Differential uptake, binding, and metabolism of retinol and retinoic acid by 10T1/2 cells.
Previous studies have shown that all-trans-retinoic acid fails to inhibit chemically induced transformation in 10T1/2 cells except at toxic levels, whereas retinol and many synthetic retinoids are potent inhibitors. In contrast, in many systems retinoic acid is a more effective modulator of differentiation and carcinogenesis than is retinol. In any attempt to explain this anomaly, we have studied the differential metabolism of retinoic acid and retinol by 10T1/2 cells and by their initiated and transformed derivatives, and have also reexamined these cells for the presence of retinoid-binding proteins. Whereas retinoic acid was depleted from the medium bathing 10T1/2 and initiated 10T1/2 cells within 48 h of treatment, retinol was concentrated 500-fold by these cells, and disappeared from the culture medium no faster than from cell-free medium. Retinoic acid metabolism by a number of transformed cell lines varied widely. There was no apparent correlation between metabolizing ability and transforming agent (methylcholanthrene, X-rays, fission spectrum neutrons, and plasmid oncogene transfection). Uptake of retinoic acid was seen in all cell lines and was not correlated with its metabolism. Retinol was metabolized minimally by all cell lines tested; metabolism of retinol was not correlated with retinoic acid metabolizing ability. Retinoic acid-induced growth inhibition and cytotoxicity were not correlated with metabolizing ability, suggesting that the rate of metabolism of retinoic acid is not a major determinant of its acute biological effects. Using sensitive radioimmunoassays, cellular retinoic acid- (CRABP) and retino-binding proteins (CRBP) were both detected in 10T1/2 and initiated 10T1/2 cells. CRABP levels of about 16 ng/10(6) cells were about 4-fold higher than CRBP levels. Therefore, lack of CRABP does not explain the failure of retinoic acid to inhibit carcinogen-induced transformation in these cells. These studies suggest that the inability of retinoic acid to inhibit transformation in the 10T1/2 cell system may be due to its rapid metabolism and clearance from the medium. On the other hand, the high cellular uptake and stability of retinol in these cells could be an important factor in the inhibition of neoplastic transformation by this retinoid.
Rundhaug
J
J
Cancer Research Center of Hawaii, University of Hawaii at Manoa, Honolulu 96813.
Gubler
M L
ML
Sherman
M I
MI
Blaner
W S
WS
Bertram
J S
JS
D002352:Carrier Proteins / Q000032:analysis; D002461:Cell Line, Transformed; D002471:Cell Transformation, Neoplastic / Q000187:drug effects; D002478:Cells, Cultured; D018168:Receptors, Retinoic Acid; D013997:Time Factors; D014212:Tretinoin / Q000378:metabolism*; D014801:Vitamin A / Q000378:metabolism*
D002352:Carrier Proteins; D018168:Receptors, Retinoic Acid; D014801:Vitamin A; D014212:Tretinoin