FoxM1 transcription factor (previously called HFH-11B, Trident, FoxM1b, Win, and MPP2) is expressed in actively dividing cells and critical for cell cycle progression. FoxM1 expression is induced in a variety of tissues during embryogenesis, and Foxm1-/- mice exhibit embryonic lethal phenotype due to multiple abnormalities in the liver, heart, lung and blood vessels. FoxM1 levels are dramatically decreased in adult tissues, but FoxM1 expression is re-activated during organ injury and numerous cancers. In this review, we discussed the role of FoxM1 in different cell lineages using recent data from transgenic mouse models with conditional “gain-of-function” and “loss-of-function” of FoxM1, as well as tissue samples from human patients. In addition, we provided experimental data showing additional sites of FoxM1 expression in the mouse embryo. Novel cell-autonomous roles of FoxM1 in embryonic development, organ injury and cancer formation in vivo were analyzed. Potential application of these findings for the diagnosis and treatment of human diseases were discussed.

Alveolar epithelial cells (AECs) participate in the pathogenesis of pulmonary fibrosis, producing pro‐inflammatory mediators and undergoing epithelial‐to‐mesenchymal transition (EMT). Herein, we demonstrated the critical role of Forkhead Box M1 (Foxm1) transcription factor in radiation‐induced pulmonary fibrosis. Foxm1 was induced in AECs following lung irradiation. Transgenic expression of an activated Foxm1 transcript in AECs enhanced radiation‐induced pneumonitis and pulmonary fibrosis, and increased the expression of IL‐1 β, Ccl2 , Cxcl5 , Snail1 , Zeb1 , Zeb2 and Foxf1 . Conditional deletion of Foxm1 from respiratory epithelial cells decreased radiation‐induced pulmonary fibrosis and prevented the increase in EMT‐associated gene expression. siRNA‐mediated inhibition of Foxm1 prevented TGF‐β‐induced EMT in vitro . Foxm1 bound to and increased promoter activity of the Snail1 gene, a critical transcriptional regulator of EMT. Expression of Snail1 restored TGF‐β‐induced loss of E‐cadherin in Foxm1‐deficient cells in vitro . Lineage‐tracing studies demonstrated that Foxm1 increased EMT during radiation‐induced pulmonary fibrosis in vivo . Foxm1 is required for radiation‐induced pulmonary fibrosis by enhancing the expression of genes critical for lung inflammation and EMT.

Pulmonary fibrosis results from dysregulated lung repair and involves multiple cell types. The role of endothelial cells (EC) in lung fibrosis is poorly understood. Using single cell RNA-sequencing we identified endothelial transcription factors involved in lung fibrogenesis, including FOXF1, SMAD6, ETV6 and LEF1. Focusing on FOXF1, we found that FOXF1 is decreased in EC within human idiopathic pulmonary fibrosis (IPF) and mouse bleomycin-injured lungs. Endothelial-specific Foxf1 inhibition in mice increased collagen depositions, promoted lung inflammation, and impaired R-Ras signaling. In vitro, FOXF1-deficient EC increased proliferation, invasion and activation of human lung fibroblasts, and stimulated macrophage migration by secreting IL-6, TNFα, CCL2 and CXCL1. FOXF1 inhibited TNFα and CCL2 through direct transcriptional activation of Rras gene promoter. Transgenic overexpression or endothelial-specific nanoparticle delivery of Foxf1 cDNA decreased pulmonary fibrosis in bleomycin-injured mice. Nanoparticle delivery of FOXF1 cDNA can be considered for future therapies in IPF.

Idiopathic pulmonary fibrosis (IPF) is characterized by aberrant accumulation of collagen-secreting myofibroblasts. Development of effective therapies is limited due to incomplete understanding of molecular mechanisms regulating myofibroblast expansion. FOXF1 transcription factor is expressed in resident lung fibroblasts, but its role in lung fibrosis remains unknown due to the lack of genetic mouse models. Through comprehensive analysis of human IPF genomics data, lung biopsies, and transgenic mice with fibroblast-specific inactivation of FOXF1, we show that FOXF1 inhibits pulmonary fibrosis. FOXF1 deletion increases myofibroblast invasion and collagen secretion and promotes a switch from N-cadherin (CDH2) to Cadherin-11 (CDH11), which is a critical step in the acquisition of the pro-fibrotic phenotype. FOXF1 directly binds to Cdh2 and Cdh11 promoters and differentially regulates transcription of these genes. Re-expression of CDH2 or inhibition of CDH11 in FOXF1-deficient cells reduces myofibroblast invasion in vitro. FOXF1 inhibits pulmonary fibrosis by regulating a switch from CDH2 to CDH11 in lung myofibroblasts.

