Multiple myeloma cell killing by depletion of the MET receptor tyrosine kinase.

Multiple myeloma (MM) is an invariably fatal plasma cell malignancy, primarily due to the therapeutic resistance which ultimately arises. Much of the resistance results from the expression of various survival factors. Despite this, the ribonucleoside analogue, 8-chloro-adenosine (8-Cl-Ado), is cytotoxic to a number of MM cell lines. Previously, we established that the analogue incorporates into the RNA and inhibits mRNA synthesis. Because 8-Cl-Ado is able to overcome survival signals present in MM cells and inhibits mRNA synthesis, it is likely that the drug induces cytotoxicity by depleting the expression of critical MM survival genes. We investigated this question using gene array analysis, real-time reverse transcription-PCR, and immunoblot analysis on 8-Cl-Ado-treated MM.1S cells and found that the mRNA and protein levels of the receptor tyrosine kinase MET decrease prior to apoptosis. To determine MET's role in 8-Cl-Ado cytotoxicity, we generated MM.1S clones stably expressing a MET ribozyme. None of the clones expressed <25% of the basal levels of MET mRNA, suggesting that a threshold level of MET is necessary for their survival. Additionally, the ribozyme knockdown lines were more sensitive to the cytotoxic actions of 8-Cl-Ado as caspase-3 activation and the induction of poly-ADP-ribose polymerase (PARP) cleavage were more pronounced and evident 12 h earlier than in the parental cells. We further established MET's role in MM cell survival by demonstrating that a retroviral MET RNA interference construct induces PARP cleavage in MM.1S cells. These results show that MET provides a survival mechanism for MM cells. 8-Cl-Ado overcomes MM cell survival by a mechanism that involves the depletion of MET.