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Inducible activation of Ras and Raf in adult epidermis
Masahito Tarutani1, Ti Cai, Maya Dajee
1Veterans Affairs Valo Alto Healthcare System and the Program in Epithelial Biology, Stanford University School of Medicine, Stanford, California 94305, USA.
Abstract:
Ras effects vary with developmental setting, with oncogenic RAS activation implicated in epithelial carcinogenesis. In epidermal cells, previous studies described conflicting Ras impacts on growth and differentiation, with the only in vivo studies relying on constitutive alterations of Ras function throughout development. To study Ras effects in developmentally mature adult epidermis, we expressed a 4-hydroxytamoxifen (4OHT)-regulated Ras fusion in transgenic mice using the keratin 14 promoter. Resulting adult K14-ER:Ras mice displayed 4OHT-inducible activation of Ras as well as elements of Raf/mitogen-activated protein kinase (MAPK) but not RalGDS/Ral or phosphatidylinositol 3'-kinase (PI3K)/Akt downstream Ras effector pathways. Ras reversibly induced massive cutaneous hyperplasia and suppressed differentiation. Ras-driven hyperproliferation was accompanied by increases in beta1 and beta4 integrin epidermal progenitor markers. Epidermal expression of inducible Raf produced similar changes. These findings indicate that activation of Ras in adult epidermis promotes proliferation and inhibits differentiation and that Raf is sufficient to mediate these effects.
Insights
Activating Ras in adult skin cells triggers rapid growth and halts differentiation, findings crucial for understanding epithelial cancers. This research clarifies Ras signaling
Area of Science:
- Molecular Biology
- Dermatology
- Oncology
Background:
- Oncogenic RAS activation is linked to epithelial carcinogenesis, but its role in adult epidermis remains unclear.
- Previous in vivo studies used constitutive Ras alterations, limiting understanding of its effects in mature skin.
Purpose of the Study:
- To investigate the specific effects of Ras activation in developmentally mature adult mouse epidermis.
- To elucidate the downstream signaling pathways mediating Ras-induced changes in the skin.
Main Methods:
- Generated transgenic mice (K14-ER:Ras) with a 4-hydroxytamoxifen (4OHT)-inducible Ras fusion construct driven by the keratin 14 promoter.
- Administered 4OHT to induce Ras activation in adult K14-ER:Ras mice.
- Analyzed Ras downstream effector pathway activation (Raf/MAPK, RalGDS/Ral, PI3K/Akt) and observed phenotypic changes in the epidermis.
Main Results:
- 4OHT-inducible Ras activation led to reversible, massive cutaneous hyperplasia (skin thickening) and suppressed epidermal differentiation.
- Ras activation specifically engaged the Raf/mitogen-activated protein kinase (MAPK) pathway, but not RalGDS/Ral or PI3K/Akt.
- Ras-driven hyperproliferation correlated with increased expression of beta1 and beta4 integrin epidermal progenitor markers.
- Inducible epidermal expression of Raf alone mimicked the hyperproliferation and differentiation suppression observed with Ras activation.
Conclusions:
- Activation of Ras in the adult epidermis promotes significant proliferation and inhibits normal differentiation.
- The Raf/MAPK pathway is sufficient to mediate these Ras-induced effects on epidermal homeostasis.
- These findings provide critical insights into Ras signaling in skin development and carcinogenesis.