Downregulation of 14-3-3σ Prevents Clonal Evolution and Leads to Immortalization of Primary Human Keratinocytes

Abstract

In human epidermal keratinocytes, replicative senescence, is determined by a progressive decline of clonogenic and dividing cells. Its timing is controlled by clonal evolution, that is, by the continuous transition from stem cells to transient amplifying cells. We now report that downregulation of 14-3-3σ, which is specifically expressed in human stratified epithelia, prevents keratinocyte clonal evolution, thereby forcing keratinocytes into the stem cell compartment. This allows primary human keratinocytes to readily escape replicative senescence. 14-3-3σ–dependent bypass of senescence is accompanied by maintenance of telomerase activity and by downregulation of the p16INK4a tumor suppressor gene, hallmarks of keratinocyte immortalization. Taken together, these data therefore suggest that inhibition of a single endogenous gene product fosters immortalization of primary human epithelial cells without the need of exogenous oncogenes and/or oncoviruses.

Frequent downregulation of 14-3-3 σ protein and hypermethylation of 14-3-3 σ gene in salivary gland adenoid cystic carcinoma

“…Moreover, downregulation of 14-3-3 s was reported to allow primary human keratinocyte to escape replicative senescence accompanied by downregulation of p16 INK4A (Dellambra et al, 2000). In our present study, we could detect frequent hypermethylation of 14-3-3 s gene in ACC, but not that of p16 INK4A gene.…”

“…In several SGCs occurred at minor salivary gland, we could observe the normal gingival tissue in the surgically resected samples. We used normal gingival tissue as positive control for IHC of 14-3-3 s. In normal gingival epithelial cells, 14-3-3 s was barely detectable in the basal layer, but was increased in the suprabasal layers ( Figure 1A), as described by Dellambra et al (2000). In normal salivary gland, we found the positive staining of 14-3-3 s protein on the salivary duct cells and the mucous gland cells, but not in the myoethithelial cells ( Figure 1B).…”

Coincident inactivation of 14-3-3σ and p16INK4a is an early event in vulval squamous neoplasia

“…This bypassing of senescence is accompanied by loss of p16 INK4a expression. Primary human keratinocytes transduced by retroviruses expressing anti-sense s become immortalized (Dellambra et al, 2000). These observations suggest that inactivation of s is equivalent to expression of the E6 and E7 proteins of HPV, since these proteins co-operate to immortalize primary keratinocytes (Hawley-Nelson et al, 1987).…”

“…s functions in cell cycle checkpoint control and is a critical regulator of senescence in epithelial cells. In studies of human keratinocytes, down-regulation of s allows escape from replicative senescence by forcing cells into the stem cell compartment (Dellambra et al, 2000). This bypassing of senescence is accompanied by loss of p16 INK4a expression.…”

The dark side of a tumor suppressor: anti-apoptotic p53

“…71 Interestingly, this hypermethylation was even observed in normal adjacent breast tissue surrounding the tumor, but not in control tissue from cancer-free patients, indicating that gene silencing of 14-3-3d might be an early event during breast carcinogenesis. Together with the observation that silencing of 14-3-3d constitutes a single step that can immortalize primary epithelial cells, 72 these findings emphasize the crucial role of 14-3-3d as a tumor suppressor. In contrast, 14-3-3 proteins were also often found overexpressed in various cancers, but the significance of these increases to cancer development has not been firmly established yet.…”