Extracellular Matrix and Fibroblast Activation in Lymphangioleiomyomatosis

Lymphangioleiomyomatosis (LAM) is a rare lung disease caused by hyperactivation of the mechanistic/mammalian target of rapamycin 1 (mTORC1) growth pathway in a subset of mesenchymal lung cells. Histopathologically, LAM lesions have been described as immature smooth muscle-like cells positive for the immature melanocytic marker HMB45/PMEL/gp100 and phosphorylated ribosomal protein S6 (pS6). Advances in single cell sequencing (scRNA-seq) 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 (SLS), an intermediate state, and a low CSC-like inflammatory state (IS). We show here that in unique LAM cells many extracellular matrix (ECM) genes, including collagens and collagen triple helix repeat-containing 1 (CTHRC1), 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 (LAFs), the bi-steric mTORC1-selective inhibitor RMC-5552 blocked translation of TGF-β-induced COL1A1, COL6A1 and phosphorylation of the mTORC1 substrates ribosomal protein S6K1/ribosomal protein S6 (S6K1/S6) and eukaryotic initiation factor 4E-binding protein/translation initiation factor 4E (4E-BP1/eIF4E), whereas rapamycin, the Food and Drug Administration (FDA)-approved therapy for LAM disease, only inhibited the S6K1/S6 axis. C82, a Wnt/-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 patients.