The melanocortin 1 receptor and cutaneous biology

Abstract

Human MC1R was tagged at the C-terminus with enhanced green fluorescent protein (eGFP) and transfected into Mc1r null B16G4F melanoma cells.  Confocal microscopy and image analysis of the cell transfectants demonstrated evidence of co-localisation of MC1R-eGFP at the melanosome.  In addition, co-localisation of MC1R with melanosomes along dendrites was also observed.  Furthermore, melanoma cell transfectants containing eGFP C-terminally tagged Asp294His variant MC1R demonstrated similar co-localisation of variant MC1R and the melanosome.  Isolation of melanosomes from B16 transfectants containing wild-type MC1R-eGFP also suggested that MC1R was present in these organelles.  The results indicate that MC1R is present at the melanosome, and suggests that part of the function of MC1R may be mediated through its association with this organelle.  It may be hypothesised that MC1R could become associated with the melanosome from other organelles such as the ER and transgolgi networks since this is where the receptor is made and the melanosome is thought to incorporate other melanosomal proteins from these sites.  The presence of MC1R within endosomes and also the receptors ability to internalise has been debated, which also adds the possibility that MC1R may be associated with melanosomes by the fusion of endosomes which have internalised from the plasma membrane containing activated MC1R.  MC1R internalisation was investigated via three approaches which included eGFP tagging of MC1R, the use of a FITC-αMSH fusion protein, and also by radioligand binding and acetic acid removal of the surface associated ligand.  The latter approach successfully demonstrated the internalisation of wild-type and Asp294His variant MC1R.

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