Myeloid-specific Nrf2 deletion increases fibrosis and contributes to the development of heart failure after murine myocardial infarction

Excessive inflammation has been implicated in the mechanism of fibrosis, maladaptive left ventricular remodelling, and heart failure after myocardial infarction (MI). We previously showed that the transcription factor Nrf2 downregulates the innate immune response in experimental MI, with profound cardiac inflammation observed in global Nrf2–/– mice compared to wild-type (WT) littermates. This was associated with increased expression of Nrf2 regulated genes (NRGs) related to the regulation of fibrosis via TGF-β in leukocytes, supporting the concept that leukocyte NRGs contribute to the regulation of ventricular modelling after MI. However, whether these findings are contingent on myeloid-specific Nrf2 is not known.

To assess the contribution of myeloid-specific Nrf2 to fibrosis and maladaptive ventricular remodelling.

For myeloid-specific deletion of Nrf2, we used the Cre/loxP breeding system: LysMCre transgenic mice were bred with Nrf2 floxed (loxP flanked Nfe2l2) mice, resulting in deletion of Nrf2 by Cre recombinase in LysM+ cells. Experimental genotypes were LysMCre+/– Nrf2fl/fl and negative control Nrf2fl/fl without the LysMCre transgene (LysMCre–/– Nrf2fl/fl). Mice either underwent induced MI by permanent left anterior descending artery (LAD) ligation or had a sham operation. We assessed cardiac remodelling by echocardiography at weeks 1, 2 and 4 after MI, as well as histological staining (Picrosirius red and Masson’s trichrome) of hearts at 28 days. Collagen deposition was quantified using ImageJ. Statistical significance was assessed using a 2-way ANOVA with Bonferroni’s post hoc test (*p<0.05, **p<0.01, ***p<0.001).

In total, this study used 38 mice, divided into either experimental MI or sham groups. Analyses of Picrosirius red stained tissues displayed higher collagen deposition (signifying fibrosis) in LysMCre+/– Nrf2fl/fl compared to LysMCre–/– Nrf2fl/fl(Fig 1A), with a similar trend in tissues stained with Masson’s trichrome (Fig 1B) (n=5-8/group, p<0.05). Four weeks after MI, ejection fraction (EF) was significantly lower in LysMCre+/– Nrf2fl/fl mice than LysMCre–/– Nrf2fl/fl (n=5-13/group, p<0.05, Fig 2A). Furthermore, LysMCre+/– Nrf2fl/fl mice also exhibited a greater increase in left ventricular end-diastolic volume (LVEDV) 4 weeks after MI (n=5-15/group, p<0.05, Fig 2B). There were no significant differences in heart rate between groups. Therefore, echocardiography analyses indicated worse left ventricular function in the absence of myeloid-specific Nrf2 compared to control littermates.

These data support the notion that Nrf2 signalling in myeloid cells has an integral role in mitigating excessive cardiac fibrosis and adverse ventricular remodelling after MI, highlighting a possible therapeutic target to prevent the development of heart failure after MI.