G9a and Sirtuin6 epigenetically modulate host cholesterol accumulation to facilitate mycobacterial survival

Cholesterol derived from the host milieu forms a critical factor for mycobacterial pathogenesis. However, the molecular circuitry co-opted by Mycobacterium tuberculosis (Mtb) to accumulate cholesterol in host cells remains obscure. Here, we report that the coordinated action of WNT-responsive histone modifiers G9a (H3K9 methyltransferase) and SIRT6 (H3K9 deacetylase) orchestrate cholesterol build-up in in vitro and in vivo mouse models of Mtb infection. Mechanistically, G9a, along with SREBP2, drives the expression of cholesterol biosynthesis and uptake genes; while SIRT6 along with G9a represses the genes involved in cholesterol efflux. The accumulated cholesterol in Mtb infected macrophages promotes the expression of antioxidant genes leading to reduced oxidative stress, thereby supporting Mtb survival. In corroboration, loss-of-function of G9a in vitro and pharmacological inhibition in vivo; or utilization of BMDMs derived from Sirt6-/- mice or in vivo infection in haplo-insufficient Sirt6-/+ mice; hampered host cholesterol accumulation and restricted Mtb burden. These findings shed light on the novel roles of G9a and SIRT6 during Mtb infection and highlight the previously unknown contribution of host cholesterol in potentiating anti-oxidative responses for aiding Mtb survival. Mycobacterium tuberculosis (Mtb) rewires the host molecular machinery and several cellular and immunological pathways to survive amidst its challenging microenvironments. Of these, deregulated lipid accumulation in host cells has been shown to play a critical role in the pathogenesis of Mtb. Biochemical analysis has indicated that cholesterol and cholesteryl esters form a major part of the accumulated lipids, and several reports support the relevance of cholesterol in aiding Mtb survival. Therefore, in this study, we aimed to determine the regulatory mechanisms that lead to cholesterol accumulation in host cells during Mtb infection. We find WNT signaling-dependent epigenetic factors G9a and SIRT6 to favor cholesterol accretion during Mtb infection by differentially regulating cholesterol biosynthesis/uptake and efflux genes. We found a distinct role of the accumulated cholesterol in the expression of antioxidant genes, that consequently leads to Mtb growth. Interestingly, perturbation of WNT, G9a, SIRT6, cholesterol accumulation genes or antioxidant genes, was found to restrict Mtb burden in vitro and lack of G9a or SIRT6 was also found to limit Mtb growth and lung pathology in an in vivo mouse model of tuberculosis (TB); thereby suggesting the targeting of these epigenetic factors as potential adjuvants for TB therapy.