Effects of high density lipoprotein containing high or low β-carotene concentrations on progesterone production and β-carotene uptake and depletion by bovine luteal cells

Luteal cells were isolated from mid-luteal heifer ovaries by collagenase digestion. Cells were cultured with DMEM/Hamʹs F12 medium in serum pre-treated plastic culture dishes for periods of up to 11 days. As β-carotene is almost completely insoluble in all polar solvents, it was added to cultures in either dimethyl sulphoxide (DMSO), tetrahydrofuran (THF) or as high-density lipoprotein (HDL) containing high or low β-carotene concentrations. Medium was replaced after 24 h, thereafter medium was changed every 48 h. Treatment of cells with DMSO alone or with β-carotene (5 μmol/l) in DMSO both resulted in significant (P<0.01) stimulation of progesterone production. β-Carotene (5 μmol/l) in THF did not alter progesterone production but 50 μmol/l β-carotene in THF resulted in significant inhibition (P<0.02) of progesterone production on days 3 and 7. Cultures were also supplemented with bovine HDL preparations containing equal concentrations of cholesterol (25 μg/ml) but high or low β-carotene (12.4 or 0.44 μg/mg of cholesterol). Both HDL preparations significantly stimulated progesterone production (P<0.001) but the high β-carotene HDL was significantly (P<0.02) more effective than the low β-carotene HDL. However, when given together with bovine luteinizing hormone (bLH) or dibutyryl cAMP (dbcAMP), the high β-carotene HDL stimulated progesterone production less than did the low HDL (P<0.01). Uptake and depletion of β-carotene by luteal cells were also examined in culture. β-Carotene supplementation increased luteal cell β-carotene from an initial level of 373 ng per 106 cells to 2030 ng per 106 cells by day 6. In contrast, the levels in control cells decreased to 14% of starting values during the same period. Cells treated with HDL containing high β-carotene on day 1 or days 1 and 3 were then incubated with or without bLH or dbcAMP for a further 2 days to investigate the effect of bLH and dbcAMP on depletion of β-carotene by luteal cells. β-Carotene depletion in the luteal cells was significantly higher (P<0.05) in LH- and dbcAMP-treated cells than in the control cells in both groups. These results indicate that the use of solvents such as DMSO or THF may have undesirable effects due to alteration of cell membrane permeability. Supplementation with bLH or dbcAMP may increase the metabolism of β-carotene in luteal cells. bLH or dbcAMP together with high β-carotene HDL may, when combined with the effect of increased β-carotene metabolism, give less stimulation than with low β-carotene HDL.