Background: Bone is a preferential site for cancer metastases, especially from prostate and breast cancer. Here, metastasized cells are resistant to current therapies and create a vicious cycle of negative influences on bone metabolism that stimulate cancer cell growth. We previously found that compounds that restore bone integrity, like vitamin D, may be promising therapies in bone metastases.

Before entering the bone, cancer cells release nano-sized particles: extracellular vesicles (EVs). EVs are able to transfer information from cell to cell, and are thought to prepare a metastatic niche in the bone where metastasized cancer cells can harbor.

Objective: Unravel the role of vitamin D in bone metastatic prostate cancer cell EV interaction with the bone.

Methods: EVs from vitamin D treated and untreated GFP-expressing human prostate cancer cells (PC-3) were isolated by differential ultracentrifugation (20.000xg and 100.000xg fractions), and counted by high-resolution flowcytometry (BD influx) and microscopy-image-guided counting (EVQuant). mRNA content was analyzed by high throughput sequencing. EV uptake by human osteoblasts (SV-HFO) was measured by flowcytometry (BD Accuri C6).

Results: Vitamin D treatment of PC-3 cells resulted in a decreased number of cells with a significantly higher number of EVs produced per cell in the 100,000xg fraction only. This 100,000xg EV fraction of the vitamin D treated PC-3 cells had a different mRNA profile compared to the 100.000xg EVs of the untreated PC-3 cells. Furthermore, vitamin D treated PC-3 EV uptake by osteoblasts was strongly reduced compared to EV uptake from untreated PC-3 cells.

Conclusion: Vitamin D treatment of bone metastatic prostate cancer cells affects their extracellular vesicle count, content and contact with bone. This may be part of the mechanism of vitamin D interference in the crosstalk between bone cells and metastatic cancer cells and implies a new role for vitamin D in the prevention of a metastatic niche in the bone.