About Placental Lactogen:
Placental lactogen is also known as chorionic somatomammotropin. It is a polypeptide placental hormone and part of the somatotropin family. Its structure and function are both similar to that of growth hormone. It changes the metabolic state of the mother during pregnancy in order to supply energy to the developing fetus. Placental lactogen I and II were identified as prolactin-like molecules that have the ability to bind to prolactin receptors with high affinity. They even mimic the actions of prolactin. These hormones can contribute to lactogenesis, progesterone production in rats during the later stages of gestation and also luteal maintenance. Placental lactogen I may be an important factor in stimulating mammary cell proliferation and in stimulating some of the adaptations of the maternal lipid and carbohydrate metabolism.
Placental lactogen has anti-insulin properties and is a hormone secreted by the syncytiotrophoblast during pregnancy. Much like human growth hormone, Placental lactogen is encoded by genes on chromosome 17q22-24 and was identified in 1963.
Structure of Human Placental Lactogen
Placental lactogen molecular mass is 22,125 dalton and contains single chain that consists of 191 amino acid residues that are linked by two disulfide bonds and the structure contains 8 helices. A crystal structure of Placental lactogen was determined via X-ray diffraction to a resolution of 2.0 Å.
Levels of Human Placental Lactogen
Placental lactogen is only present during pregnancy. Maternal serum levels rise in relation to the growth of the fetus and placenta. Maximum levels of Placental lactogen are reached near term. Higher levels are noted in patients with multiple gestation. A small amount of hPL enters the fetal circulation. Its biological half-life is 15 minutes.
Physiologic Function
The following manners describe how Placental lactogen affects the metabolic system of the maternal organism:
● In a bioassay, Placental lactogen mimics the actions of prolactin. It is currently unclear whether Placental lactogen has any role in human lactation
● Lowered maternal insulin sensitivity which leads to an increase in maternal blood glucose levels
● Lowered maternal glucose utilization which helps to ensure adequate fetal nutrition
● Increased lipolysis with the release of free fatty acids
These functions help to support fetal nutrition, even during times of maternal malnutrition. Placental lactogen is a potent agonist of the prolactin receptor and a weak agonist of the growth hormone receptor.
Growth Hormone-Like Activity
Placental lactogen has weak actions like those of growth hormone. This causes the formation of protein tissues in a similar way to that of growth hormone. However, a hundred times more Placental lactogen is required to promote growth. An enhancer for the human placental lactogen gene is found 2 kb downstream of the gene and it participates in cell-specific control gene expression.