University of Kentucky

Research Overview

The melanocortin-1 receptor (MC1R) is a cell surface receptor on melanocytes and part of a broader damage-response hormonal pathway in the skin. We focus on MC1R because it is a major determinant of melanoma susceptibility. Millions of Americans have inherited MC1R variants that place them at up to a four-fold (or more) increased lifetime risk of melanoma. Such persons tend to be fair-skinned and burn rather than tan after UV exposure.

The melanocortin signaling axis. Melanocortins such as alpha melanocyte stimulating hormone (α-MSH) and adrenocorticotropic hormone (ACTH), produced in the pituitary and the epidermis, act as high-affinity agonistic ligands to the melanocortin 1 receptor (MC1R), a Gs-protein coupled receptor on the surface of melanocytes. Adenylyl cyclase is activated and cytoplasmic levels of cAMP increase, leading to activation of protein kinase A (PKA) and upregulation of the cAMP responsive binding element (CREB) and microphthalmia (MITF) transcription factors. Together, CREB and MITF stimulate melanin production through increased expression of melanin biosynthetic enzymes such as tyrosinase (Tyr). Melanin synthesis increases and the skin is better protected against UV insults as a result. In addition, cAMP signaling enhances the ability of melanocytes to resist and recover from UV damage by boosting DNA repair. Our lab determined that cAMP-enhanced DNA repair is dependent on PKA-mediated phosphorylation of ATR which induces its association with XPA, a key participant in the nucleotide excision repair pathway.

The MC1R relays survival and differentiation signals to melanocytes when it is bound to alpha melanocyte stimulating hormone (α-MSH). UV damage activates this pathway in the skin, causing keratinocytes to increase their MSH production which then leads to more MC1R signaling in melanocytes. Melanocytes with fully functional MC1R expressed on their surfaces bind α-MSH and transmit this signaling event into the cell’s cytoplasm by inducing the production of cyclic AMP (cAMP), a “second messenger” molecule that has profound impact on melanocyte physiology. cAMP causes melanocytes to manufacture higher levels of a pigment called melanin that gets deposited in the epidermis and protects the skin against UV damage by acting as a built-in sunblock. Higher cAMP levels also help melanocytes recover from UV damage by repairing UV lesions in the DNA. In people with inherited MC1R variants, there is inadequate production of cAMP downstream of α-MSH binding, suboptimal melanin production and sluggish DNA repair.

Inherited defects in MC1R signaling – a bona fide genetic melanoma risk factor – contribute to melanoma development in two major ways: (1) less melanin leaves the skin more vulnerable to the damaging (and cancer-causing) effects of incoming UV radiation, and (2) sluggish melanocyte DNA repair favors UV mutagenesis and carcinogenesis.

Our lab has been interested in the molecular mechanisms linking MC1R stimulation and cAMP signaling with the nucleotide excision repair (NER) pathway of genome maintenance. NER is the major pathway responsible for removing UV-induced DNA damage and protecting cells from UV-induced malignant changes. Our work focuses on the molecular links between cAMP signaling and NER because cAMP is a “druggable” pathway and pharmacologic manipulation of the MC1R signaling pathway holds great promise as a melanoma-preventive strategy.

Recent findings from our lab relate to the involvement of the ataxia telangiectasia and Rad3 related (ATR) protein in MC1R-enhanced DNA repair. Our major discoveries related to this are:

  1. MC1R signaling impacts nucleotide excision DNA repair by a novel and direct PKA-mediated phosphorylation of ATR on Ser435 (S435), an amino acid residue previously not known to affect ATR’s function.
  2. Rather than activate ATR to promote Chk1 phosphorylation and cell cycle arrest (ATR’s normal function in cellular damage responses), Ser435 phosphorylation by PKA promotes ATR’s physical association with xeroderma pigmentosum complement group A (XPA) protein and directs XPA to sites of UV damage in the nucleus to accelerate repair of photodamage. XPA is a core NER factor and purported to be the rate limiting factor for NER.
  3. PKA-mediated phosphorylation of ATR on S435, stimulated either by MSH-MC1R interactions or pharmacologically by forskolin-mediated adenylyl cyclase activation, accelerates repair of UV photodamage and robustly protects against UV-induced mutagenesis.
  4. A kinase anchoring protein 12 (AKAP12) is indispensable for PKA-mediated ATR phosphorylation. By interacting with PKA and with ATR, AKAP12 acts as a molecular “scaffold” for generation of p435S-ATR, the key molecular event needed for enhancement of NER by cAMP/MC1R signaling.
  5. UV induces an association between AKAP12 and ATR in the cytoplasm. If cells have been stimulated to increase cAMP levels, then PKA is activated, associates with the AKAP12, ATR complex and phosphorylates ATR on S435.
  6. Nuclear translocation of the repair enhancement complex is dependent on phosphorylation of AKAP12 on its S732 residue by ATR, which is a Ser/Thr kinase.
  7. PKA-mediated ATR phosphorylation (on S435) occurs in the cytoplasm.
  8. Together, p435S-ATR and p732S-AKAP12 translocate to the nucleus. There, XPA is recruited to the complex and together, all three moieties localize to sites of UV damage in chromatin.
  9. Mechanistically, cAMP’s acceleration of NER involves enhanced 5’ strand incision.
  10. Using a mouse model of the fair-skinned, melanoma-prone human, we demonstrated that topical application of forskolin (a drug that increases cAMP in the skin) greatly increased the efficiency of the repair of UV photodamage in the skin. This is a proof-of-concept experiment that shows that cutaneous cAMP stimulation can protect against UV mutations.

MC1R signaling enhances nucleotide excision repair (NER). cAMP increases cause PKA to phosphorylate ATR on S435, which promotes interactions with the core NER factor XPA. AKAP12 is the molecular scaffold that facilitates PKA-mediated ATR phosphorylation; it associates with ATR in the cytoplasm after UV exposure. Nuclear translocation of the AKAP12-ATR complex depends on ATR-mediate phosphorylation of AKAP12 on its S732 residue. Once in the nucleus, the AKAP12-ATR complex associates with XPA and together, the AKAP12-ATR-XPA complex localizes to UV photodamage where it facilitates the 5’strand incision step of NER.

Our work and the work of others is shedding light on how cAMP signaling is linked with NER, a critical genome maintenance pathway the directly regulates melanoma susceptibility. Our investigations have clear translational potential. Pharmacologic manipulation of the cAMP signaling pathway holds great promise as a melanoma-preventive strategy since it enhances innate DNA repair pathways central for cellular resistance to carcinogenesis. We speculate that it may be possible for fair-skinned individuals (many of whom harbor loss-of-function MC1R defects) to greatly reduce their melanoma risk by pharmacologically enhancing repair of mutagenic UV photolesions in melanocytes

Our lab is privileged to be part of an exciting and dynamic "MC1R community", which together has provided new insights into the mechanistic relationships between UV exposure and skin cancer. However, the overall picture of MC1R's influence on carcinogenesis is not yet complete and there is much more work to be done before rational development of DNA repair enhancers is feasible. Thank you for your interest in our work and, most importantly, thank you for supporting biomedical research!