3HRetinoic acid (RA) and 3Hretinol bind in an unsaturable manner to isolated nuclei from Nulli-SCC1 and PCC4.aza1R embryonal carcinoma (EC) cells. When nuclei are challenged with the same labeled retinoids on their respective binding proteins (CRABP and CRBP), much less binding is observed and the binding is saturable. RA-CRABP does not compete with 3Hretinol-CRBP for binding to specific Nulli-SCC1 nuclear sites, whereas retinol-CRBP (but not apo-CRBP) actually potentiates the binding of 3HRA-CRABP to these nuclei. The binding of 3HRA-CRABP and 3Hretinol-CRBP is not dramatically affected by prior removal of the outer nuclear membrane with Triton X-100. However, treatment with the detergent after the binding reaction is complete removes about half of the bound 3HRA-CRABP and almost all of the bound 3Hretinol-CRBP. We measured specific retinoid-binding activities in nucleoplasmic extracts of Nulli-SCC1 and PCC4.aza1R cells. The only readily detectable specific binding activity in nucleoplasmic extracts from untreated cells was for 3Hretinol in PCC4.aza1R preparations. Nucleoplasmic extracts from Nulli-SCC1 and PCC4.aza1R cells pretreated with RA had considerable levels of specific 3HRA-binding activity with little or no increase in 3Hretinol binding. By contrast, similar extracts from Nulli-SCC1 cells treated with retinol bound large amounts of both 3Hretinol and 3HRA. Under the same conditions, PCC4.aza1R extracts also contained 3HRA-binding activity with no increase in 3Hretinol binding above the high endogenous levels. Although these results might reflect translocation of binding proteins from cytoplasm to nucleus, other interpretations must be considered since we often observed an increase, rather than the expected reduction, in cytoplasmic retinoid-binding protein levels.
The Journal of Cell Biology – Rockefeller University Press
Published: Mar 1, 1987
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