Transcription regulators of hepatic heme-oxygenase-1 gene expression in IL-6 knock out mice: Comparison of two different models of acute phase reaction

Introduction: Heme-oxygenase-1 (HO-1) has a crucial role in maintaining body iron metabolism during acute phase response (APR). Its expression is principally regulated at transcriptional level, and involves complex intracellular pathways in response to external stimuli. IL-6 is one of principle mediators of APR and most of its signalling pathways, including the ones regulating HO-1 gene expression, involve STAT3 activation. In the current work, we aimed to investigate the main transcriptional regulators of HO-1 in IL-6 knock out mice using two different models of acute phase response.

Methods: Male IL-6 knock-out mice (B6.129S2-IL6tm1Kopf/J), 6–8 weeks old were injected intramuscularly 10ml/kg turpentine oil. Alternatively, mice were intraperitoneally injected 50µg of LPS. Animals were sacrificed at different time points after treatment. Livers of the animals were excised, and processed for nuclear protein isolation. STAT3 activation by phosphorylation was assessed by western blotting whereas among other main known transcriptional regulators of HO-1 gene expression, DNA binding activities of NFκB and Ap-1 were analyzed by gel mobility shift assay using DNA binding consensus sequences.

Results: In knock out mice, the constitutive expression of HO-1 was higher than their wild type counterparts. The magnitude of HO-1 expression up-regulation was comparable in two models of APR in knockout mice, but was lower than their wild type counterparts. Hepatic STAT3 phosphorylation was observed in both animal models of IL-6 knock out mice. Among other transcription regulators, considering time kinetics to give a mechanistic rationale, AP-1 appeared to be more closely involved in HO-1 gene regulation in TO-treated animals whereas NFκB was more prominent in LPS treated animals to control hepatic HO-1 gene expression.

Discussion/conclusion: Though IL-6 plays an important role in the development of acute phase response and is also considered the main regulator of HO-1 expression, the results obtained from the current work suggest, that the presence of alternative mechanisms can overcome the lack of IL-6 during APR to regulate HO-1 gene expression.