The vitamin A metabolism and expression of retinoid-binding proteins differ in HaCaT cells and normal human keratinocytes

HaCaT keratinocytes differ from normal human epidermal keratinocytes (LEK) by constitutive expression of differentiation markers which are normally su ppressed by vitamin A. In search of an explanation for this discrepancy we compared the vitamin A content, the expression of retinoid-binding proteins , and the vitamin A metabolism in the two cell types, The concentrations of retinol and 3,4-didehydroretinol in cultured HaCaT cells were less than on e-fifth those in HEK, and the content of fatty acyl esters was even lower, Similarly the concentrations of cellular retinol-binding protein and cellul ar retinoic acid-binding protein (CRBPI and CRABPII, respectively) were 10- 30 times lower in HaCaT cells than in HEK corresponding to a reduced mRNA e xpression of these proteins, Unexpectedly, HaCaT cells expressed RAR beta i n addition to RAR alpha, RAR gamma and RXR alpha, which are nuclear recepto rs normally found in HEK, Radioactive retinol added to the culture medium a ppeared only transiently in HaCaT cells, and pulse labeling confirmed a def ective cellular retention of retinyl esters, After 24 h of incubation with [H-3]retinol, cell-associated radioactivity corresponding to retinol, 3,4-d idehydroretinol, all-trans-retinoic acid and 3,4-didehydroretinoic acid was found in both HaCaT cells and LEK, [H-3]Retinoic acid showed a more rapid metabolism to 4-hydroxy/4-keto-retinoic acid in HaCaT cells than in LEK, wh ich could be explained by a higher expression of cytochrome p450RAI in the former cells, In conclusion, the abnormal uptake of vitamin A and low level s of retinoid binding proteins in HaCaT cells, linked with an aberrant meta bolism of retinol, may help to explain why these cells differentiate also i n the presence of retinoids.