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2013 Research Abstract

Interaction and Regulation of Ultraviolet B (UVB)-induced Cytokines in Human Keratinocytes

Investigator: Lisa Hisaw

Mentor: Jenny Kim, MD, PhD

Abstract: Keratinocytes are the major immunologically competent component of human skin, providing the initial barrier and response against environmental dangers such as ultraviolet B (UVB) irradiation. UVB-irradiated human keratinocytes have been shown to secrete potent inflammatory cytokines including IL-1β, IL-6, IL-8, and TNF-α. In this study, we investigate the innate immune response in human keratinocytes irradiated with UVB; specifically, we focus on how inflammasome-mediated cytokines IL-1β and IL-18 regulate IL-6 and IL-8 secretion.

Inflammasomes are self-assembling multi-protein complexes that are activated by various cell stressors. Activation of the inflammasome leads to activation of caspase-1, which processes inactive precursors pro-IL-1β and pro-IL-18 into active, mature IL-1β and IL-18, respectively, which are potent pro-inflammatory cytokines of the innate immune system. Immature, inactive pro-IL-1β and pro-IL-18 are constitutively present in the cell and can be cleaved into their active form by the inflammasome when cell stress signals are obtained. NLRP3 is the specific inflammasome that has been previously shown to be activated by UVB irradiation.

In our study, UVB induced significant IL-6 and IL-8 secretion in both immortalized human keratinocyte (HaCaT) cell line and normal human epidermal keratinocytes (NHEK) but not IL-1β, IL-18, or TNF-α. Because IL-1β and IL-18 are rapidly activated by the inflammasome when exposed to stress, they may be early signals that amplify pro-inflammatory cytokines like IL-6 and IL-8 even at low levels. Treatment of HaCaT cells with IL-1β alone induced IL-6 secretion while IL-18 did not. IL-1β had a synergistic effect on IL-6 production in UVB-irradiated HaCaT cells. Together, UVB and caspase-1 inhibitor diminished UVB-induced IL-6 secretion suggesting caspase-1 (involved in inflammasome activation) is involved in UVB-induced IL-6 secretion. These data suggest indirect involvement of the inflammasome pathway in UVB-induced IL-6 secretion in human keratinocytes. Treatment of HaCaT cells with IL-1β or IL-18 did not induce IL-8 and caspase-1 inhibitor did not diminish the level of UVB-induced IL-8.

We used qPCR to analyze Nod Like Receptor gene expression after exposing HaCaT and NHEK cells to varying UVB doses. Our data show significant dose-dependent up regulation of NLRP3 (UVB-induced inflammasome).

Our results suggest potential for cross-talk between inflammasome and non-inflammasome mediated cytokines in UVB-irradiated cells given modulation of IL-6 secretion with addition of exogenous IL-1β and inhibition of inflammasome-dependent caspases in UVB-irradiated cells. Better understanding of UVB-induced inflammasome activation and subsequent regulation of IL-6 in keratinocytes could lead to novel ways of manipulating the inflammatory response caused by UVB.

Last Updated: 4/18/14

Updated on April 18, 2014.