Estrogen Signaling and MicroRNAs in Lung Fibrosis. Sex, Hormones, and Rock Scars

Substantial epidemiological evidence suggests that sex impacts the prevalence, susceptibility, and severity of chronic lung diseases. Whether this is directly related to sex hormones is currently unknown. On the other hand, it is well known that sex hormones regulate lung development, physiology, and pathology (1). In particular, idiopathic pulmonary fibrosis (IPF), a progressive fibrotic lung disease of unknown etiology, is more prevalent and has a worse prognosis in males than in females (2). However, differences in survival in both humans (3) and experimental models (4) are still controversial. Sex differences exist in other chronic lung diseases, including idiopathic pulmonary arterial hypertension (5), lymphangioleiomyomatosis, and collagen vascular disease–associated interstitial lung diseases (6) that primarily affect young to middle-aged women, supporting a role for estrogen signaling in both disease development and progression (7). These sex differences underline the importance of considering sex hormones in the prevention and treatment of chronic lung diseases.

In this issue of the Journal, Elliot and colleagues (pp. 1246–1257) study the role of estrogen signaling in the pathogenesis of lung fibrosis and attempt to explain sex differences in disease development and prognosis (8). They describe the results of an elegant series of experiments and report the following major findings:

1. ERα (estrogen receptor α) expression was upregulated in IPF lung tissue and fibroblasts at both the mRNA and protein levels.

2. IPF lung fibroblasts exhibited increased responsiveness to estrogen compared with controls, and blockage of ER diminished this effect. In vivo pharmacologic inhibition and mimicry of ERα and ERβ, respectively, attenuated bleomycin-induced lung fibrosis, as assessed by biochemical and histological analyses of fibrotic lungs.

3. Mechanistic data demonstrated that ERα antagonism exerted its antifibrotic properties through negative regulation of profibrotic kinase-controlled signal transduction pathways, including Smad2 and AKT (protein kinase B). ERβ agonism appeared to exert minimal effects on the regulation of the Smad-mediated TGFβ (transforming growth factor β) canonical pathway.

4. The antifibrotic microRNAs let-7a (lethal-7a) and let-7d, which are known to be mediators of the ER-signaling pathway and to be reduced in IPF lungs (9), downregulated ERα and TGF-β and increased Smad7 mRNA expression, respectively, in IPF lung myofibroblasts.

5. Mice with genetic ablation of the AF2 (activation function 2) functional domain of the estrogen ligand-binding domain developed lung fibrosis in response to bleomycin, which also occurred in wild-type littermates, suggesting that ligands other than estrogens may be responsible for bleomycin-induced lung fibrosis.

6. IGF-1 (insulin-like growth factor 1), a known stimulator of the ER–signal transduction pathway, was upregulated in both IPF lungs and myofibroblasts and mediated ER transcriptional activity only in the IPF lung fibroblasts.

The idea of treating lung fibrosis with hormone analogs is both challenging and attractive. Our study group recently showed that IPF lungs are locally hypothyroid, and aerosolized administration of active thyroid hormone (T3) as well as systemic delivery of sobetirome, a thyroid-hormone receptor agonist, alleviated experimental lung fibrosis through restoration of alveolar epithelial cell mitochondrial homeostasis (10). In a similar way, a first-line antidiabetic drug called metformin exerts antifibrotic properties through positive regulation of the AMPK (adenosine monophosphate–activated protein kinase)-controlled metabolic pathway, rendering lung myofibroblasts more prone to apoptosis (11). Interestingly, and relevant to the concept of the current study, administration of danazol, a weak androgen, was shown to reconstitute telomere length in patients with manifestations of short-telomere syndrome, including bone marrow failure and pulmonary fibrosis (12). Dehydroepiandrosterone, a steroid prohormone that decreases with age, was shown to be significantly decreased in patients with IPF (an aging-related disorder) and exhibited strong antifibrotic properties through pleiotropic mechanisms such as apoptosis, proliferation, and differentiation of lung myofibroblasts (13). Finally, vitamin D, another steroid prohormone with a vital role in bone and muscle health, was found to be deficient in patients with IPF, was associated with worse prognosis, and exerted antifibrotic properties both in vitro and in vivo (14). The story of drug repurposing is rapidly expanding in chronic lung diseases, including IPF. Unfortunately, retrospective analyses of pirfenidone trials showed no effect of metformin use in patients with IPF, indicating that personalized-medicine approaches need to be applied to increase therapeutic likelihood (15). Whether hormone supplementation is effective for lung fibrosis per se or its associated comorbidities remains to be determined. Large prospective, randomized clinical trials in biologically enriched and well-defined cohorts of patients with IPF are required to address this issue. Local delivery of hormone analogs may provide clinical benefit with minimal adverse events and may lead to novel therapeutic strategies.

Although the results are cause for enthusiasm, this study has some limitations that should be adequately discussed:

1. The exact mechanism that regulates ER activation is currently unknown. Although the authors performed a thorough investigation of the ER–signal transduction pathway, genomic and nongenomic regulators of the ER pathway require further validation to identify potential therapeutic candidates. The suggested linkage of the ER pathway with microRNA mimicry appears vague and complicated given that microRNAs are undruggable targets due to their pleiotropism and complexity.

2. The phenomenon of ER-mediated attenuation of bleomycin-induced lung fibrosis does not seem to be cell specific. The authors focused on fibroblast biology; however, the effects of ER-signaling modulation in other cell types that are known to be involved in lung fibrogenesis, including structural (epithelial) and immune (macrophages and T cells) cells, need to be better clarified.

3. ERs are ubiquitously expressed receptors that are involved in physiological and pathological processes, including carcinogenesis, and therefore the side effects of modulating their activity require further consideration. In addition, pulmonary hypertension and embolism have been associated with oral contraceptives through estrogen-induced obliterative and thrombotic vascular lesions, and estrogens exert proinflammatory and oxidative stress effects (1).

Despite the above concerns, this study represents the first attempt to provide a pathogenic linkage between sex hormones and lung collagen deposition, and to explain the female survival bias in lung fibrosis. Linking ER and other hormone analogs with accelerated aging, senescence, and metabolic disorders, which are often seen in patients with fibrotic lung disease, would be an interesting future approach—the best is yet to come.