Functional characterization of the intermediate isoform of the human prolactin receptor.

Prolactin-dependent signaling occurs as the result of ligand-induced dimerization of the prolactin receptor (PRLr). While three PRLr isoforms have been characterized in the rat, studies have suggested the existence of several human isoforms in breast carcinoma species and normal tissues. Reverse transcription polymerase chain reaction was performed on mRNA isolated from the breast carcinoma cell line T47D, revealing two predominant receptor isoforms: the previously described long PRLr and a novel human intermediate PRLr. The nucleotide sequence of the intermediate isoform was found to be identical to the long isoform except for a 573-base pair deletion occurring at a consensus splice site, resulting in a frameshift and truncated intracytoplasmic domain. Scatchard analysis of the intermediate PRLr revealed an affinity for PRL comparable with the long PRLr. While Ba/F3 transfectants expressing the long PRLr proliferated in response to PRL, intermediate PRLr transfectants exhibited modest incorporation of [(3)H]thymidine. Significantly, however, both the long and intermediate PRLr were equivalent in their inhibition of apoptosis of the Ba/F3 transfectants after PRL treatment. The activation of proximal signaling molecules also differed between isoforms. Upon ligand binding, Jak2 and Fyn were activated in CHO-K1 cells transiently transfected with the long PRLr. In contrast, the intermediate PRLr transfectants showed equivalent levels of Jak2 activation but only minimal activation of Fyn. Last, Northern analysis revealed variable tissue expression of intermediate PRLr transcript that differed from that of the long PRLr. Taken together, differences in signaling and tissue expression suggest that the human intermediate PRLr differs from the long PRLr in physiological function.