STAT3-mediated signaling dysregulates lung fibroblast-myofibroblast activation and differentiation in UIP/IPF.

STAT3 is a latent transcription factor that plays a role in regulating fibroblast function in fibrotic lung diseases. To further understand the role of STAT3 in the phenotypic divergence and function of human lung fibroblasts (LFs), we investigated the effect of basal and cytokine-induced STAT3 activity on indices of LF differentiation and activation, including expression of α-smooth muscle actin (α-SMA), collagen, and adhesion molecules Thy-1/CD90 and α(v) ß(3) and ß(5) integrins. We identified a population of fibroblasts from usual interstitial pneumonia (UIP)/idiopathic pulmonary fibrosis (IPF) lungs characterized by constitutively phosphorylated STAT3, lower proliferation rates, and diminished expression of α-SMA, Thy-1/CD90, and ß(3) integrins compared with control LFs. Staining of UIP lung biopsy specimens demonstrated that phosphorylated STAT3 was not present in α-SMA-positive fibroblastic foci but was observed in the nuclei of cells located in the areas of dense fibrosis. STAT3 activation in LFs did not significantly influence basal or transforming growth factor ß(1)-induced collagen I expression but inhibited expression of α-SMA, Thy-1/CD90, and αv ß(3) integrins. Suppression of STAT3 signaling diminished resistance of IPF LFs to staurosporine-induced apoptosis and responsiveness to transforming growth factor ß(1) but increased basal α-SMA and restored ß(3) integrin expression in LFs via an ALK-5-dependent, SMAD3/7-independent mechanism. These data suggest that STAT3 activation regulates several pathways in human LFs associated with normal wound healing, whereas aberrant STAT3 signaling plays a critical role in UIP/IPF pathogenesis.

Endothelial STAT3 is essential for the protective effects of HO-1 in oxidant-induced lung injury.

Administering high levels of inspired oxygen, or hyperoxia, is commonly used as a life-sustaining measure in critically ill patients. Unfortunately, the oxidant stress generated by prolonged hyperoxia can lead to respiratory failure, multiorgan failure, and death. Although the endothelial cell is known to be a target for hyperoxia-induced injury, its precise role is unclear. Heme oxygenase-1 (HO-1) and "signal transducer and activator of transcription 3" (STAT3) have been found to confer protection against endothelial cell injury. We sought to elucidate the specific roles of HO-1 and STAT3 in hyperoxic lung and endothelial cell injury. Mice or murine lung endothelial cells (MLEC) administered HO-1 siRNA exhibited marked injury and death compared with nonspecific siRNA. Overexpression of either HO-1 or STAT3 confers protection. However, HO-1 and its reaction product carbon monoxide (CO) lose their protective effects in the presence of STAT3 siRNA in MLEC or in endothelial-specific, STAT3-deficient mice. STAT3 overexpression is able to partially rescue HO-1-deficient MLEC from hyperoxia-induced cell death. Our results demonstrate 1) the importance of the endothelium in lethal hyperoxic injury, 2) HO-1 and CO require endothelial STAT3 for their protective effects, and 3) STAT3 confers endothelial cell protection via both HO-1-dependent and independent mechanisms.

DNA repair and cancer: lessons from mutant mouse models.

DNA damage, if the repair process, especially nucleotide excision repair (NER), is compromised or the lesion is repaired by some other error-prone mechanism, causes mutation and ultimately contributes to neoplastic transformation. Impairment of components of the DNA damage response pathway (e.g., p53) is also implicated in carcinogenesis. We currently have considerable knowledge of the role of DNA repair genes as tumor suppressors, both clinically and experimentally. The deleterious clinical consequences of inherited defects in DNA repair system are apparent from several human cancer predisposition syndromes (e.g., NER-compromised xeroderma pigmentosum [XP] and p53-deficient Li-Fraumeni syndrome). However, experimental studies to support the clinical evidence are hampered by the lack of powerful animal models. Here, we review in vivo experimental data suggesting the protective function of DNA repair machinery in chemical carcinogenesis. We specifically focus on the three DNA repair genes, O(6)-methylguanine-DNA methyltransferase gene (MGMT ), XP group A gene (XPA) and p53. First, mice overexpressing MGMT display substantial resistance to nitrosamine-induced hepatocarcinogenesis. In addition, a reduction of spontaneous liver tumors and longer survival times were evident. However, there are no known mutations in the human MGMT and therefore no associated cancer syndrome. Secondly, XPA mutant mice are indeed prone to spontaneous and carcinogen-induced tumorigenesis in internal organs (which are not exposed to sunlight). The concomitant loss of p53 resulted in accelerated onset of carcinogenesis. Finally, p53 null mice are predisposed to brain tumors upon transplacental exposure to a carcinogen. Accumulated evidence in these three mutant mouse models firmly supports the notion that the DNA repair system is vital for protection against cancer.

Cardiomyocyte-restricted knockout of STAT3 results in higher sensitivity to inflammation, cardiac fibrosis, and heart failure with advanced age.

Cytokines and inflammation have been implicated in the pathogenesis of heart failure. For example, IL-6 family cytokines and the gp130 receptor play important roles in cardiac myocyte survival and hypertrophy. Signal transducer and activator of transcription 3 (STAT3) is a major signaling protein that is activated through gp130. We have created mice with a cardiomyocyte-restricted deletion of STAT3. As measured by serial echocardiograms, mice with cardiac specific deletion of STAT3 are significantly more susceptible to cardiac injury after doxorubicin treatment than age-matched controls. Intriguingly, STAT3 appears to have a critical role in protection of inflammation-induced heart damage. STAT3-deficient mice treated with lipopolysaccharide demonstrated significantly more apoptosis than their WT counterparts. At the cellular level, cardiomyocytes with STAT3 deleted secrete significantly more tumor necrosis factor in response to lipopolysaccharide than those with WT STAT3. Furthermore, histologic examination of the cardiomyocyte-restricted STAT3-deficient mice reveals a dramatic increase in cardiac fibrosis in aged mice. Although no overt signs of heart failure are present in young STAT3-deficient mice, they spontaneously develop heart dysfunction with advancing age. These results indicate the crucial functions of STAT3 in cardiomyocyte resistance to inflammation and other acute injury and in pathogenesis of age-related heart failure.

STAT3 deletion during hematopoiesis causes Crohn's disease-like pathogenesis and lethality: a critical role of STAT3 in innate immunity.

Signal transducer and activator of transcription 3 (STAT3) is a key transcriptional mediator for many cytokines and is essential for normal embryonic development. We have generated a unique strain of mice with tissue-specific disruption of STAT3 in bone marrow cells during hematopoiesis. This specific STAT3 deletion causes death of these mice within 4-6 weeks after birth with Crohn's disease-like pathogenesis in both the small and large intestine, including segmental inflammatory cell infiltration, ulceration, bowel wall thickening, and granuloma formation. Deletion of STAT3 causes significantly increased cell autonomous proliferation of cells of the myeloid lineage, both in vivo and in vitro. Most importantly, Stat3 deletion during hematopoiesis causes overly pseudoactivated innate immune responses. Although inflammatory cytokines, including tumor necrosis factor alpha and IFN-gamma, are overly produced in these mice, the NAPDH oxidase activity, which is involved in antimicrobial and innate immune responses, is inhibited. The signaling responses to lipopolysaccharide are changed in the absence of STAT3, leading to enhanced NF-kappa B activation. Our results suggest a model in which STAT3 has critical roles in the development and regulation of innate immunity, and deletion of STAT3 during hematopoiesis results in abnormalities in myeloid cells and causes Crohn's disease-like pathogenesis.