Diagnostic and therapeutic application of ribosomal proteins in gastrointestinal cancer

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Although ribosomal proteins are major components of ribosomes, recent data have shown them to have extraribosomal functions apart from the ribosome and protein biosynthesis. In our earlier study we showed that ribosomal protein L13 mRNA was upregulated in response to DNA damage in hamster cells. We report here that L13 expression is upregulated in human gastrointestinal cancers. We also examined the biological role of L13 on human cancer cells. Knocking down L13 expression by small interfering RNA (siRNA) resulted in drastic attenuation of cancer cell growth with significant G1 and G2/M arrest of the cell cycle. Moreover, L13 siRNA significantly enhanced the cellular sensitivity to certain DNA damaging agents and, concordantly, L13-overexpressing cells demonstrated greater chemoresistance compared to parent cells, suggesting an inverse correlation between L13 expression and chemosensitivity. By using semiquantitative RT-PCR, we analyzed expression of L13 in freshly resected cancer tissue of the stomach, colorectum, and liver. Upregulation of L13 mRNA expression was observed in 10 (28%) of 36 gastric, 19 (41%) of 46 colorectal, and 5 (20%) of 25 liver cancer tissue samples compared to adjacent normal tissue samples. We also found that increased expression of the L13 gene was correlated with clinical staging in gastric cancers. The results of this study suggest that L13 plays an essential role in the progression of some gastrointestinal malignancies.