Vitamin K2 enhances the tumor suppressive effects of 1,25(OH)2D3 in triple negative breast cancer cells

Abstract

K vitamins are well known as essential cofactors for hepatic γ-carboxylation of coagulation factors, but their potential role in chronic diseases including cancer is understudied. K2, the most abundant form of vitamin K in tissues, exerts anti-cancer effects via diverse mechanisms which are not completely understood. Our studies were prompted by previous work demonstrating that the K2 precursor menadione synergized with 1,25 dihydroxyvitamin D3 (1,25(OH)₂D₃) to inhibit growth of MCF7 luminal breast cancer cells. Here we assessed whether K2 modified the anti-cancer effects of 1,25(OH)₂D₃ in triple negative breast cancer (TNBC) cell models. We examined the independent and combined effects of these vitamins on morphology, cell viability, mammosphere formation, cell cycle, apoptosis and protein expression in three TNBC cell models (MDA-MB-453, SUM159PT, Hs578T). We found that all three TNBC cell lines expressed low levels of the vitamin D receptor (VDR) and were modestly growth inhibited by 1,25(OH)₂D₃ in association with cell cycle arrest in G0/G1. Induction of differentiated morphology by 1,25(OH)₂D₃ was observed in two of the cell lines (MDA-MB-453, Hs578T). Treatment with K2 alone reduced viability of MDA-MB-453 and SUM159PT cells but not Hs578T cells. Co-treatment with 1,25(OH)₂D₃ and K2 significantly reduced viable cell number relative to either treatment alone in Hs578T and SUM159PT cells. The combination treatment induced G0/G1 arrest in MDA-MB-453 cells, Hs578T and SUM159PT cells. Combination treatment altered mammosphere size and morphology in a cell specific manner. Of particular interest, treatment with K2 increased VDR expression in SUM159PT cells suggesting that the synergistic effects in these cells may be secondary to increased sensitivity to 1,25(OH)₂D₃. The phenotypic effects of K2 in TNBC cells did not correlate with γ-carboxylation suggesting non-canonical actions. In summary, 1,25(OH)₂D₃ and K2 exert tumor suppressive effects in TNBC cells, inducing cell cycle arrest leading to differentiation and/or apoptosis depending on the specific cell line. Further mechanistic studies to clarify common and unique targets of these two fat soluble vitamins in TNBC are warranted.

Keywords: Breast cancer; Menaquinone-4; TNBC; Vitamin D; Vitamin K2.