Jakob Triebel

Prolactin promotes cartilage survival and attenuates inflammation in inflammatory arthritis

Chondrocytes are the only cells in cartilage, and their death by apoptosis contributes to cartilage loss in inflammatory joint diseases, such as rheumatoid arthritis (RA). A putative therapeutic intervention for RA is the inhibition of apoptosis-mediated cartilage degradation. The hormone prolactin (PRL) frequently increases in the circulation of patients with RA, but the role of hyperprolactinemia in disease activity is unclear. Here, we demonstrate that PRL inhibits the apoptosis of cultured chondrocytes in response to a mixture of proinflammatory cytokines (TNF-α, IL-1β, and IFN-γ) by preventing the induction of p53 and decreasing the BAX/BCL-2 ratio through a NO-independent, JAK2/STAT3-dependent pathway. Local treatment with PRL or increasing PRL circulating levels also prevented chondrocyte apoptosis evoked by injecting cytokines into the knee joints of rats, whereas the proapoptotic effect of cytokines was enhanced in PRL receptor-null (Prlr(-/-)) mice. Moreover, eliciting hyperprolactinemia in rats before or after inducing the adjuvant model of inflammatory arthritis reduced chondrocyte apoptosis, proinflammatory cytokine expression, pannus formation, bone erosion, joint swelling, and pain. These results reveal the protective effect of PRL against inflammation-induced chondrocyte apoptosis and the therapeutic potential of hyperprolactinemia to reduce permanent joint damage and inflammation in RA.

Investigation of prolactin-related vasoinhibin in sera from patients with diabetic retinopathy

OBJECTIVE In vitro experiments and in vivo studies on rodents demonstrate that N-terminal 14, 15, 16, 17, and 18 kDa fragments prolactin-related vasoinhibin (PRL-V) of human PRL are natural inhibitors of neovascularization in the retina and elsewhere. These N-terminal PRL fragments belong to a family of peptides named vasoinhibins, which act as endogenous regulators of angiogenesis and vascular function. These observations led to the hypothesis that PRL-V could play a role in the pathophysiology of diabetic retinopathy in humans. The purpose of this study was to investigate whether patients with diabetes mellitus and diabetic retinopathy have aberrant concentrations of PRL-V in the circulating blood. RESEARCH DESIGN We performed a case-control study and developed a new technique to semi-quantitatively determine PRL-V in serum samples from 48 male subjects. The case group consisted of 21 patients with diabetes mellitus and proliferative or non-proliferative diabetic retinopathy. The control group consisted of 27 healthy subjects with no history of diabetes mellitus. METHODS For the detection of PRL-V, we developed a new analytical method, consisting of immunologic and laser-induced fluorescence techniques. Results The case group had significantly lower PRL-V serum concentrations than the control group (P=0.041). There was no significant difference between patients with proliferative and those with non-proliferative diabetic retinopathy. CONCLUSION We conclude that given the antiangiogenic and antivasopermeability actions of PRL-V, the decreased serum levels of PRL-V in patients with diabetes mellitus could contribute to the development and progression of diabetic retinopathy.

Melanocortin receptors in rat liver cells: change of gene expression and intracellular localization during acute-phase response.

MCRs are known to be expressed predominantly in the brain where they mediate metabolic and anti-inflammatory functions. Leptin plays an important role in appetite and energy regulation via signaling through melanocortin receptors (MCRs) in the brain. As serum levels of MCR ligands are elevated in a clinical situation [acute-phase response (APR)] to tissue damage, where the liver is responsible for the metabolic changes, we studied hepatic gene expression of MCRs in a model of muscle tissue damage induced by turpentine oil (TO) injection in rats. A significant increase in gene expression of all five MCRs (MC4R was the highest) in liver at the RNA and protein level was detected after TO injection. A similar pattern of increase was also found in the brain. Immunohistology showed MC4R in the cytoplasm, but also in the nucleus of parenchymal and non-parenchymal liver cells, whereas MC3R-positivity was mainly cytoplasmic. A time-dependent migration of MC4R protein from the cytoplasm into the nucleus was observed during APR, in parallel with an increase in α-MSH and leptin serum levels. An increase of MC4R was detected at the protein level in wild-type mice, while such an increase was not observed in IL-6ko mice during APR. Moreover, treatment of isolated liver cells with melanocortin agonists (α-MSH and THIQ) inhibited the endotoxin-induced upregulation of the acute-phase cytokine (IL-6, IL1β and TNF-α) gene expression in Kupffer cells and of chemokine gene expression in hepatocytes. MCRs are expressed not only in the brain, but also in liver cells and their gene expression in liver and brain tissue is upregulated during APR. Due to the presence of specific ligands in the serum, they may mediate metabolic changes and exert a protective effect on liver cells.

