Extracellular Matrix and Fibroblast Activation in Lymphangioleiomyomatosis

Lymphangioleiomyomatosis (LAM) is a rare lung disease caused by hyperactivation of the mTORC1 (mechanistic/mammalian target of rapamycin 1) growth pathway in a subset of mesenchymal lung cells. Histopathologically, LAM lesions have been described as immature smooth muscle-like cells that are positive for the immature melanocytic marker HMB45/PMEL/gp100 and phosphorylated ribosomal protein S6 (pS6). Advances in single-cell sequencing technology allowed us to group LAM cells according to their expression of cancer stem cell (CSC) genes and identify three clusters: a high CSC-like state (i.e., stem-like state), an intermediate state, and a low CSC-like inflammatory state. We show here that, in unique LAM cells, many extracellular matrix (ECM) genes, including collagens and CTHRC1 (collagen triple helix repeat-containing 1), are expressed in the high and intermediate CSC-like LAM clusters and suggest that, as is observed in CSCs, the ECM may provide a shield for LAM lesions against immunosurveillance. In LAM-associated fibroblasts, the bisteric mTORC1-selective inhibitor RMC-5552 blocked translation of TGF-β (transforming growth factor-β)-induced COL1A1, COL6A1, and phosphorylation of the mTORC1 substrates ribosomal protein S6K1/S6 (S6K1/ribosomal protein S6) and 4E-BP1/eIF4E (eukaryotic initiation factor 4E-binding protein/translation initiation factor 4E), whereas rapamycin, the U.S. Food and Drug Administration-approved therapy for LAM disease, inhibited only the S6K1/S6 axis. C82, a Wnt/β-catenin transcription inhibitor, prevented TGF-β-induced collagens but not pS6 or p4E-BP1. This demonstrates that mTORC1-driven 4E-BP1/eIF4E rapamycin-insensitive translational control overrides transcriptional control of ECM genes. Inhibition by RMC-5552 of ECM and fibroblast activation may result in destruction of CSC-like LAM cells and provide more enduring therapy for LAM.