Leukemia is a heterogenic group of diseases characterized by infiltration of neoplastic cells of the hematopoietic system into the blood, bone marrow, and other tissues [1, 2]. Leukemia is the most common malignancy among people aged <20 years. In the last decade, these diseases have exhibited a clear ascending pattern in the morbidity index, becoming a great challenge to health institutions .
The main treatment for this disease is chemotherapy. However, its results are very often limited due to the treatment resistance that the neoplastic cells develop [4, 5]. In an attempt to increase the efficiency of antileukemic treatments, higher doses of the cytotoxic agents have been used or different combinations of them [6, 7], but in the majority of the cases, higher doses have been put into effect in an empirical manner without good results and incrementing side effects.
Given this situation, our research team has developed the concept of chemotherapy with a rational molecular basis. The former is based on the premise that chemotherapy acts mainly to induce a genetically programmed death of the cell called apoptosis, and that this depends in turn on the synthesis of proteins de novo and the activation of biochemical factors as a result of a modification in the balance between expression of pro- and antiapoptotic genes in response to treatment [8, 9]. The cells undergoing apoptosis show internucleosomal fragmentation of the DNA, followed by nuclear and cellular morphologic alterations, which leads to a loss of the integrity of the membrane and the formation of apoptotic bodies. All of these processes are mediated by caspases, which are the main enzymes that act as apoptosis initiators and effectors. Some of these molecules can active themselves, while others require other caspases in order to acquire biological activity. This proteolytic cascade breaks down specific intracellular proteins including nuclear proteins of the cytoskeleton, endoplasmic reticulum, and cytosol, finally hydrolyzing the DNA [10–12].
On the other hand, it is noteworthy that upon apoptotic stimulus such as that generated by chemotherapy, this not only induces apoptosis but can also activate antiapoptotic mechanisms [13, 14]. Similarly, the nuclear factor-kappa B (NF-κB) transcription factor plays an important role in tumor cell growth, proliferation, invasion, and survival. In inactive cells, this factor is linked with its specific inhibitor I-kappa B (IκB), which sequesters NF-κB in the cytoplasm and prevents activation of target genes [15–18]. In this respect, NF-κB can activate antiapoptotic genes such as Bcl-2, Bcl-XL, and survivin, affecting chemotherapy efficiency, even if the chemotherapy itself or the radiotherapy itself can activate the NF-κB factor [19–21]. Blast cells exhibit overexpression of antiapoptotic proteins (Bcl-2 and Bcl-XL), which increase resistance to antitumor therapy .
In this regard, the drug PTX can prevent the phosphorylation of serines 32 and 36 of IκB, and we have found that PTX in combination with antitumor drugs such as adriamycin and cisplatin induced in vitro and in vivo a significant increment of apoptosis in fresh leukemic human cells , lymphoma murine models , and cervical cancer cells . Similar results have also been observed with PTX in other studies . PTX is a xanthine and a competitive nonselective phosphodiesterase inhibitor that inhibits tumor necrosis factor (TNF) and leukotriene synthesis and reduces inflammation [25, 26]. The MG132 proteasome inhibitor is another drug that decreases NF-κB activity . Proteasome inhibitors are becoming possible therapeutic agents for a variety of human tumor types that are refractory to available chemotherapy and radiotherapy modalities [28, 29]. The proteasome is a multicatalytic complex that is responsible for regulating apoptosis, cell cycle, cell proliferation, and other physiological processes by regulating the levels of important signaling proteins such as NF-κB, IκB, and the MG132 proteasome inhibitor have been shown to induce apoptosis in tumor cells [30, 31]. This is important because apoptosis is regulated by the ubiquitin/proteasome system at various levels . The aim of the present work was to study in vitro in U937 leukemic cells the effects on viability, apoptosis, cell cycle, caspases cleavage, cytochrome c release and mitochondrial membrane potential (ΔΨm), the Bcl-2 and Bcl-XL antiapoptotic proteins, and related genes activated by the PTX and/ or MG132 proteasome inhibitor, compounds that possess a NF-κB-mediated inhibitory effect.