Characteristics of retinol accumulation from serum retinol-binding protein by cultured Sertoli cells

Janet L Shingleton; Michael K Skinner; David E Ong

doi:10.1021/bi00451a015

The uptake of retinol was examined in cultured Sertoli cells when retinol was provided as a complex with the transport protein retinol-binding protein (RBP). Sertoli cells accumulated [3H]retinol in a time- and temperature-dependent manner. At 32 °C, the rate of retinol accumulation was biphasic. Accumulation was linear for approximately 1 h, but then accumulation continued at a linear but decreased rate for 23 h. The change in rate of retinol accumulation occurred when the cells had accumulated approximately 0.53 pmol of retinol/Mg of cellular DNA. This amount of retinol was approximately equal to the cellular content of cellular retinol-binding protein (CRBP). Extraction and HPLC analysis of the cell-associated radioactivity yielded retinol and retinyl esters, indicating that a significant proportion of the accumulated retinol was esterified. Excess unlabeled retinol-RBP competed with [3H]retinol-RBP for [3H]retinol delivery to the cells, indicating that RBP delivery of retinol was a saturable and competable process. However, free [3H]retinol associated with Sertoli cells in a noncompetable manner. The transport constant for specific retinol accumulation from RBP was 3.0 μ , suggesting that any change in the normal circulating retinol-RBP level (approximately 2 μ ) would directly affect the rate of retinol accumulation. Neither iodinated nor reductively methylated RBP was accumulated by or tightly bound to Sertoli cells. In addition, energy inhibitors and lysosomal poisons had no effect on [3H] retinol accumulation, indicating that RBP delivery of retinol to Sertoli cells did not occur by endocytosis of the retinol-RBP complex. Competition studies indicated, however, that protein recognition is important in the retinol uptake process. RBP, CRBP, and CRBP(II) competed with [3H] retinol-RBP for [3H] retinol accumulation, but free retinol, retinol-bovine serum albumin, and retinol-/3-lactoglobulin did not. Transthyretin, bound to [3H] retinol-RBP in the physiological 1:1 ratio, decreased [3H]retinol accumulation by the cells by 25-30% compared to [3H]retinol accumulation from [3H]retinol-RBP. These studies indicated that Sertoli cell uptake of retinol involved recognition of the retinol-RBP complex at the cell surface with subsequent internalization of retinol, but not RBP. The fate of the internalized retinol may first have involved binding by CRBP, but eventually a significant portion of the accumulated retinol was esterified.

Characteristics of retinol accumulation from serum retinol-binding protein by cultured Sertoli cells

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