There is a vicious circle in psoriasis that disrupts epidermal homeostasis through alterations in keratinocytes (hyperproliferation, parakeratosis) and immunocytes (infiltration and activation). While it is well known that uncontrolled keratinocyte proliferation is largely driven by pro-inflammatory cytokines from the immunocytes, the functional role of keratinocytes in the recruitment and activation of immunocytes is poorly understood. We have discovered intriguing links between Trim32 (an E3 ubiquitin ligase), its substrate Piasy (an E3 SUMO ligase), and psoriasis. Trim32 is elevated in psoriasis tissue samples compared to non-lesional control epidermis. Trim32 negatively regulates the pro-apoptotic Piasy protein, a repressor of NF-kB, STAT, and SMAD transcription factors that have been implicated in the pathogenesis of psoriasis. The Piasy gene resides in the PSORS6 susceptibility locus on chromosome 19p13, although the significance of this remains to be determined. We have found that Trim32 activates and Piasy inhibits keratinocyte production of CCL20, a chemokine increased in psoriatic lesions that is a major factor in recruitment of dendritic cells and Th17 lymphocytes to the skin. The CCL20 induction by TNFa and IL17 cytokines is mediated through the activation of NF-kB. These findings lead us to hypothesize that Trim32 and Piasy are part of a positive feedback loop of CCL20 overproduction by keratinocytes and Th17 activation that contributes to the cycle of psoriasis. Initial evidence suggests that Trim32 is not simply a general marker of epidermal hyperplasia because its elevation in psoriasis, recognized as a Th17 disease, is not shared by atopic dermatitis, recognized as a Th2 cell disease, and because upregulation of CCL20 in keratinocytes responds to Th17 but not Th1 or Th2 cytokines. We propose to define the role of Trim32 and Piasy in psoriasis according to the following aims: 1) determine molecular pathways of Trim32 and Piasy regulation of CCL20 production in keratinocytes in response to Th17 cytokines, in particular through the NF-kB pathway, and evaluate the effects of Trim32 and Piasy on the dermal recruitment of CD11c+ dendritic cells and Th17 cells;2) explore the functional role of Trim32 and Piasy in keratinocyte survival and epidermal acanthosis in response to Th17 activation, using in vitro and in vivo approaches, and determine the impact of Trim32 KO and Piasy KO on the severity of phenotypes in two mouse models of psoriasiform dermatitis;and 3) evaluate the role of Trim32 and Piasy in CCL20 expression, inflammation and keratinocyte apoptosis in psoriasis and atopic dermatitis. Ultimately, these studies may impact our understanding of the molecular mechanisms of psoriasis as distinct from atopic dermatitis and lead to rational improvement of treatment strategies for psoriasis patients.
Psoriasis is a skin disease in which skin cells lose the proper balance between their growth and death rates, and certain immune system cells are over-active. We have found that the enzyme Trim32 is overly active in psoriasis, causing loss of the protein Piasy, a major inhibitor of certain genes that are active in human psoriasis and that can cause psoriasis-like symptoms in mice, leading to the production of proteins that attract more immune cells to the skin, making the symptoms worse. Understanding how Trim32 and Piasy control skin cell survival and skin inflammation promises new ways to treat psoriasis and other human diseases of the skin, as well as inflammatory diseases in other organs of the body.
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