A relationship between retinol and cellular retinol-binding protein concentrations in human squamous cell carcinomas

Abstract The retinol and retinyl ester concentrations in human xenografted squamos cell carcinomas, with various concentrations of cellular retinol-binding protein (CRBP), were studied, as well as the in vivo uptake and esterification in these tumours of labelled retinol, presented as a complex with plasma RBP. The mean retinol concentration in the different tumours was in the range 3.7–6.2 nmol/g protein, and the mean CRBP concentration was between 16 and 69 nmol/g protein. There was a statistically significant correlation between the retinol and the CRBP concentrations in the same tumour ( P r = 0.622). Calculation of the maximal extent of retinol-saturation of CRBP showed low values (range: 9–26%). Retinyl palmitate, the predominant retinyl ester, comprised approx. 70% of the retinyl esters in the tumours. There was no correlation between the concentration of CRBP and that of retinyl palmitate. The uptake of [ 3 H]retinol from intravenously injected retinol-RBP complex was similar in the four human squamous cell carcinomas studied, and not related to their CRBP concentration. 20% of the radioactivity in tumour specimens was lipid soluble, as compared to 96% in liver specimens, showing that in the former a higher fraction metabolised to polar compounds. Taken together, our results suggest that in these squamous carcinoma cells, factors other than cellular CRBP content are the major determinants of net cellular uptake and esterification of retinol. The cellular retinol concentration, on the other hand, appears proportional to CRBP content.