The metabolic role of prolactin: systematic review, meta-analysis and preclinical considerations

Drug induced hypoprolactinemia

Prolactin levels can be influenced by multiple medications primarily through the interaction with dopamine receptors which regulate its secretion. Unlike hyperprolactinemia which has a well-defined clinical phenotype, the effects of hypoprolactinemia beyond inability to lactate are incompletely understood. Recent studies have raised concerns regarding detrimental changes in glucose metabolism, sexual function and psychological profile in patients with low prolactin levels. In contrast with anatomic and genetic etiologies, drug-induced hypoprolactinemia is usually reversible after dose reduction of the offending medication. The most common clinical scenario of drug-induced hypoprolactinemia in the endocrine clinic pertains to patients treated with cabergoline or bromocriptine for prolactin-secreting or other types of pituitary adenomas. Also, data has accumulated regarding hypoprolactinemia in patients receiving aripiprazole for schizophrenia and other psychiatric disorders. These patients warrant careful evaluation for comorbidities. This review aims to increase awareness about the potentially detrimental effects of drug-induced hypoprolactinemia, which should be considered in clinical practice decisions.

Isolated hypoprolactinemia: The rarest of the rare?

Isolated hypoprolactinemia (IHP) can be defined as the presence of consistently low serum levels of prolactin in the absence of other anterior pituitary hormone abnormalities. It is an extremely rare condition, and consequently incompletely understood and unrecognized. A recent study has reported the first cases of IHP caused by mutation in the PRL gene. There are also cases where the pathogenesis of IHP is likely secondary to an autoimmune response against the PRL-secreting cells. But most published cases are acquired and idiopathic. The best characterized function of PRL is to facilitate milk production in the puerperium. Analysis of the GTEX data repository, however, shows that PRL is the most abundantly expressed gene in the human pituitary, independently of gender and age, suggesting the presence of additional roles for PRL. Newer studies have indeed revealed a much larger spectrum of PRL functions and will likely uncover novel clinical phenotypes associated with severe PRL deficiency.

The homeo-FIT-prolactin hypothesis: the role of prolactin in metabolic homeostasis - association or causality?

The homeo-fit-prolactin hypothesis proposes a causal metabolic role for prolactin with hypoprolactinemia and hyperprolactinemia leading to adverse metabolic alterations. However, prolactin within the normal range and up to four times the upper reference limit may be a consequence of metabolic adaption and have a positive metabolic role similar to increased insulin in pre-diabetes. As a consequence, drugs that would increase prolactin levels within this threshold may hold promising effects, particularly for patients with type 2 diabetes. A documented positive metabolic effect of prolactin just above the normal threshold would not just be of benefit to patients with diabetes but assist in the decision to treat mild hyperprolactinemia in other patient groups as well, e.g. drug-induced hyperprolactinemia or idiopathic hyperprolactinemia. Prolactin receptors are present in the pancreas, liver, and adipose tissue, and pre-clinical studies suggest a positive and causal effect of prolactin on the gluco-insulinemic profile and lipid metabolism. This narrative review examines the evidence for the homeo-fit-prolactin hypothesis with a particular focus on results from human studies.

Acquired hypoprolactinemia in men, possible phenotype

The physiological role of prolactin (PRL) in men is still not well defined. The pathological increase is characterized by sexual function impairment along with possible negative consequences in body composition and metabolic profile. Conversely, the clinical significance of reduced PRL levels was only partially investigated or mainly neglected. The present paper aims to summarize and critically discuss possible phenotypes characterizing male subjects with reduced PRL levels. When possible, meta-analytic results were provided. Available data derived from patients seeking medical care for sexual dysfunction as well as from cross-sectional and longitudinal studies showed that low PRL in males is associated with a worse metabolic phenotype (including diabetes mellitus), mood disturbances (including anxiety and depression), and sexual dysfunctions (including psychogenic erectile and ejaculatory dysfunctions). Whether or not these features are direct consequences of reduced PRL levels or whether the latter reflect other pathway impairments such as serotoninergic failure cannot be clarified. The present data, however, emphasize that a deficiency of PRL should be taken into account and need further investigations.

The beneficial and detrimental effects of prolactin hormone on metabolic syndrome: A double-edge sword

The metabolic syndrome (MetS) is a clustering of abdominal obesity, hypertension, hyperglycaemia, hypertriglyceridemia and low high-density lipoprotein (HDL) level. MetS development is affected by endocrine hormones such as prolactin (PRL) hormone which induce insulin resistance and central obesity because PRL is implicated in the pathogenesis of MetS. Pituitary PRL controls mammary gland, however extra-pituitary PRL is highly intricate in the regulation of adipose tissue function. In addition, cAMP activators enhance expression and release of PRL which involved in the control of lipogenesis and energy homeostasis. Consequently, hyperprolactinaemia may be associated with the development of MetS. However, normal physiological level of PRL is essential for insulin sensitivity and regulation of adipose tissue function and energy metabolism. Therefore, PRL has dual effects on the components of MetS. Hence, the present review aims to discuss the modulatory mechanistic role of PRL on MetS regarding its beneficial and detrimental effects.

