Vitamin D signaling in
mammary cancer stem cells (MCSCs), which are implicated in the initiation and progression of
breast cancer, is poorly understood. In this study, we examined
vitamin D signaling in mammospheres which are enriched in MCSCs from established
breast cancer cell lines.
Breast cancer cells positive for
aldehyde dehydrogenase (ALDH(+)) had increased ability to form mammospheres compared to ALDH(-) cells. These mammospheres expressed MCSC-specific markers and generated transplantable xenografts in nude mice.
Vitamin D receptor (VDR) was significantly down-regulated in mammospheres, as well as in ALDH(+)
breast cancer cells. TN aggressive human
breast tumors as well as transplantable xenografts obtained from SKBR3 expressed significantly lower levels of VDR but higher levels of CD44 expression. Snail was up-regulated in mammospheres isolated from
breast cancer cells. Inhibition of VDR expression by
siRNA led to a significant change in key EMT-specific
transcription factors and increased the ability of these cells to form mammospheres. On the other hand, over-expression of VDR led to a down-regulation of Snail but increased expression of E-cad and significantly compromised the ability of cells to form mammospheres. Mammospheres were relatively insensitive to treatment with
1,25-dihydroxyvitamin D (1,25D), the active form of
vitamin D, compared to more differentiated
cancer cells grown in presence of serum. Treatment of H-Ras transformed HMLE(HRas) cells with
DETA NONOate, a
nitric oxide (NO)-donor led to induction of
MAP-kinase phosphatase -1 (MKP-1) and dephosphorylation of ERK1/2 in the mammospheres. Combined treatment of these cells with 1,25D and a low-concentration of
DETA NONOate led to a significant decrease in the overall size of mammospheres and reduced
tumor volume in nude mice. Our findings therefore, suggest that combination
therapy using 1,25D with drugs specifically targeting key survival pathways in MCSCs warrant testing in prospective clinical trial for treatment of aggressive
breast cancer.