Abstract #2725: Reduced expression of Raf-1 kinase inhibitor protein (RKIP) in human lung tumors: Prognostic significance

Objective and Rationale: Raf-1 kinase inhibitor protein (RKIP) has been reported to inhibit both the Raf-1/MEK/ERK and NF-kB signaling pathways. Its low expression or absence in some metastatic cancers suggested that RKIP is a metastatic suppressor gene product. Recent studies have reported that RKIP expression is of prognostic significance in some cancers such as breast, colon, prostate, and ovarian. RKIP can be phosphorylated at Ser153 and the phosphorylation results in its inability to inhibit the Raf-1/MEK pathway. Therefore, we investigated whether RKIP expression is altered in clinical specimens of human lung tumors and its potential prognostic significance. Experimental Designs and Methods: We analyzed lung tissue microarrays (TMA) with 5,079 cores derived from 671 patients representing various stages of tumor stage and histopathologies. The expression of RKIP and phospho-RKIP (pRKIP) was examined by immunohistochemistry. The specificity of the antibodies was validated by competitive inhibition with purified peptides used for immunization. In addition, control isotype antibodies were used. The anti-RKIP antibody detects both RKIP and p-RKIP while the anti-p-RKIP antibody is specific for p-RKIP detection only. Immunostaining was scored blindly by pathologists and recorded for both the percentage of cells positively stained as well as the intensity of staining. The immunohistochemistry and analyses were performed more than once for reproducibility and the data were analyzed statistically. Results: The findings revealed that RKIP expression in malignant cells was significantly lower than adjacent normal cells. Most of the RKIP expression was cytoplasmic. The intensity of the positively stained cells was moderate to weak. The spectrum of RKIP expression was moderate in small cell lung cancer and non-small cell lung cancer, weak in adenocarcinoma, weak or negative in squamous cell carcinoma and variable in other cell types. RKIP expression in metastatic malignant cells was low or absent compared to expression in primary tumors and the staining was predominantly cytoplasmic. Analysis of p-RKIP expression revealed in all cases weak to very low levels and primarily was cytoplasmic. Conclusion: This is the first study of RKIP and p-RKIP expression in a large scale cohort of human lung tumors. The low expression or RKIP and p-RKIP in primary tumors and low or absent expression in metastatic tumors suggest their potential prognostic significance independent of other biomarkers for lung tumor cancer progression. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 2725.