BRAFE600-associated senescence-like cell cycle arrest of human naevi

Abstract

Most normal mammalian cells have a finite lifespan, thought to constitute a protective mechanism against unlimited proliferation. This phenomenon, called senescence, is driven by telomere attrition, which triggers the induction of tumour suppressors including p16(INK4a) (ref. 5). In cultured cells, senescence can be elicited prematurely by oncogenes; however, whether such oncogene-induced senescence represents a physiological process has long been debated. Human naevi (moles) are benign tumours of melanocytes that frequently harbour oncogenic mutations (predominantly V600E, where valine is substituted for glutamic acid) in BRAF, a protein kinase and downstream effector of Ras. Nonetheless, naevi typically remain in a growth-arrested state for decades and only rarely progress into malignancy (melanoma). This raises the question of whether naevi undergo BRAF(V600E)-induced senescence. Here we show that sustained BRAF(V600E) expression in human melanocytes induces cell cycle arrest, which is accompanied by the induction of both p16(INK4a) and senescence-associated acidic beta-galactosidase (SA-beta-Gal) activity, a commonly used senescence marker. Validating these results in vivo, congenital naevi are invariably positive for SA-beta-Gal, demonstrating the presence of this classical senescence-associated marker in a largely growth-arrested, neoplastic human lesion. In growth-arrested melanocytes, both in vitro and in situ, we observed a marked mosaic induction of p16(INK4a), suggesting that factors other than p16(INK4a) contribute to protection against BRAF(V600E)-driven proliferation. Naevi do not appear to suffer from telomere attrition, arguing in favour of an active oncogene-driven senescence process, rather than a loss of replicative potential. Thus, both in vitro and in vivo, BRAF(V600E)-expressing melanocytes display classical hallmarks of senescence, suggesting that oncogene-induced senescence represents a genuine protective physiological process.

Transgenic expression of oncogenic BRAF induces loss of stem cells in the mouse intestine, which is antagonized by β-catenin activity

“…Mutational activation of BRAF elicits oncogene-induced senescence and apoptosis in melanoma. 25,53,54 However, in agreement with another recent publication, 18 we observed neither senescence nor apoptosis-related cleaved caspase3 in the intestine after oncogenic BRAF induction, although it has been reported before under similar conditions. 26 Strikingly, our data suggest the existence of a novel alternative fail-safe mechanism that may block tumor development after mutational activation of BRAF, that is, by oncogene-induced loss of stem cells.…”

Pirin Inhibits Cellular Senescence in Melanocytic Cells

“…Furthermore, PIR levels rapidly and significantly decrease on expression of activated BRAF, which is an extremely frequent event in nevi and is directly associated with the onset of oncogene-induced senescence. 8 PIR expression appears to be required to overcome the senescence barrier in PIR ϩ melanomas, given that its ablation in these models rapidly results in a senescencelike phenotype. Instead, PIR Ϫ melanomas likely reacquire the capacity to proliferate through mechanisms that do not involve PIR.…”

“…3,10 Nevi frequently harbor oncogenic mutations of the tyrosine kinase BRAF gene, particularly V600E, 11 and BRAF V600E is also found in approximately 70% of cutaneous melanomas. 12 Expression of BRAF V600E in human melanocytes leads to oncogene-induced senescence, 8 which can be considered as a mechanism that protects from malignant progression. In time, some cells may eventually escape senescence, probably through the acquisition of additional genetic abnormalities, thus favoring transformation to melanoma.…”

Harnessing the complexity of DNA-damage response pathways to improve cancer treatment outcomes

“…Like apoptosis, senescence has been proposed as a tumor suppressor mechanism (Braig et al, 2005;Chen et al, 2005;Collado et al, 2005;Lazzerini Denchi et al, 2005;Michaloglou et al, 2005). In normal cells, genotoxic and oncogenic stress activate the p53-p21 and p16-pRB pathways, respectively, leading to transient or permanent growth arrest.…”