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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Sci Transl Med. Author manuscript; available in PMC 2013 June 17.
Published in final edited form as:
PMCID: PMC3684391
NIHMSID: NIHMS481379

Prevention of Alveolar Destruction and Airspace Enlargement in a Mouse Model of Pulmonary Lymphangioleiomyomatosis (LAM)

Abstract

Pulmonary lymphangioleiomyomatosis (LAM) is a rare genetic disease characterized by neoplastic growth of atypical smooth muscle–like LAM cells, destruction of lung parenchyma, obstruction of lymphatics, and formation of lung cysts, leading to spontaneous pneumothoraces (lung rupture and collapse) and progressive loss of pulmonary function. The disease is caused by mutational inactivation of the tumor suppressor gene tuberous sclerosis complex 1 (TSC1) or TSC2. By injecting TSC2-null cells into nude mice, we have developed a mouse model of LAM that is characterized by multiple random TSC2-null lung lesions, vascular endothelial growth factor–D expression, lymphangiogenesis, destruction of lung parenchyma, and decreased survival, similar to human LAM. The mice show enlargement of alveolar airspaces that is associated with progressive growth of TSC2-null lesions in the lung, up-regulation of proinflammatory cytokines and matrix metalloproteinases (MMPs) that degrade extracellular matrix, and destruction of elastic fibers. TSC2-null lesions and alveolar destruction were differentially inhibited by the macrolide antibiotic rapamycin (which inhibits TSC2-null lesion growth by a cytostatic mechanism) and a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, simvastatin (which inhibits growth of TSC2-null lesions by a predominantly proapoptotic mechanism). Treatment with simvastatin markedly inhibited MMP-2, MMP-3, and MMP-9 levels in lung and prevented alveolar destruction. The combination of rapamycin and simvastatin prevented both growth of TSC2-null lesions and lung destruction by inhibiting MMP-2, MMP-3, and MMP-9. Our findings demonstrate a mechanistic link between loss of TSC2 and alveolar destruction and suggest that treatment with rapamycin and simvastatin together could benefit patients with LAM by targeting cells with TSC2 dysfunction and preventing airspace enlargement.

INTRODUCTION

Pulmonary lymphangioleiomyomatosis (LAM), a rare lung disease affecting predominantly women of childbearing age (1), is caused by mutational inactivation of the tumor suppressor gene tuberous sclerosis complex 1 (TSC1) or TSC2. LAM can be sporadic (LAM-S) or associated with hamartoma syndrome tuberous sclerosis (LAM-TS) and is characterized by neoplastic growth of smooth muscle (SM)–like LAM cells, destruction of lung parenchyma, formation of lung cysts, and obstruction of lung lymphatics. Patients exhibit shortness of breath, spontaneous pneumothoraces (lung rupture and collapse), and chylothorax (obstruction of the thoracic duct and leakage of lymphatic fluid into the pleural space) (1). TSC1 mutations cause a less severe clinical phenotype than do TSC2 mutations (2). In addition, about 40% of LAM-S and 80% of LAM-TS patients develop angiomyolipomas (AMLs)—benign tumors of SM, blood vessels, and fat cells in the kidney (1). It is not known how LAM cells deficient for TSC2 cause destruction of lung parenchyma or whether lung destruction in LAM can be ameliorated.

TSC1/TSC2 regulates mammalian target of rapamycin (mTOR), which forms two functionally distinct complexes: rapamycin-sensitive mTORC1 and rapamycin-insensitive mTORC2 (3). Rapamycin-insensitive regulation of the actin cytoskeleton occurs through mTORC2-dependent regulation of RhoA and Rac1 guanosine triphosphatases (GTPases) (3, 4), and Rac1 is required for mTOR activation (5). In TSC2-null and human LAM cells, Rho GTPase activity is required for cell adhesion, motility, proliferation, and survival (68). The invasive cell phenotype is associated with up-regulation of matrix metalloproteinases (MMPs), and loss of TSC2 causes up-regulation of MMPs (911).

The discovery that TSC2 functions as a negative regulator of mTORC1 (3, 1214) led to clinical trials that tested the effect of the rapamycin analog sirolimus on LAM disease (15, 16). At nanomolar concentrations, rapamycin forms a complex with the immunofilin FKBP12 that allosterically inhibits mTORC1 (3). In some patients with LAM, sirolimus slows disease progression (15). Cessation of therapy, however, is associated with regression of pulmonary function (15). Thus, there is a need for alternative therapies to treat pulmonary LAM that target LAM cell survival and the cystic airspace enlargement.

