Background
OPN, a phosphorylated glycoprotein, is detected in body fluids as well as in tumor tissues. Initially, it was suggested that OPN is expressed by macrophages infiltrating the tumor rather than by tumor cells themselves [
1]. However, elevated serum or plasma levels of OPN have been detected in a variety of human cancers, which has been correlated with tumor progression and metastasis [
2‐
4]. Moreover, tumor OPN protein over expression is also linked to a worse prognosis in different tumor entities such as breast cancer [
5,
6], lung cancer [
7], cervical cancer [
8,
9], prostate cancer [
10] and head and neck cancer [
11‐
13]. Furthermore, previous studies suggest that high tumor OPN mRNA expression also correlates with an unfavorable prognosis [
14‐
18].
In support of its prognostic value, direct or indirect inhibition of the OPN signaling pathway results in reduced malignant tumor potential with effects on cell survival, migration, invasion, tumor growth and metastasis (reviewed in [
4,
19,
20]). However, only a few studies have investigated the role of osteopontin in tumor progression of sarcoma patients. For example, in primary sarcomas of the pulmonary artery, OPN protein could be detected in tumor cells and in macrophages and it is potentially involved in tumor progression and metastasis [
21]. A further immunohistochemical study of STS showed that OPN in tumor tissue is associated with tumor stage, grade and overall survival of STS patients at 5-years [
22]. However, another study using immunohistochemical evaluation of OPN expression did not provide predictive information on the outcome of osteosarcoma patients [
23]. We therefore analyzed OPN mRNA and protein levels of tumor samples and OPN serum levels of 93 histological verified adult STS patients. Moreover, to characterize the role of OPN in soft tissue sarcoma, we correlated OPN levels with clinical parameters and prognosis.
Discussion
OPN overexpression is linked to an unfavorable prognosis in a variety of human cancers. However, only a few studies to date have investigated the prognostic impact of osteopontin in sarcoma patients. In the present study, we analyzed OPN mRNA and protein levels in tumors and the OPN protein levels from the serum of 93 soft tissue sarcoma patients and correlated OPN levels with clinical parameters and prognosis.
In our study no correlation was found between OPN levels in serum and tumor tissue. It is well known that extracellular OPN is involved to tumor progression with signaling to proliferation, migration and survival. Also intracellular OPN is involved in cell migration [
29]. Additionally, there is also evidence that suggests that intracellular OPN has other cellular functions, particularly in relation to cell proliferation [
30]. But regulation between intra- and extracellular OPN forms remains unknown. Nordsmark et al. (2007) assume a relation between plasma concentration of OPN and intratumoral OPN production [
31]. However, in agreement with our results Nordsmark et al. (2007) detected no correlation between plasma OPN and tumor OPN protein levels in patients with head and neck cancer (p = 0.75). Furthermore, own in vitro studies detected a clear decrease of extracellular OPN protein levels after transfection with OPN siRNA in MDA-MB231 cells. In contrast, the intracellular OPN protein level was only partially decreased (Hahnel
et al., manuscript submitted). More studies on the functions of intracellular OPN forms are necessary to understand their relationship with extracellular OPN and their tumor biological role.
Our data suggest that increased serum and tumor OPN protein levels are significantly associated with clinical parameters, such as tumor stage, tumor grade, subtype of tumor, tumor size and the rate of relapse (Table
3). In the Kaplan-Meier analysis, a significantly decreased overall survival was observed for patients with strong OPN protein expression in their serum (p = 0.04; Table
3) and tumor tissue (p = 0.007; Table
3). Furthermore, the univariate Cox's regression hazard model demonstrated that STS patients with a high serum and tumor OPN protein level have an increased risk of tumor-related death of 2.2 (p < 0.05) and 3.7 (p = 0.01; Fig.
2), respectively (Table
3). This is in agreement with a previously reported immunohistochemical study of 33 soft tissue sarcoma patients that showed an increased OPN level significantly correlated with higher tumor stage, grade and overall survival [
22]. Furthermore, primary sarcomas of the pulmonary artery display an abundant immunohistochemical OPN protein staining of tumor cells and extracellular matrix. Therefore, OPN protein may play a substantial role in tumor progression of vessel sarcomas [
21]. However, an immunohistochemical OPN study did not find any correlation between OPN protein expression and outcome of osteosarcoma patients [
23]. In this regard, Luo et al. were able to show that different osteosarcoma cell lines had significantly lower OPN levels than mature osteoblasts [
32]. This may indicate that OPN has no significance for osteosarcoma tumor growth. However, antisense oligodeoxynucleotides against human OPN reduced the tumorigenicity of xenotransplanted osteosarcoma tumors in nude mice [
33]. Additionally, our analysis shows that siRNA-induced osteopontin inhibition results in reduced clonogenic survival and migration of soft tissue sarcoma and breast cancer cell lines (Hahnel
et al., manuscript submitted).
Our analysis of tumor OPN mRNA expression of 68 soft tissue sarcoma patients did not show any association with clinical parameters or prognosis. In contrast, another study of 41 osteosarcoma patients did show that a high OPN mRNA level is correlative with overall survival, event-free survival and relapse-free survival [
34]. Moreover, a study of 15 adult soft tissue sarcoma patients showed a significant increase of OPN mRNA levels compared to normal tissue [
22]. In addition, OPN splice variants may be involved in tumor progression. In particular, osteopontin-c has been postulated to be a significant factor for glioma [
35], breast cancer [
17,
36] and hepatocellular carcinoma [
37]. Clearly, more data are needed to characterize the role of osteopontin mRNA in sarcomas.
Conclusion
In the present study, we analyzed OPN mRNA and protein levels in tumors and the OPN protein levels from the serum of 93 soft tissue sarcoma patients. Our data suggest that increased serum and tumor OPN protein levels are significantly associated with clinical parameters, such as tumor stage, tumor grade, subtype of tumor, tumor size and the rate of relapse. In addition, using a univariate Cox's proportional hazards regression model, we found that an elevated OPN protein level in the serum and tumor tissue extracts is a significant negative prognostic factor for patients with STS. Our data suggest OPN protein in serum as well as in tumor tissue extracts is an important prognostic factor for soft tissue sarcoma patients.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
MB and DV designed the study, collected data, performed statistical analysis and drafted the manuscript. MKa, HW, SR, AH, TG, HMS and MKo and HT made substantial contributions acquisition of data, and analysis and interpretation of data. PW treated the patients, collected material and data and reviewed the manuscript. All authors read and approved the final manuscript.