Abstract
Melanoma risk is increased in patients with mutations of melanocortin 1 receptor (MC1R) yet the basis for the increased risk remains unknown. Here we report in vivo evidence supporting a critical role for MC1R in regulating melanoma tumor growth and determining overall survival time. Inhibition of MC1R by its physiologically relevant competitive inhibitor, agouti signaling protein (ASIP), reduced melanin synthesis and morphological heterogeneity in murine B16-F10 melanoma cells. In the lungs of syngeneic C57BL/6 mice, mCherry-marked, ASIP-secreting lung tumors inhibited MC1R on neighboring tumors lacking ASIP in a dose dependent manner as evidenced by a proportional loss of pigment in tumors from mice injected with 1:1, 3:1 and 4:1 mixtures of parental B16-F10 to ASIP-expressing tumor cells. ASIP-expressing B16-F10 cells formed poorly pigmented tumors in vivo that correlated with a 20% longer median survival than those bearing parental B16-F10 tumors (p=0.0005). Mice injected with 1:1 mixtures also showed survival benefit (p=0.0054), whereas injection of a 4:1 mixture showed no significant difference in survival. The longer survival time of mice bearing ASIP-expressing tumors correlated with a significantly slower growth rate than parental B16-F10 tumors as judged by quantification of numbers of tumors and total tumor load (p=0.0325), as well as a more homogeneous size and morphology of ASIP-expressing lung tumors. We conclude that MC1R plays an important role in regulating melanoma growth and morphology. Persistent inhibition of MC1R provided a significant survival advantage resulting in part from slower tumor growth, establishing MC1R as a compelling new molecular target for metastatic melanoma.
Highlights
Normal human and mouse melanocytes and melanoma cells express melanocortin 1 receptor (MC1R), a G-protein coupled receptor that regulates the biogenesis and maintenance of melanosomes, the specialized lysosomal compartments within which melanin pigments are synthesized
In this report we describe the generation of B16 cells that stably express the MC1R inverse agonist agouti signaling protein (ASIP) in amounts sufficient to suppress melanogenesis when these cells are grown in culture, as well as when introduced into animals either intravenously or when implanted subcutaneously
There was no difference in engraftment efficiency as determined by enumeration of physical tumors found in the lungs after systemic administration, but a notable difference was that the individual ASIP-secreting tumors were smaller and more dispersed throughout the lung and dermal tissues compared to either the parental B16-F10 tumors or those formed by the B16-GFP cells, which were generally larger, more densely packed, and more heterogeneously dispersed within tumors
Summary
Normal human and mouse melanocytes and melanoma cells express MC1R, a G-protein coupled receptor that regulates the biogenesis and maintenance of melanosomes, the specialized lysosomal compartments within which melanin pigments are synthesized. MC1R mutations associated with fair skin and red hair [20] may increase melanoma risk through insufficient pigment protection of genomic DNA from ultra-violet light damage; some MC1R mutations that increase melanoma risk are not associated with fair skin and red hair and do not alter pigment synthesis [21] These observations indicate that a pigment-independent mechanism exists, but the basis for this increased risk has not yet been identified and it is not known which MC1R signaling pathway is involved in the pigment-independent mechanism, α-MSH agonist- or ASIP-inverse agonist induced signaling. While ASIP stimulates opposite effects, that is, de-differentiation of melanocytes, human melanoma and murine B16-F1 melanoma cells, and increases their migration in wound healing assays [24, 27], the effect of ASIP induced MC1R signaling on melanoma colonization in vivo is not known
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