In a recent study, we used the combination of rapamycin and simvastatin to abrogate TSC2-null tumor recurrence in mice carrying TSC2-deficient xenographic flank tumors (8). Simvastatin, a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor that modulates lipid metabolism, shows pleiotropic effects including inhibition of Rho GTPases, prevention of cancer (17), and prevention of experimental emphysema (18). However, the generalizability of drug effects on the TSC2-null flank tumors is limited and is unlikely to predict responses of the LAM lung.

A major limitation in understanding the mechanism of lung destruction in LAM and in identifying new therapeutic strategies has been the lack of a good animal model (19). Homozygous TSC1−/− and TSC2−/− mice are embryonic lethals (19), and the major spontaneous tumors in heterozygous TSC1+/− and TSC2+/− mice are kidney cystadenomas and liver hemangiomas (19). The spontaneous occurrence of lung tumors without apparent destruction of lung parenchyma in heterozygous TSC1+/− and TSC2+/− mice and in Eker rats with naturally occurring TSC2 mutations is extremely rare and only occurs late in the animals' life (19). Thus, an experimental mouse model with TSC2-null lesions that shows lung parenchymal changes would be useful to identify mechanisms of alveolar destruction in LAM and to develop therapeutic strategies to prevent these changes. Here, we report the development of a TSC2-null mouse LAM model that can be used to investigate the link between TSC2 loss and cystic destruction in LAM. We also characterize the effect of two drugs (rapamycin and simvastatin) on emphysematous lung destruction.

RESULTS

TSC2 loss induces lung lesions

Circulating LAM cells have been isolated from peripheral blood (20) and chylous effusions (21), suggesting that LAM cells may behave like metastatic tumor cells. However, the TSC2-related renal lesions such as AMLs, renal oncocytoma, and renal cell carcinoma (RCC) in human LAM (22) or cystadenoma and RCC in mice (19) are considered benign and cannot be classified as true RCC because they are not of renal epithelial origin (23).

To develop an animal model of LAM, we used TSC2-null cells derived from mouse kidney lesions that can spontaneously develop in heterozygous TSC2+/− mice (19, 24). Because these cells rarely formed lung lesions when directly injected into the tail vein of nude mice, we enhanced their neoplastic characteristics as follows (fig. S1). Cells from kidney lesions were injected into the flanks of athymic nude mice. The resulting tumors were dissociated, and cells were passaged. These TSC2-null tumor cells show mTORC1 activation, high proliferation rate without growth factor stimuli, migration, invasiveness, and SM α-actin expression similar to human LAM cells (Figs. 1, A and B, and and3A3A and fig. S2A) (7, 12, 25, 26). Injection of the TSC2-null tumor cells into the tail veins of nude mice induced growth of multiple lesions in the lung (Figs. 1, C and D, and and22 and figs. S3 and S4). No visible tumors were observed in kidney, spleen, liver, heart, intestine, or uterus.

Fig. 1
TSC2-null lung lesions induce alveolar destruction. (A) Immunoblot analysis of TSC2-null and TSC2-positive LLC cells with specific antibodies to detect indicated proteins. (B) DNA synthesis and invasion of TSC2-null, control epithelial NMuMG (Contr), ...
Fig. 2
Time-dependent alveolar airspace enlargement is associated with TSC2-null lesion growth in the lung. (A) Representative images of H&E-stained lungs collected at days 0, 10, 15, and 20 after injection. Scale bar, 200 μm. (B and C) Lesion/lung ...
Fig. 3
SM α-actin–positive TSC2-null lesions show mTORC1 activation, increased VEGF-D, and lymphangiogenesis. (A) Fluorescence-activated cell sorting (FACS) analysis of epithelial NMuMG cells (Control), TSC2-null cells, control HLF cells, and ...

TSC2-null lesions induce alveolar destruction and airspace enlargement in lungs

In pulmonary LAM, it is not known whether airspace enlargement occurs as a result of TSC2 loss in LAM cells. To address this question, we performed morphometric analysis of lungs from control mice and mice with TSC2-null lesions. To ensure that the lungs were processed under the same experimental conditions and to preserve the lung architecture, we inflated lungs from control and experimental animals at a constant 25-cm H2O pressure, followed by fixation as described in Materials and Methods. Hematoxylin and eosin (H&E) staining of control lungs showed typical lung structure with conducting airways, branching bronchioles, and alveoli (Figs. 1D and 2 and fig. S3). Lungs with TSC2-null lesions showed multiple lesions surrounded by thin-walled alveoli with multiple enlarged airspaces (Figs. 1D and and22 and fig. S3). The TSC2-null lesions stained positive for SM α-actin, as is also seen in human LAM lungs, and for phospho-S6 (P-S6), the molecular signature of mTORC1 activation in LAM (12) (Fig. 3, B and C, respectively).

