Decreased human epidermal antigen-presenting cell activity after ultraviolet A exposure: dose-response effects and protection by sunscreens.

BACKGROUND: Ultraviolet (UV) exposure of human skin causes immunosuppression that contributes to the growth of skin cancer. The contribution of UVA in these processes is still a matter of debate. OBJECTIVES: The purpose of our study was first to find a dose-response effect of UVA exposure on human epidermal antigen-presenting cell (APC) activity and to evaluate the protective capacity of two sunscreen formulations against a high level of acute UVA exposure. We also tried to evaluate the protective capacity afforded by the same sunscreens against UVA-induced clinical changes such as redness and pigmentation. METHODS: The functional assessment of the alloantigen-presenting capacity of epidermal cells prepared from skin keratotome samples 3 days after UVA exposure was measured with a mixed epidermal cell-lymphocyte reaction (MECLR) in each healthy volunteer (n = 16). Redness and pigmentation were assessed by chromametry 24 h after exposure to a single UVA dose. RESULTS: In vivo UVA exposure to 15, 30 and 60 J cm(-2) resulted in a dose-dependent decrease in purified allogeneic T cell (CD4+ T cells) proliferation induced by UVA-irradiated epidermal cells. The epidermal APC function was significantly decreased with a suberythemal exposure corresponding to 15 J cm(-2). The decrease, partial and not statistically different between 30 and 60 J cm(-2), exhibits a plateau-response effect. There was no correlation between the decrease of the epidermal APC function and the intensity of erythema and persistent pigment darkening. Both sunscreen formulations strongly inhibited the UVA-induced reduction of MECLR at 90 J cm(-2). CONCLUSION: Our results clearly demonstrate that UVA impairs the APC activity of the epidermal cells and thus may contribute to UV-induced immunosuppression in humans. They also indicate that erythema and immunosuppression have different dose-response curves in the UVA range. The two sunscreen formulations afforded a significant protection against the decrease in epidermal APC activity induced by exposure to a high UVA dose (90 J cm(-2)).

The third intracellular loop of the rat and mouse cholecystokinin-A receptors is responsible for different patterns of gene activation.

It has previously been reported that the cholecystokinin analog JMV-180 behaves differently on the rat and the mouse cholecystokinin-A receptor (CCK-AR). In mice this analog acts as an agonist on low- and high-affinity sites of the CCK-AR, whereas in rats this compound acts as an agonist on high-affinity sites and as an antagonist on low-affinity sites. In an attempt to understand why the same compound behaves differently on these two CCK-A receptors, we cloned the cDNA encoding the mouse CCK-AR. We then investigated a cellular model able to mimic the effect that was observed in rats and mice. HeLa cells were transiently cotransfected with plasmids leading to expression of the rat or mouse CCK-AR in the presence of pFos-Luc as reporter plasmid; such a plasmid placed the regulatory part of the human c-Fos gene upstream from the firefly luciferase structural gene (Luc). We then observed that the two CCK-A receptors behaved differently, not only in the presence of compound JMV-180 but also in the presence of cholecystokinin or even in absence of ligand; the rat CCK-AR was 2 to 3 times more potent than the mouse CCK-AR in inducing the reporter protein, whatever the ligand studied. This result was confirmed using the same kind of experiment with the reporter plasmid p(TRE)(3)-tk-Luc. Using various mutated receptors, we investigated the role of the putative third intracellular loop. We concluded that both the primary structure of the receptor and the cellular context are in part responsible for the differential behavior of these CCK-A receptors.

Melanin-concentrating hormone binding sites in human SVK14 keratinocytes.

Melanin concentrating hormone (MCH) is a cyclic peptide which regulates a broad array of functions in the mammalian brain and it may act as a paracrine factor in peripheral organs. In these studies a radiolabeled MCH derivative, the [125I]-[Phe13, Tyr19]-MCH, was synthesized and used as a tracer to perform binding experiments. A number of human or rodent cell lines displayed specific binding with [125I]-[Phe13, Tyr19]-MCH, the highest binding capacity being observed with human SVK14 keratinocytes. Saturation binding analysis with SVK14 cells indicated about 10,000 MCH binding sites per cell and a Kd of 0.7 nM for [125I]-[Phe13, Tyr19]-MCH. Surprisingly, the iodinated [Phe13, Tyr19]-MCH displayed about 10-fold higher affinity (Ki approximately 3.0 nM) for the putative MCH receptor than the noniodinated form (Ki approximately 25-30 nM). Competition binding analyses comparing various MCH-related peptides revealed a similar low binding potency for all these peptides (Ki approximately 65-160 nM). Strikingly, rat ANP and rat/human CNP but not rat BNP displaced [125I]-[Phe13, Tyr15]-MCH with Ki approximately 210-365 nM and may be due to topological similarities instead of partial sequence identities between MCH and some of the natriuretic peptides. However, other peptides such as CRF, alpha MSH, Arg-vasopressin, and MGOP-peptide I did not compete with the radioligand. Finally, the molecular mass of the MCH binding sites on SVK14 cells was estimated to be 47 kDa by crosslinking and SDS-PAGE experiments. Taken together, our data revealed the widespread expression of MCH binding sites on mammalian cells, particularly on skin carcinoma cells. However, the low affinity of these sites for the native MCH and MCH-related peptides as well as competitivity with ANP and CNP indicates that further biochemical and functional characterizations are needed to validate them as genuine physiological MCH receptors.