The metastasis-promoting P1597L mutation in PlexinB1 enhances Ras activity

Metastatic prostate cancer is a leading cause of cancer-related morbidity and mortality in men, yet the underlying molecular mechanisms are poorly understood. Plexins are transmembrane receptors for semaphorins with divergent roles in many forms of cancer. We recently found that a single clinically relevant specific amino acid change (Proline1597Leucine, (P1597L)), found in metastatic deposits of prostate cancer patients, converts PlexinB1 from a metastasis suppressor to a gene that drives prostate cancer metastasis in vivo. However, the mechanism by which PlexinB1(P1597L) promotes metastasis is not known.

These results suggest that the clinically relevant P1597L mutation in PlexinB1 may transform PlexinB1 from a suppressor to a driver of metastasis in mouse models of prostate cancer by reducing the RapGAP activity of PlexinB1, leading to Ras activation. These findings highlight the PlexinB1-Rap-Ras pathway for therapeutic intervention in prostate cancer.

Pull down assays using GST-RalGDS or -GSTRaf1-RBD were used to reveal the effect of mutant or wild-type PlexinB1 expression on Rap and Ras activity respectively. Protein-protein interactions were assessed in GST pulldown assays, Akt/ERK phosphorylation by immunoblotting and protein stability by treatment with cycloheximide. Rho/ROCK activity was monitored by measuring MLC2 phosphorylation and actin stress fiber formation. PlexinB1 function was measured using cell-collapse assays.

We show here that the single clinically relevant P1597L amino acid change converts PlexinB1 from a repressor of Ras to a Ras activator. The PlexinB1(P1597L) mutation inhibits the RapGAP activity of PlexinB1, promoting a significant increase in Ras activity. The P1597L mutation also blocks PlexinB1-mediated reduction in Rho/ROCK activity, restraining the decrease in MLC2 phosphorylation and actin stress fiber formation induced by overexpression of wild-type PlexinB1. PlexinB1(P1597L) has little effect on the interaction of PlexinB1 with small GTPases or receptor tyrosine kinases and does not inhibit PlexinB1-stimulated Akt or ERK phosphorylation. These results indicate that the mutation affects Rho signalling via the Rap/Ras pathway. The PlexinB1(P1597L) mutation inhibits morphological cell collapse induced by wild-type PlexinB1 expression, suggesting that the mutation induces a loss of an inhibitory tumour suppressor function.