Retinol mobilization from cultured rat hepatic stellate cells does not require retinol binding protein synthesis and secretion

Retinol mobilization from retinyl esters stores of hepatic stellate cells (HSCs) is a key step in the regulation of mammalian retinol homeostasis, but the precise mechanisms of such a mobilization are still poorly understood. Using primary cultures of HSCs, we first demonstrated that HSCs expressed immunoreactivity against retinol-binding-protein (RBP) when cultured in a medium containing RBP but were unable to synthesize RBP transcripts and proteins. Using pulse and chase-type experiments, we demonstrated that radioactive retinol was released in culture medium without binding proteins. Inhibition of protein secretion by brefeldin A did not modify quantitatively retinol release. This data ruled out, for the first time, the direct involvement of RBP in retinol mobilization from HSCs. Moreover, HSCs co-cultured with primary isolated hepatocytes displayed an increase of retinol transfer from HSCs to hepatocytes when they established direct physical contacts, as compared with co-cultures without contact. Based on this latter data, a mechanism of retinol mobilization from HSCs via the hepatocytes using retinol transfer through cellular membranes is proposed.