The antiapoptotic DeltaNp73 is degraded in a c-Jun–dependent manner upon genotoxic stress through the antizyme-mediated pathway

Abstract

p73, the structural and functional homologue of p53, exists as two major forms: the transactivation-proficient, proapoptotic TAp73 or the transactivation-deficient, antiapoptotic DNp73. Expectedly, expression of both these major forms has to be coordinated precisely to achieve the desired cellular outcome. Genotoxic insults resulting in cell death lead to the stabilization of TAp73, mainly through posttranslational modifications, and the concomitant degradation of DNp73, through poorly understood mechanisms. We have therefore investigated the possible mechanisms of stress-induced DNp73 degradation and show here that c-Jun, the AP-1 family member activated by stress signals and involved in stabilizing TAp73, promotes DNp73 degradation. Genotoxic stress-mediated DNp73 degradation was found to occur in a c-Jun–dependent manner through a ubiquitin-independent but proteasome-dependent mechanism. Absence or down-regulation of c-Jun expression abrogated the reduction of DNp73 levels upon stress insults, whereas overexpression of c-Jun led to its degradation. c-Jun controlled DNp73 degradation through the nonclassical, polyamine-induced antizyme (Az) pathway by regulating the latter’s processing during stress response. Consistently, expression of c-Jun or Az, or addition of polyamines, promoted DNp73 degradation, whereas silencing Az expression or inhibiting Az activity in cells exposed to stress reduced c-Jun–dependent DNp73 degradation. Moreover, Az was able to bind to DNp73. These data together demonstrate the existence of a c-Jun–dependent mechanism regulating the abundance of the antiapoptotic DNp73 in response to genotoxic stress.

Antizyme Affects Cell Proliferation and Viability Solely through Regulating Cellular Polyamines

“…However, it was suggested recently that Az might exert its antiproliferative effect not only through targeting ODC to ubiquitin-independent degradation and to inhibiting polyamine uptake but also by stimulating degradation of the growth-regulating proteins cyclin D1, Aurora-A, and ⌬Np73, which do not belong to polyamine metabolism (23)(24)(25). However, in these studies, the ability of Az to stimulate the degradation of these proteins was not compared with its ability to stimulate ODC degradation.…”

“…However, several recent studies put forward an alternative possibility, suggesting that Az might also inhibit cell proliferation by targeting to ubiquitin-independent degradation growth-regulating proteins that do not belong to the cellular polyamine metabolic pathway. These include Smad1, a key transducer of the bone morphogenetic proteins (20 -22); the cell cycle regulators cyclin D1 and Aurora-A (23,24); and the anti-apoptotic N-terminally truncated form of p73 (⌬Np73) (25).…”

Suppression of acetylpolyamine oxidase by selected AP-1 members regulates DNp73 abundance: mechanistic insights for overcoming DNp73-mediated resistance to chemotherapeutic drugs

“…31 In this respect, we have previously shown that DNp73 is degraded by c-Jun upon exposure to genotoxic stress via the Az-mediated degradation pathway. 20 However, the exact mechanisms for this process as well as its relevance to DNp73 in mediating chemoresistance was unclear. Az has been shown to degrade several cell-cycle regulatory proteins, including cyclin D1, Smad1 and Aurora A kinase, as well as Mps1, a protein that regulates centrosome (e) Schematic illustrates the proposed mechanism by which genotoxic stresses can induce the degradation of anti-apoptotic DNp73 by activating c-Jun, FosB and JunB.…”

“…[15][16][17][18][19] However, we have previously shown that the selective degradation of DNp73 by genotoxic stresses occur through the ubiquitin-independent, polyamine-induced antizyme (Az) pathway that is regulated by c-Jun, a member of the activator protein-1 (AP-1) family of transcription factors. 20 Az1 is a small inhibitory protein that binds to its target proteins and directs them to the proteasome for degradation. 21,22 Az1 transcript exists with two overlapping open reading frames, and translation of a fully functional Az1 protein requires a unique þ 1 ribosomal frameshift mechanism, which is dependent on the level of polyamines, a group of cationic compounds such as putrescine, spermidine and spermine present in the cells 21,22 ( Figure 1a, lower panel).…”

Role of polyamines, their analogs and transglutaminases in biological and clinical perspectives

“…ODC antizyme inhibitors act mainly as positive regulators of polyamine levels by binding to antizymes (Kahana 2009;Mangold 2006;López-Contreras et al 2010). There is also evidence that they may affect other important cellular proteins (Kahana 2009;Dulloo et al 2010). In mammals, there are three antizymes (AZ1, AZ2 and AZ3) and two antizyme inhibitors (AZIN1 and AZIN2).…”