Prolactin Receptors. In: Horseman ND, editor. Prolactin. Boston: Springer US; 2001. pp. 341-53. [DOI: 10.1007/978-1-4615-1683-5_17]

Molecular evolution of prolactin in primates

Pituitary prolactin, like growth hormone (GH) and several other protein hormones, shows an episodic pattern of molecular evolution in which sustained bursts of rapid change contrast with long periods of slow evolution. A period of rapid change occurred in the evolution of prolactin in primates, leading to marked sequence differences between human prolactin and that of nonprimate mammals. We have defined this burst more precisely by sequencing the coding regions of prolactin genes for a prosimian, the slow loris (Nycticebus pygmaeus), and a New World monkey, the marmoset (Callithrix jacchus). Slow loris prolactin is very similar in sequence to pig prolactin, so the episode of rapid change occurred during primate evolution, after the separation of lines leading to prosimians and higher primates. Marmoset prolactin is similar in sequence to human prolactin, so the accelerated evolution occurred before divergence of New World monkeys and Old World monkeys/apes. The burst of change was confined largely to coding sequence (nonsynonymous sites) for mature prolactin and is not marked in other components of the gene sequence. This and the observations that (1) there was no apparent loss of function during the episode of rapid evolution, (2) the rate of evolution slowed toward the basal rate after this burst, and (3) the distribution of substitutions in the prolactin molecule is very uneven support the idea that this episode of rapid change was due to positive adaptive selection. In the slow loris and marmoset there is no evidence for duplication of the prolactin gene, and evidence from another New World monkey (Cebus albifrons) and from the chimpanzee and human genome sequences, suggests that this is the general position in primates, contrasting with the situation for GH genes. The chimpanzee prolactin sequence differs from that of human at two residues and comparison of human and chimpanzee prolactin gene sequences suggests that noncoding regions associated with regulating expression may be evolving differently from other noncoding regions.

Prolactin, a lymphocyte growth and survival factor

Evidence accumulated over the past 20 y indicates that the anterior pituitary hormone, prolactin (PRL), is a critical, physiologically relevant immunomodulator. Results from early hormone-ablation studies in animals implicated PRL as a factor that contributes to maintenance of immunocompetence. However, the discovery of PRL receptors on T and B lymphocytes and the observation that these cells synthesize and secrete PRL spurred intensive investigation into the actions and underlying mechanisms triggered by the hormone in the immune system. In numerous cell culture systems, PRL was found to act as a co-mitogen, enhancing the efficacy of plant lectins and cytokines in the stimulation of lymphocyte proliferation. In addition, results from more recent studies suggest that PRL may promote survival of certain lymphocyte subsets presumably due to its capacity to augment expression of anti-apoptotic genes. In this review, we focus on the proliferative actions of PRL and its survival promoting properties in immune cells.