The influence of UV irradiation on pigmentation is well established, but the molecular and cellular mechanisms controlling dendrite formation remain incompletely understood. MicroRNAs (miRNAs) are a class of small RNAs that participate in various cellular processes by suppressing the expression of target mRNAs. In this study, we investigated the expression of miRNAs in response to UVB irradiation using a microarray screen and then identified potential mRNA targets for differentially expressed miRNAs among the genes governing dendrite formation. We subsequently determined the ability of miRNA 340 (miR-340) to suppress the expression of RhoA, which is a predicted miR-340 target gene that regulates dendrite formation. The overexpression of miR-340 promoted dendrite formation and melanosome transport, and the downregulation of miR-340 inhibited UVBinduced dendrite formation and melanosome transport. Moreover, a luciferase reporter assay demonstrated direct targeting of RhoA by miR-340 in the immortalized human melanocyte cell line Pig1. In conclusion, this study has established an miRNA associated with UVB irradiation. The significant downregulation of RhoA protein and mRNA expression after UVB irradiation and the modulation of miR-340 expression suggest a key role for miR-340 in regulating UVB-induced dendrite formation and melanosome transport. Melanocytes originate from neural crest-derived melanoblasts, which migrate and differentiate in the basal layer of the epidermis (1). A hallmark of melanocytes is their ability to form dendrites, which are specialized cell structures that transport melanosomes to their tips for transfer to the surrounding keratinocytes in response to growth factors and UV irradiation. Following skin penetration by UV rays and subsequent DNA damage, thymidine dinucleotide fragments induce melanogenesis and cause the melanocyte to produce melanosomes (2).These melanosomes are then transferred to neighboring keratinocytes through the intricate network of melanocyte dendrites, contributing to skin darkening and thereby providing protection from UV radiation (3, 4). Melanocyte dendrites can vary markedly in length and number in response to different growth factors and, in a manner analogous to the way in which neural cells seek out target neurons, these dendrites form growth cone-like structures that attach to keratinocytes. Melanocyte-keratinocyte adhesion is a prerequisite for the transfer of melanosomes to keratinocytes; therefore, the formation of melanocyte dendrites, particularly of the appropriate length and number, is essential for efficient melanosome transfer. Due to the importance of dendricity for melanocyte activity, cutaneous pigmentation, and photoprotection, it is critical to determine the precise mechanisms involved in the regulation of melanocyte dendrite formation.It is well known that melanocytes are sensitive to UV irradiation, with substantial evidence suggesting that it plays a pivotal role in regulating melanocyte dendricity. Studies have shown that UV irradiation induce...

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