Gonadal dysfunction in women with diabetes mellitus

It is well known that both type 1 and type 2 diabetes mellitus (DM) are related to increased risk for cardiovascular (CV) and chronic kidney disease (CKD). However, besides these prominently presented complications, DM has also been associated with reproductive dysfunctions. It seems that these disorders are met in up to 40% of women with DM and consist of delayed menarche, all types of menstrual disorders, such as amenorrhea, oligomenorrhea, menstrual irregularity, as well as menorrhagia, infertility, characteristics of polycystic ovary syndrome (PCOS) and early (or rarely late) menopause. In type 1 DM (T1DM), insulin treatment, although it has reduced the rates of insulinopenic-induced hypogonadotropic hypogonadism, an entity commonly presented in many women with the disease in the past decades, when it is used in excess it can also promote hyperandrogenism. Regarding type 2 DM (T2DM), insulin resistance (IR) and hyperinsulinemia have mainly been implicated in the pathogenesis of reproductive dysfunctions, as insulin can act as gonadotropin on the theca cells of the ovary and can lead to hyperandrogenism and inhibition of proper ovulation. This review aims to detail the reproductive dysfunctions associated with DM and provide scientific data to enlighten the underlying pathogenetic mechanisms.

Serum Prolactin Levels and Mortality in Adults Without Prolactinoma: A Meta-Analysis

CONTEXT

Prolactin (PRL) is a highly versatile, multifunctional hormone synthesized and secreted by lactotroph cells of the anterior pituitary. Its metabolic role has been extensively studied even in normoprolactinemic populations. Recently, a wealth of observational data have outlined the potential prognostic value of PRL in various clinical settings.

OBJECTIVE

This systematic review aims to systematically evaluate and quantitatively synthesize the association between serum PRL levels and risk of mortality in adults without prolactinoma.

METHODS

A systematic literature search was conducted up to June 10, 2023, to identify studies reporting the association of serum PRL levels with clinical outcomes of adults without prolactinoma. A random-effects meta-analysis was conducted to quantify the adjusted hazard ratios [(a)HRs] for all-cause and cardiovascular death (CVD) during follow-up.

RESULTS

Twenty-eight studies were deemed eligible reporting the outcomes of adults without prolactinoma, in whom serum PRL levels were measured for risk-stratification. Fourteen studies reported appropriate data for meta-analysis encompassing a total of 23 596 individuals. Each unit of PRL increase was independently associated with increased risk of all-cause (pooled aHR = 1.17 [1.08-1.27]; I2 = 48%) and CV mortality (pooled aHR = 1.54 [1.14-2.09]; I2 = 89%). Individuals belonging to the highest PRL category had significantly higher risk for all-cause (pooled aHR = 1.81 [1.43-2.30]; I2 = 65%) and CV (pooled aHR = 1.59 [1.04-2.42]; I2 = 82%) mortality compared to their lowest-PRL category counterparts. The association between PRL levels and in-hospital death did not reach statistical significance.

CONCLUSION

PRL levels seem to be an independent predictor for mortality. Further validation is warranted before its role as a risk-stratification tool can be delineated in clinical practice.

Hyperprolactinemia and male sexual function: focus on erectile dysfunction and sexual desire

The present paper aims to analyze and discuss the available evidence supporting the relationship between male sexual function and elevated prolactin (PRL) levels (HPRL). Two different sources of data were analyzed. Clinical data were derived from a series of patients seeking medical care for sexual dysfunction at our Unit. Out of 418 studies, 25 papers were used with a meta-analytic approach to evaluate the overall prevalence of HPRL in patients with erectile dysfunction (ED) and to study the influence of HPRL and its treatment on male sexual function. Among 4215 patients (mean age 51.6 ± 13.1 years) consulting for sexual dysfunction at our Unit, 176 (4.2%) showed PRL levels above the normal range. Meta-analytic data showed that HPRL is a rare condition among patients with ED (2 [1;3]%). Either clinical and meta-analytic data confirm a stepwise negative influence of PRL on male sexual desire (S = 0.00004 [0.00003; 0.00006]; I = -0.58915 [-0.78438; -0.39392]; both p < 0.0001 from meta-regression analysis). Normalization of PRL levels is able to improve libido. The role of HPRL in ED remains inconclusive. Data from a meta-analytic approach showed that either HPRL or reduced T levels were independently associated with ED rates. The normalization of PRL levels only partially restored ED. HPRL did not significantly contribute to ED severity, in our clinical setting. In conclusion, treating HPRL can restore normal sexual desire, whereas its effect on erection is limited.

Current Insights in Prolactin Signaling and Ovulatory Function

Prolactin (PRL) is a pleiotropic hormone released from lactotrophic cells of the anterior pituitary gland that also originates from extrapituitary sources and plays an important role in regulating lactation in mammals, as well as other actions. Acting in an endocrine and paracrine/autocrine manner, PRL regulates the hypothalamic-pituitary-ovarian axis, thus influencing the maturation of ovarian follicles and ovulation. This review provides a detailed discussion of the current knowledge on the role of PRL in the context of ovulation and ovulatory disorders, particularly with regard to hyperprolactinemia, which is one of the most common causes of infertility in women. Much attention has been given to the PRL structure and the PRL receptor (PRLR), as well as the diverse functions of PRLR signaling under normal and pathological conditions. The hormonal regulation of the menstrual cycle in connection with folliculogenesis and ovulation, as well as the current classifications of ovulation disorders, are also described. Finally, the state of knowledge regarding the importance of TIDA (tuberoinfundibular dopamine), KNDγ (kisspeptin/neurokinin B/dynorphin), and GnRH (gonadotropin-releasing hormone) neurons in PRL- and kisspeptin (KP)-dependent regulation of the hypothalamic-pituitary-gonadal (HPG) axis in women is reviewed. Based on this review, a rationale for influencing PRL signaling pathways in therapeutic activities accompanying ovulation disorders is presented.