Abnormality of the hepatocyte growth factor/MET pathway in pulmonary adenocarcinogenesis
Introduction
Lung carcinoma is the leading cause of cancer death worldwide [1], and one of the most common histologic types is adenocarcinoma, which has been showing a continuous increase in incidence in both Japan and Western countries. In particular, more cases of small peripheral adenocarcinoma are now being found as a result of technological advances in computed tomography [2]. Surgical resection remains the standard treatment for non-small cell lung carcinoma (NSCLC), including small peripheral adenocarcinomas. Noguchi et al. [3] examined a number of surgically resected small adenocarcinomas of the lung, and demonstrated that there are definite cases showing a very favorable outcome. According to their criteria, localized bronchioloalveolar carcinoma (LBAC, type A) and LBAC with alveolar collapse (type B) are defined as non-invasive adenocarcinoma and show a 100% 5-year survival rate. Therefore, type A and B tumors can be candidates for reduction surgery. On the other hand, LBAC with foci of active fibroblastic proliferation (type C) is a type of adenocarcinoma showing lepidic growth, but it represents a large category containing both minimally invasive and invasive adenocarcinomas. Some patients with type C tumors who have no detectable metastasis at the time of surgery die of their disease because type C tumors include adenocarcinomas with various prognoses. Therefore, there is a definite need for prognostic markers that can be used to determine which patients can be treated with reduction surgery or those who require the standard operation.
In the 2004 World Health Organization (WHO) classification [1], major adenocarcinoma was divided into five subtypes, including bronchioloalveolar carcinoma (BAC), acinar, papillary, solid with mucin production, and mixed subtypes. However, more than 90% of adenocarcinomas fall into the mixed subtype. In addition, although the term BAC is defined as non-invasive adenocarcinoma, it is used loosely for a broad spectrum of tumors showing BAC-type spread. In order to resolve these issues, a multidisciplinary adenocarcinoma classification has been recently proposed by the International Association for the Study of Lung Cancer in 2011 [4]. In this new classification, adenocarcinomas were fundamentally classified into three categories based on progression: (1) preinvasive lesions including atypical adenomatous hyperplasia (AAH) and adenocarcinoma in situ (AIS), (2) minimally invasive adenocarcinoma (MIA), and (3) invasive adenocarcinoma including lepidic-predominant, acinar-predominant, papillary-predominant, micropapillary-predominant, and solid-predominant with mucin production. AIS is considered to be synonymous with BAC, according to the 2004 WHO classification, and with type A and B tumors in the Noguchi classification. MIA is a small, solitary adenocarcinoma with a predominantly lepidic pattern and showing less than 5 mm of invasion in greatest dimension. Therefore, like AIS, MIA is expected to show an extremely favorable outcome. MIA is thought to include a proportion of adenocarcinomas of mixed subtype with BAC (the 2004 WHO classification) and also type C tumors (Noguchi classification). On the other hand, adenocarcinoma of mixed subtype with BAC (2004 WHO classification) includes any adenocarcioma subtype that includes a lepidic pattern (2011 proposed new classification) and type C adenocarcinoma (Noguchi classification).
Hepatocyte growth factor (HGF), also known as scatter factor, was originally and independently isolated as a hepatic regeneration factor [5]. HGF is a pleiotropic cytokine, whose biological effects are mediated by activation of the MET proto-oncogene tyrosine kinase receptor [6]. HGF is currently considered to be both an autocrine and a paracrine mediator produced by mesenchymal cells, including fibroblasts [7], [8], [9]. The MET oncogene was originally isolated from a human osteogenic sarcoma cell line that had been subjected to chemical mutagenesis in vitro [10]. MET-receptor tyrosine kinase is activated by its cognate ligand HGF, and receptor phosphorylation activates the downstream pathways [11]. Signaling mediated by HGF/MET promotes multiple biological activities, including cell proliferation, motility, invasion, angiogenesis, and morphogenesis in a wide variety of normal and neoplastic cells [12], [13], [14], [15].
Alteration of the MET gene, including amplification, overexpression, and mutation, has been described in various cancers [16], [17]. Expression of MET and HGF proteins assessed by immunohistochemistry has been reported to predict poor outcome in patients with resected lung cancer [18], [19], [20], [21]. Recently, MET amplification has been identified as one of the mechanisms of acquired resistance to epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitor (TKI) [22], [23], and has been found not only in tumors with acquired resistance to EGFR-TKI but also in primary untreated NSCLC [23]. Several studies have demonstrated that an increase in the MET gene copy number is associated with poor outcome in patients with NSCLC [24], [25], [26], [27]. However, all of the patients analyzed in these studies had undergone resection of advanced NSCLC, and no previous investigations have focused on HGF/MET in small adenocarcinomas, including both non-invasive and invasive cases.
