Elafin in Pulmonary Arterial Hypertension. Beyond Targeting Elastases

More than a quarter of a century of research has surrounded the serine elastase inhibitor elafin. Initially identified in 1990 as an endogenously produced low-molecular-weight elastase inhibitor derived from psoriatic skin, elafin has been extensively studied in multiple disease contexts primarily involving inflammatory and infectious etiologies (1). It is predominantly expressed by the epithelial lining of multiple tissue beds, including the skin, female reproductive tract, upper gastrointestinal tract, and lungs. It consists of two functional domains: the N-terminal domain contains four transglutaminase substrate repeats, and the C-terminal whey acidic protein domain harbors the protease inhibitory function, which is known to inhibit a number of endogenously expressed elastases (2). Elafin-based therapy is currently being tested in clinical trials in the context of improving clinical outcomes in patients undergoing coronary artery bypass surgery, esophagectomy for esophageal cancers, and kidney transplantation (3). These studies have centered around elafin’s known inhibitory effects on proteases that contribute to inflammation that accompanies many of these conditions.

In the context of the cardiopulmonary system, studies dating back more than a decade have elaborated on the therapeutic promise of elafin. Work predominantly from the Rabinovitch group has demonstrated protective effects of elafin in multiple disease contexts, including transplant coronary arteriopathy (4), viral myocarditis (5), and arterial and vein graft injury models (6, 7). In the context of the pulmonary vasculature, initial studies demonstrated increased vascular elastase activity in the pulmonary arteries of experimental pulmonary hypertension (PH) models, and the promise and efficacy of elastase inhibitors were demonstrated using several agents (8–11). Supported by these studies, transgenic mice overexpressing elafin were tested in the hypoxia PH model and were found to be markedly protected (12). Mechanistically, the protective effects were thought to involve the inhibition of the elevated matrix metalloproteinase activity by elafin, with suppression of matrix metalloproteinase-9 activity in the elafin transgenic mice. The promise of this as a therapy led to orphan drug designation for elafin in both the European Union and the United States.

In this issue of the Journal, Nickel and colleagues (pp. 1273–1286) identify a novel, endothelial-based mechanism of elafin that may prove to be of key value in the context of pulmonary arterial hypertension (PAH) (13). Advancing the prior finding that transgenic mice overexpressing elafin were protected against hypoxia-induced PH, the current work demonstrates the efficacy of subcutaneous elafin administration in rescuing the SU-5416/hypoxia rat model of PH, as described by improved right ventricular systolic pressure, amelioration of pulmonary artery occlusive changes, and improved right ventricular hypertrophy. Interestingly, the observation was made in elafin-treated rats that lung expression of apelin, previously described by the authors to be a transcriptional target of BMPR2-mediated signaling (14), was modestly but significantly increased in response to elafin treatment. Given that apelin is predominantly expressed by endothelial cells, additional investigation of the effects of elafin focused on its potential role as a modulator of endothelial bone morphogenetic protein (BMP) signaling. Elafin was found to improve the dysregulated tube formation in PAH pulmonary artery endothelial cells and augment the downstream targets of BMP signaling, namely, SMAD1/5 phosphorylation and Id1 (inhibitor of DNA binding 1) expression, when concurrently used with exogenous BMP ligands. Moreover, these effects were found to be dependent on caveolin-1 by augmenting BMP-mediated redistribution of caveolin-1 to the plasma membrane. These studies raise the exciting possibility of a novel mechanistic interaction among BMP signaling, caveolin-1, and endothelial apelin expression circumscribed by an endogenous inhibitor of vascular elastase activity.

Interestingly, supported by previous findings that lung section tissues can be maintained in culture for days and exhibit treatment responses (15), the authors demonstrate for the first time the use of human lung tissue sections from patients with PAH to investigate the response to elafin. Elafin induced increases in lumen area and diameter and also induced apoptosis of pulmonary arterial smooth muscle cells. Such use of ex vivo human tissue for testing therapeutic response represents a novel paradigm for future studies in PAH research, although the paucity of available tissue specimens would limit widespread use of such a method.

The present study provides a number of key insights, as well as a number of questions that should be the topic of future investigations. First, the in vivo efficacy of elafin in rescuing the SU-5416/hypoxia model reaffirms its therapeutic efficacy in experimental PAH; the mechanistic insights with respect to its augmentation of BMP signaling provide an additional mechanism, which may be a key component of its efficacy in the rescue of PH models, in addition to its known inhibition of elastase activity. However, the contribution of each of these mechanisms to the disease-ameliorating process remains to be fully defined. Certainly, both of the described mechanisms of elafin are likely contributing to its therapeutic efficacy, but further delineation of these effects by testing its efficacy in additional genetic contexts, such as by using the caveolin-1 knockout mice, apelin knockout mice, and Bmpr2 mutant models, would shed key insights into which mechanism of action of elafin has the most important role. Ascertaining whether or not elafin rescues pulmonary vascular BMP signaling in vivo and discerning how elafin promotes Bmpr2–caveolin-1 interactions could help elucidate its mechanism of action further and could identify novel interventions. Second, elafin therapy is being tested in a multitude of disease contexts, suggesting its systemic administration for PAH will likely affect the function of other vascular beds and target organs. Advancing the technology to limit potential off-target effects through methods such as inhalational delivery or other means to provide localized effects to the lungs could improve the likelihood of achieving therapeutic efficacy of this promising therapy.