Keratin 17 (K17) is a type I Intermediate filament which is highly expressed in psoriatic epidermis. Studies on K17 uncover several role for K17, including mechanical support, regulation of cell proliferation and differentiation, and apoptosis. We previously proposed a K17/T-cell/cytokine autoimmune loop in which K17 could be induced by inflammatory cytokine stimulation in psoriasis. Yet, little is known about the regulation of K17 expression via post-translational modification. Ubiquitination is one of the most important post-translational modifications that modulate the functions of proteins. UbiNet software predicated several ubiquitination sites on human K17. We sought to determine whether K17 underwent ubiquitination by dysregulated E3 ligase in psoriasis, which regulated the protein levels of K17 and contributed to the pathogenensis of psoriasis. To assess whether K17 is ubiquitinated in human keratinocytes, we employed human keratinocyte cell line HaCaT cells. Furthermore, we tested K17 ubiquitination levels in normal human keratinocyte and psoriatic keratinocytes by immunoprecipitation. Mass spectrometry was used to identify the E3 ligases of K17. We initially found that K17 is ubiquitinated in human keratinocytes, while with a higher level of ubiquitination observed in psoriatic keratinocytes. Moreover, immunoprecipitation coupled with mass spectrometry identified the candidate E3 ligase for K17, Trim21. We further confirmed the direct interaction between Trim21 and K17 by immunoprecipitation and immunofluorescence in HaCaT cells. Importantly, Trim21 mRNA and protein levels were significantly elevated in psoriatic epidermis comparing with normal epidermis, as assessed by RT-PCR, western blotting, immunofluorescence and immunohistochemestry. Overexpression of Trim21 led to activation of K17, but not degradation, which can be partly illustrated by our finding that K17 is ubiquitinated by Trim21 via K63-linkage. Taken together, our data suggest a previously unrecognized role for ubiquitination in regulation protein level of K17 in psoriasis, further indicating a novel mechanism in the pathogenesis of psoriasis. Trim21, which regulated K17 activation by ubiquitination, provides a potential target for interrupting K17 activation in the treatment of psoriasis.

Keratin 17 is ubiquitinated by Trim21 in human keratinocytes in psoriasis