The
cortactin oncoprotein is frequently overexpressed in
head and neck squamous cell carcinoma (
HNSCC), often due to amplification of the encoding gene (CTTN). While
cortactin overexpression enhances invasive potential, recent research indicates that it also promotes cell proliferation, but how
cortactin regulates the cell cycle machinery is unclear. In this article we report that stable
short hairpin RNA-mediated
cortactin knockdown in the 11q13-amplified cell line FaDu led to increased expression of the
Cip/Kip cyclin-dependent kinase inhibitors (CDKIs) p21(WAF1/Cip1), p27(Kip1), and p57(Kip2) and inhibition of S-phase entry. These effects were associated with increased binding of p21(WAF1/Cip1) and p27(Kip1) to
cyclin D1- and E1-containing complexes and decreased
retinoblastoma protein phosphorylation.
Cortactin regulated expression of p21(WAF1/Cip1) and p27(Kip1) at the transcriptional and posttranscriptional levels, respectively. The direct roles of p21(WAF1/Cip1), p27(Kip1), and p57(Kip2) downstream of
cortactin were confirmed by the transient knockdown of each CDKI by specific small interfering RNAs, which led to partial rescue of cell cycle progression. Interestingly, FaDu cells with reduced
cortactin levels also exhibited a significant diminution in RhoA expression and activity, together with decreased expression of Skp2, a critical component of the
SCF ubiquitin ligase that targets p27(Kip1) and p57(Kip2) for degradation. Transient knockdown of RhoA in FaDu cells decreased expression of Skp2, enhanced the level of Cip/Kip CDKIs, and attenuated S-phase entry. These findings identify a novel mechanism for regulation of proliferation in 11q13-amplified
HNSCC cells, in which overexpressed
cortactin acts via RhoA to decrease expression of Cip/Kip CDKIs, and highlight Skp2 as a downstream effector for RhoA in this process.