IKKalpha, a critical regulator of epidermal differentiation and a suppressor of skin cancer

Abstract

IkappaB kinase alpha (IKKalpha), one of the two catalytic subunits of the IKK complex involved in nuclear factor kappaB (NF-kappaB) activation, also functions as a molecular switch that controls epidermal differentiation. This unexpected function requires IKKalpha nuclear translocation but does not depend on its kinase activity, and is independent of NF-kappaB signalling. Ikkalpha(-/-) mice present with a hyperproliferative and undifferentiated epidermis characterized by complete absence of a granular layer and stratum corneum. Ikkalpha-deficient keratinocytes do not express terminal differentiation markers and continue to proliferate even when subjected to differentiation-inducing stimuli. This antiproliferative function of IKKalpha is also important for the suppression of squamous cell carcinogenesis. The exact mechanisms by which nuclear IKKalpha controls keratinocyte proliferation and differentiation remained mysterious for some time. Recent studies, however, have revealed that IKKalpha is a major cofactor in a TGFbeta-Smad2/3 signalling pathway that is Smad4 independent. This pathway controls cell cycle withdrawal during keratinocyte terminal differentiation. Although these are not the only functions of nuclear IKKalpha, this multifunctional protein is a key regulator of keratinocyte and epidermal differentiation and a critical suppressor of skin cancer.

Figure 1

Macroscopical presentation of WT and Ikkα^–/– mice.

Figure 2

(A) Haematoxylin and eosin staining of skin sections from WT and Ikkα^–/– mice. (B) Schematic representation of normal and Ikkα-deficient epidermis with expression profile of molecular markers of proliferation and differentiation (BL: basal layer; SP: spinous layer; GR: granular layer; SC: stratum corneum; hf: hair follicle; de: dermis; magnification × 100 in (A)).

Figure 3

Schematic representation of epidermal development stages. We propose that the main function of IKKα is to induce cell cycle exit of intermediate keratinocytes when they mature into spinous cells.

Figure 4

TGFβ-related signalling pathway controlling Myc activity in keratinocytes. The IKKα–Smad2/3 axis induces Mad1 and Ovol 1 expression on TGFβ stimulation. These proteins may inhibit the activity and expression of Myc, inducing in turn keratinocyte cycle exit and differentiation. Interestingly, a TGFβ–Smad3/4 signalling pathway, which is not associated with IKKα, but functions in cooperation with E2F4/5 transcription factors, has also been shown to negatively control c-Myc expression in keratinocytes (Chen et al, 2002).

Figure 5

Mad1 expression is not induced by kDIF-mediated keratinocyte differentiation. Conditioned medium from WT keratinocytes, which contains kDIF as shown earlier (Hu et al, 2001), failed to induce Mad1 expression in Ikkα^–/– keratinocytes while leading to keratinocyte differentiation as indicated by filaggrin expression. Only the re-expression of IKKα in Ikkα^–/– keratinocytes infected with adenovirus encoding this protein (Ad-IKKα) induces Mad1 expression.

Figure 6

(A) In the canonical TGFβ signalling pathway, ligands signal through type I and II transmembrane protein kinase receptors. After TGFβ binding, type II receptor recruits and phosphorylates type I receptor, which in turn activates R-Smads. Phosphorylated R-Smads oligomerize with the co-Smad Smad4 and accumulate in the nucleus where they interact with DNA and transcription factors to regulate the expression of target genes. (B) During epidermal terminal differentiation, TGFβ1 stimulation of keratinocytes induces the formation of a complex between activated Smad2/3 and IKKα. This complex accumulates in keratinocyte nuclei independently of the presence of Smad4 and controls the transcriptional activation of Mad1 and Ovol1 probably with the cooperation of other transcription factors such as IRF6. The Smad4-independent TGFβ–Smad2/3–IKKα axis is required for cell cycle exit and induction of terminal differentiation of keratinocytes.