Differential Expression of Axl and Gas6 in Renal Cell Carcinoma Reflecting Tumor Advancement and Survival

Purpose: Overexpression of the receptor tyrosine kinase Axl is implicated in several cancers. Therefore, we conducted this study to determine the expression of Axl and its ligand Gas6 in various renal cell carcinoma (RCC) types and in oncocytoma. Experimental Design: Real-time quantitative reverse transcription-PCR was used to quantify tumor mRNA levels for Axl and Gas6 in a cohort (n = 221) of RCC patients. Serum levels of soluble sAxl and Gas6 proteins were measured using specific ELISA assays (n = 282). The presence of Axl protein in tumor tissue was evaluated by immunohistochemistry (n = 294). Results were correlated to tumor-associated variables, clinical biochemical tests, and patient survival. Results: Tumor Axl mRNA levels correlated independently to survival when assessed against tumor stage and grade. In the study group, the median cancer-specific survival of all RCC patients during 307 months of follow-up was 55 months (confidence interval, +/-40.4). The 25% of patients with lowest tumor Axl mRNA levels had significantly better survival than the rest (P = 0.0005), with 70% of the patients still alive at the end of follow-up. In contrast, in patients with medium-high Axl mRNA, only 25% were alive at the end of follow-up. Tumor Gas6 mRNA levels correlated to survival, tumor-associated variables, and disease severity as did serum levels of soluble sAxl and Gas6 protein. However, no correlation between Axl protein in tumor tissue and survival was found. Conclusions: Axl and Gas6 expression in RCC are associated with tumor advancement and patient survival. In particular, low tumor Axl m RNA levels independently correlated with improved survival.