Rap1 stabilizes beta-catenin and enhances beta-catenin-dependent transcription and invasion in squamous cell carcinoma of the head and neck.
OBJECTIVE
In head and neck squamous cell carcinoma (HNSCC) cells, Rap1 shuttles between the nucleus and cytoplasm. Prior findings suggested that Rap1 may modulate the beta-catenin-independent Wnt pathway in some settings, but the role of Rap1 in beta-catenin-dependent Wnt signaling remains undefined.


RESULTS
We observed that beta-catenin bound to active Rap1 in vitro and Rap1 activated beta-catenin/T-cell factor (TCF)-dependent transcription. Immunofluorescence studies showed that ectopic expression of Rap1 increased nuclear translocation of beta-catenin. Overexpression of active Rap1 facilitated an increase in beta-catenin-mediated transcription that was abrogated by dominant-negative TCF4. Conversely, small interfering RNA-mediated inhibition of endogenous Rap1 expression inhibited beta-catenin/TCF-mediated transcription as well as invasion of HNSCC. Furthermore, inhibition of Rap1 expression downregulated the expression of matrix metalloproteinase 7, a transcriptional target of beta-catenin/TCF. In HNSCC cells stably transfected with beta-catenin or treated with lithium chloride or Wnt3A to stabilize endogenous beta-catenin, inhibition of Rap1 expression led to decreases in the free pool of beta-catenin. Immunohistochemical studies of tissue from HNSCC patients revealed that increased beta-catenin intensity correlated with higher tumor stage. Furthermore, the prognostic effect of active Rap1 on tumor N stage was found to depend on cytosolic beta-catenin expression (P < 0.013). When beta-catenin is high, higher Rap1GTP intensity is associated with more advanced N stage.


CONCLUSIONS
The findings suggest that Rap1 enhances beta-catenin stability and nuclear localization. In addition to indicating that Rap1 has a significant role in regulating beta-catenin and beta-catenin-dependent progression to more advanced N-stage lesions, these data highlight Rap1 as a potential therapeutic target in HNSCC.
Goto
Mitsuo
M
Departments of Periodontics and Oral Medicine and Biologic and Materials Science, University of Michigan School of Dentistry, Ann Arbor, Michigan 48109-1078, USA.
Mitra
Raj S
RS
Liu
Min
M
Lee
Julia
J
Henson
Bradley S
BS
Carey
Thomas
T
Bradford
Carol
C
Prince
Mark
M
Wang
Cun-Yu
CY
Fearon
Eric R
ER
D'Silva
Nisha J
NJ
D002294:Carcinoma, Squamous Cell / Q000235:genetics; D045744:Cell Line, Tumor; D015972:Gene Expression Regulation, Neoplastic; D006258:Head and Neck Neoplasms / Q000235:genetics; D006801:Humans; D000089804:Shelterin Complex; D015398:Signal Transduction; D034501:Telomere-Binding Proteins / Q000378:metabolism / Q000502:physiology*; D015533:Transcriptional Activation; D014162:Transfection; D051153:Wnt Proteins / Q000378:metabolism; D051176:beta Catenin / Q000378:metabolism*
D000089804:Shelterin Complex; C440809:TERF2IP protein, human; D034501:Telomere-Binding Proteins; D051153:Wnt Proteins; D051176:beta Catenin