Multi-walled carbon nanotubes inhibit estrogen receptor expression in vivo and in vitro through transforming growth factor beta1

Exposure to multi-walled carbon nanotubes (MWCNTs) is suspected to contribute to pulmonary fibrosis through modulation of transforming growth factor betal (TGF-beta 1). There is growing evidence that estrogen signaling is important in pulmonary function and modulates pro-fibrogenic signaling in multiple models of pulmonary fibrosis, however an interaction between MWCNT exposure and estrogen signaling in the lung is not known. The purpose of this work was to determine whether estrogen signaling in the lung is a target for MWCNTs and to identify potential signaling mechanisms mediating MWCNT-induced responses using a whole-body inhalation mouse model and an in vitro human lung cell model. Mice exposed to MWCNTs had reduced mRNA expression of estrogen receptor alpha and beta (Esr1 and Esr2, respectively) in lung tissue at multiple time-points post exposure, whereas expression of G-protein coupled estrogen receptor 1 (Gper1) was more variable. We localized ESR1 protein expression as primarily associated with bronchioles and within inflammatory macrophages. The reduction in estrogen receptor expression was concomitant to an increase in TGF-beta 1 levels in the bronchoalveolar lavage fluid (BALF) of MWCNT-exposed animals. We confirmed a role for TGF-beta 1 in mediating MWCNT-induced repression of ESR1 mRNA expression using a TGF-beta type-I receptor inhibitor in bronchial epithelial cells in vitro. Overall these results highlight a novel mechanism of MWCNT-induced signaling where MWCNT-induced regulation of TGF-beta 1 represses estrogen receptor expression. Dysregulated estrogen signaling through altered receptor expression may have potential consequences on lung function.