Global gene expression profiling studies of human HNSCC tumors have identified genes associated with tumorigenesis and metastatic potential [8
]. In addition, molecular classification of HNSCC has also been proposed based on gene expression patterns [11
]. It has been reported that gene expression profile can distinguish clinically distinct subgroups of HNSCC, identify genes potentially associated with advanced-stage of the disease, and predict outcome of treatment [9
]. In our study, in order to identify novel therapeutic targets for diagnosis or treatments, we compared gene expression in HNSCC with normal tissues and further characterized protein expression of over expressed genes in HNSCC samples. We identified 192 up-regulated genes in HNSCC compared with normal tissues. Based on our results, published literature, and availability of antibody, five candidate genes (CD24, CD44, CD74, FABP5, and HSP27) were selected for further characterization in HNSCC samples. These gene products were expressed in most tumor samples but not in normal tissues. Although various studies have reported on the expression of these genes in various tissues, our study presents their significance in HNSCC tumors.
Among selected genes, CD24, CD44, and CD74 are cell surface adhesion molecules shown to be over expressed in various cancers [15
]. Among these, CD44 was most strongly expressed in all HNSCC samples in tissue arrays. Interestingly, CD44+ subpopulation of cells with cancer stem cell properties has been identified in HNSCC and various other cancer types [21
]. CD24 has also been shown to be expressed in various tumors. Its expression has been used as a prognostic indicator of poor survival in breast cancer, non-small cell lung carcinoma, and prostate cancers [15
]. However, in contrast to CD44, only a small portion of CD24+ cells were positive in HNSCC cancer cells in our study. These results suggest that a subset of CD24+/CD44+ cells exists, which may represent cancer stem cells in HNSCC. This hypothesis is supported by a report which showed that a highly tumorigenic subpopulation of pancreatic cancer cells express cell surface markers CD44, CD24, and epithelial-specific antigen (ESA) [23
]. This subpopulation of pancreatic cancer cells with CD44+/CD24+/ESA+ phenotype (only 0.2 to 0.8% of pancreatic cancer cells) had a 100-fold increased tumorigenic potential compared with other cancer cells [23
]. Future studies will focus on the isolation and characterization of the CD24+/CD44+ and other marker positive tumor subpopulations from HNSCC.
CD74 is another cell surface membrane protein identified in this study, which was not previously reported in HNSCC. However, the over expression of CD74 has been reported in many different malignancies, including gastric tumors, renal epithelial neoplasms, pancreatic cancers, certain types of sarcoma and skin cancer, and non-small cell lung cancers [26
]. CD74 has been identified as the high-affinity receptor for the macrophage migration inhibitory factor [31
]. The over expression of macrophage migration inhibitory factor has also been reported in various cancers, including lung cancer, breast cancers, prostate cancers, ovarian cancers, colorectal cancers, and HNSCC tumors in this study (Table ) [30
]. Overexpression of tumor cell-derived macrophage migration inhibitory factor (MIF) in solid tumors is related to tumor growth, progression, and angiogenesis [32
]. Recent study on non-small cell lung cancers suggested that the co-expression of MIF and its receptor CD74 is associated with greater tumor vascularity and greater of angiogenic CXC chemokines [30
]. In vitro inhibition of MIF or its receptor resulted in reduced production of angiogenic CXC chemokines by lung cancer cells [30
]. In addition, the clinical study in colorectal cancer patients found that the serum level of MIF was significantly increased in cancer patients, suggesting that MIF could be used as a diagnostic marker in colorectal cancers [35
]. In our study, since both MIF and its receptor CD74 are highly expressed in HNSCC, these results suggest that both MIF and CD74 may potentially serve as valuable biomarkers in HNSCC. In addition, as a cell membrane protein, CD74 may serve as a new target for anti-cancer therapy of HNSCC. MIF, on the other hand, may serve as a new diagnostic marker for HNSCC.
FABP5 is a fatty acid-binding protein and is expressed in epidermis and endothelial cells of the microvasculature of different organs [37
]. FABP5 has also been identified as a tumor-associated antigen, which is highly expressed in various cancers [38
]. FABP5 was detected in the sera of HNSCC patients with early stage cancer [40
]. Antibodies specific for FABP5 were significantly increased in a substantial amount in patients, suggesting that FABP5 may be a potential diagnostic biomarker for HNSCC [40
]. As FABP5 is highly expressed at both mRNA and protein levels in HNSCC compared to normal tissues, our results support previous suggestion that FABP5 may serve as a biomarker for HNSCC.
Another interesting gene identified in HNSCC is heat shock protein (HSP27). Over expression of HSP27 in HNSCC was not reported previously, although HSPs have been reported to be over-expressed in a wide range of human cancers, which suggest that HSPs may serve as an effective target for therapy [41
]. In addition, over expression of HSP has been correlated with a poor prognosis in terms of survival and response to therapy in specific cancer types [41
]. HSP27 is associated with poor prognosis in gastric cancer, liver cancer, prostate carcinoma, and non-small cell lung carcinoma [41
]. HSP27 is also implicated in resistance to chemotherapy in breast cancer [41
]. Several small-molecule drugs that target the HSP27 have been identified as potential anticancer agents [45
]. Thus, up-regulation of HSP27 in HNSCC tumors identified in our current study provides opportunities for a new biomarker of disease monitoring and development of new targeted therapy for HNSCC.