Targeting Insulin-like Growth Factor in Multiple Myeloma: Novel Strategies in the Treatment of Proteasome Inhibitor Resistant Cells

Introduction: Multiple myeloma (MM) is one of the hematological malignancy that affects the plasma cells in bone marrow. The treatment of MM patients has been dramatically changed by new agents such as proteasome inhibitors and immunomodulatory drugs. However, most patients will relapse even if new agents provide therapeutic advantages. Therefore, a new strategy is still needed to increase MM patient survival. Insulin-like growth factor (IGF) cause intracellular signaling that ultimately results in cellular growth and proliferation. Because IGF signaling pathways have crucial functions in hematological malignancies and solid tumors, IGF pathways might regulate myeloma cell survival.

Materials and Methods: In this study, we investigated whether IGF1R inhibitor could suppress myeloma cells. We also investigated whether IGF1 was involved in proteasome sensitivity against myeloma cells and cytokines were induced by IGF1 in human umbilical vein endothelial cells (HUVEC).

Results: We first investigated the IGF signaling pathways by microarray gene expression data from the online Gene Expression Omnibus (GEO). IGF is tightly regulated by six related IGF-binding proteins (IGFBPs). Gene expression of IGF1, IGF1R, IGFBP2, IGFBP5, IGFBP7 and IGF2 are involved in myeloma and normal control samples from the public microarray datasets of GSE39754. We next investigated the proteasome inhibitor, bortezomib or carfilzomib activity. Proteasome inhibitors exhibit cell growth inhibition against myeloma cells. However, in the presence of IGF1, proteasome inhibitors activity was reduced. Linsitinib is an orally bioavailable small molecule inhibitor of the IGF1R. We investigated whether linsitinib could inhibit the proliferation of myeloma cells. We found linsitinib treatment inhibited the growth of myeloma cells in a dose dependent manner. Activity of Caspase 3/7 and cytotoxicity were also increased. Combined treatment of myeloma cells with carfilzomib and linsitinib caused more cytotoxicity than each drug alone. Caspase 3/7 activity and cellular cytotoxicity was also increased. We examined the intracellular signaling. Activity of caspase 3 and poly (ADP-ribose) polymerase (PARP) was increased after carfilzomib and linsitinib treatment. We next blocked IGF1R function by small interfering RNA (siRNA). We found drug sensitivity of the cells to the carfilzomib was increased compared to mock-transfected cells. Caspase 3/7 activity and cytotoxicity were increased after carfilzomib treatment in siRNA transfected cells.

We next investigated the in vitro effects of IGF1 on cultured HUVEC. Because the entire bone marrow cavity is hypoxic condition, we investigated the IGF signaling pathways in the hypoxic condition by microarray gene expression data from GEO. Gene expression of IGF1R and IGF2 are increased in the hypoxia from the public microarray datasets of GSE36837. Gene amplification was confirmed by RT-PCR analysis. In the immunoblot analysis, p38 mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase (JNK) phosphorylation was increased by IGF1 stimulation in the time dependent manner. We also found that gene expression of monocyte chemotactic protein-1 (MCP-1) was enhanced by IGF1 stimulation.

Conclusion: The IGF1 signaling pathway is involved in proteasome inhibitor sensitivity and plays a key role in chemokine production of the HUVEC. Our data also suggested that administration of IGF1R inhibitor, linsitinib, might be a powerful strategy against myeloma cells and enhance cytotoxic effects of proteasome inhibitors in those residual MM cells.