Expression of Osteopontin in selected benign and malignant salivary gland neoplasms

Osteopontin (OPN) is a phosphorylated acidic glycoprotein involved in the maturation of various hard tissues in the body.  It stabilises osteoblast function, influences the migration of osteoclasts, and acts as a chemoattractant in inflammatory responses.   Over-expression of OPN has also been shown to play a vital role in tumourigenesis and metastases in various cancers.  Coppola et al., (2004) has shown that OPN is expressed in a variety of human tumours.  In breast cancer, high levels of OPN expression within tumours correlate with poor prognosis (Tuck et al., 1998), and high plasma OPN levels have been shown to be associated with increased tumour burden and poor outcome (Singhal et al., 1997; Bramwell et al., 2006).  Recent animal studies have attempted to localize OPN production within salivary glands (Asaka et al., 2006).  OPN is expressed differently in various salivary gland tumours (Darling et al., 2006), but its role has not yet been elucidated.  The objective of this study was to determine OPN expression in several salivary gland cancers, and to correlate it with the clinical parameters such as histological grade, age of diagnosis, and stage of diseases.

Immunohistochemical techniques were used to semi-quantify the level of OPN  expression in normal salivary gland (NSG) (n = 20) and 5 different types of salivary gland tumour: (1) pleomorphic adenoma (PA) (n = 20), (2) acinic cell carcinoma (ACA) (n = 11), (3) adenoid cystic carcinoma (ACC) (n = 22), (4) polymorphous low grade adenocarcinoma (PLGA) (n = 16), and (5) mucoepidermoid carcinoma (MEC) (n= 29).  These specimens were obtained from archived samples from resected primary tumours of salivary gland origin. 

Statistical analysis of the data was performed using Mann-Whitney U test and Kruskal Wallis test where applicable for comparative analysis, and Spearman rank correlation for correlation study.

Our results for OPN expression show that low levels were detected in benign tumours and normal salivary gland, while significant higher levels of OPN expression were found in malignant tumours.  As well, early results show that OPN expression level moderately correlate with pathological grade of the neoplasms in MEC and ACC.  Further analysis is underway to examine the correlation between the expression of OPN, its interacting molecules and clinical outcome, as markers or predictors of tumour behaviour.

References

1. Coppola D, Szabo M, Boulware D, et al. Correlation of osteopontin protein expression and pathological stage across a wide variety of tumor histologies. Clin Cancer Res. 2004;10:184

2. Asaka M, Ohta K, Muramatsu T, et al. The expression and localization of osteopontin in the mouse major salivary glands. Arch Histol Cytol 2006;69:181.