V600EBraf induces gastrointestinal crypt senescence and promotes tumour progression through enhanced CpG methylation of p16INK4a.

The majority of human colorectal cancers (CRCs) are initiated by mutations arising in the adenomatous polyposis coli (APC) tumour suppressor gene. However, a new class of non-APC mutated CRCs has been defined that have a serrated histopathology and carry the (V600E)BRAF oncogene. Here we have investigated the pathogenesis of serrated CRCs by expressing (V600E)Braf in the proliferative cells of the mouse gastrointestinal tract. We show that the oncogene drives an initial burst of Mek-dependent proliferation, leading to the formation of hyperplastic crypts. This is associated with ß-catenin nuclear localization by a mechanism involving Mapk/Erk kinase (Mek)-dependent, Akt-independent phosphorylation of Gsk3ß. However, hyperplastic crypts remain dormant for prolonged periods due to the induction of crypt senescence accompanied by upregulation of senescence-associated ß-galactosidase and p16(Ink4a). We show that tumour progression is associated with down-regulation of p16(Ink4a) through enhanced CpG methylation of exon 1 and knockout of Cdkn2a confirms this gene is a barrier to tumour progression. Our studies identify (V600E)BRAF as an early genetic driver mutation in serrated CRCs and indicate that, unlike APC-mutated cancers, this subtype arises by the bypassing of a (V600E)Braf driven oncogene-induced senescence programme.

CRAF autophosphorylation of serine 621 is required to prevent its proteasome-mediated degradation.

The CRAF protein kinase regulates proliferative, differentiation, and survival signals from activated RAS proteins to downstream effectors, most often by inducing MEK/ERK activation. A well-established model of CRAF regulation involves RAS-mediated translocation of CRAF to the plasma membrane, where it is activated by a series of events including phosphorylation. Here we have discovered a new mode of regulation that occurs prior to this step. By creating a kinase-defective version of CRAF in mice or by use of the RAF inhibitor sorafenib, we show that CRAF must first undergo autophosphorylation of serine 621 (S621). Autophosphorylation occurs in cis, does not involve MEK/ERK activation, and is essential to ensure the correct folding and stability of the protein. In the absence of S621 phosphorylation, CRAF is degraded by the proteasome by mechanisms that do not uniquely rely on the E3 ubiquitin ligase CHIP.

AP-1 differentially expressed proteins Krp1 and fibronectin cooperatively enhance Rho-ROCK-independent mesenchymal invasion by altering the function, localization, and activity of nondifferentially expressed proteins.

The transcription factor AP-1, which is composed of Fos and Jun family proteins, plays an essential role in tumor cell invasion by altering gene expression. We report here that Krp1, the AP-1 up-regulated protein that has a role in pseudopodial elongation in v-Fos-transformed rat fibroblast cells, forms a novel interaction with the nondifferentially expressed actin binding protein Lasp-1. Krp1 and Lasp-1 colocalize with actin at the tips of pseudopodia, and this localization is maintained by continued AP-1 mediated down-regulation of fibronectin that in turn suppresses integrin and Rho-ROCK signaling and allows pseudopodial protrusion and mesenchyme-like invasion. Mutation analysis of Lasp-1 demonstrates that its SH3 domain is necessary for pseudopodial extension and invasion. The results support the concept of an AP-1-regulated multigenic invasion program in which proteins encoded by differentially expressed genes direct the function, localization, and activity of proteins that are not differentially expressed to enhance the invasiveness of cells.