Human placental lactogen mRNA and its structural genes during pregnancy: quantitation with a complementary DNA.

A complementary DNA (cDNA) strand was transcribed from human placental lactogen (hPL) mRNA. Based on alkaline sucrose gradient centrifugation, the size of the cDNA was about 8 S, which would represent at least 80% of the hPL mRNA. Previously we showed that four to five times more hPL was synthesized in cell-free extracts derived from term as compared to first trimester placentas. Hybridization of the cDNA with RNA derived from placental tissue revealed that there was about four times more hPL mRNA sequences in total RNA from term placenta than in a comparable quantity of total first trimester RNA. Only background hybridization was observed when the cDNA was incubated with RNA prepared from human kidney. To test if this differential accumulation of hPL mRNA was the result of an amplification of hPL genes, we hybridized the labeled cDNA with cellular DNA from first trimester and term placentas and with DNA isolated from human brain. In all cases, the amount of hPL sequences was approximately two copies per haploid genome. Thus, the enhanced synthesis of hPL mRNA appears to result from a transcriptional activation rather than an amplification of the hPL gene. The increase likely reflects placental differentiation in which the proportion of syncytial trophoblast increases at term.