Retinoic Acid Potentiates TNF- -Induced ICAM-1 Expression in Normal Human Epidermal Keratinocytes Sophie Janssens,* Luc Bols,† Marc Vandermeeren,† Guy Daneels,† Marcel Borgers,† and Johan Geysen† †Department of Cell Biology and Developmental Genetics, Janssen Research Foundation, Beerse, Belgium; and *Department of Biology, University of Antwerp (UIA), Antwerp, Belgium Received December 23, 1998 ICAM-1 protein in keratinocytes is thought to con- tribute to cutaneous inflammatory reactions. Its in- duction depends—among others— on cytokines such as TNF-, IFN-, IL-1 or on retinoic acid (RA), a key regulator of epidermal homeostasis. We investigated the effect of treatments with TNF-, RA or their com- bination on ICAM-1 expression on proliferative or dif- ferentiating keratinocytes over an 8 day culture pe- riod. Basal ICAM-1 levels were undetectable at low (30 M) and standard (88 M) Ca 2 and RA alone did not induce ICAM-1. However, at high Ca 2 (1500 M), ICAM-1 levels were augmented in response to RA- treatment. TNF- induced a transient ICAM-1 increase in NHK, which reached peak-levels 2-4 days post cyto- kine stimulus. RA potentiated the TNF--induced ICAM-1 response in all Ca 2 -concentrations. This potentiating effect of RA was confirmed at the mRNA level. In summary, our results establish retinoic acid as an enhancer of TNF--induced ICAM-1 levels in NHK. © 1999 Academic Press Intercellular adhesion molecule-1 (ICAM-1), a cell surface glycoprotein of 80 –114 kDa which belongs to the immunoglobulin family (1), plays a prominent role in keratinocyte mediated immune reactions (reviewed in 2, 3). As ligand for lymphocyte-function associated antigen-1 (LFA-1) (4), which is expressed on the plasma membrane of leukocytes, ICAM-1 is critical for the interaction between leukocytes and keratinocytes (5) and thus for the migration of activated T cells from the vasculature to the epidermis during inflammation (2, 6). The ICAM-1/LFA-1 interaction has proven to be essential for the accessory cell function of the keratin- ocyte in the costimulation of helper T cells (7) and is necessary in CD8 + -cell mediated cytotoxicity (8). In normal, uninflamed skin, keratinocytes (NHK) show minimal basal ICAM-1 levels (5). However, in many cutaneous diseases, such as psoriasis (9), allergic con- tact dermatitis (10), delayed hypersensitivity reaction (11) or graft-vs-host disease (12), ICAM-1 levels are markedly upregulated and associated with an in- creased epidermal T cell infiltrate (10, 13). A variety of stimuli induce ICAM-1 expression in keratinocytes: pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-) (10), interferon-gamma (IFN-) (10, 14) or interleukin-1 (IL-1) (14) as well as many non-cytokine agents such as UV (15), substance P or histamine (3). In many tumor cell lines, retinoic acid (RA) is described as a potent inducer of ICAM-1 (16 –18). Retinoids exert many functions in skin through bind- ing to and activation of their nuclear receptors (19, 20). The predominant receptors in skin are RAR- and RXR- (21). Because of their inhibitory effects on ker- atinocyte differentiation (22) and on proliferation (9), retinoids are applied as therapeutic agents in many skin diseases (19, 20). Topical treatment with RA often results in erythema and inflammation (23). It is sug- gested that this can be partly explained by the induc- tion of ICAM-1 by RA (19). However, literature data about RA induction of ICAM-1 in keratinocytes are conflicting. Aoudjit et al. (17) and Kashihara-Sawami and Norris (24) did not observe any effect of RA on basal ICAM-1 expression, in contrast to earlier reports of Fisher et al. (25) and Barker et al. (6). RA is also able to modulate cytokine-induced ICAM-1-expression. Pre- vious investigations described the synergistic effect of RA on IFN--induced ICAM-1 expression in keratino- cytes (24, 26). To our knowledge, no reports have been published about the modulation by RA of TNF-- induced ICAM-1 in keratinocytes. In tumor cells, such as human thyroid carcinoma cells, tumor necrosis fac- tor increased ICAM-1 surface expression to a greater extent in combination with RA (18). In contrast, in human dermal microvascular endothelial cells, no ef- Abbreviations used: ICAM-1, intercellular adhesion molecule-1; IFN-, interferon-gamma; NHK, normal human keratinocytes; RA, all-trans-retinoic acid; TNF-, tumor necrosis factor-alpha. Biochemical and Biophysical Research Communications 255, 64 – 69 (1999) Article ID bbrc.1999.0147, available online at http://www.idealibrary.com on 64 0006-291X/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.