Prolactin Receptor Signal Transduction

Exploring the association of serum prolactin with serum glucose levels and clinical findings in a cohort of patients with early rheumatoid arthritis

BACKGROUND

In the context of rheumatoid arthritis and its systemic inflammatory implications, there is an increasing interest in investigating the role of prolactin in the clinical and metabolic aspects of the disease. This study aimed to explore the potential links between serum prolactin levels, serum glucose levels, and the clinical manifestations of arthritis.

METHODS

This exploratory, cross-sectional, observational study focused on women diagnosed with rheumatoid arthritis. The research involved assessing prolactin and blood glucose concentrations, alongside specific clinical traits such as disease-related inflammation, morning stiffness, and fatigue intensity. The presence of changes in serum prolactin (PRL) was initially compared among the groups based on disease activity intensity. Using a multinomial regression analysis, the study analyzed the impact of predetermined clinical and metabolic factors on various categories of prolactin concentration.

RESULTS

Out of the 72 participants included in the study, hyperprolactinemia was detected in 9.1% of the sample. No differences in serum PRL were identified among the evaluated groups based on disease activity. Following multivariate analysis, no statistically significant differences were identified for the outcomes of inflammatory activity and morning stiffness within each PRL category when compared to the reference category for PRL. There was no increased likelihood of encountering blood glucose levels below 100 mg/dl among individuals with higher prolactin concentrations compared to those in the lowest prolactin category (OR 5.43, 95% CI 0.51-58.28). The presence of clinically significant fatigue revealed a higher likelihood of encountering this outcome among patients with intermediate PRL values (prolactin categories 7.76-10.35 with OR 5.18, 95% CI 1.01-26.38 and 10.36-15.29 with OR 6.25, 95% CI 1.2-32.51) when compared to the reference category.

CONCLUSIONS

The study found no discernible correlation between prolactin concentrations and worse scores for inflammatory activity of the disease, nor between prolactin concentrations and serum glucose levels. The findings regarding fatigue should be approached with caution given the exploratory nature of this study.

A Novel Mechanism of hPRL-G129R, a Prolactin Antagonist, Inhibits Human Breast Cancer Cell Proliferation and Migration

Prolactin (PRL) and its receptor, PRLR, are closely related to the occurrence and development of breast cancer. hPRL-G129R, an hPRLR antagonist, has been found to induce apoptosis in breast cancer cells via mechanisms currently unknown. Recent studies have indicated that PRLR exhibits dual functions based on its membrane/nucleus localization. In that context, we speculated whether hPRL-G129R is a dual-function antagonist. We studied the internalization of the hPRLR-G129R/PRLR complex using indirect immunofluorescence and Western blot assays. We found that hPRL-G129R not only inhibited PRLR-mediated intracellular signaling at the plasma membrane, but also blocked nuclear localization of the receptor in T-47D and MCF-7 cells in a time-dependent manner. Clone formation and transwell migration assays showed that hPRL-G129R inhibited PRL-driven proliferation and migration of tumor cells in vitro. Further, we found that increasing concentrations of hPRL-G129R inhibited the nuclear localization of PRLR and the levels of signal transducer and activator of transcription (STAT) 5 in tumor-bearing mice and hPRL-G129R also exerted an antiproliferative effect in vivo. These results indicate that hPRL-G129R is indeed a dual-function antagonist. This study lays a foundation for exploring and developing highly effective agents against the proliferation and progression of breast malignancies.

Unmodified Prolactin (PRL) and S179D PRL-Initiated Bioluminescence Resonance Energy Transfer between Homo- and Hetero-Pairs of Long and Short Human PRL Receptors in Living Human Cells

We have used bioluminescence resonance energy transfer (BRET) to examine the interaction between human prolactins (PRLs) and the long (LF) and two short isoforms (SF1a and SF1b) of the human PRL receptor in living cells. cDNA sequences encoding the LF, SF1a, and SF1b were subcloned into codon-humanized vectors containing cDNAs for either Renilla reniformis luciferase (Rluc) or a green fluorescent protein (GFP2) with a 12- or 13-amino acid linker connecting the parts of the fusion proteins. Transfection into human embryonic kidney 293 cells demonstrated maintained function of Rluc and GFP2 when linked to the receptors, and confocal microscopy demonstrated the localization of tagged receptors in the plasma membrane by 48 h after transfection. All three tagged receptors transduced a signal, with the LF and SF1a stimulating, and SF1b inhibiting, promoter activity of an approximately 2.4-kb β-casein-luc construct. Both unmodified PRL (U-PRL) and the molecular mimic of phosphorylated PRL, S179D PRL, induced BRET with all combinations of long and short receptor isoforms except SF1a plus SF1b. No BRET was observed with the site two-inactive mutant, G129R PRL. This is the first demonstration, 1) that species homologous PRL promotes both homo- and hetero-interaction of most long and short PRLR pairs in living cells, 2) that both U-PRL and S179D PRL are active in this regard, and 3) that there is some aspect of SF1a-SF1b structure that prevents this particular hetero-receptor pairing. In addition, we conclude that preferential pairing of different receptor isoforms is not the explanation for the different signaling initiated by U-PRL and S179D PRL.

Effects of prolactin on signal transduction and gene expression: possible relevance for systemic lupus erythematosus

Receptors for prolactin (PRL-R) are expressed in normal leukocytes from rat and man. PRL signals through PRL-R associated Janus tyrosine kinase (Jak)-2 and signal transducers and activators of transcription (Stat). In addition, in human leukocytes PRL also activates the p38 MAP kinase pathway. PRL, at physiological concentrations, stimulates the expression of the interferon regulatory factor (IRF)-1 gene in rat spleen and bone marrow cells. In man, genes induced by PRL include several members of the 'suppressors of cytokine signaling' (SOCS) family and inducible nitric oxide synthase (iNOS; in mononuclear cells and in granulocytes) and IRF-1 (in granulocytes). Thus, in normal leukocytes, PRL induces the expression of several genes relevant to innate and acquired immune responses. Sex hormones, such as estrogen and PRL, have been implicated in the pathogenesis of murine and human SLE. Also defective signaling in leukocytes is a feature of the disease. What the origin is of aberrant signaling processes in SLE lymphocytes and how they relate to tolerance breakdown and immunopathology is still unknown. It is not unlikely that PRL is a player at some level. The exact contribution of PRL to immune responses in normal subjects and in SLE patients is not known. Further work should also indicate whether PRL might contribute to the onset or progression of the disease and assess the possible benefits of manipulating PRL concentrations in patients.