Rhamnazin suppresses melanosome transport by promoting the ubiquitin-mediated proteasomal degradation of melanophilin.

BACKGROUND: Melanosomes are intracellularly transported from the perinuclear region to the cell periphery and then to neighboring keratinocytes. We recently reported that the flavonoid rhamnazin suppresses melanosomal transport within pigment cells, yet the action mechanism remained unclear. OBJECTIVE: Our aim was to elucidate how rhamnazin influences the intracellular transport of melanosomes. METHODS: A melanosome distribution assay and immunostaining were performed using B16F10 mouse melanoma cells and normal human epidermal melanocytes, respectively. Expression levels of melanosome transport-related proteins, including melanophilin (MLPH), RAB27A, and myosin VA (MYO5A), were analyzed by immunoblotting. Ubiquitinated MLPH was detected using a commercial ubiquitin detection kit. To investigate the interaction between rhamnazin and MLPH, we prepared rhamnazin conjugated with magnetic FG beads. RESULTS: Immunoblotting analysis revealed that rhamnazin specifically reduces the expression of MLPH but not RAB27A or MYO5A proteins. The ubiquitin detection assay, which made use of a proteasome inhibitor, showed that MLPH accumulated as a polyubiquitinated protein after treatment with rhamnazin. We speculated that the affinity of rhamnazin for the components of the melanosome transport-related tripartite complex may alter the stability of the formation of the tripartite assembly. By using affinity-based techniques with B16F10 whole cell lysates or recombinant MLPH and RAB27A proteins, we revealed the interaction of rhamnazin with the components of the tripartite complex. CONCLUSION: We found that rhamnazin inhibits intracellular transport of melanosomes through proteasomal degradation of MLPH. Our results suggest that topical application of rhamnazin may provide a new approach for treating skin pigmentation disorders.

Alternate expression of PEPT1 and PEPT2 in epidermal differentiation is required for NOD2 immune responses by bacteria-derived muramyl dipeptide.

Peptide transporters 1 and 2 (PEPT1 and PEPT2) are proton-coupled oligopeptide transporter members of the solute carrier 15 family and play a role in the cellular uptake of di/tri-peptides and peptidomimetics. Our previous work showed that PEPT2 is predominantly expressed within undifferentiated keratinocytes. Here we show that PEPT2 expression decreases as keratinocyte differentiation progresses and that PEPT1 alternately is expressed at later stages. Absolute quantification using quantitative polymerase chain reaction revealed that the expression level of PEPT1 is about 17 times greater than that of PEPT2. Immunohistochemical study of human skin provided evidence of PEPT1 in the epidermis. The uptake of glycylsarcosine into keratinocytes was significantly blocked by PEPT inhibitors, including nateglinide and glibenclamide. Moreover, we found that PEPT1 knockdown in differentiated keratinocytes significantly suppressed the influence of a bacterial-derived peptide, muramyl dipeptide (MDP), on the production of proinflammatory cytokine interleukin-8, implying that bacteria-derived oligopeptides can be transported by PEPT1 in advanced differentiated keratinocytes. Taken together, PEPT1 and PEPT2 may concertedly play an important role in MDP-NOD2 signaling in the epidermis, which provides new insight into the mechanisms of skin homeostasis against microbial pathogens.

Bifunctional effects of O-methylated flavones from Scutellaria baicalensis Georgi on melanocytes: Inhibition of melanin production and intracellular melanosome transport.

The growing interest in skin lightening has recently renewed attention on the esthetic applications of Chinese herbal medicine. Although Scutellaria baicalensis Georgi is used for antipyretic and antiinflammatory purposes, its whitening effect remains unclear. This study reports three major findings: (1) S. baicalensis has a potent inhibitory effect on melanogenesis; (2) wogonin and its glycoside are the active components of S. baicalensis; and (3) O-methylated flavones from S. baicalensis, such as wogonin, inhibit intracellular melanosome transport. Using a melanin quantification assay, we showed that S. baicalensis potently inhibits melanogenesis in B16F10 cells. Componential analyses revealed that the main components of S. baicalensis are baicalin, wogonoside, baicalein, wogonin, and oroxylin A. Among these five flavones, wogonin and wogonoside consistently inhibited melanogenesis in both B16F10 melanoma cells and primary melanocytes. Wogonin exhibited the strongest inhibition of melanin production and markedly lightened the color of skin equivalents. We identified microphthalmia-associated transcription factor and tyrosinase-related proteins as potential targets of wogonin- and wogonoside-induced melanogenesis suppression. In culture, we found that the melanosomes in wogonin-treated B16F10 cells were localized to the perinuclear region. Immunoblotting analyses revealed that wogonin significantly reduced in melanophilin protein, which is required for actin-based melanosome transport. Other actin-based melanosome transport-related molecules, i.e., Rab27A and myosin Va, were not affected by wogonin. Cotreatment with MG132 blocked the wogonin-induced decrease in melanophilin, suggesting that wogonin promotes the proteolytic degradation of melanophilin via the calpain/proteasomal pathway. We determined that the structural specificities of the mono-O-methyl group in the flavone A-ring and the aglycone form were responsible for reducing melanosome transport. Furthermore, wogonin and two wogonin analogs, mono-O-methyl flavones, strongly suppressed melanosome transport. Our findings suggest the applicability of S. baicalensis in the esthetic field. Thus, we propose a novel pharmacologic approach for the treatment of hyperpigmentation.

H(+)/peptide transporter (PEPT2) is expressed in human epidermal keratinocytes and is involved in skin oligopeptide transport.

Peptide transporter 2 (PEPT2) is a member of the proton-coupled oligopeptide transporter family, which mediates the cellular uptake of oligopeptides and peptide-like drugs. Although PEPT2 is expressed in many tissues, its expression in epidermal keratinocytes remains unclear. We investigated PEPT2 expression profile and functional activity in keratinocytes. We confirmed PEPT2 mRNA expression in three keratinocyte lines (normal human epidermal keratinocytes (NHEKs), immortalized keratinocytes, and malignant keratinocytes) by reverse transcription-polymerase chain reaction (RT-PCR) and quantitative real-time RT-PCR. In contrast to PEPT1, PEPT2 expression in the three keratinocytes was similar or higher than that in HepG2 cells, used as PEPT2-positive cells. Immunolocalization analysis using human skin showed epidermal PEPT2 localization. We studied keratinocyte transport function by measuring the oligopeptide content using liquid chromatography/tandem mass spectrometry. Glycylsarcosine uptake in NHEKs was pH-dependent, suggesting that keratinocytes could absorb small peptides in the presence of an inward H(+) gradient. We also performed a skin-permeability test of several oligopeptides using skin substitute, suggesting that di- and tripeptides pass actively through the epidermis. In conclusion, PEPT2 is expressed in keratinocytes and involved in skin oligopeptide uptake.