DEVELOPMENTAL INCREASE IN LOW-DENSITY-LIPOPROTEIN RECEPTOR MESSENGER-RIBONUCLEIC-ACID LEVELS IN PLACENTAL SYNCYTIOTROPHOBLASTS DURING BABOON PREGNANCY

We have previously shown that there was an estrogen-regulated developm ental increase in low density lipoprotein (LDL) uptake by placental sy ncytiotrophoblasts during baboon pregnancy. To determine whether this reflected enhanced expression of the LDL receptor, the levels of LDL r eceptor messenger RNA (mRNA) were determined by Northern blot and reve rse transcription-polymerase chain reaction in placental tissue obtain ed from baboons (Papio anubis) in early (days 58-64; pooled to yield 5 samples), mid- (days 97-110; pooled to yield 12 samples), and late (d ays 161-175; pooled to yield 15 samples) gestation (term = 184 days). Whole villous tissue and a trophoblast cell fraction isolated by 50% P ercoll gradient centrifugation were analyzed. The latter cell fraction was equally comprised predominantly of syncytiotrophoblasts at early, mid-, and late gestation as determined by extensive immunocytochemica l reactivity with antisera to syncytiotrophoblast-specific peptides. T issues were extracted with guanidine isothiocyanate and 5 mu g polyade nylated-enriched RNA hybridized to a P-32-labeled human LDL receptor c omplementary DNA (cDNA). A major 6.2-kilobase LDL receptor mRNA transc ript was expressed in syncytiotrophoblasts and whole villous tissue, a s determined by Northern blot. In the syncytiotrophoblast-rich cell fr action, LDL receptor mRNA levels, analyzed by Northern blot and autora diodensitometry and expressed as a ratio of beta-actin, were similarly low in early (0.66 +/- 0.12 arbitrary units) and mid- (1.15 +/- 0.23) gestation, then increased to a level in late gestation (2.71 +/- 0.33 ) that was over 4- and 2-fold greater (P < 0.01) than that in early or midgestation, respectively. In contrast, in whole villous tissue, LDL receptor and beta-actin mRNA levels exhibited no consistent change or decreased slightly with advancing pregnancy, so that when corrected f or beta-actin, LDL receptor mRNA levels were similar in early (1.53 +/ - 0.33), mid- (1.44 +/- 0.16), and late (2.32 +/- 0.29) gestation. The unchanged levels of LDL receptor mRNA in whole placental villous tiss ue with advancing primate gestation may reflect potential confounding effects that nontrophoblast, e.g. vascular, components of the developi ng placenta may have on assessing trophoblast endocrine function in wh ole villous tissue. Amplification of trophoblast RNA by reverse transc ription-polymerase chain reaction with LDL receptor primers generated a single cDNA product of approximately 258 base pairs. The level of LD L receptor mRNA, semiquantified by serial dilution of input trophoblas t cDNA in representative samples, was 3.7-fold greater in trophoblast cells of late vs. mid gestation, a result similar to the 3.2-fold diff erence in LDL receptor mRNA levels determined by Northern blot on the same RNA samples. Therefore, we conclude that there is a developmental increase in expression of the LDL receptor and, thus, in receptor-med iated LDL uptake by syncytiotrophoblasts with advancing baboon pregnan cy.