MTOR Inhibitors Activate the AKT Kinase in Multiple Myeloma Cells by Upregulating the IGF-1/IRS-1/PI-3 Kinase Cascade.

MTOR inhibitors, such as rapamycin and CCI-779, have shown pre-clinical potential as therapy for multiple myeloma (MM). By inhibiting expression of cell cycle proteins, these agents induce G1 arrest. However, by also inhibiting an mTOR-dependent phosphorylation of insulin receptor substrate-1 (IRS-1), they may alter its subcellular localization and/or prevent its degradation which could enhance IGF-1 signaling and downstream PI3-kinase/AKT activation. This may be a particular problem in MM where IGF-1-induced activation of AKT is an important anti-apoptotic cascade. We, thus, studied PI3-kinase/AKT activation in MM cells treated with mTOR inhibitors. Rapamycin enhanced basal AKT activity, AKT phosphorylation and PI3-kinase activity in MM cell lines. Both PTEN-null as well as PTEN-wild type myeloma lines were similarly affected. Rapamycin also significantly prolonged activation of AKT induced by exogenous IGF-1. CCI-779, used in a xenograft model, also resulted in MM cell AKT activation in vivo. Blockade of IGF-1 receptor function prevented rapamycin’s activation of AKT. Furthermore, rapamycin prevented serine phosphorylation of IRS-1 and IRS-1 degradation. Though similarly blocking IRS-1 degradation, proteasome inhibitors did not activate MM cell AKT. Although rapamycin sensitized MM cells for dexamethasone-induced apoptosis, it protected against PS-341-induced apoptosis. Thus, mTOR inhibitors activate PI3-K/AKT in MM cells and activation depends on basal IGF-1/IGF-R signaling. As activated AKT may protect against apoptosis, future use of mTOR inhibitors in myeloma patients will have to carefully consider the types of anti-myeloma agents used in combination.