Expression Of Truncated Protein Tyrosine Phosphatase, Receptor Type, O (PTPROt) Influences Multiple Myeloma Sensitivity To Bortezomib Through Akt Signaling, and Is a Favorable Clinical Prognostic Factor

Proteasome inhibitors such as bortezomib and carfilzomib are an important part of our current chemotherapeutic armamentarium against multiple myeloma, and have improved outcomes in the up-front, relapsed, and relapsed/refractory settings. Their efficacy has been demonstrated both as single agents, and as part of rationally designed combination regimens, but they are at this time used empirically, since biomarkers to identify patients who would most or least benefit from their application have not been clinically validated. Moreover, the vast majority of patients eventually develop drug-resistant disease which precludes further proteasome inhibitor use through mechanisms that have not been fully elucidated.

We compared gene expression profiles (GEPs) of a panel of bortezomib-resistant myeloma cell lines and their vehicle-treated, drug-naïve counterparts to identify significant changes associated with drug resistance. The list of genes whose expression was changed by at least 2-fold was compared with independent RNA interference studies whose goal was to identify genes whose suppression conferred drug resistance. Further validation of genes of interest was pursued in a panel of myeloma cell lines, and in clinically annotated GEP databases.

Suppression of PTPROt expression was noted in bortezomib-resistant RPMI 8226 and ANBL-6 myeloma cells compared to isogenic, drug-naïve controls, and this was confirmed by quantitative PCR. Overexpression of PTRPOt in RPMI 8226, ANBL-6 and other myeloma cell lines was by itself sufficient to increase the level of apoptotic, sub-G0/G1 cells compared to vector controls, or cells expressing a phosphatase-dead PTPROt mutant. Moreover, PTPROt enhanced the ability of bortezomib to reduce myeloma cell viability, in association with increased activation of caspases 8 and 9. Exogenous over-expression of PTPROt was found to reduce the activation status of Akt, a known anti-apoptotic pathway that reduces bortezomib activity, based on Western blotting with antibodies to phospho-Akt (Ser473), and Akt kinase activity assays. Notably, we also found that exogenous over-expression of PTPROt resulted in increased expression levels of p27Kip1. Interestingly, array CGH data from studies of myeloma cell lines and primary cells showed that the PTPROt gene was located in a genomic region with a high propensity for loss. Analysis of the Total Therapy databases of GEP and patient outcomes available on the Multiple Myeloma Genomics Portal showed that higher than median expression of PTPROt was associated with better long-term survival (P=0.0175). Finally, analysis of the Millennium Pharmaceuticals database of studies of bortezomib in the relapsed and relapsed/refractory setting showed high PTRPOt expression was more frequently seen in patients who achieved complete remission (P<0.01), and was associated with a better median overall survival (P=0.0003).

Taken together, the data support the possibility that high expression of PTPROt is a good prognostic factor for response to bortezomib-containing therapies, and that this may occur through modulation by PTPROt of the Akt pathway. Moreover, they suggest that strategies to enhance the expression of PTPROt should be investigated to restore bortezomib sensitivity in patients with proteasome inhibitor-resistant disease.

Orlowski:Bristol-Myers Squibb: Honoraria, Membership on an entity’s Board of Directors or advisory committees, Research Funding; Celgene: Honoraria, Membership on an entity’s Board of Directors or advisory committees, Research Funding; Millennium: The Takeda Oncology Company: Honoraria, Membership on an entity’s Board of Directors or advisory committees, Research Funding; Onyx: Honoraria, Membership on an entity’s Board of Directors or advisory committees, Research Funding; Resverlogix: Research Funding; Array Biopharma: Honoraria, Membership on an entity’s Board of Directors or advisory committees; Genentech: Honoraria, Membership on an entity’s Board of Directors or advisory committees; Merck: Membership on an entity’s Board of Directors or advisory committees.