Received for publication August 17, 2007.
Revised November 16, 2007.
Accepted for publication December 18, 2007.

Interferon augments TSC2-dependent inhibition of TSC2-null ELT3 and human LAM-derived cell proliferation

Abstract

Lymphangioleiomyomatosis (LAM), a rare pulmonary disorder, manifests as an abnormal neoplastic growth of smooth muscle-like cells within the lungs. Mutational inactivation of tumor suppressor tuberous sclerosis complex 2 (TSC2) in LAM constitutively activates the mammalian target of rapamycin (mTOR)/p70 S6 kinase 1 (S6K1) signaling pathway and promotes neoplastic growth of LAM cells. In many cell types, type I interferon (IFN) inhibits proliferation and induces apoptosis through STAT-dependent and STAT-independent signaling pathways, one of which is mTOR/S6K1 signaling pathway. Our study shows that IFN is expressed in LAM tissues and LAM-derived (LAM) cell cultures; however, IFN attenuates LAM-derived (LAM) cell proliferation only at high concentrations, 100 and 1000 U/ml (IC₅₀ for IFN is 20 U/ml compared to 1 U/ml for normal human mesenchymal cells, human bronchus fibroblasts (HBFs) and human airway smooth muscle (HASM) cells. Similarly, IFN only attenuates proliferation of smooth muscle TSC2-null ELT3 cells. Analysis of IFN signaling in LAM cells showed expression of IFN receptor (IFNR) and IFNR, activation and nuclear translocation of STAT1, and phosphorylation of STAT3 and p38 MAPK, but IFN had little effect on S6K1 activity. However, the re-expression of TSC2 or inhibition of mTOR/S6K1 with rapamycin (sirolimus) augmented anti-proliferative effects of IFN in LAM and TSC2-null ELT3 cells. Our study demonstrates that IFN-dependent activation of STATs and p38 MAPK is not sufficient to fully inhibit proliferation of cells with TSC2 dysfunction, and that TSC2-dependent inhibition of mTOR/S6K1 cooperates with IFN in inhibiting human LAM and TSC2-null ELT3 cell proliferation.

Key words: PDGF, Interferons, Jak/Stats, Protein targets, Mechanisms of cell killing/apoptosis, Tumor suppressors