Conditional ablation of Ikkb inhibits melanoma tumor development in mice
Jinming Yang, Ryan Splittgerber, Fiona E. Yull, Sara Kantrow, Gregory D. Ayers, Michael Karin, Ann Richmond
Jinming Yang, Ryan Splittgerber, Fiona E. Yull, Sara Kantrow, Gregory D. Ayers, Michael Karin, Ann Richmond
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Research Article Oncology

Conditional ablation of Ikkb inhibits melanoma tumor development in mice

Abstract

Several lines of evidence suggest that tumor cells show elevated activity of the NF-κB transcription factor, a phenomenon often resulting from constitutive activity of IκB kinase β (IKKβ). However, others have found that loss of NF-κB activity or IKKβ is tumor promoting. The role of NF-κB in tumor progression is therefore controversial and varies with tumor type. We sought to more extensively investigate the role IKKβ in melanoma tumor development by specifically disrupting Ikkb in melanocytes in an established mouse model of spontaneous melanoma, whereby HRasV12 is expressed in a melanocyte-specific, doxycycline-inducible manner in mice null for the gene encoding the tumor suppressor inhibitor cyclin-dependent kinase 4/alternative reading frame (Ink4a/Arf). Our results show that Ink4a/Arf–/– mice with melanocyte-specific deletion of Ikkb were protected from HRasV12-initiated melanoma only when p53 was expressed. This protection was accompanied by cell cycle arrest, with reduced cyclin-dependent kinase 2 (Cdk2), Cdk4, Aurora kinase A, and Aurora kinase B expression. Increased p53-mediated apoptosis was also observed, with decreased expression of the antiapoptotic proteins Bcl2 and survivin. Enhanced stabilization of p53 involved increased phosphorylation at Ser15 and reduced phosphorylation of double minute 2 (Mdm2) at Ser166. Together, our findings provide genetic and mechanistic evidence that mutant HRas initiation of tumorigenesis requires Ikkβ-mediated NF-κB activity.

Authors

Jinming Yang, Ryan Splittgerber, Fiona E. Yull, Sara Kantrow, Gregory D. Ayers, Michael Karin, Ann Richmond

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Figure 7

Schematic illustration of oncogenic HRasV12-induced and Ikkβ/NF-κB–mediated melanoma tumorigenesis.

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Schematic illustration of oncogenic HRasV12-induced and Ikkβ/NF-κB–media...
Mutation-activated HRasV12 in Ink4a/Arf–/– melanocytes triggers the Ikkβ/NF-κB signal transduction pathway, leading to inhibition of the apoptosis machinery (p53, p21, Bcl2, caspase 3 cleavage), a transcriptional burst of IL6 accompanied by activation of Stat3, cell cycle progression (with induction of Rb phosphorylation, aurora kinase, Cdk4, survivin), cell proliferation, and immortalization when p16/p19 are deleted. Deletion of Ikkb in these cells blocks cell cycle progression (reduced Rb phosphorylation, aurora kinase, Cdk4, and survivin expression), reduces proliferation, and enhances p53-mediated apoptosis. When p53 is knocked down in cells null for Ikkb, the induction of apoptosis is reversed, indicating that p53 induction (indicated by bold font) is required for the effects of loss of Ikkb on inhibition melanoma tumor growth. Therefore, deletion of Ikkb emerges as an effective target for treating melanoma tumors that express wild-type p53.