Targeting Myeloma Cells and Their Progenitors By Pim Kinase Inhibition

Myeloma (MM) cells grow and expand almost exclusively in the bone marrow while creating a cellular microenvironment suitable for MM cell growth and survival (MM niche). In pursuing the molecular mechanisms whereby MM cells gain drug resistance in the “MM niche”, we have found that the serine/threonine kinase Pim-2 is constitutively over-expressed in MM cells, and further up-regulated by co-cultures with bone marrow stromal cells (BMSCs) as well as osteoclasts (Leukemia, 2011), and that Pim-2 is an important therapeutic target in MM for the progression of MM tumor and bone disease (Leukemia, 2014). The ABC transporter BCRP is preferentially expressed in drug resistant MM cells as well as in MM progenitors or stem cells. BCRP has been demonstrated to be phosphorylated by Pim kinases to trigger its dimerization and function; Pim inhibition may suppress the BCRP function to sensitize BCRP-expressing MM cells to chemotherapeutic agents. In the present study we therefore explored whether Pim inhibition is able to target and impair BCRP-expressing drug-resistant MM cells and MM progenitors. We analyzed an ABC transporter activity in BCRP-expressing RPMI8226 and KMS11 cells by intracellular accumulation and retention of BCRP substrates with auto-fluorescence emission, mitoxantrone and doxorubicin, in flow cytometry. Treatment with Pim inhibitors, SMI-16a or SMI-4a, increased the incorporation of these drugs into the MM cells and enhanced their subsequent intracellular retention after 6-hour incubation without these drugs, although BCRP expression on their surface was only marginally affected by the Pim inhibition. Interestingly, acidic conditions up-regulated Pim-2 expression while reducing the accumulation and retention of these drugs in BCRP-expressing RPMI8226 and KMS11 cells. However, the Pim inhibitors efficaciously restored the drug accumulation and retention reduced by extracellular acidification, and enhanced the cytotoxic activity of the BCRP substrate doxorubicin against RPMI8226 cells rather preferentially in acidic conditions. Furthermore, the Pim inhibition minimized the sizes of “side populations”, highly drug-resistant fractions with enhanced BCRP activity, and the ability of colony formation in RPMI8226 and KMS11 cells, which was more marked in acidic conditions. We previously demonstrated the in vivo effects of the Pim inhibitors in human INA-6 cell-bearing SCID-rab MM models and syngeneic mouse MM models with an intra-tibial inoculation of 5TGM1 MM cells (Leukemia, 2014). To further examine the acid-tropism of anti-tumorigenic activity of Pim inhibition, we pretreated murine 5TGM1 MM cells in vitro with or without SMI16a at pH6.8 for 24 hours, and transplanted to the tibiae in mice the same numbers of viable MM cells remaining in each treatment group. Treatment with SMI16a at pH6.8 almost completely abrogated in vivo tumorigenic capacity of 5TGM1 cells, while MM cells without the treatment rapidly grew and expanded in and outside of the tibiae, suggesting targeting clonogenic MM cells by Pim inhibition preferentially in acidic conditions. Taken together, Pim-2 may become an important therapeutic target of drug-resistant BCRP-expressing MM cells and their progenitors which appear to gain more drug resistance in acidic bone lesions. Combinatory treatment with Pim inhibitors warrants further study to overcome drug resistance in MM cells, including their tumorigenic cancer stem cells.