A New Experimental Tool Toward Understanding the Regulation of Human Prolactin Secretion and Functions.

The regulation of prolactin (PRL) secretion by the hypothalamo-pituitary–PRL axis shapes patterns in physiology (1). Originally named for its role promoting lactation, PRL is known to exert a plethora of biological actions, including effects on osmoregulation, growth and development, immune function, brain and behavior, angiogenesis, and metabolism (2). The mean concentration of PRL in blood is around 9.9 μg/L in women and around 8.4 μg/L in men. Disruption of the PRL secretion that results in large increases in the circulation (> 25 μg/L), conventionally defined as hyperprolactinemia by means of a distinct clinical entity, has adverse effects, particularly on reproductive function (3). However, the concept of a dyad of PRL levels (normoprolactinemia [< 25 μg/L] vs hyperprolactinemia [> 25 μg/L] with positive vs negative clinical implications, respectively) has recently been challenged by the description of a triad of PRL levels with important homeostatic outcomes in metabolic health (4). Indeed, blood PRL levels ranging between 25 μg/L and 100 μg/L occurring physiologically both in women and men were defined as homeostatic functionally increased transient prolactinemia (HomeoFIT-PRL), since they contribute to maintain and promote metabolic homeostasis (4). This new paradigm, which also challenges the long-held notion that PRL is diabetogenic, is supported by much clinical and experimental evidence showing that lower (< 25 μg/L) and higher (> 100 μg/L) PRL levels are associated with metabolic alterations, whereas HomeoFIT-PRL levels promote metabolic homeostasis in diseases such as diabetes and obesity (4). The conflict between these 2 concepts has brought back the need to better understand the mechanisms responsible for the regulation of PRL secretion, particularly in humans.

The hypothalamo–pituitary PRL axis is quite distinct from other hypothalamo-pituitary systems. PRL secretion is kept at basal levels most of the time by a tonic inhibitory input by means of dopamine (DA) released from tuberoinfundibular dopamine neurons within the arcuate nucleus that reaches the pituitary gland via portal vessels and binds to DA receptors on the surface of lactotrophs. The transient escape of this tonic inhibitory input, triggered by various physiological stimuli including suckling and stress (5), activates multiple signaling pathways enabling sustained stimulatory responses to main PRL secretagogues (thyrotropin-releasing hormone and vasoactive intestinal peptide) that last well beyond the period of decreased DA concentration (6). DA concentration in portal blood is sufficient to occupy approximately 80% of D2 DA receptors in anterior pituitary lactotrophs (7), and their dissociation is followed by the crosstalk of signaling pathways at every level, from proximal effects on receptor affinity to distal actions on the phosphorylation/activation of transcriptional factors (6). Furthermore, by not having an endocrine target tissue, the hypothalamo-pituitary–PRL axis is not regulated by a classical hormonal feedback system, but by a short-loop feedback in which PRL itself stimulates DA secretion (1).

The unique mechanisms involved in the operation of the hypothalamo-pituitary–PRL axis from fish to mammals, the fact that PRL has the widest range of physiological actions of any hormone, and the incomplete understanding of the physiopathological role of PRL in humans underscore the need to address the neuroendocrinology of PRL with new and powerful experimental tools. There are several in vitro models of animal lactotrophs, but the study of human PRL-producing cells has been limited to the use of primary pituitary adenoma cell cultures.

A recent study developed a well-defined approach to differentiate human induced pluripotent stem cells (hiPSCs) into lactotrophs. In this report, published in Endocrinology, Miyake et al (8) differentiated pituitary cells from hiPCSs using a serum/free-floating culture of embryoid-like aggregates with a quick reaggregation method. They showed the presence of PRL-producing cells containing secretory granules by fluorescence immunostaining and immunoelectron microscopy; evaluated the secretion of PRL induced by relatively high concentrations of various known PRL secretagogues (the PRL-releasing peptide PrRP31, vasoactive intestinal peptide, and thyrotropin-releasing hormone) as well as the inhibition of PRL release by the DA agonist, bromocriptine. Finally, they demonstrate tyrosine hydroxylase–positive cells connected to lactotrophs. These findings revealed that it was possible to generate functional pituitary lactotrophs from hiPSCs that recreate some features of a functional hypothalamo-pituitary–PRL axis.

While the study demonstrates a potentially useful model for evaluating human lactotrophs, relevant issues need to be resolved. Validation of this model awaits addressing the heterogeneity of lactotroph-like cells within the pituitary organoids and between organoids from different populations of hiPSCs. Also, the duration of the lactotroph-like phenotype should be investigated, as well as whether the lactotrophs are tonically inhibited by DA and respond to DA escape with increased sensitivity toward PRL secretagogues. Finally, the organoids have tyrosine hydroxylase–positive cells whose projections impinge on the lactotrophs. As mentioned, DA released from tuberoinfundibular dopamine neurons accesses lactotrophs through the portal circulation and, therefore, dopaminergic fiber juxtaposition on lactotrophs is not physiological.

In conclusion, the novel technique reported by Miyake and colleagues (8) allowing the differentiation of lactotrophs from hiPSCs may provide a much-needed model to improve our understanding of the complex hypothalamic regulation of PRL secretion, particularly in humans. While these findings generate more questions than they answer, they open the possibility of new and exciting studies investigating the complexity and diversity of PRL regulation and function in humans.