Alternatively, our results show that treatment with vitamin D analogues primarily caused an increase in apoptosis. It has been shown that apoptosis occurs naturally in human retinoblastoma,
40 but this event is exacerbated after vitamin D treatment. The ability of vitamin D to trigger apoptosis in retinoblastoma is similar to results obtained in other cancer cells such as in breast cancer,
32 glioma,
33 34 and colon cancer.
35 Vitamin D affects gene expression by to binding and activating its receptor (VDR). The activated VDR then translocates to the nucleus where it interacts with VDREs present in the promoter of target genes. Numerous target genes are sensitive to vitamin D
2 and some of them are involved in apoptosis. The p21 gene, for example, possesses a VDRE
30 and induces apoptosis in human malignant glioma cell lines
45 and in WERI-RB1 and Y79 retinoblastoma cell lines
46 in cell transfection and transduction studies. In addition, p53, one of the first tumor-suppressor genes linked to apoptosis, plays a major role in regulating apoptotic cell death in retinoblastoma.
41 47 It too has been shown to be upregulated after treatment with vitamin D in glioma cell lines
34 although, because this gene does not contain a VDRE, the mechanism of its upregulation is not clear. The roles that p21 and p53 play in the vitamin D-mediated cell death in retinoblastoma is unknown. Convention dictates that p53 activates p21 expression.
47 However, because vitamin D is able to increase p21 expression directly, it is possible that the toxic effects of p21 in retinoblastoma cells subsequently stimulate the activation of p53. Apoptosis then ensues by p53-mediated upregulation of cell death regulatory genes such as BAX.
48 The exact sequence of the molecular events that occurs in retinoblastoma cells treated with vitamin D is the subject of ongoing studies in our laboratory.