To determine that enlarged airspaces were induced specifically by loss of TSC2 in the lesions, we reexpressed TSC2 in TSC2-null cells to use as a control. Reexpression of TSC2 not only inhibited cell growth but also prevented the ability of these cells to form tumors. As an alternative approach, we used Lewis lung carcinoma (LLC) cells, an established model of mouse lung cancer (27). LLC cells expressed TSC2 (Fig. 1A), formed multiple lung lesions (Fig. 1D), and were negative for P-S6 and SM α-actin (Fig. 3, B and C). Lung parenchyma surrounding TSC2-expressing LLC lesions is comparable to that found in control animals (Fig. 1D). Morphometric analysis of H&E sections, assessed by measurement of the mean linear intercept (MLI) (a mean of chord lengths between intersections with alveoli) and mean alveolar airspace area (MAAA) (28) (the average area of alveoli in examined fields), showed a statistically significant increase in alveolar airspace in lungs with TSC2-null lesions compared to lungs with LLC lesions and control lungs (Fig. 1, E and F). As an additional control, we measured MLI and MAAA in mouse lungs bearing TSC2+/+p53−/− lesions and found no alveolar space enlargement (Fig. 1, D to F). In immunocompetent C57BL/6 mice, TSC2-null cells also formed lung lesions that induced alveolar space enlargement (Fig. 1, E and F). Increases in airspace enlargement dependent on the percentage of the TSC2-null lesion area to the total lung area indicate that progressive destruction of lung parenchyma is induced by growth of TSC2-null but not TSC2-expressing lesions (Fig. 2).

Comparison of lung morphology with human LAM shows that TSC2-null lesions in mice appear to contain more cells than human LAM lesions and that TSC2-null lesions in mice tend to accumulate around veins, arteries, or airways (fig. S4), a feature present but less prominent in human LAM lesions. Also, a predominant feature in human LAM is holes/cysts with modest increase in alveolar size (29), whereas in the mouse model, only enlargement of alveolar spaces was observed. TSC2-null lesions in mice show SM α-actin expression (Fig. 3B), a characteristic feature of human LAM (1). Furthermore, in both TSC2-null lesions in mice and in human LAM, a biomarker of TSC2 loss, mTORC1 activation, was detected by phosphorylation of ribosomal protein S6 (Fig. 3C). Collectively, these data demonstrate that although the mouse model may not be a perfect morphological model for human LAM, this model has proliferating SM α-actin-positive TSC2-null cells inducing progressive alveolar destruction in the lung.

TSC2-null lesions express vascular endothelial growth factor–D and promote lymphangiogenesis

Lymphatic involvement in human LAM has been demonstrated by abundant lymphatics in human LAM lungs and LAM lesions (30) and is associated with high levels of lymphatic growth factor vascular endothelial growth factor–D (VEGF-D) in the serum of subjects with LAM disease (31). Lungs from mice with TSC2-null lesions showed marked immunoreactivity for VEGF-D, similar to VEGF-D immunoreactivity in human LAM tissue samples as shown in Fig. 3D. Nodules from LAM patients have pronounced lymphatic channels lined by endothelial cells (30) (Fig. 3E). In the mouse model, numerous lymphatic vessels were detected in TSC2-null lesions with antibody for the hyaluronan receptor (LYVE-1), a marker of lymphatic vessels (Fig. 3E). These data demonstrate increased VEGF-D expression and lymphangiogenesis in the TSC2-null mouse model of LAM, which have also been observed in human LAM.

TSC2-null lung lesions induce MMP expression and elastin fiber degradation

The prevailing hypothesis for the mechanism of parenchymal destruction and cystic lung formation in LAM is that it is mediated by MMPs that particularly target elastin. In LAM, MMP-2—which acts on substrates including elastin and collagens I, III, and IV—is up-regulated in an mTORC1-independent manner without significant change in expression of tissue inhibitors of metalloproteinases (TIMPs) (9). Here, MMP-2 and MMP-3 (elastin and collagens III and IV) and MMP-9 (elastin and collagen IV), analyzed by enzyme-linked immunosorbent assay (ELISA), were significantly increased in bronchoalveolar lavage (BAL) fluid from mice with TSC2-null lesions compared with age-matched controls (Fig. 4, A to C) (32). Immunohistochemical analysis of lungs with TSC2-null lesions also showed significant increases in MMP-7 (elastin and collagens I, III, and IV), MMP-9, and MMP-12 (elastin and collagens I and IV) (fig. S5, A, C, and D). MMP-8 (collagens I and III) immunostaining was also increased in lungs with TSC2-null and TSC2-expressing lesions compared to control lungs (fig. S5B). These data demonstrate differential expression of MMPs in lungs with TSC2-null and TSC2-expressing lesions, in particular of those MMPs that target elastin and collagens of various types. Notably, these changes in MMP levels occurred in a time-dependent manner, consistent with increases in lesion growth and lung destruction (Fig. 2).

