1 alpha,25-Dihydroxyvitamin D3 [1,25(OH)2D3] exerts antiproliferative actions in colorectal cancer, but their underlying molecular mechanisms have not been determined. 1,25(OH)2D3 regulates target gene transcription via a specific nuclear vitamin D receptor (VDR), which mediates hormone action preferentially as a heterodimer with 9-cis-retinoic acid receptors (RXRs). We investigated the actions of 1,25(OH)2D3 and 9-cis-retinoic acid (RA) in two human colon cancer cell lines, HT-29 and Caco-2. Both expressed mRNAs encoding VDR, RXR alpha, and RXR gamma, and VDR was regulated posttranscriptionally in Caco-2 cells. There was an antiproliferative response of both cell lines to 1,25(OH)2D3. 9-cis-RA exerted antiproliferative effects on Caco-2 cells but blocked 1,25(OH)2D3 actions in HT-29 cells. The 1,25(OH)2D3-responsive gene 25-hydroxyvitamin D3 24-hydroxylase was induced in both cell lines b 1,25(OH)2D3 but in only HT-29 cells by 9-cis-RA. 1,25(OH)2D3 and 9-cis-RA cotreatment enhanced 24-hydroxylase expression in HT-29 cells only. The 24-hydroxylase enzyme is known to result in catabolism of 1,25(OH)2D3 and attenuation of its actions. Increased 24-hydroxylase activity in HT-29 cells, but not in Caco-2 cells, in response to 9-cis-RA may account for some of the complex cell-specific responses demonstrated in these studies.