The effect of the cyclin-dependent kinase inhibitor flavopiridol on anaplastic large cell lymphoma cells and relationship with NPM-ALK kinase expression and activity

Background
The loss of cell cycle regulation due to abnormal function of cyclin-dependent kinases(cdk) occurs in tumors and leads to genetic instability of chemotherapy-resistant cells. In
this study, we investigated the effect of the cdk inhibitor flavopiridol in anaplastic large cell lymphomas, in which unrestrained proliferation depends on NPM-ALK tyrosine
kinase activity.
Design and Methods
Effects of flavopiridol were examined in ALK-positive and -negative anaplastic large cell lymphoma cells by means of immunoblotting and immunofluorescence analyses to assess
cdk expression and activity, quantitative real time reverse transcriptase polymerase chain
reaction to measure drug-induced changes in transcription, and FACS analyses to monitor changes in proliferation and survival.
Results
Treatment with flavopiridol resulted in growth inhibition of anaplastic large cell lymphoma
cells, along with accumulation of subG1 cells and disappearance of S phase without cell cycle arrest. Consistent with flavopiridol activity, phosphorylation at cdk2, cdk4,
cdk9 sites on RB and RNA polymerase II was inhibited. This correlated with induction of cell death through rapid mitochondrial damage, inhibition of DNA synthesis, and downregulation of anti-apoptotic proteins and transcripts. Notably, flavopiridol was less active in ALK-positive cells, as apoptosis was observed at higher concentrations and later time
points, and resistance to treatment was observed in cells maintaining NPM-ALK signaling. NPM-ALK inhibition affected proliferation but not survival of anaplastic large cell lymphoma
cells, whereas it resulted in a dramatic increase in apoptosis when combined with flavopiridol.
Conclusions
This work provides the first demonstration that targeting cdk is effective against anaplastic
large cell lymphoma cells, and proves the critical role of NPM-ALK in the regulation of responsiveness of tumor cells with cdk dysregulation.