cis-Urocanic Acid Stimulates Primary Human Keratinocytes Independently of Serotonin or Platelet-Activating Factor Receptors

PD-L1 Enhanced by cis-Urocanic Acid on Langerhans Cells Inhibits Vγ4+ γδT17 Cells in Psoriatic Skin Inflammation

Psoriasis is an IL-23/IL-17-mediated inflammatory autoimmune dermatosis and ultraviolet B (UVB) may contribute to immunosuppression and ameliorate associated symptoms. One of the pathophysiology underlying UVB therapy is through the production of cis-urocanic acid (cis-UCA) from keratinocytes. However, the detailed mechanism is yet to be fully understood. In the current study, we found filaggrin expression and serum cis-UCA levels were significantly lower in psoriasis patients than in healthy controls. We also noted that cis-UCA application inhibited psoriasiform inflammation through the reduction of Vγ4⁺ γδT17 cells in murine skin and draining lymph nodes. Meanwhile, CCR6 was down-regulated on γδT17 cells, which would suppress the inflammatory reaction at a distal skin site. We revealed that 5-HT_2A receptor (HTR2A), the known cis-UCA receptor, was highly expressed on Langerhans cells (LCs) in the skin. cis-UCA also inhibited IL-23 expression and induced PD-L1 on LCs, leading to the attenuated proliferation and migration of γδT cells. Compared to the isotype control, α-PD-L1 treatment in vivo could reverse the anti-psoriatic effects of cis-UCA. PD-L1 expression on LCs was sustained through cis-UCA-induced MAPK/ERK pathway. These findings uncover the cis-UCA-induced PD-L1-mediated immunosuppression on LCs, which facilitates the resolution of inflammatory dermatoses.

Serotonin and 5-HT3 receptors sensitize human skin cells to direct irradiation cell death but not to soluble radiation-induced bystander signals

Ionizing radiation (IR) is an environmental carcinogen and the biological damages it elicits are mechanistically distinct between high and low doses. Non-targeted effects occurring in nonirradiated cells such as the radiation-induced bystander effect predominate at low doses of IR. However, the role of non-targeted effects in environmental radiation protection is often overlooked because the governing mechanisms are complex and multifactorial. An improved understanding of the signaling molecules and their capacity to sensitize specific cell types are essential in establishing environmental IR risks. In particular, serotonin (5-HT) has been identified to exacerbate both direct irradiation and bystander-induced cell death (CD) in certain cell types, although not all cell types are responsive to 5-HT in this respect. In this study, we further characterize the role of 5-HT and 5-HT receptors (5-HTR) in the amplification of CD following IR exposure in human keratinocytes. We examined the survival of HaCaT cells treated with 5-HT and the 5-HTR antagonists ketanserin (5-HT_2A) and ondansetron (5-HT₃) following exposure to direct IR and irradiated cell condition medium (ICCM). Nonirradiated cell survival was consistent with the vehicle control among 5-HT concentrations ranging from 0.001 to 100 μM. Significant 5-HT concentration-dependent increases in CD occurred following direct IR exposure. Nonirradiated ICCM-recipient CD was not altered by 5-HT (0.001-100 μM) when present during donor cell irradiation among all IR doses. Increases in direct irradiation CD evoked by 5-HT were significantly attenuated by ondansetron, blocking the effect of 5-HT, whereas ketanserin did not alter CD. Western blotting of these target 5-HTRs revealed protein expression of the 5-HT₃ receptor, while the 5-HT_2A receptor was not detected. We have demonstrated a definitive role for 5-HT in the exacerbation of CD following direct IR exposure and identified the 5-HT₃ receptor as a potential target for ameliorating radiation damage in keratinocytes.

Involvement of TRPV4 in Serotonin-Evoked Scratching

Several thermo-sensitive TRP channels (TRPV1, -3; TRPA1) have been implicated in itch. In contrast, the role of transient receptor potential vanilloid type-4 (TRPV4) in itch is unknown. Therefore, we investigated if TRPV4, a temperature-sensitive cation channel, plays an important role in acute itch in mice. Four different pruritogens including serotonin (5-hydroxytrytamine, 5-HT), histamine, SLIGRL (PAR2/MrgprC11 agonist) and chloroquine (MrgprA3 agonist) were intradermally injected and itch-related scratching behavior was assessed. TRPV4 knockout (TRPV4KO) mice exhibited significantly fewer 5-HT-evoked scratching bouts compared to wild-type (WT) mice. Notably, no differences between TRPV4KO and WT mice were observed in the number of scratch bouts elicited by SLIGRL and histamine. Pretreatment with a TRPV4 antagonist significantly attenuated 5-HT-evoked scratching in vivo. Using calcium imaging in cultured primary murine dorsal root ganglion (DRG) neurons, the response of neurons after 5-HT application, but not other pruritogens, was significantly lower in TRPV4KO compared to WT mice. A TRPV4 antagonist significantly suppressed 5-HT-evoked responses in DRG cells from WT mice. Approximately 90% of 5-HT-sensitive DRG neurons were immunoreactive for an antibody to TRPV4, as assessed by calcium imaging. These results indicate that serotonin-induced itch is linked to TRPV4.

