Rap1GAP Inhibits Tumor Growth in Oropharyngeal Squamous Cell Carcinoma

Abstract

Rap1, a growth regulatory protein that is strongly expressed in human squamous cell carcinoma (SCC), is inactivated by rap1GAP. Recent evidence in normal rat cells suggests that rap1GAP regulates proliferation. The objective of the current study was to investigate whether rap1GAP functions as a tumor suppressor in SCC. Using a pull-down assay, active GTP-bound rap1 was up-regulated in SCC compared to normal or immortalized keratinocytes. Because both rap1A and rap1B isoforms of rap1 are expressed in SCC, the rap1GAP inactivation of both rap1 isoforms was verified using cells transfected with EGFP-rap1A or EGFP-rap1B or co-transfected with FLAG-tagged rap1GAP. The results demonstrate that expression of rap1GAP in oropharyngeal SCC down-regulated active rap1, ERK activation, and proliferation. Rap1A and rap1B are ras-like proteins that are ubiquitously expressed. Rap1A, which is also referred to as smg p21 or Krev1, shares 95% homology with rap1B, differing by only nine amino acids (of a total of 184), six of which are located at the C-terminus between amino acids 171 to 184.1 Active rap1 modulates cellular functions by regulating the translocation of other proteins or by shuttling between intracellular compartments.

Prostaglandin E2 Regulates Renal Cell Carcinoma Invasion through the EP4 Receptor-Rap GTPase Signal Transduction Pathway

“…Rap1GAP protein levels are decreased in several types of cancer, including pancreatic adenocarcinoma (55), papillary thyroid carcinoma (56,57), colorectal carcinoma (58), and melanoma (59). The implication of Rap1GAP as a tumor suppressor was largely deduced based upon experimental results using forced overexpression of full-length Rap1GAP (60,61). In addition to its GAP domain, the Rap1GAP protein encompasses regulatory N terminus and C terminus domains (34,38), rendering it capable of exerting other functions, like regulation of heterotrimeric G z protein signaling (62) or serving as a docking site for binding partner proteins due to phosphorylation-dependent modification of its C terminus (63,64).…”

Mesenchymal high-grade glioma is maintained by the ID-RAP1 axis

“…Conversely, preventing inactivation of RAP1-GTPase through a constitutively active RAP1 protein rescues Id-depleted GICs from the inhibitory effects of Rap-1GAP. Interestingly, RAP1GAP has recently emerged as a candidate tumor suppressor gene in other tumor types (41)(42)(43)(44). Besides repressing Rap1GAP, the expression of ID proteins in malignant glioma is essential to prevent expression of cyclin-dependent kinase inhibitors (e.g., p57 Kip2 ), thus explaining the proliferative arrest precipitated by acute Id ablation in GICs.…”

Preventing the Activation or Cycling of the Rap1 GTPase Alters Adhesion and Cytoskeletal Dynamics and Blocks Metastatic Melanoma Cell Extravasation into the Lungs

“…Thus, Rap1 likely regulates multiple steps in the metastatic cascade. Indeed, altered expression of Rap1, the exchange factors that activate Rap1, or the GTPase-activating proteins (GAP) that convert Rap1 to the inactive GDP-bound state, has been observed in several tumor types, including melanoma (14)(15)(16)(17)(18)(19). However, how altered Rap1 activation affects specific aspects of the metastatic cascade is cell type-dependent.…”