In recent years, a chemical inhibitor of p53, Pifithrin-α (PFT-α), has been identified and used both in vitro and in vivo to investigate p53 function [ 4 ]. PFT-α reversibly inhibits p53-transcriptional activity, inhibiting p53-induced apoptosis, cell cycle arrest and DNA-synthesis block [ 4 – 9 ].
Full Answer
Does PFT-α inhibit IR-induced apoptosis in p53-independent manner?
PFT- α inhibits IR-induced apoptosis in a p53-independent manner. (a) PFT- α blocks the processing of caspase-9 and -3. Wild-type and p53-deficient HCT116 cells were exposed to IR in the absence or presence of PFT- α (30 μ M) and analyzed after the indicated times by western blotting for the processing of caspase-9 and caspase-3.
What is the mechanism of action of p53 inhibitor Pifithrin-Alpha?
The p53 inhibitor pifithrin-alpha is a potent agonist of the aryl hydrocarbon receptor. J Pharmacol Exp Ther 2005; 314: 603–610. Komarova EA, Neznanov N, Komarov PG, Chernov MV, Wang K, Gudkov AV . p53 inhibitor pifithrin alpha can suppress heat shock and glucocorticoid signaling pathways.
How effective is Pifithrin-α for fibrosis?
Furthermore, pifithrin-α administered on a PA schedule actually produced worse fibrosis compared with vehicle control animals after ischemic injury [21%/area (SD4.4) vs.16%/area (SD3.6)] as well as under sham conditions [2.6%/area (SD1.8) vs. 4.7%/area (SD1.3)].
How does PFT-α protect cells from DNA damage-induced apoptosis?
In summary, we have shown that PFT- α protects cells from DNA damage-induced apoptosis also by a p53-independent mechanism that takes place downstream of mitochondria and that might involve cyclin D1.
What are the consequences of phosphorylation of serine residues in p53?
Phosphorylation of serine residues in p53 by PIKKs has two important consequences: activation of p53 signaling and increased resistance of p53 to proteasomal degradation thereby enhancing its stability ( Culmsee and Mattson, 2005 ). The case for involvement of p53 in neuronal apoptosis following genotypic stress is undeniable. Once activated, p53 enhances transcription of a number of genes involved in the process of programmed cell death including pro-apoptotic members of the Bcl-2 family (Bax, Bad, Bid, Noxa, Puma), apoptotic protease-activating factor 1 (APAF1; an essential mediator of apoptosome formation), and the apoptosis regulatory protein SIVA1 ( Chatoo et al., 2011; Culmsee and Mattson, 2005 ). Interestingly, p53 also appears to have a variety of pro-oxidant activities in the CNS which may further exacerbate cellular injury associated with oxidative stress. In support of a role for p53 in cell death after CNS injury, increased levels of total p53 and phosphorylated p53 were noted in the contused rat spinal cord ( Kotipatruni et al., 2011 ). These changes were associated with increased p53-mediated transcriptional activity and up-regulation of its downstream targets including Bax and Apaf1. p53 signaling in this study was also associated with elevation in classical markers of apoptosis such as increased numbers of TUNEL+ cells in the caudal penumbra and cleavage/activation of both caspase-3 and caspase-9. Similar studies using experimental models of TBI and cerebral ischemia have also described a role for neuron-specific induction of p53 activity in neuronal death ( Plesnila et al., 2007; Yamaguchi et al., 2002 ). Recently, p53 has received attenuation as a therapeutic target in CNS injuries. Using pifithrin-α (PFT-α), a specific inhibitor of p53 transcriptional activity, Plesnila et al. demonstrated reduced lesion volume and improved neurological function (beam-walking and novel object recognition tasks) in a mouse model of CCI ( Plesnila et al., 2007 ). Similar findings have also been reported in other experimental models of brain injury using both pharmacological and genetic approaches to reduce p53 activity ( Leker et al., 2004; Nijboer et al., 2011; Yonekura et al., 2006 ). However, it should be noted that the role of p53 in the pathophysiology of CNS injuries is complex and remains to be fully characterized. A growing body of work suggests that p53 may also promote neuronal and axonal regeneration after cellular insult ( Tedeschi and Di Giovanni, 2009 ). These differences may attributed to a variety of factors including the type of injury, severity of injury, time point at which p53 activity is examined, and involvement of other signaling pathways. Interestingly, recent studies have also defined a new role for p53 in initiating necrosis in the context of cerebral ischemia-reperfusion injury ( Vaseva et al., 2012 ).
Does TQ induce apoptosis?
Both p53-dependent and -independent mechanisms of apoptosis induction by TQ have been reported. TQ selectively induced apoptosis in SiHa cells through the induction of p53 [75]. Likewise, TQ induced cell cycle arrest and apoptosis in HCT116 cells by elevating the mRNA levels of p53 and its target gene p21, which was abrogated by pretreatment with a p53 inhibitor pifithrin-α. Moreover, p53-null HCT116 cells were less sensitive to TQ-induced growth arrest and apoptosis, partly due to the upregulation of a cell survival gene CHEK1, further affirming the requirement of p53 for the induction of apoptosis by this compound [92]. Besides the transcriptional activation of p53, TQ restored p53 level by blocking proteasomal degradation of this protein in malignant glioma (U87) cells [93]. In contrast, TQ induced apoptosis more significantly in several cancer cells lacking functional p53, such as osteosarcoma (MG63) [67] and myoblastic leukemia (HL-60) cells [86]. The p53-mutant osteosarcoma (MNNG/HOS) cells were less sensitive to apoptosis than the p53-null MG63 cells upon TQ treatment, partly due to the more pronounced expression of an oxidative stress marker gamma-H2AX, the DNA repair enzyme NBS1, and mutant p53-mediated overexpression of p21 [67]. Moreover, the apoptotic effects of TQ in p53-mutated acute lymphocytic leukemia (Jurkat) cells was mediated through upregulation of p73, a member of p53 family protein, and the downregulation of anti-apoptotic and epigenetic integrator protein ubiquitin-like PHD and ring finger-1 (UHRF1), and two of its partners DNA methyl transferase-1 (DNMT1) and histone deactylase-1 (HDAC1) [94]. Knockdown of p73 restored the level of UHRF1, reactivated cell cycle progression, and abrogated TQ-induced apoptosis in Jurkat cells [94]. The induction of p73 and the downregulation of UHRF1 by TQ in Jurkat cells were mediated through the inhibition of phosphodiesterase (PDE)-1A, an enzyme involved in the modulation of cyclic-AMP- or cyclic-GMP-mediated apoptosis, in Jurkat cells [95].