MicroRNA let-7 downregulates ligand-independent estrogen receptor–mediated male-predominant pulmonary fibrosis

Rationale: The relevance of hormones in idiopathic pulmonary fibrosis (IPF), a predominantly male lung disease, is unknown. Objectives: To determine whether the ER (estrogen receptor) facilitates the development of pulmonary fibrosis and is mediated in part through microRNA regulation of ERa and ERa-activated profibrotic pathways. Methods: ER expression in male lung tissue and myofibroblasts from control subjects (n = 6) and patients with IPF (n = 6), aging bleomycin (BLM)-treated mice (n = 7), and BLM-treated AF2ERKI mice (n = 7) was determined. MicroRNAs that regulate ER and fibrotic pathways were assessed. Transfections with a reporter plasmid containing the 39 untranslated region of the gene encoding ERa (ESR1) with and without miRNA let-7 mimics or inhibitors or an estrogen response element–driven reporter construct (ERE) construct were conducted. Measurements and Main Results: ERa expression increased in IPF lung tissue, myofibroblasts, or BLM mice. In vitro treatment with let-7 mimic transfections in human myofibroblasts reduced ERa expression and associated fibrotic pathways. AF2ERKI mice developed BLM-induced lung fibrosis, suggesting a role for growth factors in stimulating ER and fibrosis. IGF-1 (insulin-like growth factor 1) expression was increased and induced a fourfold increase of an ERE construct. Conclusions: Our data show 1) a critical role for ER and let-7 in lung fibrosis, and 2) that IGF may stimulate ER in an E₂-independent manner. These results underscore the role of sex steroid hormones and their receptors in diseases that demonstrate a sex prevalence, such as IPF.