Retinol-binding protein mediates uptake of retinol to cultured human keratinocytes.

Retinol (vitamin A) circulates in the blood bound to retinol-binding protein (RBP). The process by which target cells acquire retinol is not fully elucidated, although a cell surface receptor for RBP has recently been identified. We show here that retinol is at least an order of magnitude more efficient at blocking the terminal differentiation of cultured normal human keratinocytes when administered as a complex with RBP than when administered free in solution. This inhibition of differentiation by RBP can be reversed by monoclonal antibody P142, reactive toward the RBP-binding membrane protein p63. These results demonstrate, at least in this in vitro system, the importance of the RBP receptor in the generation of a cellular response to retinol.