Cancer-selective therapy of the future: Apoptin and its mechanism of action

Classical chemotherapy, that specifically targets rapidly proliferating cells, has been in existence for over eighty years and has proven to be fully successful in only a limited number of cancers. Thus, this review focuses on a novel, emerging approach for cancer therapy that uses alternative, and more unique features of cancer cells. This new approach facilitates the selective targeting of cancer, while sparing normal, non-transformed cells. Examples of molecules that kill cancer cells selectively are: apoptin, E4orf4, viral protein R (VpR), and Brevinin-2R. Below we focus on apoptin, a product of the third open reading frame (VP3) of the chicken anemia virus. Besides discussing apoptin's mechanism of action, we also provide concise insight into the biology of a chicken anemia virus infection. Since apoptin's cancer-selective toxicity depends on its nuclear localization, we broadly discuss mechanism(s) involved in its nuclear retention (both nuclear import and export). We also discuss recent findings on apoptin's molecular mechanism of action, with a focus on the role of Nur77 in apoptin's nucleo-cytoplasmic signaling. Finally, we compare the current findings on apoptin to the mechanism of cancer selective toxicity of E4orf4. In the 'summary' -section, besides highlighting important issues related to cancer-selective therapy, we also discuss concurrent approaches towards therapy personalization, particularly those related to the in vivo-, and real time cancer-therapy efficacy monitoring, using "lab-on-the-chip" and other emerging technologies.

Cancer stem cells as targets for cancer therapy: selected cancers as examples

It is becoming increasingly evident that cancer constitutes a group of diseases involving altered stem-cell maturation/differentiation and the disturbance of regenerative processes. The observed malignant transformation is merely a symptom of normal differentiation processes gone astray rather than the primary event. This review focuses on the role of cancer stem cells (CSCs) in three common but also relatively under-investigated cancers: head and neck, ovarian, and testicular cancer. For didactic purpose, the physiology of stem cells is first introduced using hematopoietic and mesenchymal stem cells as examples. This is followed by a discussion of the (possible) role of CSCs in head and neck, ovarian, and testicular cancer. Aside from basic information about the pathophysiology of these cancers, current research results focused on the discovery of molecular markers specific to these cancers are also discussed. The last part of the review is largely dedicated to signaling pathways active within various normal and CSC types (e.g. Nanog, Nestin, Notch1, Notch2, Oct3 and 4, Wnt). Different elements of these pathways are also discussed in the context of therapeutic opportunities for the development of targeted therapies aimed at CSCs. Finally, alternative targeted anticancer therapies arising from recently identified molecules with cancer-(semi-)selective capabilities (e.g. apoptin, Brevinin-2R) are considered.

Cell survival, cell death and cell cycle pathways are interconnected: implications for cancer therapy

The partial cross-utilization of molecules and pathways involved in opposing processes like cell survival, proliferation and cell death, assures that mutations within one signaling cascade will also affect the other opposite process at least to some extent, thus contributing to homeostatic regulatory circuits. This review highlights some of the connections between opposite-acting pathways. Thus, we discuss the role of cyclins in the apoptotic process, and in the regulation of cell proliferation. CDKs and their inhibitors like the INK4-family (p16(Ink4a), p15(Ink4b), p18(Ink4c), p19(Ink4d)), and the Cip1/Waf1/Kip1-2-family (p21(Cip1/Waf1), p27(Kip1), p57(Kip2)) are shown both in the context of proliferation regulators and as contributors to the apoptotic machinery. Bcl2-family members (i.e. Bcl2, Bcl-X(L) Mcl-1(L); Bax, Bok/Mtd, Bak, and Bcl-X(S); Bad, Bid, Bim(EL), Bmf, Mcl-1(S)) are highlighted both for their apoptosis-regulating capacity and also for their effect on the cell cycle progression. The PI3-K/Akt cell survival pathway is shown as regulator of cell metabolism and cell survival, but examples are also provided where aberrant activity of the pathway may contribute to the induction of apoptosis. Myc/Mad/Max proteins are shown both as a powerful S-phase driving complex and as apoptosis-sensitizers. We also discuss multifunctional proteins like p53 and Rb (RBL1/p107, RBL2/p130) both in the context of G1-S transition and as apoptotic triggers. Finally, we reflect on novel therapeutic approaches that would involve redirecting over-active survival and proliferation pathways towards induction of apoptosis in cancer cells.