Sensitization to TNF-induced apoptosis by 1,25-dihydroxy vitamin D3 involves up-regulation of the TNF receptor 1 and cathepsin B

The active form of vitamin D-3, 1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3), induces caspase-independent apoptosis in MCF-7 and T47D breast cancer cells. Before the appearance of apoptotic cells at Day 4 after the addition of 1,25(OH)(2)D-3, the MCF-7 cells are sensitized to the caspase-mediated apoptosis induced by TNF. We studied the mechanism underlying the cross talk between these 2 distinct death pathways in MCF-7 and T47D cells. Whereas 1,25(OH)(2)D-3 pre-treatment enhanced TNF-induced apoptosis of TNF sensitive MCF-7 cells, it failed to render TNF resistant T47D cells sensitive to this cytokine, Opposing to an earlier report suggesting that cytosolic phospholipase A(2) (cPLA(2)) mediates the 1,25(OH)(2)D-3-induced sensitization to TNF, we could not detect any cPLA(2) protein in MCF-7 cells and its overexpression had no effect on cellular sensitivity to 1,25(OH)(2)D-3 or the combination with TNF. The sensitization of MCF-7 cells to TNF-induced apoptosis by pre-treatment with 1,25(OH)(2)D-3 may instead be partially explained by an increased surface expression of the TNF receptor (TNF-R1), In line with this, not only the TNF-induced activation of caspases and apoptosis but also that of NF-kappaB was enhanced by 1,25(OH)(2)D-3 pre-treatment. Furthermore, 1,25(OH)(2)D-3 enhanced TNF-induced NF-kappaB activation in T47D cells suggesting that it potentiates TNF signaling in general. Interestingly, the lysosomal protease cathepsin B, which expression is up-regulated by 1,25(OH)(2)D-3, was released from the lysosomes upon TNF treatment, and inhibition of its activity rescued 1,25(OH)(2)D-3 treated MCF-7 cells from TNF-induced apoptosis, In conclusion, 1,25(OH)(2)D-3 may enhance TNF-induced apoptosis by increasing the expression of both the TNF-R1 and cathepsin B. (C) 2001 Wiley-Liss, Inc.