1. ¹Institute of Basic Medical Sciences, College of Medicine, National Cheng Kung University, Tainan, Taiwan
2. ²Department of Surgery, College of Medicine, National Cheng Kung University, Tainan, Taiwan
3. ³Institute of Biotechnology, College of Bioscience and Biotechnology, National Cheng Kung University, Tainan, Taiwan
4. ⁴Department of Biotechnology, College of Medicine, Chia-Na University of Pharmacy and Science, Tainan, Taiwan
5. ⁵Department of Parasitology, College of Medicine, National Cheng Kung University, Tainan, Taiwan
6. ⁶Department of Biochemistry and Molecular Biology, College of Medicine, National Cheng Kung University, Tainan, Taiwan

Correspondence: Dr M-C Chang, Department of Biochemistry and Molecular Biology, Basic Medical Science, College of Medicine, National Cheng Kung University, 138 Sheng-Li Road, Tainan 704, Taiwan. E-mail: mcchang@mail.ncku.edu.tw

Received 25 September 2006; Revised 9 May 2007; Accepted 3 June 2007; Published online 9 July 2007.

Abstract

The ribosomal acidic P0 protein, an essential component of the eukaryotic ribosomal stalk, was found to interact with the helix–loop–helix protein human Grap2 and cyclin D interacting protein (GCIP)/D-type cyclin-interacting protein 1/human homolog of MAID protein. Using in vivo and in vitro binding assays, we show that P0 can interact with the N and C termini of GCIP via its N-terminal 39–114 amino-acid residues. Although the P0–GCIP complex was detected mainly in cytoplasmic fraction, polysome profile analysis indicated that the P0–GCIP complex did not coelute with either polysomes or 60S ribosomes, suggesting that GCIP associates with the free form of P0 in the cytoplasm. Transfection of GCIP into MCF-7 cells resulted in decreased levels of pRb phosphorylation. Cotransfection of P0 with GCIP, however, resulted in GCIP-mediated reduction of pRb phosphorylation level which was repressed by P0. Furthermore, overexpression of P0 in breast cancer and hepatocellular cancer cell lines promoted cell growth and colony formation compared to control transfectants. Overexpression of P0 also increased cyclin D1 expression and phosphorylation of pRb at Ser780. Interestingly, P0 mRNA was overexpressed in 12 of 20 pairs of breast cancer/ normal breast specimens (60%). Together, these data indicate that P0 overexpression may cause tumorigenesis in breast and liver tissues at least in part by inhibiting GCIP-mediated tumor suppression.