Gene amplification and overexpression of protein phosphatase 1α in oral squamous cell carcinoma cell lines

Gene amplification of chromosomal band 11q13 is observed frequently in oral squamous cell carcinomas (OSCC). Several genes have been identified in the 11q13 amplicon, including FGF3, FGF4, CCND1, EMS1 and TAOS1. Some of these genes show good correlation between gene copy number and gene expression, and are thought to play a role in driving 11q13 amplification. The PPP1CA gene, which encodes the catalytic subunit of serine/threonine protein phosphatase protein phosphatase 1 alpha (PP1 alpha), is also located in 11q13. Protein phosphatase 1a, one of the isoforms of PP1, regulates critical cellular events, such as cell cycle progression, and apoptosis. We sought to explore the possibility that PPP1CA was amplified and overexpressed in OSCC cells. Indeed, some OSCC cell lines had PPP1CA gene amplification, as analysed by fluorescence in situ hybridization. We have also demonstrated that PPP1CA gene copy number is increased in 21% of the OSCC cell lines determined by quantitative microsatellite analysis. PP1a RNA expression determined by quantitative reverse transcription-polymerase chain reaction was significantly higher in OSCC cell lines with 11q13 amplification compared to those without 11q13 amplification (P= 0.011). The difference was even more significant between cell lines with at least three copies of the PPP1CA gene and those with less than three copies of the gene (P= 0.00045). Relative PP1a protein levels were also significantly associated with PPP1CA gene copy number (P= 0.014). Furthermore, knockdown of PP1 alpha and/or cyclin D1 by small interfering RNA suppressed OSCC cell growth, at least in part by modulating pRB phosphorylation, resulting in G0 growth arrest. These data suggest that like the cyclin D1 gene, CCND1, amplification and overexpression of the PP1a gene, PPP1CA, may be involved in OSCC tumorigenesis and/or progression.