Prolactin
Introduction
Prolactin is a peptide hormone primarily associated with lactation, but it also plays a significant role in various other physiological processes. It is produced by the anterior pituitary gland and, to a lesser extent, by other tissues including the immune system, the brain, and the uterus. This article delves into the multifaceted roles of prolactin, its regulation, its clinical significance, and its broader implications in human health.
Structure and Synthesis
Prolactin is a single-chain polypeptide consisting of 199 amino acids. It is synthesized in the lactotroph cells of the anterior pituitary gland. The gene encoding prolactin is located on chromosome 6 in humans. The synthesis of prolactin involves transcription of the prolactin gene into mRNA, which is then translated into the prolactin protein. Post-translational modifications, including glycosylation, can occur, affecting the hormone's stability and activity.
Regulation of Prolactin Secretion
The secretion of prolactin is primarily regulated by the hypothalamus through the release of dopamine, which inhibits prolactin secretion. Conversely, thyrotropin-releasing hormone (TRH) and estrogens stimulate prolactin release. During pregnancy and lactation, prolactin levels are significantly elevated due to the increased production of estrogens and the suckling stimulus, which reduces dopamine inhibition.
Physiological Roles
Lactation
Prolactin's most well-known function is its role in lactation. It stimulates the mammary glands to produce milk (lactogenesis) after childbirth. Prolactin levels rise during pregnancy, but the high levels of progesterone and estrogen inhibit milk production until after delivery, when these hormone levels drop.
Reproductive System
Prolactin has various roles in the reproductive system. It influences the menstrual cycle and has been implicated in the regulation of gonadotropin-releasing hormone (GnRH) secretion, which affects luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels. Elevated prolactin levels can lead to anovulation and amenorrhea.
Immune System
Prolactin acts as an immunomodulator, influencing the immune system by promoting the proliferation of lymphocytes and the production of cytokines. It has been shown to affect both innate and adaptive immune responses.
Metabolism
Prolactin has metabolic effects, including the regulation of lipid metabolism, carbohydrate metabolism, and osmoregulation. It influences adipose tissue and has been linked to insulin resistance and obesity.
Clinical Significance
Hyperprolactinemia
Hyperprolactinemia is a condition characterized by elevated levels of prolactin in the blood. It can result from various causes, including prolactinomas (prolactin-secreting tumors), hypothyroidism, and certain medications. Symptoms in women include galactorrhea, amenorrhea, and infertility. In men, it can cause hypogonadism, erectile dysfunction, and gynecomastia.
Hypoprolactinemia
Hypoprolactinemia is a condition of abnormally low prolactin levels. It is less common and can be associated with pituitary disorders, Sheehan's syndrome, or the use of dopamine agonists. Symptoms may include lactation failure and infertility.
Prolactin and Cancer
Prolactin has been implicated in the development and progression of certain cancers, particularly breast cancer and prostate cancer. It can promote cell proliferation and inhibit apoptosis, contributing to tumor growth.
Measurement and Diagnosis
Prolactin levels are measured using immunoassays, such as enzyme-linked immunosorbent assay (ELISA) or radioimmunoassay (RIA). Blood samples are typically taken in the morning, as prolactin levels exhibit diurnal variation. Elevated prolactin levels require further investigation to determine the underlying cause, which may include imaging studies like magnetic resonance imaging (MRI) of the pituitary gland.
Treatment
Treatment of prolactin-related disorders depends on the underlying cause. Dopamine agonists, such as bromocriptine and cabergoline, are commonly used to reduce prolactin levels in cases of hyperprolactinemia. Surgery or radiation therapy may be necessary for large prolactinomas. Hormone replacement therapy may be indicated for hypoprolactinemia.
Research and Future Directions
Ongoing research is exploring the broader roles of prolactin in health and disease. Studies are investigating its involvement in autoimmune diseases, metabolic syndrome, and neuropsychiatric disorders. Advances in understanding prolactin's molecular mechanisms may lead to new therapeutic targets and treatments.