TSC1 and TSC2 mutations cause neoplasms in rare disease pulmonary LAM and neuronal pathfinding in hamartoma syndrome TSC. The specific roles of TSC1 and TSC2 in actin remodeling and the modulation of cell motility, however, are not well understood. Previously, we demonstrated that TSC1 and TSC2 regulate the activity of small GTPases RhoA and Rac1, stress fiber formation and cell adhesion in a reciprocal manner. Here, we show that Tsc1(-/-) MEFs have decreased migration compared to littermate-derived Tsc1(+/+) MEFs. Migration of Tsc1(-/-) MEFs with re-expressed TSC1 was comparable to Tsc1(+/+) MEF migration. In contrast, Tsc2(-/-) MEFs showed an increased migration compared to Tsc2(+/+) MEFs that were abrogated by TSC2 re-expression. Depletion of TSC1 and TSC2 using specific siRNAs in wild type MEFs and NIH 3T3 fibroblasts also showed that TSC1 loss attenuates cell migration while TSC2 loss promotes cell migration. Morphological and immunochemical analysis demonstrated that Tsc1(-/-) MEFs have a thin protracted shape with a few stress fibers; in contrast, Tsc2(-/-) MEFs showed a rounded morphology and abundant stress fibers. Expression of TSC1 in either Tsc1(-/-) or Tsc2(-/-) MEFs promoted stress fiber formation, while TSC2 re-expression induced stress fiber disassembly and the formation of cortical actin. To assess the mechanism(s) by which TSC2 loss promotes actin re-arrangement and cell migration, we explored the role of known downstream effectors of TSC2, mTORC1 and mTORC2. Increased migration of Tsc2(-/-) MEFs is inhibited by siRNA mTOR and siRNA Rictor, but not siRNA Raptor. siRNA mTOR or siRNA Rictor promoted stress fiber disassembly in TSC2-null cells, while siRNA Raptor had little effect. Overexpression of kinase-dead mTOR induced actin stress fiber disassembly and suppressed TSC2-deficient cell migration. Our data demonstrate that TSC1 and TSC2 differentially regulate actin stress fiber formation and cell migration, and that only TSC2 loss promotes mTOR- and mTORC2-dependent pro-migratory cell phenotype.

Spontaneous pneumothoraces due to lung cyst rupture afflict patients with the rare disease Birt-Hogg-Dubé (BHD) syndrome, which is caused by mutations of the tumor suppressor gene folliculin (FLCN). The underlying mechanism of the lung manifestations in BHD is unclear. We show that BHD lungs exhibit increased alveolar epithelial cell apoptosis and that Flcn deletion in mouse lung epithelium leads to cell apoptosis, alveolar enlargement, and an impairment of both epithelial barrier and overall lung function. We find that Flcn-null epithelial cell apoptosis is the result of impaired AMPK activation and increased cleaved caspase-3. AMPK activator LKB1 and E-cadherin are downregulated by Flcn loss and restored by its expression. Correspondingly, Flcn-null cell survival is rescued by the AMPK activator AICAR or constitutively active AMPK. AICAR also improves lung condition of Flcn(f/f):SP-C-Cre mice. Our data suggest that lung cysts in BHD may result from an underlying defect in alveolar epithelial cell survival, attributable to FLCN regulation of the E-cadherin-LKB1-AMPK axis.