Reply: Promise and Limitations of Relaxin-based Therapies in Chronic Fibrotic Lung Diseases

From the Authors:

We thank Profs. Royce, Bathgate, and Samuel for their comments. It is an honor to receive feedback from the leaders in understanding relaxin biology. We share their exhortation to the research community to study how relaxin/insulin-like family peptide receptor 1 (RXFP1) gene expression is regulated. We believe that this knowledge will be critical to realizing the promise of relaxin-based therapies in fibrotic disease. Our finding, that pulmonary RXFP1 gene expression in 134 patients with idiopathic pulmonary fibrosis (IPF) is directly associated with both FVC (the best-validated measure of prognosis in patients with IPF [1]) and diffusing capacity of the lung for carbon monoxide (2), belies the importance of the relaxin–RXFP1 signaling axis in IPF. We also found that IPF fibroblasts were less sensitive to a relaxin-like peptide, CGEN25009, than donor lung fibroblasts, as measured by the phosphorylation of myosin light chains. To our knowledge, this is the first head-to-head comparison between donor and IPF lung fibroblasts in terms of sensitivity to relaxin signaling. We concluded that patients with IPF with the highest expression of RXFP1 would be predicted to be most sensitive to relaxin-based therapies, and, by extension, that low expression of RXFP1 may compromise sensitivity to relaxin-based therapies in IPF. The authors suggest that our data are, perhaps, discordant with multiple pieces of preclinical data that have shown the efficacy of relaxin in fibrosis. Rather, we submit that our data are supportive of previous preclinical data by demonstrating a clinical correlation between RXFP1 expression and IPF severity. Therefore, we suggest that relaxin remains a compelling therapeutic target to be considered for further testing in IPF.

However, before we can translate relaxin into an effective therapy for fibrotic lung disease, we are forced to grapple with the “elephant in the room,” that is, understanding why recombinant relaxin failed in a clinical trial for systemic sclerosis (3). Although we cannot answer this question definitively based on our study of IPF, we speculate that the loss of RXFP1 expression is common to many forms of fibrotic disease and that this loss of RXFP1 may compromise the potential effectiveness of relaxin. Therefore, as we reconsider relaxin-based therapies in fibrotic disease, we must contend with how to enhance sensitivity of IPF and systemic sclerosis lungs to relaxin. Although we show that the sensitivity of IPF fibroblasts to CGEN25009 was restored by enhanced expression of RXFP1 (2), we agree with the authors’ point that the functional activity of RXFP1 in fibroblasts is more complex than simply gene expression alone.

Some additional points are worth mentioning. First, we agree with the authors that differences in the cellular composition of IPF compared with donor lungs will have dramatic effects on mRNA detected by microarray. We argue, however, based on our in vitro data, where we have shown that RXFP1 expression is decreased in lung fibroblasts treated with transforming growth factor-β at the level of both mRNA and protein (Figures 3A–3C in Reference 2), that reductions in RXFP1 mRNA in fibroblasts are responsible, at least in part, for the decreased RXFP1 gene expression observed by microarray of whole–IPF lung homogenates. Second, we agree that the effect of relaxin on cells in the lung other than fibroblasts is of great interest. Further study is needed to understand how relaxin signals in epithelial cells and macrophages, for example, and how this impacts cellular cross-talk with fibroblasts.

Again, we thank the authors for the very important questions that they have raised and hope that we can collaboratively study the regulation of RXFP1 expression to achieve the ultimate goal of bringing relaxin to patients who suffer from IPF.