IKK2 hepatocyte specific deletion accelerates the liver regeneration and enhances the innate immune response in mice

After injury, the liver has the ability to restore its mass and function through a process called “liver regeneration“. In resting cells, NF-κB is confined inactive in the cytoplasm, bound to its inhibitor IκB. After stimulation, IκB is phosphorylated by the IKK complex (composed of two catalytic subunits IKK1 and IKK2 and a regulatory subunit NEMO) leading IκB to a proteasomal degradation and thus liberating NF-κB to translocate into the nucleus. As NF-κB is involved in early events of hepatocyte proliferation, we therefore studied the impact of hepatocyte-specific IKK2 deletion on liver regeneration. IKK2 constitutive knock-out (ko) mice die in-utero. Hence we generated C57/BL6 hepatocyte-specific IKK2 deletion (IKK2Δhepa) mouse using the Cre/LoxP system. 70% partial hepatectomy (PH) was performed on IKK2f/f mice (wt) and IKK2Δhepa mice (knock-out), and liver regeneration was studied. All mice survived after PH. However, we observed impaired NF-κB activation in hepatocytes of IKK2Δhepa mice. IL–6 and TNF-α are two main cytokines driving the priming of hepatocytes after PH. Whereas IL–6 showed similar protein levels in wt and ko mice, TNF-α protein expression showed a earlier and stronger up-regulation 1h after PH in ko mice, while wt animals reached similar result at only 6h after PH. TNF induces matrix remodelling via MMP–9 and we observed earlier MMP–9 protease activity in ko mice. These data suggest earlier priming in the ko animals. Serum Amyloid A, marker of the Acute Phase Response in mice exhibited a strong increase in mRNA expression as early as 6h in ko animals compared to wt. Neutrophil chemoattractant chemokine CXCL–1 mRNA expression was analysed by real time PCR and the kinetic revealed an overexpression of CXCL–1 in ko mice with a peak of expression at 6h. This result directed us to analyse the involvement of the innate immune response after PH. We examined blood and liver cells 12h after PH via flow cytometry, and ko mice exhibited higher number of polymorphonuclear (PMN) cells in both blood and liver compared to wt animals. As a result of all these early events, ko animals presented earlier hepatocytes proliferation compared to wt mice, confirmed by BrdU analysis and cyclins expression. Our data revealed that deleting IKK2 in hepatocyte caused a faster priming phase, an increased acute phase response and a stronger PMN cells recruitment in the liver leading to an earlier hepatocyte proliferation in IKK2Δhepa mice during liver regeneration.