Prolactin and Vasoinhibins: Endogenous Players in Diabetic Retinopathy

Diabetic retinopathy is a disease of the retinal microvasculature that develops as a complication of diabetes mellitus and constitutes a major cause of blindness in adults of all ages. Diabetic retinopathy is characterized by the loss of capillary cells leading to increased vasopermeability, ischemia, and hypoxia that trigger the excessive formation of new blood vessels in the retina. The influence of the pituitary gland in the pathophysiology of diabetic retinopathy was recognized nearly six decades ago, but the contribution of pituitary hormones to this disease remains unclear. Recent studies have shown that the pituitary hormone prolactin is proteolytically cleaved to vasoinhibins, a family of peptides with potent antivasopermeability, vasoconstrictive, and antiangiogenic actions that can protect the eye against the deleterious effects of the diabetic state. In this review, we summarize what is known about the changes in the circulating levels of prolactin and vasoinhibins during diabetes and diabetic retinopathy as well as the implications of these changes for the development and progression of the disease with particular attention to hyperprolactinemia in pregnancy and postpartum. We discuss the effects of prolactin and vasoinhibins that may impact diabetic retinopathy and suggest these hormones as important targets for therapeutic interventions.

Prolactin promotes normal liver growth, survival, and regeneration in rodents: effects on hepatic IL-6, suppressor of cytokine signaling-3, and angiogenesis

Prolactin (PRL) is a potent liver mitogen and proangiogenic hormone. Here, we used hyperprolactinemic rats and PRL receptor-null mice (PRLR(-/-)) to study the effect of PRL on liver growth and angiogenesis before and after partial hepatectomy (PH). Liver-to-body weight ratio (LBW), hepatocyte and sinusoidal endothelial cell (SEC) proliferation, and hepatic expression of VEGF were measured before and after PH in hyperprolactinemic rats, generated by placing two anterior pituitary glands (AP) under the kidney capsule. Also, LBW and hepatic expression of IL-6, as well as suppressor of cytokine signaling-3 (SOCS-3), were evaluated in wild-type and PRLR(-/-) mice before and after PH. Hyperprolactinemia increased the LBW, the proliferation of hepatocytes and SECs, and VEGF hepatic expression. Also, liver regeneration was increased in AP-grafted rats and was accompanied by elevated hepatocyte and SEC proliferation, and VEGF expression compared with nongrafted controls. Lowering circulating PRL levels with CB-154, an inhibitor of AP PRL secretion, prevented AP-induced stimulation of liver growth. Relative to wild-type animals, PRLR(-/-) mice had smaller livers, and soon after PH, they displayed an approximately twofold increased mortality and elevated and reduced hepatic IL-6 and SOCS-3 expression, respectively. However, liver regeneration was improved in surviving PRLR(-/-) mice. PRL stimulates normal liver growth, promotes survival, and regulates liver regeneration by mechanisms that may include hepatic downregulation of IL-6 and upregulation of SOCS-3, increased hepatocyte proliferation, and angiogenesis. PRL contributes to physiological liver growth and has potential clinical utility for ensuring survival and regulating liver mass in diseases, injuries, or surgery of the liver.

Principles of the prolactin/vasoinhibin axis

The hormonal family of vasoinhibins, which derive from the anterior pituitary hormone prolactin, are known for their inhibiting effects on blood vessel growth, vasopermeability, and vasodilation. As pleiotropic hormones, vasoinhibins act in multiple target organs and tissues. The generation, secretion, and regulation of vasoinhibins are embedded into the organizational principle of an axis, which integrates the hypothalamus, the pituitary, and the target tissue microenvironment. This axis is designated as the prolactin/vasoinhibin axis. Disturbances of the prolactin/vasoinhibin axis are associated with the pathogenesis of retinal and cardiac diseases and with diseases occurring during pregnancy. New phylogenetical, physiological, and clinical implications are discussed.

The role of the prolactin/vasoinhibin axis in rheumatoid arthritis: an integrative overview.

