**4.3 Up-regulation of Bcl-2 (pro-apoptotic) family proteins to overcome resistance in cancer cells**

The up-regulated expression level of anti-apoptotic Bcl-2 proteins might be possible be neutralized by increasing the expression of pro-apoptotic Bcl-2 proteins. Puma is an important BH-3 protein, having a key role in mediating pro-apoptotic p53 function and help to release p53 from inactive forms with Bcl-Xl. Hereby, freed cytosolic p53 can activate Bax and Bak, which results in stimulation of apoptotic

signaling through MOMP [44, 151]. p53 increases the expression of pro-apoptotic protein level, called ARTS, and counteracts Bcl-xl to induce apoptosis [152]. Hence, the inhibition of p53 antagonists such as Mdm-2 can cause an increased level of p53, induce pro-apoptotic protein expressions. Additionally, reduction of pro-apoptotic BH-3 only protein levels can also lead to drug resistance. This effect might be due to its interaction with BCL-2 anti-apoptotic proteins in cancer cells [153].

#### **4.4 Therapeutic approach to target death receptor**

There are different reasons and ways cause the failure of apoptosis or leading to resistance. Including altered death receptor signaling, an unbalanced level of pro-and anti-apoptotic proteins lowered caspase role and disrupted p53 function. Impairment of the death receptor signaling pathway is associated with many sorts of tumors, showing the importance of the Fas–FasL system and/or the abnormal level of cytosolic components of this pathway such as FADD can improve the tumor conversion [154, 155]. Transcriptionally silencing of Fas is familiar with oncogenic events in the epithelial conversion, the mutation in this gene linked with B-cell germinal center-derived lymphomas [156]. However, the lack of expression level of FADD is a common sign in acute myelogenous leukemia (AML) which leads to chemotherapy resistance [154, 155, 157]. Furthermore, the low and/or lack of caspase-8 expression have been reported in numerous other cancers including neuroblastoma and small cell lung cancer [158–160]. However, the induction of apoptosis by triggering the death receptor pathway might be an important promising approach to overcome the resistance to therapeutic agents. An important therapeutic approach that confers the TRAIL ligand which induces apoptosis in many different cancer cell lines [161]. TRAIL-induced apoptosis, an important mechanistic strategy that has a harmful effect against cancer cells and leads to several different approaches to develop agonistic-featured antibodies against TRAIL death receptors and/or several soluble recombinant derivatives of TRAIL [162, 163]. On the other side, increasing the sensitivity of malignant tumors to TRAIL-induced apoptosis is crucial to design new promising molecules to target and activate caspase-8 [164]. Importantly, the validation of in silico screening some small molecules which promote caspase-8 activation have been reported. In many different cancer cell lines including leukemic cells, the CaspPro molecule is shown to be very promising, and shown to induce not only caspase-8 activation but also caspase 3 activation, and leading to apoptosis in the presence of TRAIL [163–165].

Among other therapeutic approaches, another important strategy is targeting IAPs. Through the use of antisense oligonucleotides and small-molecule inhibitors very potential anticancer therapeutic approach has been developed. In ovarian cancer, the supplementation of antisense agents targeting XIAP through an adenoviral vector was displayed to reduce the expression level of XIAP and underlined its remarkable effect to induce apoptosis [166]. Besides, in lung cancers also its effect has been proven by showing that it increases cell sensitivity to radiation treatment [167]. In this regard, by inhibition of XIAP through the use of specifically designed oligomers also displayed to promote the activation of caspase-3, increasing the apoptotic effect of TRIAL in different cancers such as prostate cancer [168]. Furthermore, the SMAC mimetics are also shown to trigger apoptosis and/ or increase the sensitivity of cancer cells to different therapies. These molecules induce caspase activation through the neutralization of XIAP-mediated caspase inhibition [169]. The successful therapeutic approach relies on the capability of the tool to trigger apoptosis mainly by two approaches, i) targeting overexpressed antiapoptotic Bcl-2 family proteins, or ii) triggering the activation of pro-apoptotic molecules.
