Prolactin receptor subtypes: a possible mode of tissue specific regulation of prolactin function.

Prolactin mediates its effect on target cells through an interaction with membrane-anchored receptors. In the last decade, several subtypes of the receptor have been isolated from different species. This has generated a great deal of interest in the roles of the receptor subtypes and the possible divergent signalling pathways in mediating the pleiotropic effects of prolactin on target tissues. Our current knowledge of the signalling pathway of prolactin is derived mainly from the interaction of the hormone with one of its receptor subtypes (the long form) isolated from rats. In vitro expression studies have led to the identification of the regions within the long form prolactin receptor that are essential for the association of the tyrosine kinase Jak-2, and the phosphorylation events leading to activation of the prolactin responsive beta-casein promoter. To date, a specific target gene that may be activated after interaction of prolactin with the short form of the receptor has not been identified. However, the different receptor subtypes are present in the same cell type in vivo and their expression is hormone regulated, possibly through multiple promoters that control transcription of the prolactin receptor gene. Comparative studies suggest that the signalling pathways and the relevance of different receptor subtypes on prolactin function may vary between species.