Fig. 4
Increases in MMPs, inflammatory cells, and cytokines and decrease in alveolar elastin are associated with TSC2-null lung lesion growth. (A to C) MMP expression assessed in the cell-free supernatant of the BAL fluid at the indicated time points. A multiplex ...

Because lung destruction in LAM is a result, at least in part, of elastic fiber degradation (33), we examined elastin and collagen composition using Verhoeff's elastin and Picro-Ponceau counterstaining (34). In normal mouse lungs, elastin (Fig. 4, D and F, black) and collagen (Fig. 4, D and F, red) fibers are especially concentrated around the rim or neck of each alveolus (arrows in Fig. 4D). The alveoli of animals with TSC2-null lesions exhibited a significant loss of elastin in the neck region as shown by a decrease in black staining and statistical analysis (Fig. 4, E to G). Morphometric analysis confirmed a statistically significant decrease in elastin but not collagen content in alveoli of mouse lungs with TSC2-null lesions (Fig. 4E). These data demonstrate that growth of TSC2-null lesions is preferentially associated with elastic fiber loss, consistent with the increase in MMPs known to target elastin (35).

Neoplastic lesion progression is also characterized by a time-dependent activation of innate immune-derived mediators. Although no LAM-specific cytokine/chemokine profile has been identified to date, human LAM has been associated with increased CXCL1 (KC), CCL2 (MCP1), and CXCL5 (36). These chemokines could be up-regulated by interleukin-1β (IL-1β), tumor necrosis factor–α (TNF-α), and/or IL-6. IL-1β and IL-6 are strongly associated with tumor growth, and TNF-α signaling is directly linked to the TSC1/TSC2-mTOR through nuclear factor κB (NF-κB)/inhibitor of NF-κB kinase β (IKKβ) (37). To investigate the inflammatory response to growth of TSC2-null lesions in the lung, we measured TNF-α, IL-1β, IL-6, and the chemokines KC/CXCL1 (the human IL-8 equivalent) and eotaxin/CCL11, the receptor of which (CCR3) occurs in primary LAM tissue (36). We noted modest, but significant, release of each of these proinflammatory mediators during lesion progression (Fig. 4, H to L). TNF-α, IL-1β, and KC expression peaked 2 weeks after inoculation of tumor cells and showed a decline by day 20. IL-6 and eotaxin levels continuously increased up to day 20. Consistent with the fact that the tumor recipient mice were nude mice, there was no T cell–derived cytokine [interferon-γ (IFN-γ) and IL-4] expression, with the exception of minimally elevated IL-13 levels. Immunostaining with the macrophage marker F4/80 showed influx of macrophages into TSC2-null lesions (fig. S6). The kinetics of the release of proinflammatory mediators (Fig. 4, H to L) coincided with the influx of inflammatory cells into the airways (Fig. 4M and fig. S6), suggesting that these mediators may drive a proinflammatory response during tumor cell invasion of the lung tissue.

Rapamycin plus simvastatin prevents TSC2-null lesion growth and lung parenchyma destruction

We assessed whether rapamycin or simvastatin, alone or in combination, could prevent tumor growth of TSC2-null lesions and alveolar destruction in our mouse model of LAM. Before in vivo experiments, background cell culture analysis was performed to determine growth-inhibitory effects of rapamycin and simvastatin on TSC2-null cells used to establish the LAM mouse model. Rapamycin or simvastatin alone markedly inhibited TSC2-null proliferation (fig. S2, B and C), but the combination of these two drugs induced greater inhibition of TSC2-null cell proliferation than either drug alone (fig. S2D). The rapamycin and simvastatin doses and treatment schedules were selected on the basis of our published data (8). The drug treatment started at day 3 after TSC2-null cells were injected into mice (Fig. 5A).

Fig. 5
Rapamycin plus simvastatin rescues animal survival, prevents lesion growth and lung destruction, and abrogates MMP induction. Mice injected with diluent (Control) or TSC2-null cells were treated with vehicle, rapamycin (Rapa), simvastatin (Simva), and ...

