Abstract
2655
Gene amplification of chromosomal band 11q13 is frequently observed in human cancers including oral squamous cell carcinomas (OSCC). Several genes have been identified in the 11q13 amplicon including FGF3, FGF4, CCND1, EMS1, and TAOS1. We found recently that the PPP1CA gene, which also maps to 11q13 and encodes the catalytic subunit of serine/threonine protein phosphatase 1α (PP1α), was amplified and overexpressed in a subset of OSCC cell lines. Here we sought to test whether PP1α regulated OSCC cell growth and contributed to tumorigenesis. Knockdown of PP1α by siRNA transfection was performed in two OSCC cell lines that overexpressed PP1α, and cell growth was determined by the MTT assay or by cell counting. PP1α siRNA transfected cells consistently grew slower than control siRNA transfected cells, suggesting that knockdown of PP1α suppressed OSCC cell growth. Knockdown of cyclin D1, the gene product of CCND1, which is frequently amplified and overexpressed in human cancers, also suppressed OSCC cell growth, consistent with its role in tumorigenesis. Interestingly, when cells were cotransfected with PP1α siRNA and cyclin D1 siRNA, the growth suppression effect was more dramatic, indicating that PP1α cooperates with cyclin D1 to promote OSCC cell growth. It has been demonstrated that PP1α regulates the cell cycle and is involved in mitotic progression. PP1α can also dephosphorylate and activate the tumor suppressor pRB. Cyclin D1 is the regulatory subunit of a CDK holoenzyme that phosphorylates and inactivates pRB, and promotes the G1/S cell cycle progression. Surprisingly, PP1α and cyclin D1 siRNA transfection resulted in accumulation of hypophosphorylated pRB and an increase in G0/G1 cells. Thus, PP1α and cyclin D1 may have a combinatorial effect on OSCC cell growth by regulating pRB phosphorylation and promoting G1/S cell cycle progression. Taken together, our data suggest that the PPP1CA gene may also be a target for gene amplification and overexpression, and in cooperation with other genes co-amplified in 11q13, such as CCND1, may be involved in OSCC tumorigenesis.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]