Flavopiridol Induces p53 via Initial Inhibition of Mdm2 and p21 and, Independently of p53, Sensitizes Apoptosis-Reluctant Cells to Tumor Necrosis Factor

Abstract

Flavopiridol (FP) inhibits gene expression and causes apoptosis, and these effects cannot be explained by inhibition of cyclin-dependent kinases that govern cell cycle. The simple and established notion that FP is an inhibitor of transcription predicts its effects. Because Mdm-2 targets p53 for degradation, FP, as predicted, dramatically induced p53 by inhibiting Mdm-2. Once p53 was induced, restoration of transcription (by removal of FP) resulted in superinduction of p21 and Mdm-2. Similarly, low concentrations of FP (50 nM) induced p21 and Mdm-2 because of their initial down-regulation. A sustained decrease of Mdm-2/p21 expression and accumulation of p53 coincided with near-maximal cytotoxicity of FP at concentrations >100 nM. Induction of p53 was a marker, not a cause, of cytotoxicity. FP caused rapid apoptosis (caspase-dependent cell death) in p53-null leukemia cells. In these cells, FP-induced apoptosis was converted to growth arrest by inhibitors of caspases. In apoptosis-reluctant A549 and PC3M cancer cells, FP inhibited cell proliferation but did not cause apoptosis. Like typical inhibitors of transcription, FP sensitized cells to apoptotic stimuli, allowing tumor necrosis factor to cause rapid and massive apoptosis in otherwise apoptosis-reluctant cells. We discuss that, as a reversible inhibitor of transcription, FP can be used clinically in novel rational drug combinations.

Colocalisation of CD9 and mortalin in CD9-induced mitotic catastrophe in human prostate cancer cells

“…Although mitotic catastrophe is sometimes followed by apoptosis (Jordan et al, 1996), it is not required for the lethal effect of mitotic catastrophe. It thus seems that CD9-induced mitotic catastrophe is not followed by apoptosis, a result consistent with the p53-null status of PC-3 cells (Isaacs et al, 1991), and with the reported apoptosis-reluctant status of PC-3M (a derivative of PC-3) cells (Demidenko and Blagosklonny, 2004). We followed the fate of 61 CD-9-transfected PC-3 clones and found that 33% underwent mitotic catastrophe (Table 1).…”

Cancer-specific enhancement of cisplatin-induced cytotoxicity with triptolide through an interaction of inactivated glycogen synthase kinase-3β with p53

“…In our condition, CDDP/triptolide combination attenuated p53 downstream molecules such as p21, MDM2 and Puma which have short half-lives for mRNA and protein, but had no remarkable influence on the expression of Bax, which has relatively longer half-life, and activated JNK evoked mitochondrial apoptosis using Bax transactivation as a proapoptotic mediator. Previously, flavopiridol, a transcriptional inhibitor, showed a similar change of p53-p21 by downregulating thousands of mRNAs that have a short half-life (Demidenko and Blagosklonny, 2004). However, as triptolide had no suppressive effect on transcription of XIAP and CIAP2 (Supplementary Figure 1) and slightly increased p53 protein level, we consider that triptolide functioned by modulating p53 transcriptional activity but not as a pantranscriptional inhibitor such as flavopiridol.…”

Updates on p53: modulation of p53 degradation as a therapeutic approach

“…However, neither the p53 -Mdm2 binding nor the nucleocytoplasmic shuttling of p53 or Mdm2 is directly affected by these inhibitors. Flavopiridol, another CDK inhibitor, has also been shown to activate WT p53 by initially inhibiting Mdm2 (Demidenko and Blagosklonny, 2004). These studies show the potential of CDK inhibitors to affect p53 activation and degradation by downregulating Mdm2 (Lu et al, 2001).…”