Xanthohumol inhibits IL-12 production and reduces chronic allergic contact dermatitis

Abstract

Xanthohumol (XN) and its related compounds were evaluated for their effects on modulating the production of interleukin (IL)-12, the most important factor driving T helper 1 immune responses. XN showed the strongest inhibitory effect on IL-12 production in macrophages stimulated by lipopolysaccharide (LPS) or LPS/interferon-γ. Xanthohumol 4′-O-β-D-glucopyranoside (XNG) inhibited IL-12 production less effectively than XN. Isoxanthohumol and 8-prenylnaringenin showed comparatively lower inhibitory effects on IL-12 production than XNG. (2S)-5-methoxy-8-prenylnaringenin 7-O-β-D-glucopyranoside did not exert any effect on IL-12 production. We then tested how these compounds affected NF-κB binding activity to the κB site in the nucleus. The compounds inhibited κB binding in macrophages with the same potency order as IL-12 inhibition. Furthermore, we investigated whether XN, which showed the most effective reduction of IL-12 production, attenuated skin inflammation. Chronic allergic contact dermatitis, an experimental model for psoriasis, was used to determine the anti-inflammatory effects of XN in vivo. XN treatment reduced the degree of ear thickening induced by oxazolone. Taken together, XN might be effective as an anti-inflammatory agent to reduce skin inflammation by inhibiting IL-12 production.

Introduction

IL-12 is a heterodimeric cytokine composed of two disulfide-linked subunits of 35 (p35) and 40 (p40) kDa encoded by two separate genes [1]. The expression of p35 mRNA is ubiquitous in many cell types, but the relatively restricted expression of p40 subunits limits potential IL-12-producing cells to phagocytic cells and antigen presenting cells [2], [3], [4]. Binding of IL-12 to IL-12Rβ1/IL-12Rβ2 phosphorylates receptor-associated tyrosine kinases, Jak2 and Tyk2, and subsequently activates signal transducer and activator of transcription (STAT) molecules to promote differentiation into the T helper (Th) 1 immune system [5], [6], [7]. The differentiation of naïve CD4⁺ Th cells into Th1 or Th2 effector cells is a critical process in regulating immune responses. The imbalance between Th1 and Th2 is closely correlated with the outcome of many diseases. Excess Th1 responses predominate in organ-specific autoimmune diseases, acute allograft rejection, recurrent abortions, and inflammatory disorders [8], [9], [10], [11], [12], [13], [14], [15], [16].

Xanthohumol (XN) is a major prenylated chalcone found in hops (Humulus lupulus, Cannabaceae), which is used to add bitterness and flavor to beer during the brewing process [17], [18]. XN has an exceptionally broad spectrum of inhibitory activity in carcinogenesis during the initiation, promotion, and progression phases [19]. At the initiation stage of carcinogenesis, XN suppresses cytochrome P450 activity, which is involved in the metabolic activation of carcinogens and induces NAD(P)H:quinone reductase activity, which regulates detoxification [20], [21], [22]. As a repressive tumor-promoting mechanism, XN exerts anti-inflammatory properties by inhibiting the activity of cyclooxygenase (COX)-1 and COX-2 as well as the production of nitric oxide [19], [23]. In the progression phase of carcinogenesis, XN prevents proliferation of tumor cells through inhibiting DNA synthesis and inducing cell cycle arrest in the S phase, followed by terminal cell differentiation or apoptosis [19], [24]. XN also inhibits angiogenesis, which is essential for solid tumor growth and dissemination [25]. Moreover, XN inhibits HIV, diacylglycerol acyltransferase, and bone resorption [26], [27], [28]. Recently, we reported that XN regulates IL-1 and tumour necrosis factor-alpha (TNF-α) production [29]. However, the in vivo immunoregulatory effects of XN are not known.

We investigated the effects of XN on IL-12 production in LPS/IFN-γ or LPS-stimulated macrophages and the regulatory mechanism underlying this effect. Moreover, we compared IL-12 regulatory effect of XN with its related compounds. Considering the pivotal role of IL-12 in Th1 dominant immune responses, the application of XN on Th1 cell-mediated inflammatory disease in skin was examined.