Rheumatoid arthritis (RA) is a chronic, autoimmune, inflammatory disease destroying articular cartilage and bone. The female preponderance and the influence of reproductive states in RA have long linked this disease to sexually dimorphic, reproductive hormones such as prolactin (PRL). PRL has immune-enhancing properties and increases in the circulation of some patients with RA. However, PRL also suppresses the immune system, stimulates the formation and survival of joint tissues, acquires antiangiogenic properties upon its cleavage to vasoinhibins, and protects against joint destruction and inflammation in the adjuvant-induced model of RA. This review addresses risk factors for RA linked to PRL, the effects of PRL and vasoinhibins on joint tissues, blood vessels, and immune cells, and the clinical and experimental data associating PRL with RA. This information provides important insights into the pathophysiology of RA and highlights protective actions of the PRL/vasoinhibin axis that could lead to therapeutic benefits.

On the Path toward Classifying Hormones of the Vasoinhibin-Family.

Prolactin and growth hormone are produced in the anterior pituitary gland and secreted into the circulation to execute their diverse physiological effects. They are also produced at various extrapituitary sites. Apart from the traditional, well-known effects mediated by the full-length hormones, they serve as the source for vasoinhibins, which are generated by proteolytic cleavage of prolactin and growth hormone, and also of placental lactogen (1, 2). Prominent, name-giving biological effects of vasoinhibins include inhibition of angiogenesis, vasodilation, and vasopermeability (1–3). As this set of vascular effects is unique, and entirely different from the characteristics of their precursors, vasoinhibins constitute a separate hormonal family (1, 2). Apart from physiological functions, vasoinhibins seem to be involved in the pathogenesis of diabetic complications (4, 5), cancer (6, 7), and pregnancy-associated diseases (8–10). At present, approximately one and a half dozen proteins have been ascribed to belong to the vasoinhibin-family. Each of them differs by precursor, the enzyme responsible for proteolytic generation of the respective vasoinhibin-isoform, and consequently by amino acid sequence and molecular mass. The generation of vasoinhibins has thoroughly been demonstrated in vitro, and ongoing research has led to accumulating information about their generation in vivo and their pathophysiological role and clinical significance in the aforementioned diseases. However, as more information surfaced, the need for a precise terminology designating a specific vasoinhibin-isoform under study rose in parallel. For example, the term “16K PRL,” often used to describe 16 kDa-like prolactin-derived vasoinhibins, does not discriminate between the different vasoinhibin-isoforms present. Of note, prolactin-derived vasoinhibins alone include several proteins with <4 kDa difference in molecular mass between 14 and 18 kDa. This is of relevance as it remains to be shown whether and to what extent each of the different proteases contributes to the physiological release of vasoinhibins and how the generation of vasoinhibins is modified under disease conditions. Accordingly, the total composition of endogenous vasoinhibins in the circulation or at the target-tissue level has yet to be determined. To address the need for a terminology with which it is possible to precisely differentiate between proteins ascribed to the vasoinhibin-family, Vazquez Rodriguez et al. proposed a classification according to precursor, proteolytic enzyme involved in the generation of the respective protein, its sequence and theoretical molecular mass, considering a variety of 20 proteins derived from prolactin, growth hormone, and placental lactogen (11). However, because of the presence of important limitations in understanding and a number of unresolved issues, which are presently inherent to the field, the proposed classification can only be considered as premature. It remains to be demonstrated if fragments derived from growth hormone and placental lactogen are generated in vivo, exert antiangiogenic effects and can consequently be classified as vasoinhibins. Also, there is neither understanding as to which differences in molecular mass impact function of vasoinhibins nor is there any evidence of a clinical relevance of vasoinhibins derived from growth hormone and placental lactogen. Lastly, significant differences in sequence, structure, and function of prolactin, growth hormone, and placental lactogen exist between species and naturally, the smallest fraction of the total body of evidence derives from studies in humans. Accordingly, a sustainable classification that provides orientation in future research and in the clinical context can only be proposed on the basis of substantial information on the structure and biological function or dysfunction of vasoinhibins in human health and disease. Further, a classification of hormones of the vasoinhibin-family should be proposed after being subject of a consensus conference of experts in the field. We suggest that an appropriate occasion for this consensus conference are the FASEB-Conferences on “The Growth Hormone/Prolactin Family in Biology and Disease,” during which new evidence can be reviewed and the possibility of a sustainable classification of hormones of the vasoinhibin-family can be reevaluated. Until such agreement is met, the field will benefit from the precise description of each vasoinhibin tested and discovered.

