Human placental lactogen (hPL), also called human chorionic somatomammotropin (HCS), is a

polypeptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. A p ...

placental hormone, the human form of

placental lactogen Placental lactogen, also called chorionic somatomammotropin, is a polypeptide placental hormone, part of the somatotropin family. Its structure and function is similar to that of growth hormone. It modifies the metabolic state of the mother durin ...

( chorionic somatomammotropin). Its structure and function are similar to those of human growth hormone. It modifies the metabolic state of the mother during pregnancy to facilitate energy supply to the fetus. hPL has anti-

insulin Insulin (, from Latin ''insula'', 'island') is a peptide hormone produced by beta cells of the pancreatic islets encoded in humans by the ''INS'' gene. It is considered to be the main anabolic hormone of the body. It regulates the metabolism o ...

properties. hPL is a hormone secreted by the

syncytiotrophoblast Syncytiotrophoblast (from the Greek 'syn'- "together"; 'cytio'- "of cells"; 'tropho'- "nutrition"; 'blast'- "bud") is the epithelial covering of the highly vascular embryonic placental villi, which invades the wall of the uterus to establish nut ...

during pregnancy. Like

human growth hormone Growth hormone (GH) or somatotropin, also known as human growth hormone (hGH or HGH) in its human form, is a peptide hormone that stimulates growth, cell reproduction, and cell regeneration in humans and other animals. It is thus important in h ...

, hPL is encoded by genes on chromosome 17q22-24. It was identified in 1963.

Structure

hPL molecular mass is 22 125 Da and contains single chain consisting of 191 amino acid residues that are linked by two disulfide bonds and the structure contains 8 helices. A crystal structure of hPL was determined by

X-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

to a resolution of 2.0 Å.

Levels

hPL is present only during pregnancy, with maternal serum levels rising in relation to the growth of the fetus and placenta. Maximum levels are reached near term, typically to 5–7 mg/L. Higher levels are noted in patients with multiple gestation. Little hPL enters the fetal circulation. Its biological half-life is 15 minutes.

Physiologic function

hPL affects the metabolic system of the maternal organism in the following manners: * In a bioassay, hPL mimics the action of prolactin, yet it is unclear whether hPL has any role in human lactation. * Metabolic: ** ↓ maternal

sensitivity (insulin resistance), leading to an increase in maternal blood glucose levels. ** ↓ maternal glucose utilization, which helps ensure adequate fetal nutrition (the mother responds by increasing beta cells). Chronic hypoglycemia leads to a rise in hPL. ** ↑ lipolysis with the release of free fatty acids. With fasting and release of hPL, free fatty acids become available for the mother as free fatty acids do not cross the placenta, so that relatively more glucose can be utilized by the fetus. With sustained fasting, maternal

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bo ...

s formed from free fatty acids can cross the placenta and be used by the fetus. These functions help support fetal nutrition even in the case of maternal malnutrition. hPL is a potent

agonist An agonist is a chemical that activates a receptor to produce a biological response. Receptors are cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an antagonist blocks the action of the ago ...

of the prolactin receptor and a weak agonist of the growth hormone receptor.

Prolactin-like activity

hPL has been found to bind to the prolactin receptor with equal

affinity Affinity may refer to: Commerce, finance and law * Affinity (law), kinship by marriage * Affinity analysis, a market research and business management technique * Affinity Credit Union, a Saskatchewan-based credit union * Affinity Equity Partn ...

to that of prolactin in rabbit milk fat globule membrane, and hPL and prolactin have been found to possess very similar lactogenic activity '' in vitro'' in mouse and rat mammary gland

explant In biology, explant culture is a technique to organotypically culture cells from a piece or pieces of tissue or organ removed from a plant or animal. The term ''explant'' can be applied to samples obtained from any part of the organism. The extracti ...

s. In addition, hPL has been found to stimulate DNA synthesis in human mammary

fibroadenoma Fibroadenomas are benign breast tumours characterized by an admixture of stromal and epithelial tissue. Breasts are made of lobules (milk producing glands) and ducts (tubes that carry the milk to the nipple). These are surrounded by glandular, ...

cells transplanted into mice, which suggests that hPL promotes the growth of the human mammary gland similarly to prolactin. As hPL circulates at concentrations that are 100-fold higher than those of prolactin during pregnancy, these findings suggest that hPL may play an important role in human

mammogenesis Breast development, also known as mammogenesis, is a complex biological process in primates that takes place throughout a female's life. It occurs across several phases, including prenatal development, puberty, and pregnancy. At menopause, bre ...

during this time. However, the relative affinities of hPL and prolactin for the human prolactin receptor have yet to be published and the effects of hPL on normal human mammary epithelial tissue have not yet been investigated, and so a definitive role of hPL in human mammary gland development during pregnancy has not been established at present.

Growth hormone-like activity

hPL has weak actions, similar to those of growth hormone, causing the formation of protein tissues in the same way that growth hormone, but 100 times more hPL than growth hormone is required to promote growth. However, hPL has a blood level of more than 50 times that of hGH, hence its effects must not be ignored. An enhancer for the human placental lactogen gene is found 2 kb downstream of the gene and participates in the cell-specific control gene expression.

Clinical measurement

While hPL has been used as an indicator of fetal well-being and growth, other

fetal testing Prenatal testing consists of prenatal screening and prenatal diagnosis, which are aspects of prenatal care that focus on detecting problems with the pregnancy as early as possible. These may be anatomic and physiologic problems with the health ...

methods have been found to be more reliable. Also, normal pregnancies have been reported with undetectable maternal levels of hPL.

References

* *

External links

* {{Prolactin receptor modulators Peptide hormones Hormones of the placenta Hormones of the pregnant female Human female endocrine system