Biological and Therapeutic Significance of Thioredoxin-1 (Trx-1) In Diffuse Large B-Cell Lymphomas

Abstract 2004

Diffuse large B cell lymphomas (DLBCL) are the most common non-Hodgkin (NHL-B) lymphoma. Current treatment is fairly successfully (∼70-80% remission with R-CHOP frontline chemotherapy), but relapse is common (∼50% after 2–3yrs) with poor salvage therapy options and short survival in relapsed/refractory (r/r) DLBCL. The greatest challenge in improving survival of DLBCL patients is overcoming chemo-resistance, which we currently have very little understanding of the basis. Elucidation of molecular pathways and tumor-encoded genes whose expression contribute to the intrinsic resistance and rapid cell growth of lymphoma cells could yield immediate clinical benefits and reveal new therapeutic targets for effective control and treatment of r/r lymphomas. Thioredoxin (Trx) family members play critical roles in the regulation of cellular redox homeostasis. Cancer cells exist in a stressed environment and rely on the Trxs for protection against stress-disregulated redox signaling. The most extensively studied member of the family is Trx-1 whose levels are increased in many human cancers, most likely in direct response to stress. Trx-1 contributes to many of the hallmarks of cancer including increased proliferation, resistance to cell death and increased angiogenesis. Trx-1 is a validated cancer drug target associated with aggressive tumor growth, resistance to standard therapy and decreased patient survival. In this study we showed that the majority of the DLBCL-derived cell lines express high basal levels of thioredoxin-1 (Trx-1) when compared to normal B cells by both Western blotting and real-time PCR. Tissue microarray (TMA) and immunohistochemistry analysis of 100 cases of DLBCL showed that 77% of the cases were positive for Trx-1 and 40% of the cases over-expressed Trx-1. Based on these observations, we hypothesize that aberrant expression of Trx-1 contributes to the development of drug resistance and cell growth and survival phenotype in DLBCL. Inhibition of Trx-1 by small interfering RNA (siRNA) inhibited DLBCL cell line growth in vitro. Down-regulation of Trx-1 also sensitized lymphoma cells to doxorubicin-induced cell growth inhibition. A doxorubicin-resistant human DLBCL cell line (McA), that was highly sensitive to doxorubicin, has been selected by continuously exposing cells to gradually increasing doses of doxorubicin. The resistant phenotype has been retained for over 6 months despite growth in drug-free medium. Western blot analysis detected increased expression of the Trx-1 in the resistant subline. These results indicate that Trx-1 plays a key role in growth and survival, as well as chemoresistance in the pathophysiology of DLBCL. Based on these data, we plan to use small molecule inhibitors in vitro as a proof of principle for possible future human phase I studies.