A synergistic increase of apoptosis utilizing Fas antigen expression induced by low doses of anticancer drug

Anticancer drugs have been known to enhance both Fas receptor and Fas ligand expression on tumor cells. Recently, low doses of cytosine arabinoside (ara-C) were reported to enhance Fas antigen expression in the human myeloid leukemia cell line HL60. Here, we showed that low doses of ara-C (LD-ara-C) and etoposide (LD-VP-16) but not vincristine (LD-VCR) induce Fas expression in the human monocytic leukemia cell line U937. We determined the concentrations of ara-C, VP-16 and VCR as 10, 100 and 1 ng/ml, respectively. The ratios for Fas antigen expression induced in non-treated U937 by 24 h incubations with ara-C, VP-16 or VCR were 1.90, 1.36 and 1.00, respectively. Utilizing the Fas antigen expression induced by low doses of anticancer drugs, we examined whether anti-Fas IgM monoclonal antibody (CH-11) combined with LD-ara-C, LD-VP-16 or LD-VCR enhances apoptosis. When CH-11 and LD-anticancer drug were added simultaneously, the ratios of annexin V positive cells were 67.8 +/- 2.4% with ara-C, 70.0 +/- 1.6% with VP-16 and 54.2 +/- 1.3% with VCR. Thus, the ratios of annexin V positive cells significantly increased when CH-11 was simultaneously added to the cells with ara-C (p < 0.0001) and VP-16 (p < 0.0001), but not with VCP (p = 0.5559), compared with the sums of annexin V positive ratios of CH-11 and LD-anticancer drug added separately. We examined whether a broad-range caspase inhibitor (C.I.) can inhibit the Fas expression enhanced by LD-anticancer drugs. However, the Fas expression enhanced by LD-ara-C or LD-VP-16 was not inhibited by a broad-range caspase inhibitor. We demonstrated that apoptosis induced by LD-ara-C or LD-VP-16 is synergistically increased by the addition of CH-11 in U937.