Abstract
Calcitriol, the hormonally-active metabolite of Vitamin D3, plays critical roles in calcium homeostasis, cell growth and differentiation, and immunoregulation. The anti-tumor activities of high-dose calcitriol have been demonstrated in a variety of preclinical models of solid tumors, leukemias and lymphomas. Recently, a new dose-intense formulation of calcitriol, termed DN-101 (Asentar™), was developed specifically for cancer therapy which allows for supraphysiological concentrations of calcitriol to be safely delivered in vivo to patients with cancer. In a recent Phase 2 clinical trial, DN-101 significantly increased overall survival and also reduced the incidence of thromboembolic events in men with androgen-independent prostate cancer receiving docetaxel-based chemotherapy. Based on previous observations we hypothesized that calcitriol’s anti-thrombotic effects in vivo may be due to the downregulation of Tissue Factor (TF) antigen and activity and/or upregulation of Thrombomodulin (TM). To test this hypothesis, we incubated A549 lung carcinoma, A375-C15 metastatic melanoma, THP-1 monocytic leukemia, and Eahy926 endothelial cells with increasing concentrations of calcitriol for 24 hrs. For TF induction, tumor cells were stimulated with TNFα for 5 hrs and activity was measured by a clotting assay and a thrombin generation assay (TGA). TM activity was measured by a chromogenic assay. TF and TM surface antigen were assessed by flow cytometry. Calcitriol prevented the induction of TF in TNFα-stimulated THP-1 cells in a dose-dependent manner (from 33% at 1 nM to 94% at 100 nM) as evidenced by a prolongation of plasma clotting time, a decrease in endogenous thrombin potential (ETP), and a reduction of surface TF antigen. In addition, the activity and surface expression of TM on THP-1 cells was increased significantly (40% and 3-fold respectively, P < 0.01) following 100 nM calcitriol treatment. Similarly, in TNFα-stimulated melanoma cells, calcitriol prevented the induction of TF activity (from 26% at 1 nM to 60% at 1 μM) and expression in a dose-dependent manner. High-dose calcitriol treatment also increased melanoma cell TM activity between 8% and 62%. In contrast, constitutively expressed TF activity and antigen were less affected by calcitriol in A549 lung carcinoma cells (12 to 28% reduction at concentrations between 1–100 nM) whilst TM activity and antigen were unaffected. In comparison to the tumor cells, calcitriol had no significant effect on TM or TF activity or antigen in TNFα-stimulated EAhy926 endothelial cells. In conclusion, we have demonstrated that high concentrations of calcitriol inhibit the induction of surface TF expression and upregulates TM in multiple tumor cell lines in vitro. The degree of the inhibition is proportional to the extent of TF induction by TNF-α. These in vitro results provide further support for the anticoagulant properties associated with high concentrations of calcitriol and may provide a rationale for understanding the lower incidence of thromboembolic complications observed in patients with metastatic prostate cancer treated with DN-101.
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