A Side-by-Side Comparison of Wildtype and Variant Melanocortin 1 Receptor Signaling with Emphasis on Protection against Oxidative Damage to DNA

Abstract

Common variants of the MC1R gene coding the α-melanocyte stimulating hormone receptor are associated with light skin, poor tanning, blond or red hair, and increased melanoma risk, due to pigment-dependent and -independent effects. This complex phenotype is usually attributed to impaired activation of cAMP signaling. However, several MC1R variants show significant residual coupling to cAMP and efficiently activate mitogenic extracellular signal-regulated kinase 1 and 2 (ERK1/2) signaling. Yet, residual signaling and the key actions of wildtype and variant MC1R have never been assessed under strictly comparable conditions in melanocytic cells of identical genetic background. We devised a strategy based on CRISPR-Cas9 knockout of endogenous MC1R in a human melanoma cell line wildtype for BRAF, NRAS and NF1, followed by reconstitution with epitope-labeled MC1R constructs, and functional analysis of clones expressing comparable levels of wildtype, R151C or D294H MC1R. The proliferation rate, shape, adhesion, motility and sensitivity to oxidative DNA damage were compared. The R151C and D294H RHC variants displayed impaired cAMP signaling, intracellular stability similar to the wildtype, triggered ERK1/2 activation as effectively as the wildtype, and afforded partial protection against oxidative DNA damage, although less efficiently than the wildtype. Therefore, common melanoma-associated MC1R variants display biased signaling and significant genoprotective activity.

The Keratinocyte in the Picture Cutaneous Melanoma Microenvironment

“…MC1R nonsynonymous variants are largely present in Asian and European populations but not in African ones [ 36 , 37 ]. However, increased melanoma risk for individuals carrying polymorphic forms of MC1R is not restricted to cAMP signaling and resultant weakened pigment-dependent function (prevalent synthesis of reddish pheomelanin having photosensitizer properties and insufficient UV-ray absorption), but is additionally attributed to reduced protection against oxidative damage, attenuated MAPKs (mitogen-activated protein kinases), AKT (also known as protein kinase B, PKB) and PI3K (phosphoinositide 3-kinase) signaling activation [ 38 , 39 , 40 , 41 , 42 ], immune responses, glucose metabolism, and energy homeostasis modulation [ 43 ]. The increased risk associated with MC1R variants spans all histological subtypes and it occurs on both chronically and intermittently sun-exposed skin [ 44 ].…”