Amylase-Producing Myeloma Cells Reduced Sensitivity to Dexamethasone and Bortezomib.

Despite recent progress in treatment for multiple myeloma (MM), a complete cure remains elusive. To further improve the therapeutic outcome of patients with MM, elucidation of the pathology of refractory cases is important. Hyperamylasemia, which is associated with ectopic amylase (AMY) production by MM cells, is a rare condition, and it has been reported to present with poor prognosis showing rapid tumor growth, extramedullary tumor mass formation, and refractoriness of the condition. However, to date, there have been no biological analyses of MM cells ectopically producing AMY. In this study we generated transfectants that stably expressed AMY with human MM cells, and investigated the impact that ectopic AMY production has on tumor proliferation and changes in drug susceptibility in vitro and in vivo.

Two human MM cell lines (RPMI8226 and KMS11) and the cDNA encoding AMY1 were used to establish transfectants with ViraPower™ Lentiviral Gateway Expression Kit (Invitrogen), because the increased AMY isotype was salivary type, which is coded in AMY1, in all MM patients previously reported. The constitutive expression and production of AMY1 were confirmed in the AMY-transfectants (8226/AMY and KMS11/AMY), while they were not in the mock controls. These transfectants were assayed for proliferation and apoptosis after exposure to dexamethasone (Dex), bortezomib (Bz) and lenalidomide (Len) in vitro. The anti-myeloma activity of Bz was also tested in vivo in a xenograft model generated by injecting 8226/AMY or the mock cells into NOD-SCID mice.

8226/AMY had no growth advantage in vitro but grew rapidly when subcutaneously transplanted in mice compared with the mock control (2,177±878 vs 970±131 mm³, p = 0.044). 8226/AMY showed a higher cell proliferation rate than the mock control in vitro when treated with Dex (40uM), Bz (2nM), and Len (1mM). The number of apoptotic 8226/AMY cells decreased after exposure to Bz and Len, but the number after exposure to Dex was equivalent compared with the mock control by the Annexin / Propidium Iodide assay. Therefore, 8226/AMY became less sensitive to Bz and Len partly through the inhibition of apoptosis induced by these drugs. 8226/AMY grew rapidly subcutaneously in mice compared with the mock control when treated with Bz (0.3mg/kg, twice weekly) (p = 0.017). As for KMS11/AMY, the AMY-transfectant showed a higher proliferation rate than the mock control in vitro. KMS11/AMY showed reduced susceptibility to Dex, no change in the susceptibility to Bz, and an enhanced susceptibility to Len unexpectedly in comparison with the mock control. The reason for a difference in the effect of ectopic AMY expression on the susceptibility to anti-MM drugs between 8226/AMY and KMS11/AMY is unclear; however, it might be due to the nature of their parental cells.

No significant difference was observed in the gene expression profiling between both AMY-transfectants and each of the respective mock controls, except for AMY1, suggesting that ectopic AMY expression did not affect the expression level of the specific gene in MM.

In conclusion, we found that 8226/AMY had reduced susceptibility to Dex, Bz, and Len in vitro and also rapid tumor growth with a weakened anti-tumor effect of Bz in vivo. All of these were consistent with the clinical course of previously reported patients with ectopic AMY-producing MM. On the other hand, KMS11/AMY showed an enhanced susceptibility to Len compared with the mock control, indicating that Len might be effective for some patients with AMY-producing MM. Our data provided beneficial clues for elucidating the molecular pathology and developing a treatment strategy for this clinical setting.