Gonadotropin surge increases fluorescent-tagged low-density lipoprotein uptake by macaque granulosa cells from preovulatory follicles

In the primate ovary, luteal steroidogenesis is largely dependent upon cholesterol derived from receptor-mediated uptake of circulating low-density lipoprotein (LDL). However, granulosa cells (GC) of preovulatory follicles possess few LDL binding sites compared to those present in developing and mature corpora lutea. We recently reported (Endocrinology 1991; 129:3247- 3253) that uptake of LDL tagged with the fluorescent probe 1,1'-dioctadecyl- 3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) can be monitored in macaque luteal cells by fluorescence-activated flow cytometry. This study was designed to determine whether an ovulatory stimulus induced uptake of DiI-LDL in GC aspirated from preovulatory follicles of macaques undergoing ovarian stimulation. Development of multiple large follicles was stimulated in adult rhesus macaques with human gonadotropin treatment for 9 days. On Day 10, monkeys received either no ovulatory stimulus or 1000 IU hCG to initiate ovulatory events. GC were aspirated on Day 10 in monkeys receiving no ovulatory stimulus (nonluteinized GC) or 27 h or 34 h after hCG injection (luteinizing GC). GC were resuspended in Ham's F-10 medium + 0.1% BSA and incubated with several concentrations (0-25 μg/ml) of DiI-LDL (Biomedical Technologies, Stoughton, MA) for various time intervals (2-60 min). DiI-LDL uptake by GC was time- and concentration-dependent. Coincubation of cells with DiI-LDL and unlabeled LDL dose-dependently suppressed the percentage of fluorescent cells. In contrast, coincubation with up to a 250-fold excess of acetylated LDL or high-density lipoprotein did not alter the percentage of fluorescent GC. Whereas 89.4 ± 1.1% (n = 12 monkeys) of luteinizing GC incubated with DiI-LDL were fluorescent, only 32.2 ± 7.8% (p < 0.05; n = 5) of nonluteinized GC fluoresced. Moreover, the mean fluorescence intensity of DiI-LDL-positive luteinizing GC (111 ± 5 units) exceeded (p < 0.05) that of DiI-LDL-positive nonluteinized cells (37 ± 6 units). The percentage of DiI- LDL-labeled luteinizing GC did not correlate with follicular or luteal phase serum estradiol or progesterone levels or luteal phase length. However, the percentage of fluorescent nonluteinized GC correlated positively (p < 0.05) with follicular estradiol secretion. These results suggest that induction of LDL receptor expression occurs in GC during the early stages of luteinization in response to the ovulatory gonadotropin surge.