Expression and function of a scavenger lipoprotein pathway in porcine luteal cells

A pathway(s) for uptake of modified (e.g., acetylated, oxidized) low-density lipoprotein (LDL) moieties has been recently discovered on luteal cells of certain species. The expression and function of this pathway during the life span of the corpus luteum (CL) have not been investigated. Aims of the present study were the following: 1) to determine whether porcine small and large luteal cells take up modified LDL; if so, 2) to compare uptake of native and modified LDL by luteal cell subpopulations during the luteal life span and 3) to compare effects of native and modified LDL on luteal steroidogenesis during the estrous cycle. Collagenase-dispersed luteal cells were prepared from porcine ovaries at various stages of the estrous cycle: early (E, Days 4-6), mid (M, Days 8-10), mid-late (ML, Day 13-14), or late (L, Day 16-17) (estrus = Day 0). Cells were incubated with fluorescent-tagged LDL (Dil-LDL; 0-10 microg/ml) or acetylated LDL (Dil-AC-LDL; 0-10 microg/ml). Fluorescence was analyzed by multiparameter flow cytometry in each of three subpopulations of cells: small (SLC) and large (LLC) luteal cells and nonsteroidogenic cells. The percentage of LLC taking up Dil-LDL remained relatively constant (65-75%) from E to ML cycle and then declined (13.3 +/- 4.1%; p < 0.05); these findings were consistent with previous data. In contrast, the percentage of LLC taking up Dil-AC-LDL gradually increased from E (29.8 +/- 8.5%) to ML (68.3 +/- 5.9%) stage and then declined (17.1 +/- 2.3%; p < 0.05). Similarly, Dil-LDL uptake by SLC was relatively constant (15-20%) from E to M cycle, declining to 2.9 +/- 0.5% at L cycle. Dil-AC-LDL uptake by SLC progressively increased from E (5.4 +/- 2.6%) to ML (24.3 +/- 6.9%) and then fell somewhat (12.9 +/- 6.7%) in L cycle. Few (< 2%) nonsteroidogenic cells labeled for Dil-LDL at all stages, whereas Dil-AC-LDL uptake by nonsteroidogenic cells was similar to that by SLC. Dual-uptake experiments revealed subtypes of LLC that took up either native LDL only or both native and modified LDL. Progesterone (P4) production by E and M luteal cell cultures was dose-dependently increased (p < 0.05) by both native and modified LDL. In contrast, modified LDL suppressed (up to 40-50%, p < 0.05) steroidogenesis by ML and L cultures, whereas LDL had no significant effect. Only native LDL stimulated P4 production by isolated SLC, although both native and modified LDL similarly increased P4 production by LLC. In conclusion, expression of one or more scavenger LDL pathways on porcine luteal cell subtypes is dynamic. Moreover, modified LDL can exert both stimulatory and inhibitory effects on luteal steroidogenesis in vitro. Differential expression and function of a scavenger LDL pathway by subpopulations of cells within the CL may play a novel role in luteal development, function, and/or regression.