Nutritional abrogation of photoimmunosuppression: in vivo investigations

Skin cancer is a major public health concern, and the primary aetiological factor in the majority of skin cancers is ultraviolet radiation (UVR) exposure. UVR not only induces potentially mutagenic DNA damage but also suppresses cell-mediated immunity (CMI), allowing cancerous cells to escape destruction and progress to tumours. A considerable proportion of an individual's annual sun exposure is obtained outside the vacation period when topical and physical measures for photoprotection are irregularly used. Certain nutrients could provide an adjunctive protective role, and evidence is accruing from experimental studies to support their use in abrogation of photoimmunosuppression. Moreover, developments in clinical research methods to evaluate impact of solar-simulated radiation on cutaneous CMI allow the immune protective potential of nutritional agents to be examined in humans in vivo. This article summarises the mediation of CMI and its suppression by UVR, evaluates the methodology for quantitative assessment in vivo, reviews the human studies reported on nutritional abrogation of photoimmunosuppression including recent randomized controlled trials and discusses the mechanisms of photoprotection by the nutrients. This includes, in addition to antioxidants, novel studies of omega-3 polyunsaturated fatty acids and nicotinamide.

cis-Urocanic acid enhances prostaglandin E2 release and apoptotic cell death via reactive oxygen species in human keratinocytes

Urocanic acid (UCA) is a major UVR-absorbing skin molecule that undergoes trans to cis photoisomerization in the epidermis following UVR exposure. Murine studies have established that cis-UCA is an important mediator of UVR-induced immune suppression, but little is known about its signaling pathway. We have previously demonstrated that treatment of normal human epidermal keratinocytes with cis-UCA resulted in increased synthesis of prostaglandin E(2) (PGE(2)) and cell death. Here, using immortalized human keratinocytes, we report that cis-UCA but not trans-UCA generates reactive oxygen species (ROS) in a dose-dependent manner and that the natural antioxidant α-tocopherol can reduce this ROS generation, subsequent PGE(2) release, and apoptotic cell death. Western blot analysis revealed that cis-UCA leads to a transient phosphorylation of EGFR as well as downstream mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinase (ERK) and p38. Pharmacological inhibition of their activity attenuated PGE(2) release induced by cis-UCA. After transient activation, cis-UCA downregulated EGFR protein expression that corresponded to activation of caspase-3. In addition, pretreatment with α-tocopherol inhibited EGFR downregulation and caspase-3 activation induced by cis-UCA. These results suggest that cis-UCA exerts its effects on human keratinocytes via intracellular ROS generation that modulates EGFR signaling and subsequently induces PGE(2) synthesis and apoptotic cell death.

Control of immune response by amino acid metabolism

The interaction between pathogenic microorganisms and their hosts is regulated by reciprocal survival strategies, including competition for essential nutrients. Though paradoxical, mammalian hosts have learned to take advantage of amino acid catabolism for controlling pathogen invasion and, at the same time, regulating their own immune responses. In this way, ancient catabolic enzymes have acquired novel functions and evolved into new structures with highly specialized functions, which go beyond the struggle for survival. In this review, we analyze the evidence supporting a critical role for the metabolism of various amino acids in regulating different steps of both innate and adaptive immunity.

Protodynamic intracellular acidification by cis-urocanic acid promotes apoptosis of melanoma cells in vitro and in vivo

The extracellular tumor microenvironment is acidified, whereas the intracellular pH of tumor and stromal cells is neutral. cis-Urocanic acid (cis-UCA), an endogenous compound of the skin, can acidify the cytosol by transporting protons into the cells. This phenomenon, termed the protodynamic concept, was studied here in human cancer cells. cis-UCA dose-dependently reduced the number of viable human melanoma, cervical carcinoma, and fibrosarcoma cells at weakly acidic extracellular pH. The intracellular pH decreased by up to 0.5 pH units in a concentration-dependent manner with 0.3-30  m cis-UCA at extracellular pH 6.5 but not at pH 7.4. Under the same conditions, 30  mM cis-UCA induced annexin-V binding and activation of caspase-3 in A2058 melanoma cells as signs of apoptotic cell death. Finally, growth of human melanoma xenografts in SCID mice was suppressed by 60% following intratumoral injection of cis-UCA. Accordingly, the percentage of tumor necrosis and active caspase-3-immunopositive cells increased, whereas proliferation activity decreased. These results identify cis-UCA as an anticancer agent inhibiting melanoma growth by immediate intracellular acidification followed by apoptotic cell death in vivo.