In
breast cancer,
cell-free DNA (
cfDNA) has been proven to be a diagnostic and prognostic
biomarker. However, there have been few studies on the origin and biological significance of
cfDNA. In this study, we assessed the release pattern of
cfDNA from
breast cancer cell lines under different culture conditions and investigated the biological significance of
cfDNA. The
cfDNA concentration increased rapidly (6 h) after passage, decreased gradually, and was then maintained at a relatively stable level after 24 h. In addition, the
cfDNA concentration did not correlate with the amount of apoptotic and necrotic cells. Interestingly, if more cells were in the G1 phase, more
cfDNA was detected (p < 0.01) and the
cfDNA concentration correlated positively with the percent of cells in the G1 phase (p < 0.05). We observed that cells could release
cfDNA actively, but not exclusively, via exosomes. Furthermore, we showed that
cfDNA could stimulate
hormone receptor-positive
breast cancer cell proliferation by activating the TLR9-NF-κB-cyclin D1 pathway. In conclusion,
cfDNA is released from
breast cancer mainly by active secretion, and
cfDNA could stimulate proliferation of
breast cancer cells.