Elevated circulating level of osteopontin is associated with advanced disease state of non-small cell lung cancer
Introduction
Over one million new cases of lung cancer are diagnosed worldwide each year and lung cancer is the leading cause of cancer-related death in men and women, globally and in Korea. Despite intensive efforts to control lung cancer mortality with standard surgery, radiation and chemotherapy, the 5-year lung cancer patient survival rate of 7% in 1970 has only recently improved to 14% [1]. A better understanding of the molecular pathogenesis of this devastating disease is thus urgently required for therapeutic breakthroughs to reduce the number of lung cancer-related deaths.
A secreted phosphorylated acidic glycoprotein, osteopontin (OPN), is a member of the small integrin-binding ligand N-linked glycoprotein (SIBLING) family that includes bone sialoprotein (BSP), dentin matrix protein-1 (DMP1), dentin sialophosphoprotein (DSPP), enamelin (ENAM), and matrix extracellular phosphoglycoprotein (MEPE). The gene encoding OPN [MIM #166490] maps to human chromosome 4q21-q25, together with other members of the SIBLING family of proteins [2]. OPN functions in various physiological and pathological processes including bone remodeling, cell-mediated immunity, maintenance or reconfiguration of tissue integrity during inflammatory processes, coronary restenosis, and tumor cell metastasis [3]. OPN has a highly conserved protease-hypersensitive arginine-serine-lysine (RSK) sequence that separates the integrin- and CD44-binding domains [4]. A C-terminal fragment of OPN binds to CD44 (v6 and v3) and mediates chemotaxis and IL-10 inhibition. OPN also contains an arginine-glycine-aspartate (RGD)-containing N-terminal domain, which is engaged to the integrin receptor (αvβ, αvβ5, α9β1, and α4β1) and relays signal to IL2, cell attachment, extravasations, and activation of matrix degrading proteases [5], [6].
Increased OPN expression is associated with tumor invasion and metastasis in cancers of the breast, prostate, colon, and, lung and serves as a predictor for poor prognosis [7], [8], [9], [10], [11]. Recent reports on the population with history of asbestos exposure and pleural mesothelioma showed that OPN level is elevated in the cases with significant radiologic findings and pleural mesothelioma comparing the cases with history of asbestos exposure, suggesting that it can be used for diagnostic marker for asbestos related mesothelioma [12]. Thus understanding the regulatory mechanisms for OPN expression draws much interest for medical scientists. Besides posttranslational regulation of OPN through phosphorylation, glycosylation, and proteolytic cleavage by thrombin and matrix metalloproteinases [13], [14], the OPN gene has many different mRNA transcripts [15] and several SNPs in the promoter that are associated with diseases such as lupus erythematosus, multiple sclerosis, and autoimmune lymphoproliferative syndrome [16], [17], [18]. Recently variation at nts −443, −156, and −66 in OPN gene promoter were shown to influence the binding affinity of MYT1 zinc finger factor, CBFA1/RUNX2, and SP1 [3], and to be one of regulatory mechanisms for its expression. Here we confirmed variations in the circulating levels of OPN in NSCLC patients and tried to identify factors that influence circulating OPN level including clinical, pathologic parameters, and the SNPs between nt +1 and nt −474 of the OPN gene promoter (relative to transcription start site) where locates important regulatory elements for OPN gene expression.
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Study population and samples
One hundred and thirty patients were diagnosed with NSCLC at the Severance and the Yongdong Severance hospital in Seoul, Korea from March 2004 to June 2006 and were enrolled in this study. All patients were pathologically proven to have NSCLC and had not received any cancer treatment at the time of sampling. The study population consisted of 96 males and 34 females and the mean age of the patients was 64.9 ± 11.2 years and 63.6 ± 11.51 years, respectively. Fifty-two patients had still smoked at the
OPN concentration is elevated in the plasma of patients with advanced disease stage
To assess whether the OPN concentration is associated with the disease stage, we compared NSCLC cases after stratification based on the TNM (tumor-node-metastasis) status (Table 1). Regarding the tumor stage, there were 32 patients with stage I (IA and IB), 8 with stage II (IIA and IIB), 43 with stage III (IIIA and IIIB), and 47 with stage IV. Analysis of the OPN concentration according to the TNM status revealed that the T4 had a higher level of circulating OPN than T1–3, N3 also has higher
Discussion
OPN, an extracellular protein produced by osteoblasts, has been suggested to be involved in the attachment of osteoclasts to the bone surface through binding to OPN receptors such as αvββ integrin and CD44 during bone resorption, to play a role in osteogenesis by attachment of osteoblasts when they form bone matrix, and to act regulating crystal size during bone mineralization [22]. Earlier reports on OPN derived from immunohistochemical analysis on surgically resected stage I–IIIA NSCLC
Conflict of interest
There is no conflict of interests to be disclosed in the authors.
Acknowledgements
This study was supported by Brain Korea 21 Project for Medical Science, the Korean Science and Engineering Fund through the Cancer Metastasis Research Center at Yonsei University College of Medicine.
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