Conversion of
retinol to
3,4-didehydroretinol is probably a rate-limiting step in the formation of
3,4-didehydroretinoic acid, a candidate
ligand for nuclear
retinoid receptors in human epidermal keratinocytes. To investigate whether this metabolic pathway also exists in other cell systems, we compared the
retinoid concentrations and the bioconversion of [3H]
retinol to [3H]3,4-didehydroretinol in human primary keratinocytes, human cervical
carcinoma (HeLa) cells, human
melanoma (JKM86-4) cells, monkey kidney epithelium (CV-1) cells, and murine
teratocarcinoma (F9) cells. The cellular
retinol concentration ranged from 2.33 to 99.1 pmol/mg
protein with the highest values observed in keratinocytes.
3,4-Didehydroretinol was only detected in cells of human origin and its concentration ranged from 0.24 pmol/mg in HeLa to 34.6 pmol/mg in the keratinocytes. Incubation with [3H]
retinol for 1-24 h resulted in a rapid appearance of [3H]3,4-didehydroretinol in human keratinocytes, and to a lesser extent in HeLa and
melanoma cells, but not in the other cells. Analysis of cellular
retinol- and
retinoic acid-
binding protein concentrations showed a correlation to the cells' ability to accumulate
3,4-didehydroretinol, suggesting a role for these
proteins in the 3,4-didehydro metabolic pathway. The combined results suggest that although
3,4-didehydroretinol is most typical for human keratinocytes, studies of its metabolism are also feasible in HeLa cells which contain low levels of
retinoid-binding proteins.