Animals treated with vehicle progressively lost weight after day 11 and were sacrificed at day 20 (Fig. 5A). Vehicle-treated animals developed multiple large lung lesions with increased alveolar spaces in the surrounding parenchyma (Fig. 5B). All mice were sacrificed at day 20. Mice treated with rapamycin and simvastatin alone or in combination did not lose weight compared to vehicle-treated mice (Fig. 5A) and were comparable in weight to control mice given the same treatment (Fig. 5A). Rapamycin alone and in combination with simvastatin prevented TSC2-null lesion growth (Fig. 5, B and C). Simvastatin alone decreased the amount of lesion/lung compared to vehicle-treated mice (Fig. 5, B and C). Treatments with rapamycin, simvastatin, or both also prevented alveolar space enlargement (Fig. 5D).

Both rapamycin and simvastatin significantly decreased MMP-2 levels in BAL (Fig. 5E), whereas MMP-3 and MMP-9 expression was markedly and significantly reduced in simvastatin- but not rapamycin-treated animals (Fig. 5, F and G), demonstrating that rapamycin and simvastatin have differential inhibitory effects on MMP-3 and MMP-9. The combined treatment abrogated the increases in both MMP-3 and MMP-9 (Fig. 5, F and G).

Rapamycin and simvastatin treat established TSC2-null lesions and lung parenchyma destruction

To determine whether rapamycin and simvastatin could inhibit the growth of well-developed TSC2-null lesions, we treated TSC2-null lung lesion–bearing mice at day 10, a time when mice have begun to develop TSC2-null lung lesions and show destruction of lung parenchyma, but before the changes are statistically significant (Fig. 2B). Treatment with each drug alone on day 10 reduced TSC2-null cell proliferation as detected by immunostaining with Ki67 (Fig. 6A and fig. S7A) and by a decreased proportion of the lung occupied by lesion (Fig. 6D). As we showed previously (8), rapamycin but not simvastatin inhibited mTORC1-dependent S6 phosphorylation in TSC2-null lesions (Fig. 6B and fig. S7B), but rapamycin did not promote apoptosis, whereas simvastatin did (Fig. 6C and fig. S7C). Simvastatin alone inhibited lesion growth (Fig. 6A and fig. S7A) and induced apoptosis (Fig. 6C and fig. S7C) but had little effect on P-S6 levels (Fig. 6B and fig. S7B). Rapamycin and simvastatin combined showed modest effect on DNA synthesis and cell apoptosis in TSC2-null lesions (Fig. 6, A and C, and fig. S7, A and C).

Fig. 6
Rapamycin and simvastatin differentially affect TSC2-null lesion growth and airspace enlargement. Mice, injected with TSC2-null cells, were treated with vehicle (−), rapamycin (Rapa), simvastatin (Simva), and rapamycin + simvastatin (Rapa + Simva) ...

Nevertheless, morphometric analyses of lung tissue sections treated under the same experimental conditions showed a significant decrease in alveolar space enlargement in animals treated with simvastatin alone (Fig. 6, E and F). Rapamycin alone did not significantly decrease MLI and MAAA compared to untreated animals (Fig. 6, E and F) and only modestly improved the beneficial effect of simvastatin on preventing an alveolar space enlargement (Fig. 6, E and F). The simvastatin-induced reduction in alveolar destruction occurred when the lesion sizes were equivalent to those of the rapamycin-treated mice (Fig. 6D). These data demonstrate that, whereas rapamycin and simvastatin can each inhibit TSC2-null lesion growth, simvastatin prevents alveolar destruction.

DISCUSSION

We have demonstrated that our mouse model of LAM carries TSC2-null lesions that showed SM α-actin expression, mTORC1 activation, VEGF-D expression, and increased lymphangiogenesis, as well as destruction of lung parenchyma, all of which are characteristics of human pulmonary LAM. Our data showed that progressive growth of TSC2-null lung lesions induced MMP up-regulation and elastin fiber degradation in the lung. These findings suggest that the cystic lung destruction seen in LAM is associated with loss of TSC2. We also have shown that rapamycin inhibits lesion growth, simvastatin prevents alveolar space enlargement, and treatment with a combination of rapamycin and simvastatin abolishes MMP up-regulation and TSC2-null lesion growth and prevents alveolar destruction. The results reported here thus support further investigation of the combination of rapamycin and simvastatin as a potential treatment for subjects with LAM. It is possible that other diseases associated with TSC2 deficiency could benefit from the same combinational treatment strategy. Abnormal enlargement of airspaces is a major pathological manifestation of many lung diseases including emphysema, chronic obstructive pulmonary disease (COPD), cystic fibrosis, pulmonary LAM, and Birt-Hogg-Dube syndrome. Although the etiologies of these diseases are different, alveolar destruction is a common pathobiological manifestation associated with cystic airspace enlargement.

