The caspase-1 inhibitor CARD18 is specifically expressed during late differentiation of keratinocytes and its expression is lost in lichen planus.

BACKGROUND: CARD18 contains a caspase recruitment domain (CARD) via which it binds to caspase-1 and thereby inhibits caspase-1-mediated activation of the pro-inflammatory cytokine interleukin (IL)-1ß. OBJECTIVES: To determine the expression profile and the role of CARD18 during differentiation of keratinocytes and to compare the expression of CARD18 in normal skin and in inflammatory skin diseases. METHODS: Human keratinocytes were induced to differentiate in monolayer and in 3D skin equivalent cultures. In some experiments, CARD18-specific siRNAs were used to knock down expression of CARD18. CARD18 mRNA levels were determined by quantitative real-time PCR, and CARD18 protein was detected by Western blot and immunofluorescence analyses. In situ expression was analyzed in skin biopsies obtained from healthy donors and patients with psoriasis and lichen planus. RESULTS: CARD18 mRNA was expressed in the epidermis at more than 100-fold higher levels than in any other human tissue. Within the epidermis, CARD18 was specifically expressed in the granular layer. In vitro CARD18 was strongly upregulated at both mRNA and protein levels in keratinocytes undergoing terminal differentiation. In skin equivalent cultures the expression of CARD18 was efficiently suppressed by siRNAs without impairing stratum corneum formation. Epidermal expression of CARD18 was increased after ultraviolet (UV)B irradiation of skin explants. In skin biopsies of patients with psoriasis no consistent regulation of CARD18 expression was observed, however, in lesional epidermis of patients with lichen planus, CARD18 expression was either greatly diminished or entirely absent whereas in non-lesional areas expression was comparable to normal skin. CONCLUSIONS: Our results identify CARD18 as a differentiation-associated keratinocyte protein that is altered in abundance by UV stress. Its downregulation in lichen planus indicates a potential role in inflammatory reactions of the epidermis in this disease.

Keratinocytes express functional CARD18, a negative regulator of inflammasome activation, and its altered expression in psoriasis may contribute to disease pathogenesis.

Caspase recruitment domain family member 18 (CARD18, Iceberg) is known as a negative regulatory molecule that inhibits inflammatory events by terminating inflammasome activation due to a direct interaction with pro-caspase-1. During the investigation of molecular mechanisms in keratinocytes that contribute to the pathogenesis of psoriasis, we found that CARD18 expression differs in healthy and psoriatic skin; moreover, CARD18 demonstrated altered response under inflammatory conditions in healthy and psoriatic skin. In healthy skin, low basal CARD18 expression was detected, which showed significant elevation in response to inflammatory stimuli (lymphokine treatment or mechanical injury). In contrast, higher basal expression was observed in psoriatic non-involved skin, but no further induction could be detected. We demonstrated that keratinocytes express CARD18 both at mRNA and protein levels and the expression increased in parallel with differentiation. The investigation of cellular inflammatory processes revealed that psoriasis-associated danger signals triggered the expression of inflammasome components (AIM2, Caspase-1) and CARD18 as well as IL-1ß production of keratinocytes. Furthermore, gene-specific silencing of CARD18 in cells treated with cytosolic DNA (poly(dA:dT)) resulted in increased IL-1ß secretion, suggesting a negative regulatory role for CARD18 in keratinocyte inflammatory signaling. The differential regulation of CARD18 in healthy and psoriatic uninvolved epidermis may contribute to the susceptibility of psoriasis. Furthermore, our in vitro results indicate that CARD18 may contribute to the fine tuning of keratinocyte innate immune processes.