Modulation of murine melanocyte function invitro by agouti signal protein

Abstract

Molecular and biochemical mechanisms that switch melanocytes between the production of eumelanin or pheomelanin involve the opposing action of two intercellular signaling molecules, α‐melanocyte‐stimulating hormone (MSH) and agouti signal protein (ASP). In this study, we have characterized the physiological effects of ASP on eumelanogenic melanocytes in culture. Following exposure of black melan‐a murine melanocytes to purified recombinant ASP in vitro, pigmentation was markedly inhibited and the production of eumelanosomes was decreased significantly. Melanosomes that were produced became pheomelanosome‐like in structure, and chemical analysis showed that eumelanin production was significantly decreased. Melanocytes treated with ASP also exhibited time‐ and dose‐dependent decreases in melanogenic gene expression, including those encoding tyrosinase and tyrosinase‐related proteins 1 and 2. Conversely, melanocytes exposed to MSH exhibited an increase in tyrosinase gene expression and function. Simultaneous addition of ASP and MSH at approximately equimolar concentrations produced responses similar to those elicited by the hormone alone. These results demonstrate that eumelanogenic melanocytes can be induced in culture by ASP to exhibit features characteristic of pheomelanogenesis in vivo. Our data are consistent with the hypothesis that the effects of ASP on melanocytes are not mediated solely by inhibition of MSH binding to its receptor, and provide a cell culture model to identify novel factors whose presence is required for pheomelanogenesis.

Involvement of Microphthalmia in the Inhibition of Melanocyte Lineage Differentiation and of Melanogenesis by Agouti Signal Protein

“…In vivo, ␣MSH promotes the production of eumelanin, while expression of the agouti gene promotes the production of pheomelanin. In vitro studies have shown that ASP could induce pheomelanogenesis in eumelanic melanocytes (30,33) demonstrating that ASP alone is sufficient to elicit such changes. It has also been shown that ASP could down-regulate the stimulation of melanogenesis induced by forskolin and ␣MSH (30,31).…”

“…In vitro studies have shown that ASP could induce pheomelanogenesis in eumelanic melanocytes (30,33) demonstrating that ASP alone is sufficient to elicit such changes. It has also been shown that ASP could down-regulate the stimulation of melanogenesis induced by forskolin and ␣MSH (30,31). In this report, we show that ASP can by itself inhibit spontaneous and induced-differentiation of melanoblasts into melanocytes.…”

“…This effect appears to be mediated in part by the ability of ASP to act as an inhibitor of ␣MSH binding to the MC1R (24 -26). On the other hand, some studies suggest that agouti protein may act through a receptor distinct from the MC1R (27)(28)(29)(30). Indeed ASP also inhibits the melanogenic activity of agents such as forskolin and dibutyryl cAMP, which mimic the effects of ␣MSH but act downstream of the MC1R (31).…”

“…In human or murine cultured cells, ASP inhibits eumelanin synthesis, cell proliferation, tyrosinase activity and reduces the level of TRP1 without significantly altering the level of tyrosinase (30,31,33).…”

Targeting protein-trafficking pathways alters melanoma treatment sensitivity

“…To further test the role of protein trafficking and eumelanosome formation in influencing melanoma drug sensitivity, we used an alternative method to moderate both processes. The binding of the melanocyte-stimulating hormone (MSH) to the MC1R on the cell surface of melanocytes has been shown to increase mature eumelanosome number, whereas binding of the antagonist agouti-signaling peptide (ASIP) to the receptor causes a decrease in mature eumelanosome formation (16)(17)(18). MSH binding to the MC1R has also been shown to influence the trafficking of the catalase protein (19).…”