The prevailing hypothesis is that lung destruction and cystic space formation in patients with LAM are mediated by degradation of extracellular matrix as a result of an imbalance between matrix-degrading proteases (MMPs) and their endogenous inhibitors TIMPs (38). Indeed, elastic fibers in remodeled alveoli in lungs from LAM patients are scant, and those that remain are often disrupted (33). Similarly, in our study, significant decreases in elastin fibers were seen in alveoli of mice bearing TSC2-null lesions. Increased MMP-1, MMP-2, MMP-9, MMP-11, and MMP-19 levels have been reported in lungs from patients with LAM (39), and mTORC1-independent up-regulation of MMP-2 expression was shown in TSC2-deficient cells and cells from LAM patients (911). Our data demonstrate that growth of TSC2-null lesions in murine lung is associated with an increase in MMP-2, MMP-3, MMP-7, MMP-9, and MMP-12 levels, all with known elastase activity (35). Note that these MMPs also target collagens, including type I, but when examined histologically, collagen content was not significantly decreased where it is most concentrated, in the alveolar rims. The extent to which altered amounts of TIMPs are involved in alveolar destruction in this model remains to be determined.

The increase in proinflammatory cytokines in the LAM model suggests that inflammation may also contribute to the destruction of alveoli, given cytokine stimulation of MMPs (35). This process could provide an amplifying feedback loop in the alveolar destruction pathway. Further studies are needed in the TSC2-null mouse model of LAM to determine the relative contribution of TSC2-null lesions in increased expression of MMPs and the recruitment of inflammatory cells and cytokines to alveolar destruction in LAM. These would probably be most informative in further studies in immunocompetent mice where TSC2-null lesions also induce alveolar space enlargement.

Pulmonary LAM is accompanied by increased lymphangiogenesis in the lung and LAM nodules (40) and increased concentrations of VEGF-D, a lymphangiogenic growth factor (41) that has been found in the serum of subjects with LAM (31). The increased VEGF-D and increased number of lymphatic vessels in TSC2-null lesions in our mice echo these features.

We acknowledge that the model is not a perfect replica of human LAM, but it is similar in many ways—notably, progressive growth of SM α-actin–positive TSC2-null lesions, destruction of lung parenchyma, and lymphatic involvement. In human disease, rounded cystic change is usually more pronounced and more widespread and, in some cases, associated with relatively few LAM cells (typically as scattered fascicles in the cyst walls) compared to our mouse model in which the destruction of alveoli and airspace enlargement usually directly surrounds TSC2-null lesions and large rounded cysts are not a feature. In addition, the mouse lesions contain more TSC-null cells than most human lesions, and the cells showed some tendency to accumulate in perivascular regions of both arteries and veins, a feature not prominent in human LAM but one that can be encountered and is seen in metastases of a variety of tumors to the lungs in humans (29). This relatively localized damage in mice could result from the exponential growth of the TSC2-null cells in a short period of time [about 3 weeks from injection to sacrifice, in compliance with Institutional Animal Care and Use Committee (IACUC) protocol] in the immunocompromised mice. The use of immunocompetent mice and injection of fewer TSC2-null cells might produce more diffuse airspace destruction, producing pathology closer to that of human LAM.

With these caveats in mind, we used our model to test the potential use of both rapamycin and simvastatin for combination therapy in pulmonary LAM. Despite promising results of rapamycin analog sirolimus in the clinic for LAM (15), after cessation of sirolimus therapy, pulmonary function reverts to the diminished levels observed before treatment (15), likely because sirolimus does not completely inhibit mTORC1 signaling, but only inhibits LAM cell growth without promoting cell death (8). Further, an undesirable side effect, hyperlipidemia, occurs in LAM and TS patients on sirolimus (15, 42).

