Multiple Myeloma (MM) is an incurable disease and even with the current therapies, the median lifespan after diagnosis is approximately three years; thus, it is imperative that new therapies be developed. These new therapies could be targeted on molecules that act as survival factors for MM. Previous studies have shown that MET, a receptor tyrosine kinase and its ligand hepatocyte growth factor plays an important role in survival, migration, and proliferation of MM cells. In addition, our studies suggested that in MM1.S cells, transcription inhibition of MET expression promotes cytotoxicity (Proc. AACR 46:5878, 2005). Flavopiridol is a drug that impedes transcription initiation and elongation by inhibiting the phosphorylation of the heptamer repeats at serine residues 2, 5, and 7 in the C-terminal domain (CTD) of RNA polymerase II (Pol II). In chronic lymphocytic leukemia, which also is an indolent B-cell neoplasia, RNA directed therapies, such as flavopiridol, have shown a lot of promise and are now being pursued in the clinic for the treatment of this disease (Clin. Cancer Res. 11:4176, 2005). Therefore, this current study was done using MM1.S to determine if flavopiridol can be effective in inducing apoptosis in the MM cell line by inhibiting transcription of MET. When analyzed for [3H]-uridine incorporation, 1 μM and 3 μM flavopiridol treatment of MM.1S cells inhibited total RNA synthesis, in a dose and time dependent manner. At 6 hrs with 1 μM flavopiridol, there was a 40% decrease of RNA synthesis and at a 3 μM, there was a 70% decrease. At 24 hrs for both these concentrations, there was more than a 90% reduction of RNA synthesis. Concurrent to the RNA synthesis inhibition, flavopiridol inhibited the phosphorylation of the serine residues 2 and 5 of the heptamers in CTD as demonstrated by immunoblot analysis. To determine if total RNA synthesis will diminish MET transcripts, real-time (RT) PCR assays were done on total RNA. These data showed that MET transcripts decreased with the treatment of flavopiridol. At 12 hrs of treatment with flavopiridol, there was a reduction of MET transcripts by more than 60% with both 1 μM and 3 μM. Furthermore, flavopiridol was able to rapidly induce apoptosis in MM1.S cells, which was seen as PARP cleavage in as little as 4 hrs at both concentrations. At 20 hrs, there was almost total PARP cleavage at 1 μM. Whereas, at about 16 hrs there was complete PARP cleavage at 3 μM. Consequently, flavopiridol shows strong potential as being an effective therapeutic agent for multiple myeloma that targets MET.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]