Besides HGF and MET abnormalities, a number of studies have demonstrated various gene abnormalities during the progression of adenocarcinoma [28], [29], [30], [31], [32], [33], [34]. These include mutation analyses of the EGFR, V-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog (KRAS), and p53, and copy number amplifications of genes including EGFR, thyroid transcription factor (TTF-1), MYC, and KRAS. Some chromosomal alterations and deletions have also been examined. These abnormalities were shown to be added during the course of malignant progression, although extremely early-stage tumors (AIS, MIA, etc.) were not examined.
In the present study, we examined both the copy number of the MET gene by fluorescence in situ hybridization (FISH) and the expression of MET and HGF using immunohistochemistry in small adenocarcinomas. We demonstrated that an increased MET gene copy number and high immunoreactivity for MET and HGF in tumor cells were significantly associated with factors indicative of a poor prognosis, such as lymphatic permeation and nuclear grading. An increased MET gene copy number was particularly associated with a poor outcome. Our results also suggest that abnormality of the HGF/MET pathway occurs during the course of progression from non-invasive to invasive adenocarcinoma.
Section snippets
Patients
We examined 106 consecutive small adenocarcinomas of the lung (20 mm or less in diameter) that were surgically resected at Tsukuba University Hospital (Ibaraki, Japan) between 2001 and 2008. None of the patients selected had received neoadjuvant or adjuvant chemotherapy or radiotherapy before or after surgery. The ethics committee of our institution approved this study, and informed consent for specimen collection was obtained from all patients.
In this study, we focused on genetic abnormalities
Clinical and histological findings
A total of 106 consecutive adenocarcinoma samples were examined. The overall gender composition was 51 (48%) males and 55 (52%) females. Fifty-one (48%) patients had never smoked and 55 (52%) were smokers. The median age, follow-up period, and tumor size were 64 years (range, 31–89 years), 50 months (range, 5–99 months), and 15 mm (range, 4–20 mm), respectively. All adenocarcinomas were evaluable according to the Noguchi classification [3], the 2004 WHO classification [1], and the new
Discussion
Various abnormalities in genes such as EGFR, KRAS, MYC and TTF-1 during the course of adenocarcinoma progression have been examined and reported [30], [31], [32], [33], [34]. However, in those previous studies, cases of AIS and MIA received scant attention. In the present study, we focused on cases at a very early stage, including AIS and MIA, and examined them for abnormalities of HGF and MET.
An increase of the MET gene copy number (MET FISH-positive signals) was observed in 10.4% of cases
Conflict of interest statement
None of the authors have a financial relationship with any commercial entity that has an interest in the area of study.
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Recent advances in the discovery of small molecule c-Met Kinase inhibitors
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2015, Lung CancerCitation Excerpt :Because the proportion of the amplified tumors is exceedingly prevalent in an IHC score of 3 compared with the IHC scores of 0–2, highly expressed MET protein can be a cue for recognizing amplification in the tumor (Table 2 and Fig. 4). A recent study has reported a correlation between non-lepidic predominant tumors and MET protein expression [30]. In contrast, there was a significant relationship between well-differentiated tumors and MET protein expression in our study.
Predictive and prognostic value of de novo MET expression in patients with advanced non-small-cell lung cancer
2015, Lung CancerCitation Excerpt :However, this finding has not been confirmed, as Tsuta et al. [21] showed that expression of the MET protein was not associated with survival. Tachibana et al. [35] reported no significant differences in survival according to the immunoreactivity of MET in tumor cells. A recent report showed that patients with MET H-score values below the median had shorter OS compared with patients with above-median values, and the MET status was highly heterogeneous among non-squamous NSCLC tumor areas [23].
Increased met gene copy number but not mrna level predicts postoperative recurrence in patients with non–small cell lung cancer
2014, Translational OncologyCitation Excerpt :However, no correlation between MET copy gain and KRAS dosage or mutational status was found. The association between EGFR and MET copy gains had been demonstrated previously [6,9,20] and proposed to result from frequent chromosome 7 aneuploidy in cancer cells [6]. However, a concept of the functional cross talk between MET and EGFR family receptors in cancer cells has also be suggested [10,24,25].
Association of c-Met phosphorylation with micropapillary pattern and small cluster invasion in pT1-size lung adenocarcinoma
2013, Lung CancerCitation Excerpt :In addition to only MPP and SCI, c-Met activation may be involved in the biological processes leading to lymphatic involvement in small pulmonary adenocarcinomas. In patients with small pulmonary adenocarcinomas, overexpression of c-Met and HGF is associated with factors indicative of poor prognosis, such as pleural invasion, vascular invasion, lymphatic permeation, lymph node metastasis, and nuclear grading [43]. Given that c-Met and stromal HGF are expressed more strongly in invasive than in non-invasive adenocarcinomas, dysfunction of the HGF/c-Met signaling pathway may be an important factor in multistep adenocarcinogenesis [43].