Background: NKG2D recognises several ligands, including polymorphic major histocompatibility complex class I chain-related chain-related proteins A and B (MICA/B) and unique long 16-binding proteins (ULBPs). These ligands are present on cancer cells and are recognised by NKG2D in a cell-structure-sensing manner, triggering natural killer (NK) cell cytotoxicity. However, the mechanisms that control the expression of NKG2D ligands in malignant cells are poorly understood. 1-α,25-Dihydroxyvitamin D3 (1,25(OH)2D3) was recently shown to enhance the susceptibility of melanoma cells to the cytotoxicity of NK cells. However, the function of 1,25(OH)2D3 in other cancers and its potential mechanisms of action remain unknown.
Methods: The expression levels of miR-302c and miR-520c in Kasumi-1, K562, MCF7 and MDA-MB-231 cells were evaluated using quantitative real-time PCR. The targets of miR-302c and miR-520c were confirmed by luciferase reporter assay. The killing effects of NK92 cells against Kasumi-1, K562, MCF7 and MDA-MB-231 cells were examined using the CytoTox 96 Non-Radioactive Cytotoxicity Assay. The levels of cytokines IFN-γ and granzyme B, which indicate the activation of NK cells, were also measured by enzyme-linked immunosorbent assay.
Results: Treatment with 1,25(OH)2D3 enhanced the susceptibility of both the haematological tumour cell line Kasumi-1 and solid tumour cell line MDA-MB-231 to NK92 cells. miR-302c and miR-520c expression was induced, and their levels inversely correlated with the levels of NKG2D ligands MICA/B and ULBP2 upon 1,25(OH)2D3 treatment. A luciferase reporter assay revealed that miR-302c and miR-520c directly targeted the 3'-UTRs of MICA/B and ULBP2 and negatively regulated the expression of MIA/B and ULBP2. Moreover, upregulation of miR-302c or miR-520c by transfection of their mimics remarkably reduced the viability of Kasumi-1 cells upon NK cell co-incubation. By contrast, the suppression of the activity of miR-302c or miR-520c by their respective antisense oligonucleotides improved the resistance of Kasumi-1 cells to NK cells.
Conclusion: 1,25(OH)2D3 facilitates the immuno-attack of NK cells against malignant cells partly through downregulation of miR-302c and miR-520c and hence upregulation of the NKG2D ligands MICA/B and ULBP2.