Apoptosis Deregulation and the Development of Cancer Multi-Drug Resistance

Abstract

The ability of tumor cells to evade apoptosis is established as one of the hallmarks of cancer. The deregulation of apoptotic pathways conveys a survival advantage enabling cancer cells to develop multi-drug resistance (MDR), a complex tumor phenotype referring to concurrent resistance toward agents with different function and/or structure. Proteins implicated in the intrinsic pathway of apoptosis, including the Bcl-2 superfamily and Inhibitors of Apoptosis (IAP) family members, as well as their regulator, tumor suppressor p53, have been implicated in the development of MDR in many cancer types. The PI₃K/AKT pathway is pivotal in promoting survival and proliferation and is often overactive in MDR tumors. In addition, the tumor microenvironment, particularly factors secreted by cancer-associated fibroblasts, can inhibit apoptosis in cancer cells and reduce the effectiveness of different anti-cancer drugs. In this review, we describe the main alterations that occur in apoptosis-and related pathways to promote MDR. We also summarize the main therapeutic approaches against resistant tumors, including agents targeting Bcl-2 family members, small molecule inhibitors against IAPs or AKT and agents of natural origin that may be used as monotherapy or in combination with conventional therapeutics. Finally, we highlight the potential of therapeutic exploitation of epigenetic modifications to reverse the MDR phenotype.

Keywords: Bcl-2 family of proteins; PI3K/AKT pathway; apoptosis; cancer associated fibroblasts; cancer therapy; caspase-dependent death; epigenetic modifications; multi-drug resistance; tumor-microenvironment.

Figure 1

The extrinsic and intrinsic pathways of apoptosis. The activation of the extrinsic pathway involves binding of an external ligand to a transmembrane Death Receptor which then induces its conformation as a homotrimer. In the internal part of the receptor, the exposed Death Domain (DD) recruit adaptor proteins such as TRADD. Through its Death Effector Domain (DED), TRADD can then recruit pro-Caspase-8 which is activated through self-proteolysis. Subsequently the death ligand, receptor, TRADD molecule and caspase-8 form the DISC complex. Active caspase-8 can cleave downstream effector caspases 3, 6 and 7 which degrade nuclear lamins and other cellular components. Caspase-8 connects the extrinsic pathway with intrinsic or mitochondrial apoptotic signaling, though the cleavage of Bid to truncated Bid (t-Bid). Bid, a member of the Bcl-2 family, facilitates the opening of mitochondrial pores in a process called MOMP by inducing the polymerization of Bax on the outer mitochondrial membrane. MOMP is also induced by internal signals, such as extensive DNA damage. Cytochrome-c (cyt-c) is released from mitochondria and along with Apaf-1 and pro-caspase-9 form the apoptosome heptamer structure. Following proteolysis, caspase-9 cleaves and activates effector caspases, further amplifying the apoptotic process. Apoptotic Protease Activating Factor 1 (APAF-1); Mitochondrial Outer Membrane Permeabilization (MOMP); TNFR1-Associated Death Domain protein, (TRADD).

Figure 2

Deregulation of apoptotic pathways during the MDR development in cancer. The PI₃K/AKT survival pathway is frequently overactive in MDR cancers, partly due to increased levels of growth ligands and receptors. Binding of growth factors to receptor tyrosine kinases (RTKs) stimulates PI₃K by autophosphorylation; PI₃K then catalyzes the conversion of PIP2 to PIP3 while tumor suppressor PTEN has an opposing function. PTEN is often mutated and thus inactive in MDR cancers. PI₃K mediates the activation of AKT via phosphorylation on Thr³⁰⁸ and Ser⁴⁷³ by PDK1 and PDK2, respectively. AKT can then activate MDM2 which blocks the function of p53. P53 regulates (among others) the levels and activation status of the Bcl-2 family of proteins. The balance between the levels of the pro-apoptotic and anti-apoptotic Bcl-2 family proteins, controls the release of pro-apoptotic factors from mitochondria. Once released, cytochrome-c activates firstly caspase-9 and then the executioner caspases-3, -6 and -7. Furthermore, in MDR cancers, members of the IAP family are overexpressed, blocking caspase function. SMAC/Diablo are also released from the mitochondria during apoptosis and can inhibit the function of IAPs. NFkB is indirectly activated by growth factors via the PI₃K/AKT pathway and up-regulates IAPs, e.g., survivin via regulation at the transcriptional level. Factors released by CAFs increase tumor cells survival via the activation of the PI₃K/AKT pathway and inhibit apoptotic pathways. The cellular and non-cellular components of the TME as well as modifications in metabolic pathways and mechanical stress have been also implicated in resistance to cancer-cells targeting pro-apoptotic therapeutic agents. The composition and structure of stromal components in tumors increase interstitial fluid pressure (IFP) hindering the penetration of macromolecules through tissue and influence the sensitivity of tumor cells to therapy [141]. The hypoxic TME favours cells that have lost sensitivity to p53-mediated apoptosis and that are deficient in DNA mismatch repair leading to resistance to platinum-based chemotherapeutic agents [142]. Moreover, the low extracellular pH in tumors decreases the cellular uptake of weakly basic drugs such as doxorubicin, mitoxantrone, vincristine and vinblastine [143]. Multi Drug Resistant (MDR); Receptor Tyrosine Kinases (RTKs); Phosphatidylinositol 4,5-bisphosphate PtdIns(4,5)P2, (PIP2); Phosphatidylinositol (3,4,5)-triphosphate PtdIns(3,4,5)P3, (PIP3); Phosphoinositide-dependent kinase, (PDK); Tumor Microenvironment (TME); Inhibitors of Apoptosis Proteins, (IAPs); Cancer Associated Fibroblasts, (CAFs).