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The melanocortin 1 receptor (MC1R) mediates the tanning response through induction of cAMP and downstream pigmentary enzymes. Diminished function alleles of MC1R are associated with decreased tanning and increased melanoma risk, which has been attributed to increased rates of mutation. We have found that MC1R or cAMP signaling also directly decreases proliferation in melanoma cell lines. MC1R overexpression, treatment with the MC1R ligand, or treatment with small-molecule activators of cAMP signaling causes delayed progression from G2 into mitosis. This delay is caused by phosphorylation and inhibition of cdc25B, a cyclin dependent kinase 1-activating phosphatase, and is rescued by expression of a cdc25B mutant that cannot be phosphorylated at the serine 323 residue. These results show that MC1R and cAMP signaling can directly inhibit melanoma growth through regulation of the G2/M checkpoint.

Melanoma is an aggressive and treatment resistant tumor of the melanocyte lineage. In humans, melanocytes are responsible for skin pigmentation and the tanning response. Pigmentation and the tanning response are controlled by the Melanocortin 1 Receptor (MC1R) which signals through the cAMP cascade. Individuals with defective MC1R often show the Red Hair Color (RHC) phenotype which is marked by red hair, freckling and decreased tanning response. This phenotype and the MC1R RHC genotype confer high risk to skin cancers, including melanoma. Although some of this risk is attributable to increased mutation rates in MC1R RHC cells, we sought to determine whether active MC1R and cAMP signaling could inhibit melanoma growth independently from these effects on mutation. We investigated the activity of MC1R RHC alleles in terms of cAMP signaling and investigated the role of MC1R and cAMP signaling in the growth and proliferation of normal primary melanocytes, melanocytes expressing activated oncogenes and melanoma cell lines. In primary cells, we found that MC1R RHC alleles had defective, but not complete loss of function cAMP signaling. We found that MC1R activity has a mitogenic role in normal melanocytes, but may cooperate with BRaf V600E in causing cell cycle arrest. We show that forskolin treatment slowed the growth of melanoma cell lines in culture. MC1R overexpression, treatment with the MC1R ligand or other small-molecule activators of cAMP signaling caused a delay in progression from G2 into mitosis. This delay was caused by inhibition of the CDK1 phosphatase cdc25B through phosphorylation at PKA sites and was rescued by expression of a cdc25B mutant that could not be phosphorylated by PKA. These data show that MC1R/cAMP signaling can inhibit melanoma growth outside of its role in protecting cells from mutagenesis and suggest that cAMP activators could represent a therapeutic strategy in melanoma.

Melanoma is an aggressive and treatment resistant tumor of the melanocyte lineage. In humans, melanocytes are responsible for skin pigmentation and the tanning response. Pigmentation and the tanning response are controlled by the Melanocortin 1 Receptor (MC1R) which signals through the cAMP cascade. Individuals with defective MC1R often show the Red Hair Color (RHC) phenotype which is marked by red hair, freckling and decreased tanning response. This phenotype and the MC1R RHC genotype confer high risk to skin cancers, including melanoma. Although some of this risk is attributable to increased mutation rates in MC1R RHC cells, we sought to determine whether active MC1R and cAMP signaling could inhibit melanoma growth independently from these effects on mutation. We investigated the activity of MC1R RHC alleles in terms of cAMP signaling and investigated the role of MC1R and cAMP signaling in the growth and proliferation of normal primary melanocytes, melanocytes expressing activated oncogenes and melanoma cell lines. In primary cells, we found that MC1R RHC alleles had defective, but not complete loss of function cAMP signaling. We found that MC1R activity has a mitogenic role in normal melanocytes, but may cooperate with BRaf V600E in causing cell cycle arrest. We show that forskolin treatment slowed the growth of melanoma cell lines in culture. MC1R overexpression, treatment with the MC1R ligand or other small-molecule activators of cAMP signaling caused a delay in progression from G2 into mitosis. This delay was caused by inhibition of the CDK1 phosphatase cdc25B through phosphorylation at PKA sites and was rescued by expression of a cdc25B mutant that could not be phosphorylated by PKA. These data show that MC1R/cAMP signaling can inhibit melanoma growth outside of its role in protecting cells from mutagenesis and suggest that cAMP activators could represent a therapeutic strategy in melanoma.