Growth of Caco-2 and many cancer cells is inhibited by 1,25(OH)₂D₃. Whereas TGF-1 inhibits normal colonic epithelial cell growth, most human colon cancer-derived cells, including Caco-2 and SW480 cells, are resistant to it. The mechanisms underlying these antiproliferative actions and resistance to TGF- growth inhibition are largely unknown. We observed that 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] sensitized Caco-2 and SW480 cells to TGF-1 growth inhibitory effects. Versus 1,25(OH)₂D₃ alone, the combination of 1,25(OH)₂D₃ and TGF-1 significantly reduced cell numbers. Also, the amount of active TGF-1 was increased (~4-fold) by this secosteroid in conditioned media from Caco-2 cells. The 1,25(OH)₂D₃ increased the expression of IGF-II receptors (IGF-IIR), which facilitated activation of latent TGF-1, and was found to activate TGF- signaling in Caco-2 cells. By using neutralizing antibodies to human TGF-1, we showed that this cytokine contributes to secosteroid-induced inhibition of Caco-2 cell growth. Also, 1,25(OH)₂D₃ was found to enhance the type I TGF- receptor mRNA and protein abundance in Caco-2 cells. Whereas the 1,25(OH)₂D₃-induced sensitization of Caco-2 cells to TGF-1 was IGF-IIR independent, the type I TGF-1 receptor was required for this sensitization. Thus 1,25(OH)₂D₃ treatment of Caco-2 cells results in activation of latent TGF-1, facilitated by the enhanced expression of IGF-IIR by this secosteroid. Also, 1,25(OH)₂D₃ sensitized Caco-2 cells to growth inhibitory effects of TGF-1, contributing to the inhibition of Caco-2 cell growth by this secosteroid.