The ubiquitin-proteasome pathway (UPP) has been validated as a therapeutic target with the approval of the small molecule proteasome inhibitor bortezomib (VELCADE(R)), in relapsed/refractory multiple myeloma (MM). Despite this advance, greater than half of patients have less than a partial response, while others initially respond and then progress, indicating the importance of primary, or innate, and secondary, or acquired resistance to bortezomib. The mechanisms by which plasma cells survive bortezomib therapy are not fully characterized, in part due to lack of appropriate model systems. Discovery of these resistance mechanisms will facilitate the development of strategies to circumvent them, and help to identify potential novel therapies for the treatment of patients with resistance to bortezomib. The mechanism(s) of bortezomib resistance is poorly understood to date, which is most likely do to lack of model systems. We created interleukin (IL)-6-dependent and -independent cell lines through the systematic increase of bortezomib concentrations over many months. These bortezomib-resistant (BR) cells lines high degrees of resistance. Further monoclonal BR cell lines were established from one polyclonal population. Our preliminary studies into the some phenotypic changes associated with bortezomib-resistance show that while basal levels of proteasome activity are repressed in bortezomib-resistance cells compared to wild-type cells, expression of the proteasome subunits are increased and still susceptible to inhibition by bortezomib. Preliminary assessment of transcriptional changes in bortezomib-resistance myeloma cells revealed identification of several perturbations in cellular signaling, namely insulin-like growth factor-1 (IGF-1), as well as changes to proteins involved in apoptosis, namely Bcl-2 and phosphatase and tensin homolog (PTEN). Increase mRNA expression of IGF-1 or its receptor IGF-1R was found consistently in our preliminary microarray profile studies in both polyclonal and monoclonal BR cell lines. Our bortezomib-resistance cells also display cross-resistance to other traditional chemotherapies, including doxorubicin and melphalan. However, the expression of multi-drug resistant pumps does not explain the cross resistance as treatment of BR cells with verapamil moderately abrogated bortezomib-resistant cell viability to doxorubicin, indicating that other resistance mechanisms are active in these cells. Our objectives are to fully characterize these resistance mechanisms, which will facilitate the development of strategies to circumvent them, and help to identify potential novel therapies for the treatment of patients with resistance to bortezomib. To identify these changes, our laboratory has developed a panel of multiple myeloma cell lines with high degrees of bortezomib- resistance (BR). To aid in the understanding of how these plasma cells and samples from patients overcome proteasome inhibitor treatment, we propose the following specific aims:
These studies would allow for the identification of the survival mechanisms ongoing that allow the malignant cell populations to overcome proteasome inhibitor treatment given their high dependence on the proteasome for cellular function. These studies will also point towards combination regimens that may overcome this resistance (e.g. monoclonal antibody against IGF-1R), which would allow for a greater therapeutic index, thereby improving response rates, survival, and quality of life of multiple myeloma patients.
| Kuhn, Deborah J; Orlowski, Robert Z (2012) The immunoproteasome as a target in hematologic malignancies. Semin Hematol 49:258-62 |
| Kuhn, Deborah J; Berkova, Zuzana; Jones, Richard J et al. (2012) Targeting the insulin-like growth factor-1 receptor to overcome bortezomib resistance in preclinical models of multiple myeloma. Blood 120:3260-70 |
