Valproic acid induces apoptosis in human leukemia cells by stimulating both caspase-dependent and -independent apoptotic signaling pathways

Abstract

We investigated the effects of valproic acid (VPA) on the growth and survival of human leukemia cell lines. VPA induced cell death in all of the nine cell lines tested in a dose dependent manner. VPA-treatment induced apoptotic changes in MV411 cells including DNA fragmentation, phosphatidylserine externalization, cytochrome c release from mitochondria, and activation of caspases-3, -8, and -9. A caspase inhibitor, zVAD-FMK, inhibited the DNA fragmentation induced by VPA but not cell death. These findings suggest that VPA exerts an anti-leukemic effect by both caspase-dependent and -independent apoptotic signaling pathways.

Introduction

Accumulating evidence indicates that some members of short chain fatty acids (SCFAs) affect the proliferation and/or the differentiation of a wide variety of tumor cells [1], [2], [3], [4], [5], [6]. For instance, aromatic SCFAs such as butyrate, phenylacetate, and phenylbutyrate, promote differentiation or induce apoptosis in many types of tumor cells including colon cancer [1], [2], head and neck squamous carcinoma [3], renal cell carcinoma [4], neuroblastoma [5], and leukemia [6], [7]. Valproic acid (2-propylpentanoic acid, VPA) is a member of branched SCFAs and has been widely used in the management of various types of epilepsy for decades. As similar to the other members of SCFAs, VPA affects the growth and differentiation of some types of cells. It is reported that VPA induces differentiation in neuroblastoma cells and inhibits their growth [8], [9]. Furthermore, VPA affects the growth of hematopoietic cells. VPA inhibited the formation of normal granulocyte and macrophage colonies at the higher concentration [10]. Tittle et al. showed that VPA inhibited the growth of murine B-lymphoid leukemia cell lines and a human T-lymphoblastic leukemia cell line, Jurkat [11]. The mechanism underlying these effects of VPA on normal and malignant hematopoietic cells is unknown at present.

In this study, we firstly aimed to clarify the effect of VPA on the growth of various human leukemia cell lines and the mechanism underlying it. We demonstrated that VPA induces apoptosis in a variety of human leukemia cell lines in a dose dependent manner. LD₅₀ of the four of nine cell lines tested were within the range that did not inhibit the growth of normal hematopoietic progenitors. It is proposed that there are some apoptotic signaling pathways and with a few exceptions [12], [13], [14], [15], [16], they all activate the family of cysteine proteases known as caspases. Though many anti-cancer drugs are known to induce apoptosis in tumor cells, few are reported to be independent to caspases. If a drug dose induces apoptosis independently to caspases, it might be applicable as an option for the cancer treatment. We show in this study that VPA led the cells to death by stimulating both caspase-dependent and -independent apoptotic signaling pathways. Our findings suggest that VPA may be useful as an anti-leukemic agent without serious toxicity to normal hematopoiesis.