Loss of low-density lipoprotein utilization by regressing porcine luteal cells: effects of protein kinase C activation

We recently reported a decline in low-density lipoprotein (LDL) uptake by regressing porcine luteal cells that correlated with diminished LDL stimulation of progesterone (P) production. The objectives of the present study were to determine 1) whether loss of LDL utilization is a specific lesion in the steroidogenic pathway in regressing luteal cells and 2) whether in vitro activation of protein kinase C (PKC) in mid-cycle luteal cells acutely suppresses LDL utilization. Dispersed cells (4 X 10(4)/0.02 ml) from mid- (Days 6-10, estrus = Day 0; n = 5) or late- (Days 15-18; n = 7) cycle porcine CL were cultured in Dulbecco's Modified Eagle/F-12 medium supplemented with insulin, transferrin, sodium selenite, and aprotinin for 24 h with human (h) LDL (0-100 microgram/ml), 22(R)-hydroxycholesterol (22[OH]-C; 0-25 microgram/ml), or pregnenolone (0-1000 nM). P production by mid-cycle luteal cells was dose-dependently increased (p < 0.05) by LDL (up to 2.8-fold), 22(OH)-C (up to 3.2-fold), and pregnenolone (> 3-fold). In contrast, LDL (10-100 microgram/ml) failed to stimulate P production by late-cycle luteal cells. But 22(OH)-C (up to 4-fold) and pregnenolone (> 10-fold) were as effective in promoting (p < 0.05) steroidogenesis relative to basal levels in late luteal cells as in mid-cycle cultures. The PKC activator, TPA (12-O-tetradecanoyl-phorbol-13-acetate; 10 ng/ml), inhibited (approximately 20%; p < 0.01) basal and LDL-supported steroidogenesis, but did not suppress 22(OH)-C-stimulated P secretion. Since the kinetics and degree of suppression of basal and LDL-stimulated P production were identical, the data argue against a specific action on LDL metabolism. The data suggest that regressing porcine luteal cells lose their ability to utilize LDL as a source of cholesterol in the presence of continued steroidogenic enzyme activity. PKC activation does not appear to acutely alter LDL uptake or degradation.