Multiple signaling pathways, structural proteins, and transcription factors are involved in the regulation of endothelial barrier function. The forkhead protein FOXF1 is a key transcriptional regulator of embryonic lung development, and we used a conditional knockout approach to examine the role of FOXF1 in adult lung homeostasis, injury, and repair. Tamoxifen-regulated deletion of both Foxf1 alleles in endothelial cells of adult mice (Pdgfb-iCreER/Foxf1^−/−) caused lung inflammation and edema, leading to respiratory insufficiency and death. Deletion of a single Foxf1 allele made heterozygous Pdgfb-iCreER/Foxf1^+/− mice more susceptible to acute lung injury. FOXF1 abundance was decreased in pulmonary endothelial cells of human patients with acute lung injury. Gene expression analysis of pulmonary endothelial cells with homozygous FOXF1 deletion indicated reduced expression of genes critical for maintenance and regulation of adherens junctions. FOXF1 knockdown in vitro and in vivo disrupted adherens junctions, enhanced lung endothelial permeability, and increased the abundance of the mRNA and protein for sphingosine 1-phosphate receptor 1 (S1PR1), a key regulator of endothelial barrier function. Chromatin immunoprecipitation and luciferase reporter assays demonstrated that FOXF1 directly bound to and induced the transcriptional activity of the S1pr1 promoter. Pharmacological administration of S1P to injured Pdgfb-iCreER/Foxf1^+/− mice restored endothelial barrier function, decreased lung edema, and improved survival. Thus, FOXF1 promotes normal lung homeostasis and repair, in part, by enhancing endothelial barrier function through activation of the S1P/S1PR1 signaling pathway.

Macrophages have a key role in tumor-associated pulmonary inflammation that supports the proliferation of tumor cells and promotes lung tumor growth. Although increased numbers of tumor-associated macrophages are linked to poor prognosis in lung cancer patients, little is known regarding the transcriptional mechanisms controlling recruitment of macrophages during lung tumorigenesis. Forkhead Box m1 (Foxm1) transcription factor is induced in multiple cell types within tumor lesions and its increased expression is associated with poor prognosis in patients with lung adenocarcinomas. To determine the role of Foxm1 in recruitment of tumor-associated macrophages, a mouse line with macrophage-specific Foxm1 deletion was generated (macFoxm1^−/−). Lung tumorigenesis was induced using a 3-methylcholanthrene/butylated hydroxytoluene (BHT; 3,5-di-t-butyl-4-hydroxytoluene) tumor initiation/promotion protocol. Ablation of Foxm1 in macrophages reduced the number and size of lung tumors in macFoxm1^−/− mice. Decreased tumorigenesis was associated with diminished proliferation of tumor cells and decreased recruitment of macrophages during the early stages of tumor formation. The expression levels of the pro-inflammatory genes iNOS, Cox-2, interleukin-1b (IL-1b) and IL-6, as well as the migration-related genes macrophage inflammatory protein-1 (MIP-1α), MIP-2 and MMP-12, were decreased in macrophages isolated from macFoxm1^−/− mice. Migration of Foxm1-deficient macrophages was reduced in vitro. The chemokine receptors responsible for monocyte recruitment to the lung, CX₃CR1 and CXCR4, were decreased in Foxm1-deficient monocytes. In co-transfection experiments, Foxm1 directly bound to and transcriptionally activated the CX₃CR1 promoter. Adoptive transfer of wild-type monocytes to macFoxm1^−/− mice restored BHT-induced pulmonary inflammation to the levels observed in control mice. Expression of Foxm1 in macrophages is required for pulmonary inflammation, recruitment of macrophages into tumor sites and lung tumor growth.