SRC and TKS5 mediated podosome formation in fibroblasts promotes extracellular matrix invasion and pulmonary fibrosis
The activation and accumulation of lung fibroblasts resulting in aberrant deposition of extracellular matrix components, is a pathogenic hallmark of Idiopathic Pulmonary Fibrosis, a lethal and incurable disease. Lung fibroblasts (LFs) are the main effector cells in pulmonary fibrosis, secreting exuberant amounts of extracellular matrix (ECM) components, such as different types of collagens. ECM fibrotic remodeling and resulting mechanical cues are considered as crucial stimulating and perpetuating factors for LF activation, while the chemoattraction of LFs to various signals and their resistance to apoptosis has been suggested to promote respectively their recruitment and accumulation. Fibroblast accumulation in pulmonary fibrosis has also been suggested to be mediated by their ability to invade the underlying ECM, and increased ECM invasion of fibroblasts isolated from the lung tissue of IPF patients or animal models has been reported. Activation of invasion, critical for embryonic development, is among the well-established hallmarks of cancer, and one of the many shared hallmarks between cancer cells and activated LFs. Invasion critically relies on the proteolysis of the underlying ECM via invadopodia in cancer cells and podosomes in other cell types.
In this report, increased expression of TKS5, a scaffold protein essential for the formation of podosomes, was detected in the lung tissue of Idiopathic Pulmonary Fibrosis patients and bleomycin-treated mice. Τhe profibrotic milieu is found to induce TKS5 expression and the formation of prominent podosome rosettes in lung fibroblasts, that are retained ex vivo, suggesting podosome formation as an inherent and possibly imprinted LF property. Haploinsufficient Tks5+/- mice, expressing reduced levels of Tks5, are found resistant to bleomycin-induced pulmonary fibrosis, largely attributed to diminished podosome formation in LFs and decreased extracellular matrix invasion. Expression profiling of Tks5+/- LFs, defective in podosome formation and ECM invasion, indicated a crosstalk of ECM with fibroblasts and podosomes, while connectivity MAP (CMap) analysis of the expression profile identified, among established and promising others, src inhibition as a possible treatment to limit LF invasion and therefore pulmonary fibrosis. In agreement, src inhibition was shown to reduce Tks5 levels and podosome numbers, to decrease aECM invasion and to attenuate BLM-induced pulmonary fibrosis in vivo when administered by inhalation. The aerosolized delivery, infrequent in IPF and animal models, opens new possibilities in IPF treatment, localizing treatment and avoiding systemic toxicity, as well as possibly increasing efficacy. The Src inhibitor Saracatanib, that has been previously also shown to attenuate BLM-induced pulmonary fibrosis61, has recently entered clinical trials (NCT04598919), and if therapeutic effects are as efficient as in mice, the inhibition of podosome formation in LFs qualifies as a major part of its mode of action. Moreover, targeting kinase-mediated podosome formation, an inherent pathogenic LF property as shown here, as well as structural (TKS5 and its protein-protein interactions) or effector (MMP 2/9/14) podosome components, are very promising therapeutic targets in pulmonary fibrosis.