Human placental lactogen

Human placental lactogen (HPL), also called human chorionic somatomammotropin, is a polypeptide placental hormone. Its structure and function is similar to that of human growth hormone. It modifies the metabolic state of the mother during pregnancy to facilitate the energy supply of the fetus. HPL has anti-insulin properties. HPL is a hormone secreted by the syncytiotrophoblast during pregnancy. Like human growth hormone, HPL is encoded by genes on chromosome 17q22-24. It was identified in 1963.

Structure
HPL molecular weight is 22,125 and contains one unique 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 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 biologic half-life is 15 minutes.

Function
In a bioassay HPL mimics the action of prolactin, yet it is unclear whether HPL has any role in human lactation.

HPL affects the metabolic system of the maternal organism. HPL decreases maternal insulin sensitivity, and, therefore, raises maternal blood glucose levels, whilst decreasing maternal glucose utilization, which helps ensure adequate fetal nutrition (the mother responds by pancreatic endocrine upregulation; this is thought to represent an evolutionary 'arms-race' resulting from the conflict of interest between mother and fetus). Chronic hypoglycemia leads to a rise in HPL. HPL induces lipolysis with the release of free fatty acids. With fasting and release of HPL, free fatty acids become available for the maternal organism as fuel, so that relatively more glucose can be utilized by the fetus. Also, ketones 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.

This hormone 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. . 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 of HPL
While HPL has been used as an indicator of fetal well-being and growth, other fetal testing methods have been found to be more reliable. Also, normal pregnancies have been reported with undetectable maternal levels of HPL.