Hairless and NFκB form a positive feedback loop after UVB and TNFα stimulation

Hairless and the polyamine putrescine form a negative regulatory loop in the epidermis

Hairless (HR) is a nuclear protein with corepressor activity that is highly expressed in the skin and hair follicle. Mutations in Hairless lead to hair loss accompanied by the appearance of papules (atrichia with papular lesions), and similar phenotypes appear when the key polyamine enzymes ornithine decarboxylase (ODC) and spermidine/spermine N(1) -acetyltransferase (SSAT) are overexpressed. Both ODC and SSAT transgenic mice have elevated epidermal levels of putrescine, leading us to investigate the mechanistic link between putrescine and HR. We show here that HR and putrescine form a negative regulatory network, as epidermal ODC expression is elevated when HR is decreased and vice versa. We also show that the regulation of ODC by HR is dependent on the MYC superfamily of proteins, in particular MYC, MXI1 and MXD3. Furthermore, we found that elevated levels of putrescine lead to decreased HR expression, but that the SSAT-TG phenotype is distinct from that found when HR is mutated. Transcriptional microarray analysis of putrescine-treated primary human keratinocytes demonstrated differential regulation of genes involved in protein-protein interactions, nucleotide binding and transcription factor activity, suggesting that the putrescine-HR negative regulatory loop may have a large impact on epidermal homeostasis and hair follicle cycling.

Loss of hairless confers susceptibility to UVB-induced tumorigenesis via disruption of NF-kappaB signaling

In order to model squamous cell carcinoma development in vivo, researchers have long preferred hairless mouse models such as SKH-1 mice that have traditionally been classified as ‘wild-type’ mice irrespective of the genetic factors underlying their hairless phenotype. The work presented here shows that mutations in the Hairless (Hr) gene not only result in the hairless phenotype of the SKH-1 and Hr^−/− mouse lines but also cause aberrant activation of NFκB and its downstream effectors. We show that in the epidermis, Hr is an early UVB response gene that regulates NFκB activation and thereby controls cellular responses to irradiation. Therefore, when Hr expression is decreased in Hr mutant animals there is a corresponding increase in NFκB activity that is augmented by UVB irradiation. This constitutive activation of NFκB in the Hr mutant epidermis leads to the stimulation a large variety of downstream effectors including the cell cycle regulators cyclin D1 and cyclin E, the anti-apoptosis protein Bcl-2, and the pro-inflammatory protein Cox-2. Therefore, Hr loss results in a state of uncontrolled epidermal proliferation that promotes tumor development, and Hr mutant mice should no longer be considered merely hairless 'wild-type' mice. Instead, Hr is a crucial UVB response gene and its loss creates a permissive environment that potentiates increased tumorigenesis.