Statins, well known as cholesterol-lowering drugs, also inhibit experimental emphysema (43) and MMP-9 secretion (44) and have anti-inflammatory effects in a variety of diseases such as COPD (18), cancer (45), and asthma (46). These drugs modulate lipid metabolism and disrupt the geranylgeranylation of Rho GTPases that is critical for membrane localization and activation. The safety and efficacy of statins as cholesterol-lowering drugs are well documented and indicate that simvastatin and atorvastatin are the most potent agents. Atorvastatin inhibited growth of TSC2−/−p53−/− mouse embryonic fibroblasts (MEFs) and TSC2-null ELT3 cells from Eker rat in vitro (47) but had little effect on subcutaneous tumors formed by TSC2−/−p53−/− MEFs (48). In previous studies on mice with subcutaneous tumors formed by TSC2-null ELT3 cells, we showed that loss of TSC2 activates the rapamycin-sensitive mTORC1-S6K1 (S6 kinase 1) and the rapamycin-resistant mTORC2–Rho GTPase signaling pathways (fig. S8A) [the latter is inhibited by the nonselective Rho GTPase inhibitor simvastatin (8)]. Here, treatment with rapamycin and simvastatin also prevented TSC2-null lesion development. However, in the mouse LAM model, we also saw prevention of alveolar airspace enlargement and decreased MMP expression, suggesting that the destruction of lung parenchyma is caused by TSC2-null lesion growth (fig. S8B). The doses and treatment schedule for rapamycin were selected on the basis of the pharmaco-kinetics and pharmacodynamics approved for immunosuppression after organ transplantation, clinical trials, and rodent studies (8, 15, 18). Although a relatively high dose of simvastatin was used in this study, because mice metabolize simvastatin more rapidly than humans, lower doses may be effective in humans. A retrospective study of LAM patients on statins cautioned about potential effects of statins on lung function, without taking into account intermolecular differences between statins (49). Thus, a comparison of different statins in preclinical studies on the same cell and animal models is still needed, and their pharmacological characteristics and safety in LAM remain to be determined.

Our data show that rapamycin and simvastatin have differential effects on TSC2-null lesion growth and alveolar space enlargement. (See schematic representation of TSC2-dependent signaling in LAM and its potential therapeutic targeting in fig. S8.) Rapamycin had a predominantly growth-inhibitory effect on TSC2-null lesions, whereas simvastatin inhibited alveolar airspace enlargement, suggesting that the therapeutic approach currently being used for treatment of LAM (rapamycin) may not address the TSC2-dependent pathological changes of lung destruction. Our study predicts that rapamycin alone would not be effective in preventing MMP increases and lung destruction. However, further investigation will reveal whether rapamycin or simvastatin has specific inhibitory effects on MMPs or whether other factors induce airspace enlargement in this mouse model.

In summary, this study reports an experimental model for testing treatment strategies in pulmonary LAM and provides preclinical evidence of a proof of concept that pharmacological use of rapamycin and simvastatin is a promising strategy for LAM. Both rapamycin and simvastatin are in clinical use for other indications. A phase 2 clinical trial could test whether rapamycin and simvastatin in combination has beneficial effects on LAM.

MATERIALS AND METHODS

Cell culture

TSC2-null cells, derived from kidney lesions of TSC2+/− mice (24), mouse LLC, and mouse epithelial NMuMG cells, purchased from the American Type Culture Collection, and Tsc2+/+p53−/− MEFs (provided by D. J. Kwiatkowski) were maintained in Dulbecco's modified Eagle's medium (DMEM) with 10% fetal bovine serum (FBS).

DNA synthesis analysis, cell counts, SM α-actin FACS analysis, and wound closure assay

These were performed as described (8, 12, 26, 50).

The human LAM and control lung tissue

Samples presented in Fig. 3 were obtained from the National Disease Research Interchange (NDRI) according to the approved protocol. The human LAM tissue presented in fig. S4 was obtained from the National Institutes of Health under the protocol approved by the National Heart, Lung, and Blood Institute and examined in compliance with the protocol approved by the University of Auckland Institutional Review Board (Auckland Ethics Committee, North Heath, New Zealand).

Animals

All animal procedures were performed according to a protocol approved by the University of Pennsylvania IACUC. Six- to 8-week-old female NCr athymic nu/nu mice (NCRNU-M, Taconic) were injected subcutaneously in both flanks with 5 × 106 TSC2-null mouse kidney epithelial cells (24) (fig. S1). When tumors reached ~1.5 cm in diameter, mice were sacrificed, and the tumors were removed, enzymatically digested, and plated in cell culture dishes in DMEM supplemented with 10% FBS. After 2 days in culture, the TSC2-null cells from the primary tumors were resuspended and filtered, and 106 cells were injected into the tail vein of 8-week-old NCRNU-M athymic nude mice. Three or 10 days after injection, mice were transferred to simvastatin-supplemented diet (100 mg/kg per day) or treated with rapamycin (intraperitoneal injections, 1 mg/kg, three times per week) alone or in combination with simvastatin diet. Chow containing simvastatin (ZOCOR, Merck) was prepared by Animal Specialties & Provision on the basis of regular chow JL Rat & Mouse/4F diet received by the control group. Negative controls included vehicle-injected mice treated as described above. For positive controls, the tail veins of NCRNU-M mice were injected with 106 LLC cells or Tsc2+/+p53−/− MEFs that were rederived from subcutaneous tumors as described above for TSC2-null cells. Animal weight was monitored throughout the experiment. Animals injected with TSC2-null cells from each group were euthanized at day 0, 10, 12, 15, or 20 of the experiment. Control-, LLC-, and Tsc2+/+p53−/− MEF–injected mice were euthanized at day 20 after injection or at 20% of body weight loss in the positive control group (TSC2-null vehicle). Lungs were inflated at 25-cm H2O pressure with 1:1 optimal cutting temperature (OCT) in phosphate-buffered saline for ~8 min or in formalin. The trachea was tied off, and the lungs were excised, placed in OCT, flash-frozen on dry ice, and sectioned into 5-μm-thick slices followed by H&E staining and immunohistochemical analysis. Each experimental group included a minimum of five animals per condition. The tissue samples were analyzed by three different investigators at the University of Pennsylvania and by one investigator in New Zealand. Experiments to determine that TSC2-null lesions induce alveoli space enlargement were performed twice, and experiments with treatment by rapamycin, simvastatin, and the combination of both were performed three times.

