Abstract. Although ribosomal proteins are major components of ribosomes, recent data have shown them to have extraribosomal functions apart from ribosome and protein biosynthesis. In our earlier study, we showed that ribosomal protein L13 mRNA was up-regulated in response to DNA damage in hamster cells. We report here that L13 expression is up-regulated in human gastrointestinal cancers. We also examined the biological role of L13 on human cancer cells. Knocking down L13 expression using 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. Up-regulation 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 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.

p63 and p73 show a high degree of structural homology to p53 and are members of a family of transcriptional factors that can activate transcription of p53-responsive genes. p53 is mutated in more than 50% of human cancers, whereas p63 and p73 are rarely mutated. Studies of knockout mice also revealed an unexpected functional diversity among the p53 family. To determine how p63 and p73 are involved in tumorigenesis and normal development, we used cDNA microarray to examine 9216 genes in human colorectal cancer cells. We discovered that the expression of pigment epitheliumderived factor (PEDF) was specifically induced by either p63 or p73, but not by p53. We also report here that the PEDF gene contains a response element specific for p63 and p73 in its promoter region and is a direct target of p63 and p73. Collectively, p63 and p73 may be involved in cell fate by inducing PEDF expression.

Abstractp73 and p63 are members of the p53 gene family and have been shown to play an important role in development and homeostasis mainly by regulating the transcription of a variety of genes.

Osteosarcoma (OS) is one of the most common malignancies of the bone. Although prognosis of OS has improved significantly during the past several years due to more intensive chemotherapy and radiotherapy regimens, new therapeutic approaches are needed for recurrent and inoperable cases. p73 and p63, like their homologue, the tumor suppressor p53, are able to induce apoptosis in several cell types. Here, we evaluated the antitumor effects of p73 and p63 on eleven different human OS cell lines. In vitro, adenovirus-mediated transduction of p63g induced apoptosis in OS cells that are resistant to p53-mediated apoptosis, while less effect was observed following transduction of p73a or p63a. Interestingly, the apoptotic effects of p63g were greater than those of wild-type p53 in OS cells carrying MDM2-amplification. We then determined the in vivo therapeutic effect of intratumoral injection of adenovirus-vector expressing p53 family members on xenografts derived from Saos-2 cells implanted in nude mice, and showed that infection with p63g significantly suppressed tumor growth compared with p53. In addition, exogenous p73b and p63g significantly increased the chemosensitivity of OS cells to doxorubicin and cisplatin, chemotherapeutic agents commonly used in the treatment of OS. Our results suggest that adenovirus-mediated transduction of p53 family members may have utility in gene therapy for OS, particularly in combination with chemotherapeutic agents.

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