Signal Transduction and Targeted Therapy | Bronchial epithelial cell-derived PTHrP1-34 promotes pulmonary fibrosis via the PTH1R signaling axis

This study reveals the critical pathogenic role of bronchial epithelial cell-secreted PTHrP1-34 in idiopathic pulmonary fibrosis (IPF) and validates its potential as a therapeutic target, offering a novel strategy for IPF intervention.

 

Literature Overview

The article titled 'Parathyroid hormone–related protein is a therapeutic target in idiopathic pulmonary fibrosis,' published in the journal Signal Transduction and Targeted Therapy, reviews and summarizes the pathogenic mechanisms of bronchial epithelial cell-derived parathyroid hormone-related protein (PTHrP) in idiopathic pulmonary fibrosis (IPF) and its potential as a therapeutic target. By integrating single-cell and bulk RNA sequencing data, the study found that PTHrP is significantly upregulated in lung tissues of IPF patients, primarily originating from bronchial epithelial cells. Further experiments confirmed that the active peptide fragment PTHrP1-34, generated by cleavage of PTHrP, promotes fibroblast activation and extracellular matrix deposition by activating the PTH1R/PKA signaling pathway, thereby driving pulmonary fibrosis. In a bleomycin-induced mouse model, blocking the PTHrP1-34/PTH1R axis using neutralizing antibodies, antagonistic peptides, or gene silencing strategies significantly alleviated fibrotic phenotypes, with efficacy comparable or even superior to the existing drug nintedanib. This study not only uncovers a novel epithelial-mesenchymal paracrine signaling mechanism in IPF but also identifies PTHrP1-34 as a potential therapeutic target, providing theoretical and experimental support for the development of novel anti-fibrotic therapies.

Background Knowledge

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive, and ultimately fatal interstitial lung disease characterized by alveolar structural destruction and abnormal extracellular matrix deposition due to extensive fibroblast activation. Although drugs such as pirfenidone and nintedanib can slow disease progression, they fail to reverse fibrosis or significantly improve survival, and are associated with tolerability issues, creating an urgent need for new therapeutic targets. The pathogenesis of IPF involves the release of multiple soluble mediators from injured alveolar epithelial cells, which activate fibroblasts and promote their differentiation into myofibroblasts that excessively secrete collagen and other matrix components. Current research focuses on signaling pathways such as TGF-β, IL-11, and Wnt, but clinical trials targeting these pathways have largely failed to meet efficacy expectations. PTHrP, a polypeptide encoded by the PTHLH gene, is widely involved in development, calcium homeostasis, and epithelial-mesenchymal interactions. Its cleavage product, PTHrP1-34, acts as a ligand for PTH1R, activating pathways such as cAMP/PKA. Previous studies have shown elevated PTHrP expression in acute lung injury, but its role in pulmonary fibrosis remains unclear. This study, based on re-analysis of public omics data, identifies significant upregulation of PTHLH in IPF and systematically reveals the critical role of bronchial epithelial cell-derived PTHrP1-34 in fibrosis, filling a gap in understanding epithelial-derived factors in IPF progression and offering a new perspective on targeting paracrine signaling.

 

 

Research Methods and Experiments

The study first integrated three public transcriptomic datasets to identify 714 differentially expressed genes in IPF lung tissues and found, through KEGG pathway analysis, significant enrichment in the PTH synthesis and action pathway. Further intersection with secretory protein and PTH-related gene sets identified PTHLH as a potential secretory factor. Immunofluorescence and qPCR were used to validate PTHLH mRNA and PTHrP protein expression levels in lung tissues from IPF patients and bleomycin-induced mouse models. Single-cell RNA sequencing analysis revealed that PTHLH is predominantly highly expressed in bronchial epithelial cells (especially abnormal basal-like cells), whereas its receptor PTH1R is mainly expressed in mesenchymal cells such as fibroblasts and smooth muscle cells, suggesting an epithelial-mesenchymal paracrine axis.

In vitro experiments involved treating human embryonic lung fibroblasts (MRC5), fibroblasts derived from IPF patients, and mouse primary fibroblasts with different PTHrP fragments to assess changes in fibrotic markers such as α-SMA and collagen I, and to analyze activation of signaling pathways including MAPK, PKA, and AKT via Western blot. The dependence on signaling pathways was confirmed using siRNA knockdown of PTH1R or treatment with the PKA inhibitor H89. The EC50 of PTHrP1-34 was determined using a CRE-luciferase reporter system in HEK293-PTH1R cells.