Morphometry

Images of lung tissue sections stained with H&E were acquired with a Nikon Eclipse 80i microscope under ×100 magnification. Ten randomly selected fields per slide from three nonserial sections about 50 μm apart were captured, and Image-Pro Plus 6.2 software (Media Cybernetics Inc.) was used to measure the MAAA as described (28). Airspace changes were also assessed with the MLI, a measurement of mean interalveolar septal wall distance, which is widely used to examine irregular size alveoli. The MLI was measured by dividing the length of a line drawn across the lung section by the total number of intercepts counted within this line at ×100 magnification. A total of 40 lines per slide were drawn and measured. Airway, vascular structures, and histological mechanical artifacts were eliminated from the analysis.

The volume fraction (%) of elastic fibers was determined on images of tissue sections stained with Verhoeff's elastin stain and Picro-Ponceau counterstain with a 100-point grid to record hits over elastin and total tissue hits to give percentage of area occupied by elastin, as described (34).

BAL fluid was collected from five mice per each condition at days 0, 10, 12, 15, and 20 after injection of TSC2-null cells. BAL cell counts were assessed as described (32).

ELISA was performed with a Searchlight Protein Array multiplex of cell-free supernatant of the BAL fluids at Aushon Biosystems.

Immunohistochemical and immunoblot analyses

These were performed as described (6, 8). Immunostaining was analyzed with the Nikon Eclipse TE2000-E microscope equipped with an Evolution QEi digital video camera. Tumors from a minimum of five animals per each treatment condition were analyzed. Staining was visualized with a Nikon Eclipse TE2000-E microscope under appropriate filters. Protein levels were analyzed by OD with Gel-Pro Analyzer software.

Data analysis

Data points from individual assays represent means ± SE. Statistically significant differences among groups were assessed with ANOVA (with the Bonferroni-Dunn correction), with values of P < 0.05 sufficient to reject the null hypothesis for all analyses. All experiments were designed with matched control conditions within each experiment (minimum of five animals) to enable statistical comparison as paired samples and to obtain statistically significant data.

Supplementary Material

All Suppelemental Figures

Acknowledgments

We thank D. J. Kwiatkowski (Brigham and Women's Hospital, Boston, MA) for Tsc2+/+p53−/− MEFs, H. DeLisser (University of Pennsylvania) for LYVE-1 antibodies, S. Lu for help with cell growth experiments, and the NDRI for providing us with LAM tissue.

Funding: Supported by 2RO1HL71106 (V.P.K.), RO1HL090829 (V.P.K.), RO1HL114085 (V.P.K.), Abramson Cancer Center Core Support Grant NIH P130-CA-016520-34 (V.P.K.), P30ES013508 (V.P.K. and A.H.), RO1AI072197 (A.H.), and RC1ES018505 (A.H.) from the NIH; American Lung Association (ALA) CI-9813-N (V.P.K.); LAM Foundation (V.P.K.); and Auckland Medical Research Foundation Grant 1109001 (M.J.M.).

Footnotes

SUPPLEMENTARY MATERIALS www.sciencetranslationalmedicine.org/cgi/content/full/4/154/154ra134/DC1

Author contributions: E.A.G., S.M.A., and V.P.K. designed the experiments. E.A.G., D.A.G., M.F., I.K., B.D., T.V.C., M.J.M., A.H., and V.P.K. performed the experiments and analyzed the data. O.H. provided original culture of TSC2-null cells. E.A.G., A.H., S.M.A., T.V.C., M.J.M., and V.P.K. wrote the paper.

Competing interests: The authors declare that they have no competing interests.

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