Higher prolactin and vasoinhibin serum levels associated with incidence and progression of retinopathy of prematurity

Background:Retinopathy of prematurity (ROP) is a potentially blinding, retinal neovascular disease. Systemic prolactin accesses the retina to regulate blood vessels. Prolactin is proangiogenic and can be cleaved to antiangiogenic vasoinhibins. We investigated whether circulating prolactin and vasoinhibins associate with incidence and progression of ROP.Methods:A prospective, longitudinal, case–control study covering postnatal weeks 1 to 9 measured serum prolactin, vasoinhibins, and vascular endothelial growth factor (VEGF) weekly in 90 premature infants diagnosed as ROP or control.Results:Prolactin levels were higher in ROP than in control patients before (106.2 ± 11.3 (SEM) vs. 64.7 ± 4.9 ng/ml, postnatal week 1) and during (120.6 ± 10 vs. 84.7 ± 7.5ng/ml, postnatal week 5) ROP diagnosis. Prolactin, but not gestational age, birth weight, Apgar score, sepsis, or ventilation time, correlated with ROP. The relative risk (RR) of developing ROP increased if Prolactin (PRL) levels were higher than thresholds of 80 ng/ml (RR = 1.55, 95% CI: 1.06–2.28), 100 ng/ml (RR = 1.63, 95% CI: 1.14–2.34), or 120 ng/ml (RR = 1.95, 95% CI: 1.41–2.68). Vasoinhibin levels were 39.7% higher (95% CI: 4.5–77.5) in the circulation of ROP than in control patients at postnatal week 1 and similar thereafter, whereas VEGF serum levels were always similar.Conclusion:High serum prolactin and vasoinhibin levels predict and may impact ROP progression.

Remarks on the Prolactin Hypothesis of Peripartum Cardiomyopathy

A seminal study in 2007 introduced the hypothesis that an antiangiogenic prolactin fragment with a molecular mass of 16 kDa is a key pathological mediator of peripartum cardiomyopathy (PPCM) (1). The study reported that this fragment is enzymatically generated by the cleavage of full-length prolactin with the lysosomal aspartyl protease cathepsin D. Upon excessive generation, possibly due to high pituitary prolactin secretion near term or postpartum and an enhanced oxidative micro-environment, this prolactin fragment would impair myocardial microvascularization and thereby contribute to myocardial dysfunction. Accordingly, a new therapy for PPCM was explored using the dopamine D2 receptor agonists, cabergoline and bromocriptine. Treatment with bromocriptine is currently being evaluated in a multicenter clinical trial (NCT00998556) (2). The concept underlying this putative therapy is the inhibition of the generation of the prolactin fragment by substrate depletion , i.e., the inhibition of pituitary prolactin secretion by activation of dopamine D2 receptors in lactotropes. PPCM is a rare disease which occurred with a frequency of 1 case/3189 live births and an estimated mortality of 1.36–2.05% (confidence interval 0.29–10.8%) from 1990 to 2002 in the United States (3). However, the incidence of PPCM seems to be variable, depending on the geographical region, ethnic background, and other criteria (4, 5). Since the initial discovery, several research, case report, and review articles have been published (5–10) describing signaling mechanisms mediating the deleterious action of the 16-kDa prolactin fragment and supporting the beneficial effects of treatment with dopamine D2 agonists in patients with PPCM. However, there are relevant aspects to the proposed pathological mechanism in PPCM that are absent in these studies with the consequence of limiting the field by pointing to wrong, or incomplete conclusions. In contrast to what is suggested in most of the PPCM-related literature, the 16-kDa prolactin fragment is only one of the several antiangiogenic prolactin fragments derived from prolactin via cathepsin D and other proteolytic enzymes. Altogether, these fragments of different molecular masses comprise a family of proteins termed vasoinhibins (11, 12). Cathepsin D alone can generate four more vasoinhibins by cleaving full-length prolactin at sites other than the one generating the 16-kDa fragment (13). Three of these cathepsin D-generated vasoinhibin isoforms have documented antiangiogenic activity (11, 13)—a notion that should not go unnoticed when studying the 16-kDa vasoinhibin isoform as a key pathologic mediator of PPCM. The possible contribution of other vasoinhibin isoforms to the pathophysiology of PPCM has neither been investigated …