Persistent activation of Rac1 in squamous carcinomas of the head and neck: evidence for an EGFR/Vav2 signaling axis involved in cell invasion

Abstract

The poor prognosis associated with head and neck squamous cell carcinoma (HNSCC) is primarily due to both local invasion and the regional and/or distant metastatic spread. Recent findings have provided evidence that the acquisition of a motile and invasive phenotype by cancer cells involves the dysregulated function of key intracellular molecular mechanisms together with aberrant signaling events initiated by the surrounding microenvironment. These intrinsic and extrinsic biochemical pathways in turn often converge to stimulate the activity of members of the Rho family of Ras-related guanosine triphosphate (GTP)-binding proteins, including RhoA, Rac and Cdc42, which control the organization of the actin cytoskeleton thereby regulating cell adhesion, polarity and motility. In this study, we examined the status of activation of these GTPases in a representative collection of HNSCC cell lines. Surprisingly, we found that most HNSCC cells exhibit remarkably high levels of GTP-bound Rac1. Further analysis revealed that the activation of Rac1 in these HNSCC cells could be due to two independent signaling events, an epidermal growth factor receptor (EGFR)-based autocrine loop that leads to the activation of the Rac1 exchange factor Vav2 and an EGFR/Vav2-independent pathway that arises as a consequence of the oncogenic mutation of the H-ras proto-oncogene. Indeed, we provide evidence that the EGFR/Vav2/Rac1 axis is a crucial pathway for the acquisition of motile and invasive properties of most HNSCC cells. These findings shed light onto the molecular mechanisms involved in HNSCC cell invasion, and may reveal new therapeutic opportunities to halt the metastatic spread of these aggressive malignancies.

Catch-slip behavior observed upon rupturing membrane-cytoskeleton bonds

“…This assumes the measured decrease in active RhoA (cf. 27 ± 12 %, Figure 6) is comparable for the other Rho-GTPase proteins present in this cell 38 . The absence of the mechanosensitive response in Toxin Btreated cells leads us to suggest that the linkages are different than present in the untreated cells.…”

“…and an H-ras mutation 38 that results in persistently-active Rho-GTPases. We apply a constant loading rate to rupture the bonds and detect a biphasic time-loading rate relationship.…”

Autocrine signaling via release of ATP and activation of P2X7 receptor influences motile activity of human lung cancer cells

“…In this study, PC-9 cells showed actin-rich lamellipodia at both ends of their long axis. This suggests enhanced activation of Rac1, probably because of activation of the EGFR-Ras axis by mutation of EGFR [31]. In fact, inhibition of EGFR resulted in disappearance of lamellipodia and decrease of motile activity, while formation of filopodia was observed.…”

Mechanisms of Invasion in Head and Neck Cancer

“…48,55,[90][91][92][93][94] A number of HNSCC cell lines have constitutively high levels of Rac1 activity, and inhibition of Rac activity results in reduced invasion capability. 95 In most cases, the increased Rac1 activity correlated with increased tyrosine phosphorylation of the Rac GEF Vav2 induced by EGFR, but an alternative pathway used Ras activation. The role of the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway in HNSCC invasion is complex.…”