Cathepsin K Alleviates Liver Fibrosis by Inhibiting the TGF-β/Smad Signaling Pathway and Inducing Hepatic Stellate Cell Apoptosis.

Abstract

Background and aims: Liver fibrosis is characterized by the excessive deposition of extracellular matrix, a process primarily driven by activated hepatic stellate cells (HSCs), and currently lacks effective therapy. Cathepsin K (CTSK) exhibits context-dependent roles across organ systems in fibrosis, but its function in liver fibrosis is unclear. This study aimed to investigate the role and underlying mechanisms of CTSK during liver fibrosis.

Methods: CTSK expression was analyzed in human fibrotic liver samples via transcriptomic analysis and confirmed in murine fibrosis models. The function of CTSK was investigated in both primary HSCs and LX-2 cells by assessing its effects on cell activation, proliferation, apoptosis, and the underlying signaling pathways following CTSK overexpression. The therapeutic potential was evaluated using an adeno-associated virus serotype 8 to overexpress CTSK in two etiologically distinct murine fibrosis models.

Results: CTSK was upregulated in activated HSCs and fibrotic livers. Furthermore, we discovered that it mediates a negative feedback loop to inhibit the TGF-β/Smad pathway via Smad7/Smurf2-dependent TGF-β receptor-I degradation, thereby suppressing HSC activation and proliferation. CTSK also induced mitochondrial apoptosis through Bax/Bcl-2 imbalance and caspase-3 activation. Together, these actions contribute to the anti-fibrotic effect of CTSK. Notably, adeno-associated virus serotype 8-mediated CTSK overexpression attenuated liver fibrosis across multiple murine models.

Conclusions: Our study demonstrates that elevated CTSK functions as an endogenous protective factor that attenuates liver fibrosis. CTSK mediates negative feedback inhibition of the TGF-β pathway while concurrently promoting the mitochondrial apoptosis pathway. The dual anti-fibrotic mechanisms identify CTSK as a promising therapeutic target for liver fibrosis.