Cathepsin K: both a likely biomarker and a new therapeutic target in lymphangioleiomyomatosis?

Lymphangioleiomyomatosis (LAM) is a rare disease which is characterized by cystic lung destruction and lymphangiomas, and is associated with a high risk of osteoporosis-related bone fractures. Its diagnosis is based on pulmonary anatomopathological criteria combined with chest computed tomography. VEGF-D is the only serum diagnostic biomarker used in clinic, while inhibition of the mTOR pathway by rapamycin is currently the only reference therapy for LAM. Human cathepsin K (CatK), a potent collagenase predominantly found in osteoclasts, is considered as a valuable target for anti-osteoporosis and bone cancer therapy. Recently, CatK, which is overexpressed in lung cysts, was proposed as a putative LAM biomarker. Moreover, CatK may take part in the LAM pathophysiology by participating in pulmonary cystic destruction and bone degradation. Accordingly, targeting of collagenolytic activity of CatK by exosite-binding inhibitors in combination with mTOR inhibition could represent an innovative therapeutic option in reducing lung destruction in LAM.