In vivo functional experiments used a bleomycin-induced mouse model of pulmonary fibrosis, administering PTHrP1-34 via intratracheal instillation to evaluate its effects on fibrotic genes, hydroxyproline content, histopathology, and α-SMA expression. PTHLH knockdown models in BEAS-2B and HBEpC cells were established, and conditioned media (CM) were collected to treat fibroblasts, verifying the necessity of PTHrP1-34 in epithelial-fibroblast crosstalk. Furthermore, interventions using a PTHrP-neutralizing antibody (α-PTHrP), antagonistic peptide PTHrP7-34, or lentivirus-shPthlh were performed to evaluate their preventive and therapeutic effects on fibrosis progression, with efficacy compared to nintedanib. Serum calcium levels were also monitored to assess systemic safety.

Key Conclusions and Perspectives

• Integrated transcriptomic analysis revealed significant upregulation of PTHLH in lung tissues of IPF patients, with its encoded protein PTHrP predominantly expressed in bronchial epithelial cells, especially highly enriched in abnormal basal-like cells
• PTHrP1-34, generated by cleavage of PTHrP, is elevated in lung homogenates and BALF of IPF patients and shows time-dependent upregulation in the bleomycin-induced mouse model, primarily during the fibrotic phase rather than the inflammatory phase
• PTHrP1-34 specifically activates the cAMP/PKA/CREB signaling pathway by binding to PTH1R on fibroblasts, inducing fibroblast activation, α-SMA expression, and extracellular matrix synthesis, without significant effects on bronchial smooth muscle cells
• In the bleomycin model, exogenous PTHrP1-34 exacerbates pulmonary fibrosis, whereas interventions using neutralizing antibody α-PTHrP, antagonistic peptide PTHrP7-34, or shRNA-mediated Pthlh gene silencing significantly alleviate fibrotic phenotypes, reducing hydroxyproline content and fibrotic gene expression
• The anti-fibrotic effects of α-PTHrP and PTHrP7-34 are comparable or even superior to nintedanib, and local administration does not affect systemic calcium homeostasis, suggesting favorable safety potential
• Under bleomycin stimulation or hypoxia, bronchial epithelial cells secrete PTHrP1-34, and the ability of their conditioned media to induce fibroblast activation is blocked by neutralizing antibodies or gene knockdown, confirming its functional paracrine role

Research Significance and Prospects

This study systematically elucidates the key pathogenic role of bronchial epithelial cell-derived PTHrP1-34 in IPF, revealing a novel mechanism of epithelial-fibroblast crosstalk and expanding our understanding of the IPF pathological network. The PTHrP1-34/PTH1R signaling axis, as a druggable paracrine pathway, provides a solid foundation for developing novel anti-fibrotic therapeutics. Compared to existing targets, PTHrP1-34 is specifically upregulated in fibrotic tissues, and its local blockade does not affect systemic calcium metabolism, suggesting a potentially higher therapeutic index and improved safety.

Future studies could further explore the expression profile of PTHrP1-34 in different IPF subtypes and its correlation with disease progression, assessing its potential as a biomarker. The competitive antagonistic peptide PTHrP7-34, with favorable pharmacological and safety profiles, is a strong candidate for preclinical development. Additionally, investigating interactions between PTHrP1-34 and other known pathways (e.g., TGF-β) will help construct a more comprehensive IPF signaling network. The successful validation of this target also provides a reference for studying similar mechanisms in fibrosis of other organs (e.g., liver, kidney).

 

 

Conclusion

This study, through multi-omics integration and functional validation, reveals a novel mechanism by which bronchial epithelial cells in idiopathic pulmonary fibrosis promote fibroblast activation via secretion of PTHrP1-34. The study shows that PTHLH/PTHrP is significantly upregulated in lung tissues of IPF patients and animal models, primarily originating from bronchial epithelial cells, and that its cleavage product PTHrP1-34 specifically activates the PTH1R receptor on fibroblasts, triggering the PKA signaling pathway and driving extracellular matrix deposition and fibrosis progression. In a bleomycin-induced pulmonary fibrosis model, targeting the PTHrP1-34/PTH1R axis with neutralizing antibodies, antagonistic peptides, or gene silencing strategies significantly alleviates fibrotic phenotypes, with efficacy comparable or even superior to nintedanib, and without affecting systemic calcium homeostasis. This study not only clarifies a new pathway of epithelial-mesenchymal crosstalk but also identifies PTHrP1-34 as a promising therapeutic target, offering a new direction for developing more effective and safer anti-fibrotic therapies. This discovery may help shift IPF treatment strategies from merely slowing progression toward targeted reversal of fibrosis.

 

Xue-Quan Fang, Suha Lim, Yoon-Mi Lee, Hyun-Woo Shin, and Ji-Hong Lim. Parathyroid hormone–related protein is a therapeutic target in idiopathic pulmonary fibrosis. Signal Transduction and Targeted Therapy.