Isolation and characterization of two novel forms of the human prolactin receptor generated by alternative splicing of a newly identified exon 11

We have identified a novel exon 11 of the human prolactin receptor (hPRLR) gene that is distinct from its rodent counterparts and have demonstrated th e presence of two novel short forms of the hPRLR (S1(a) and S1(b)), which a re derived from alternative splicing of exons 10 and 11. S1(a) encodes 376 amino acids (aa) that contain partial exon 10 and a unique 39-aa C-terminal region encoded by exon 11. S1(b) encodes 288 as that lack the entire exon 10 and contains 3 amino acids at the C terminus derived from exon 11 using a shifted reading frame. These short forms, which were found in several nor mal tissues and in breast cancer cell lines, were expressed as cell surface receptors and possessed binding affinities comparable with the long form. Unlike the long form, neither short form was able to mediate the activation of the beta -casein gene promoter induced by prolactin. Instead they acted as dominant negative forms when co-expressed with the long form in transfe cted cells. Due to a marked difference in the cellular levels between the t wo short forms in transfected cells, S1(b) was more effective in inhibiting the prolactin-induced activation of the beta -casein gene promoter mediate d by the long form of the receptor. The low cellular level of Sla was due t o its more rapid turnover than the S1(b) protein. This is attributable to s pecific residues within the C-terminal unique 39 amino acids of the S1(a) f orm and may represent a new mechanism by which the hPRLR is modulated at th e post-translational level. Since both short forms contain abbreviated cyto plasmic domains with unique C termini, they may also exhibit distinct signa ling pathways in addition to modulating the signaling from the long form of the receptor. These receptors may therefore play important roles in the di versified actions of prolactin in human tissues.