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PTEN as a New Agent in the Fight against Fibrogenesis

Research Institute for Diseases of the Chest, Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan

Pathways involving epithelial cells and fibroblasts have been considered to be dominant in the pathogenesis of idiopathic pulmonary fibrosis (IPF), rather than an inflammatory response, even though the latter is actually the common process of most interstitial lung diseases (1). Although there are various initiating factors or causes for IPF, the terminal stages are characterized by fibroblast proliferation and the accumulation of connective tissue replacing normal, functioning parenchyma. In this context, myofibroblasts play a crucial role in the progression of fibrosis. Myofibroblasts arise from the differentiation of fibroblasts under the effect of transforming growth factor (TGF-), platelet-derived growth factor, and other fibrogenic cytokines. They produce fibrogenic molecules, such as tissue inhibitor of metalloproteases and fibrogenic growth factor, as well as proapoptotic molecules that affect the epithelium, such as angiotensin II. In particular, fibroblastic foci, which consist mainly of myofibroblasts, are the leading edge of active fibrosis in IPF. Therefore, understanding the molecular mechanisms involved in myofibroblast differentiation may lead to the development of effective treatments against IPF.

The tumor suppressor, phosphatase and tensin homolog deleted on chromosome 10 (PTEN), dephosphorylates phosphatidylinositol-3,4,5-triphosphate (2). PTEN inhibits cell proliferation (3) and induces apoptosis (4), whereas PTEN insufficiency is associated with invasion and metastasis of various malignancies, as well as synovial cell migration in rheumatoid arthritis (5). White and colleagues previously reported that PTEN expression is diminished in fibroblasts isolated from patients with IPF, compared with those obtained from control subjects, and that a loss of integrin-4/1 signaling via PTEN confers a migratory/invasive phenotype to fibrotic lung fibroblasts (6). In this issue of AJRCCM (pp. 112–121), they extend their research to examine the role of PTEN in myofibroblast differentiation, both in vitro and in vivo (7). They demonstrate that there is an inverse correlation between PTEN and -smooth muscle actin (-SMA) expression in fibroblastic foci of lung tissue from patients with IPF. They also found that the pharmacologic inhibition of PTEN augments -SMA expression both in fibroblasts in vitro and in pulmonary fibrosis in vivo. They further show that the inhibition or loss of PTEN is both necessary and sufficient to induce myofibroblast differentiation, including proliferation, -SMA expression, and collagen production. In contrast, reconstitution of PTEN into pten^–/– cells inhibits myofibroblast differentiation, whereas the overexpression of PTEN suppresses any myofibroblast differentiation that may be induced by TGF- (7). These results suggest that PTEN plays a crucial role in myofibroblast differentiation both in vitro and in vivo.

TGF- represses pten transcription (8) and induces myofibroblast differentiation (9). It is possible that autocrine TGF- signaling may account for the myofibroblast differentiation of pten^–/– cells. However, the authors show that the increased expression of -SMA in pten^–/– fibroblasts is not dependent on TGF- stimulation. They also show that there is no difference in TGF- receptor expression and Smad 2/3 phosphorylation between wild-type and pten^–/– cells. Interestingly, Smad 7 is upregulated in pten^–/– cells, which may be the reason why TGF- exposure does not induce any further increase in -SMA expression on pten^–/– cells. Therefore, the induction of -SMA through the loss of PTEN appears to be independent of autocrine TGF- stimulation, TGF- receptor expression, or the Smad-signaling pathway. The authors show that PTEN inhibits -SMA expression by antagonizing either phosphatidylinositol-3 kinase or the focal adhesion kinase/Src pathways. Using DNA and protein arrays, it should be possible to verify the transcriptional and translational signaling pathways by which PTEN regulates myofibroblast differentiation. In addition, it should also be possible to investigate the mutation of pten and epigenetic modifications by these methods.

The current article includes beautiful photographs showing that PTEN is diminished and -SMA is inversely upregulated in fibroblastic foci in lung tissues obtained from patients with IPF. The remaining question relates to the mechanisms by which PTEN expression is stably suppressed in fibroblastic foci, whereas PTEN expression is not diminished in alveolar epithelial cells or interstitial cells in alveolar walls under the same microenvironment as fibroblastic foci in IPF. The authors initially hypothesized that autocrine TGF- may suppress PTEN expression and may increase -SMA expression, but then they were unable to support this hypothesis since the neutralizing antibody to TGF- was unable to affect -SMA expression in pten^–/– cells. However, this hypothesis should not be ignored when considering wild-type cells in human lung tissues. TGF- is highly expressed in epithelial cells, macrophages, and extracellular matrix in advanced pulmonary fibrosis (10), and the TGF- receptor is present on various kinds of cells.

What is the mechanism by which PTEN expression is inhibited, specifically on fibroblastic foci? Individual analysis of the expression profiles on epithelial cells, interstitial cells of alveolar walls, and fibroblastic foci using laser-captured microdissection may be valuable when trying to address these questions.

The cuboidal epithelium of the fibrotic human lung is composed of both proliferating and dying cells, and apoptotic and necrotic epithelial cells are observed in proximity to myofibroblasts in fibroblastic foci (11). Neither inflammation nor fibrosis is correlated with survival, and the only pathologic data that have shown a significant correlation with mortality are the numbers of areas that contain fibroblastic foci (12). PTEN deficiency makes fibroblasts differentiate into myofibroblasts, while also making them resistant to apoptosis, whereas PTEN expression in epithelial cells physiologically regulates the phenotype of these cells, but also may cause them to remain susceptible to proapoptotic stimulation. Although the role of PTEN in apoptosis of nontransformed epithelial cells is not yet known, epithelial cell fate and fibroblast phenotype regulated by PTEN may be involved in abnormal epithelial–mesenchymal interactions. Disruption of the normal function of PTEN may contribute to the pathogenesis and exacerbation of fibrotic lung disease by preventing normal epithelial repair and by allowing the progression of abnormal fibroblast proliferation. Accordingly, targeting PTEN may well prove be a novel treatment strategy for IPF.

FOOTNOTES

Conflict of Interest Statement: K.K. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

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