RNA interference against human cancers: a perspective

RNA interference (RNAi) is a natural and highly conserved cellular process for targeting and specific cleavage of mRNA through small interfering RNAs (siRNA) of 21-23 nucleotides. RNAi approaches are now emerging as a novel coadjuvant therapy to correct abnormal expression of cancer oncogenes that contribute to sustained cell growth and chemotherapeutic resistance. Cancer cells undergoing RNAi-forced expression display novel genotype/phenotype as part of global gene expression changes, thereby increasing the activity of conventional chemotherapeutic drugs used in the treatment of cancers. Many delivery systems are in development, including virus vectors (retrovirus, adenovirus and adeno-associate virus), liposome complexes, nanoparticles and monoclonal antibodies, to carry the chemically synthesized 21-23 base-pair siRNAs or short hairpin RNA (shRNA) through the cell membrane into the cytoplasm and nucleus of the tumor cells. Here, we update the current uses of this approach in basic and clinical oncology studies.

Tumor necrosis factor-á signaling cascades in apoptosis, necrosis, necroptosis and cell proliferation

Tumor necrosis factor-á (TNF-á) is a multifunctional cytokine involved in host defense, inflammation, apoptosis, autoimmunity, organogenesis and lymphoid microarchitecture. Many of these activities may be explained by the ability of this cytokine to induce distinct signal transduction pathways that recruit regulatory proteins involved in differentiation, cell death or cell proliferation. In this review, we discuss the contribution of caspases -3, -6, -7 and -8, and of cyclin-dependent kinases (CDKs), cyclin B and cyclin-dependent kinase inhibitors (CKI p21 and p27), as well as retinoblastoma tumor suppressor in the signaling cascades triggered by TNF-á to induce apoptosis, necrosis and cellular proliferation in the murine cell lines NIH3T3 and WEHI-164 and the human cervical carcinoma cell line HeLa-S3. Based on the findings of many literature reports and our own data, we discussed a model in which caspases are continuously activated throughout the cell cycle and kept at a critical threshold level by IAP (inhibitor of apoptosis) antagonists. Following the release of Smac/Diablo and HtrA2/OMI from mitochondria in response to diverse stimuli, this threshold is overcome and results in amplified caspase activation and cell death. An alternative, caspase-independent mechanism of cell death is induced in NIH3T3 fi broblasts by a combination of TNF and the pan-caspase inhibitor z-VADfmk. This cell death phenotype, known as necroptosis, displays some morphological features of apoptosis and necrosis. Although caspases are critical regulators of the TNF signaling pathway during cellular life and death, the mechanisms involved in the fine regulation of their dual effects remain to be fully elucidated.

Molecular mechanisms of mitochondrial apoptogenic factor release through pores and megachannels

Mitochondrial membrane permeabilization is a biochemically well-defined phenomenon that occurs in response to numerous physiological and pathological processes that regulate cell survival. In many situations, mitochondrial membrane permeabilization is triggered by an excess of reactive oxygen species (ROS), Ca2+ overload, and the interference of BH3-only proteins of the BCL-2 family, as well as by activated caspases that can act on components of the inner or outer membrane to cause the opening, assembly and/or activation of membrane mitochondrial permeability transition pores. These pores permit the release of apoptogenic factors such as cytochrome c, apoptosis-inducing factor, Smac/Diablo, HtrA2/Omi and endonuclease G from the intermembrane space to the cytosol where they mediate many of the biochemical and morphological features of apoptosis and necrosis. In this review, we discuss the pharmacological, genetic and biochemical evidence that proteins, protein complexes and membrane structures can form pores through which apoptogenic factors can be released from mitochondria.