia, non-Hodgkin lymphomas, Kaposi sarcoma, hairy cell leukemia, multiple myeloma, and malignant melanoma), viral infections, as well as various other diseases (5, 6). However, only a fraction of patients are responsive to IFN therapy, and many patients eventually develop resistance after chronic IFN exposure. The underlying mechanism for IFN resistance is still unclear, but it is reasonable to suggest that genetic variation and selection during prolonged IFN exposure may reflect IFN signaling defects.IFN binds to its cell surface receptor resulting in the activation of JAK1 and TYK2 nonreceptor protein-tyrosine kinases, which phosphorylate STAT proteins (7). Phosphorylated STAT1 and STAT2 in a complex with IRF9 bind to a conserved IFN stimulus-response element (ISRE) present in the promoters of hundreds of IFN-stimulated genes (ISGs) inducing their expression. Mutant cell lines with defined signaling defects have made significant contributions in elucidating the IFN-activated JAK/STAT signal transduction pathway (7,8). Such IFN-resistant mutants were isolated after multiple rounds of chemical mutagenesis and selection of IFN-resistant mutants. However, this procedure does not mimic in vivo what happens to patients who are subjected to long term IFN treatment. Moreover, these IFN-resistant cells ...

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