Three mRNA transcripts of the proopiomelanocortin gene in human placenta at term

The proopiomelanocortin (POMC) gene whose normal pituitary specific mRNA product is 1200 bases (b) is also expressed in placenta and its peptide derivatives such as ACTH and beta-endorphin may play an important role in the initiation of labor. So far, two mRNA transcripts, one small (800b) and one large (1380b) have been reported in placenta by Northern blot analysis, similar to other endocrine tissues and various extrapituitary tumors; however, it is questionable whether both of these transcripts are effectively translated to a functional protein. We examined by Northern blot analysis the size and the differential expression of placental POMC gene transcripts in pregnant women with different modes of delivery. Placental tissues were collected from two groups of pregnant women, six with vaginal delivery (VD) and five with cesarean section (CS). In both groups of placentae three POMC gene transcripts were detected of 800, 1200 and 1380 bases; the 1200b pituitary specific species often predominated and was always present. The 800b transcript was also always present, while the large transcript (1380b) was expressed in 3/6 VD and 2/5 CS placental tissues. No differences in the relative levels of any of these mRNA species showing effect of the mode of delivery were observed. We conclude that POMC gene transcription in placental tissue at term gives rise to three mRNA transcripts, thus resembling extrapituitary tumors. The reported changes in the levels of the derivative peptides according to the mode of delivery do not reflect changes in POMC mRNA levels and could be attributed to a post-translational effect.