Localisation of endothelin B receptor variants to plasma membrane microdomains and its effects on downstream signalling

The endothelin B (ETB) receptor can undergo a proteolytic cleavage resulting in an unglycosylated N-terminally truncated receptor. We investigated whether ETB receptor processing affects caveolar localisation and mitogenic signalling. Distinct subcellular localisations of ETB receptor constructs and epidermal growth factor (EGF) receptor ligands were analysed performing detergent-free caveolae preparations and total internal reflection fluorescence microscopy. ETB receptor-induced transactivation of the EGF receptor and its downstream signalling was investigated performing shedding assays and ERK1/2 phosphorylation analyses. In COS7 cells, the N-terminally truncated but not the full-length or glycosylation-deficient ETB receptor localised to caveolae. In caveolae-free HEK293 cells, only ETB receptor constructs fused to caveolin-2 localised to membrane microdomains. A caveolar accumulation of the ETB receptor disfavoured EGF receptor ligand shedding. Nonetheless, the activation of ERK1/2 was efficient and long-lasting. In HEK293 cells, the shedding activity was also impaired by N-terminal truncation. The subsequent ERK1/2 phosphorylation was long-lasting only for the full-length ETB receptor. We conclude that the ETB receptor localisation might depend on the presence of caveolae within the cell investigated. The data further suggest that caveolar enrichment of ETB receptors does not facilitate the release of EGF receptor ligands. However, independent of their localisation, ETB receptors are able to induce an ERK1/2 phosphorylation.