Placental protein and peptide hormone synthesis: impact of maternal nutrition

The human placenta is an organ with a long period of growth in cell number later succeeded by cessation of cell division but some continued growth in cell size. The RNA concentration and content per cell (RNA/DNA ratio) are reduced between the end of the first trimester and the end of pregnancy, and there is a change in availability of placental chromatin for transcription when incubated in vitro with RNA polymerase II. Synthesis and secretion of placental peptide hormones on membrane-bound polyribosomes also undergo changes during pregnancy. During early pregnancy, levels of human chorionic gonadotropin are maximal, declining in later pregnancy, levels of human chorionic gonadotropin are maximal, declining in later pregnancy. The messenger RNA for this hormone undergoes similar changes in relative amount in the placenta. In contrast, the plasma level of placenta lactogen increases progressively during pregnancy, and in parallel with this, the placenta content of mRNA for this hormone increases throughout later pregnancy. It is concluded that placental programing regulates the relative amounts of mRNA for each hormone, and this in turn determines the amounts secreted at any stage of pregnancy. Nutritional status of the mother can affect placental RNA content, most clearly established in studies on rats in which a diet low in protein or with added alcohol results in a reduced capacity to form and secrete placental lactogen. The extent of this depression parallels the reduction in placental RNA content. It is suggested that underproduction of placental lactogen may be a factor in reducing flow of nutrients from the maternal tissues to the fetus in later pregnancy, under conditions of malnutrition.