Hairless and NFκB Form a Positive Feedback Loop After UVB and TNFα Stimulation^†

Abstract

Hairless (HR) is a nuclear protein with corepressor activity whose exact function in the skin remains to be determined. Mutations in both human and mouse Hairless lead to hair loss accompanied by the appearance of papules, a disorder called atrichia with papular lesions. Furthermore, mice with mutations in HR are known to have a higher susceptibility to ultraviolet radiation-induced tumorigenesis, suggesting that HR plays a crucial role in the epidermal UVB response. Using normal human keratinocytes (NHKs) and keratinocytes containing a mutation in HR, we found that HR is an early UVB response gene that negatively regulates NFκB mRNA expression. HR mutant keratinocytes have a dysregulated UVB response that includes increased proliferation and the aberrant activation of NFκB effector genes. Additionally, we show that another UVB response gene, TNFα, negatively regulates HR mRNA expression. TNFα-induced negative regulation of HR occurs through a direct interaction of the p65 subunit with a single NFκB-binding domain located in the HR promoter region. Therefore, we show for the first time that HR and NFκB participate in a positive feedback loop that can be initiated either by UVB or TNFα.

Loss of Hairless Confers Susceptibility to UVB-Induced Tumorigenesis via Disruption of NF-kappaB Signaling

“…NFκB has long been known to play a role in UVB induced tumorigenesis and forms a nexus of proliferative and apoptotic pathways; additionally, our previous work shows that HR regulates NFκB in vitro [29]. We therefore theorized that Hr and NFκB may be mechanistically related in a similar fashion in vivo .…”

“…Both Hr −/− and SKH-1 animals exhibited significantly lower levels of Hr mRNA that equaled an ∼70% decrease in expression (Figure 1). Our previous work showed that Hr is expressed in suprabasal kerationcytes [19], [20] and is involved in the early UV response mechanism in primary human kerationcytes [29]. Therefore, we decided to focus our future experiments on the epidermis.…”

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

“…We also examined ODC expression in the converse situation when HR expression is significantly decreased due to a naturally occurring 22‐bp deletion in exon (hereafter referred to as HR keratinocytes) . As expected from the overexpression of HR , ODC expression in HR keratinocytes is significantly upregulated by approximately 2.5 times as compared to NHKs (Fig.…”

“…However, the HR mutant animals exhibit a coordinated loss of hair indicating an active process is at work while the SSAT‐TG mutants abnormally retain their club hair fibres and only shed them after mechanical stimulation, which accounts for the apparent random and individualized hair loss in the SSAT‐TG population. HR mutations are primarily considered to lead to dysregulated apoptosis and proliferation , and the downregulation of the HR undoubtedly accounts for the defective club hair formation seen in the SSAT‐TG animals. However, we hypothesize that the lack of active hair shedding is due to one of the many other pathways that putrescine effects.…”

“…Unless otherwise indicated, all cell culture supplies are from Life Technologies Grand Island, NY, USA. Normal human keratinocytes (NHKs) from human foreskin and primary keratinocytes from an APL patient were cultured on mitomycin‐C (2 μ g/ml for 2 h, Sigma‐Aldrich, St. Louis, MO, USA)‐treated 3T3‐J2 feeder layers .…”