Poster presentation, P122

Official XXIst International Pigment Cell Conference website - 21-24 Sept 2011, Bordeaux - France | updated: September 04 2011

Repeated ultraviolet exposure induces TLR4 expression of neonatal human melanocytes

Toll-like receptors (TLRs) play a critical role in cutaneous innate immunity. In invertebrates, a major aspect of the innate immune defense system against invading pathogens involves melanin. Recent studies showed that human melanocytes expressed TLR1, 2, 3, 4, 7 and 9, though their function is not yet established. TLR4 is thought to regulate UV-induced cutaneous immunosuppression. Studies have reported that in Langerhans cells, TLR4 is upregulated upon exposure to ultraviolet (UV) light. However, this phenomenon has not been studied in melanocytes. In the present study, we examine the effect of single or repeated UV exposure on TLR4 expression in human melanocytes and its relationship to pigmentation. Primary cultured neonatal human melanocytes were exposed to either a single dose of UVA (3 J/cm²) or UVB (300 mJ/cm²), or daily low dose UVA (1 J/cm2) or UVB (100 mJ/cm²) for 5 consecutive days. TLR4 expression was evaluated by flow cytometry and reverse transcription polymerase chain reaction (RT-PCR), and pigmentation was determined by melanin content assay. We found that TLR4 was not expressed in the resting state of neonatal human melanocytes. LPS stimulation increased both TLR4 and melanin content. A single dose of either UVA or UVB increased melanin content only. However, repeated exposure to either UVA or UVB increased both TLR4 and melanin content. The level of TLR4 expression from UV was lower than that of LPS. Our results suggest TLR4 is not expressed in resting neonatal human melanocytes, and its levels can be upregulated by repeated exposure to UV light. These findings differ from previous studies in that TLR4 is expressed in basal state adult human melanocytes. This study suggests human melanocytes may participate actively in UV-induced immune modulation, and UV-induced melanogenesis is not dependent on TLR4 expression.

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