Unscheduled Akt-Triggered Activation of Cyclin-Dependent Kinase 2 as a Key Effector Mechanism of Apoptin's Anticancer Toxicity

Abstract

Apoptin, a protein from the chicken anemia virus, has attracted attention because it specifically kills tumor cells while leaving normal cells unharmed. The reason for this tumor selectivity is unclear and depends on subcellular localization, as apoptin resides in the cytoplasm of normal cells but in the nuclei of transformed cells. It was shown that nuclear localization and tumor-specific killing crucially require apoptin's phosphorylation by an as yet unknown kinase. Here we elucidate the pathway of apoptin-induced apoptosis and show that it essentially depends on abnormal phosphatidylinositol 3-kinase (PI3-kinase)/Akt activation, resulting in the activation of the cyclin-dependent kinase CDK2. Inhibitors as well as dominant-negative mutants of PI3-kinase and Akt not only inhibited CDK2 activation but also protected cells from apoptin-induced cell death. Akt activated CDK2 by direct phosphorylation as well as by the phosphorylation-induced degradation of the inhibitor p27 Kip1 . Importantly, we also identified CDK2 as the principal kinase that phosphorylates apoptin and is crucially required for apoptin-induced cell death. Immortalized CDK2-deficient fibroblasts and CDK2 knockdown cells were markedly protected against apoptin. Thus, our results not only decipher the pathway of apoptin-induced cell death but also provide mechanistic insights for the selective killing of tumor cells.

Autophagy, Apoptosis, Mitoptosis and Necrosis: Interdependence Between Those Pathways and Effects on Cancer

“…One has to keep in mind that Akt does not always fulfill pro-survival functions. As recent reports have shown, Akt may become pro-apoptotic if it enters the nucleus (Chen et al 2011;Los et al 2009b;Maddika et al 2007Maddika et al , 2008Maddika et al , 2009.…”

Nuclear localized Akt enhances breast cancer stem-like cells through counter-regulation of p21Waf1/Cip1and p27kip1

“…However, under certain conditions it may also be transferred into the nucleus. 25 Here, it then triggers the regulation of downstream signaling molecules that play a role in cell survival, growth, migration, proliferation and cell death. 25 The cytoprotective function of Akt is attributed to its ability to both promote activation of anti-apoptotic molecules and to inhibiting the pro-apoptotic molecules of the B cell lymphoma 2 (Bcl2)-family member Bad and fork head transcription factor, FKHRL.…”

“…25 Here, it then triggers the regulation of downstream signaling molecules that play a role in cell survival, growth, migration, proliferation and cell death. 25 The cytoprotective function of Akt is attributed to its ability to both promote activation of anti-apoptotic molecules and to inhibiting the pro-apoptotic molecules of the B cell lymphoma 2 (Bcl2)-family member Bad and fork head transcription factor, FKHRL. 26 Akt is also involved in the phosphorylation of different cell proliferation factors such as cyclin-dependent kinase (CDK) inhibitors p21 Waf1/Cip1 and p27 kip1 and glycogen synthase kinase-3 (GSK3).…”

Human Gyrovirus-Apoptin Interferes with the Cell Cycle and Induces G2/M Arrest Prior to Apoptosis

“…Earlier research from our group shows that CAV-apoptin presence in the cell leads to the activation of CDK2, and that the activated CDK2 is responsible for the phosphorylation of CAVApoptin at critical for nuclear retention, Thr-108 (Maddika et al 2009). CAV-Apoptin also causes nuclear transfer of Akt kinase that then phosphorylates in the nucleus several proteins contributing to CAV-Apoptin's toxicity (Jain et al 2016;Maddika et al 2007).…”

“…Two viral PKTCs E4orF4 and CAV-Apoptin kill cancer cells by interference with the cell cycle, and by induction of G2/M arrest and apoptosis (Kornitzer et al 2001;Lanz et al 2013;Maddika et al 2009). Earlier research from our group shows that CAV-apoptin presence in the cell leads to the activation of CDK2, and that the activated CDK2 is responsible for the phosphorylation of CAVApoptin at critical for nuclear retention, Thr-108 (Maddika et al 2009).…”