High GRP78 (78-kDa Glucose-Regulated Protein) Expression Predicts for a Favorable Clinical Outcome in Patients with Multiple Myeloma and May be a Potentially Useful Therapeutic Target in the Treatment of Multiple Myeloma

Background: GRP78 (78-kDa glucose-regulated protein) is a molecular chaperone that is upregulated during cellular stress. It has been well demonstrated that GRP78 upregulation is associated with chemoresistance and metastasis in solid tumours. GRP78 has not been widely explored in multiple myeloma (MM), however, we and others have shown that GRP78 is much more overexpressed in myeloma cell lines compared to other cell lines. To assess the clinical relevance of GRP78 overexpression in MM, we investigated the association of plasma cell GRP78 expression on primary bone marrow (BM) trephines to clinical outcome in patients with MM, and correlate this finding to concurrent in vitro studies to investigate the potential usefulness of targeting GRP78 for the treatment of MM.

Method: The degree of GRP78 expression within CD138+ plasma cells was assessed by immunohistochemistry (IHC) on archived bone marrow trephines of patients with newly diagnosed MM, who underwent autologous stem cell transplant (ASCT) at St.Vincent's Hospital Melbourne. Independent assessment of GRP78 was performed by 3 hematopathologists, who underwent initial calibration. The degree of GRP78 expression within plasma cells was assigned as low, medium or high. Clinical data was abstracted from medical records of the corresponding patients with respect to baseline demographics, treatment-response, progression free survival (PFS), time to next treatment (TTNT) and overall survival (OS). The association GRP78 expression to each of these clinical parameters was assessed using Kaplan-Meier product limit method and the Mantel-Cox logrank test. In vitro, GRP78 expression was also quantified in various myeloma cell lines by RT-PCR and western blot. The association of GRP78 expression to MM-cell survival and drug resistance was assessed in vitro. The impact of GRP78 inhibition on reversal of drug resistance and myeloma-cell viability was investigated.

Result: Between the years 2000 to 2014, a total of 243 patients with newly diagnosed MM underwent ASCT as part of initial therapy, and were included in the study. Baseline bone marrow trephine was available for CD138 and GRP78 staining for 91 patients. Of these, 20, 42 and 34% of patients had low, medium and high expression of GRP78 within BM plasma cells, respectively. Low GRP78 expression was associated with a shorter PFS (HR 2.4, p=0.0006) and shorter TTNT (HR 2.5, p=0.008) compared to intermediate or high GRP78 expression. No significant difference was seen in OS. High GRP78 correlated with a higher probability of achieving CR (p=0.03). In vitro, inhibition of GRP78 resulted in decreased myeloma cell viability, and sensitized myeloma cells to various antimyeloma agents. As a result, synergistic anti-myeloma activity was seen when GRP78 inhibition was combined with melphalan (synergy quotient (SQ) 1.2), dexamethasone (SQ 1.97) and especially bortezomib (SQ 2.06).

Conclusion: In contrast to what is reported for solid tumours in the literature, higher GRP78 expression appeared to predict for a more favorable clinical outcome in patients with MM. In vitro, GRP78 inhibition resulted in significant anti-myeloma effects and increased the antimyeloma activity of various agents especially bortezomib. Together, these findings suggest that GRP78 is potentially a useful biomarker and therapeutic target that warrants further investigation in patients with MM.