MiRs are small (20–22 nucleotide) non-coding RNAs that degrade or inhibit translation of mRNA by binding to recognition
sequences on the mRNA sequence. One miR can modulate a number of genes and as such function as a master regulator. In the case of apoptosis signalling for instance, several miRs have been shown to imprint an apoptosis-resistant phenotype on tumour cells. Several miRs have been reported to modulate apoptotic signalling by TRAIL and other TNF family members. In GBM, a specific miR (miR21) has been reported as highly overexpressed in >90% of tumours analysed. Interestingly, inhibition of miR21 significantly blocked GBM outgrowth, while co-treatment of anti-miR21 therapy with neural stem cells expressing sTRAIL resulted in synergistic inhibition of tumour growth in vivo. An important consideration to make regarding all of these combinatorial strategies is the possible this website sensitization of normal cells. For instance, synergistic pro-apoptotic anti-cancer activity upon combination PF-562271 cell line of sTRAIL with proteasome inhibition yielded a therapeutic window in hepatoma cells, but was also associated with enhanced toxicity towards hepatocytes [71]. In addition, hepatocytes were strongly sensitized to Fas upon initial priming with TRAIL [72]. Hepatocytes indeed appear the most TRAIL-sensitive type of cell, with aggregated TRAIL preparations strongly reducing hepatocyte
viability [73]. Therefore, it is apparent that purely homogenous sTRAIL as well as the rational design of non-toxic combinatorial strategies is required for effective anti-cancer strategy in humans. From a conceptual point of view, the efficacy of sTRAIL is likely to be hampered by several factors, including rapid clearance from Dichloromethane dehalogenase the circulation by the kidney. Indeed, sTRAIL has an approximate
half-life of only 30 min in primates and a similar pharmacokinetic profile in humans in a phase I clinical trial [32,74]. Together with the ubiquitous expression of TRAIL receptors in the human body this may severely limit tumour accretion. Moreover, many tumours express higher levels of TRAIL-R2 compared with TRAIL-R1, whereas TRAIL-R2 signalling is only poorly activated by sTRAIL [75]. We and others have attempted to overcome these drawbacks by fusing sTRAIL to an antibody derivative, such as an antibody fragment. The resultant trimeric molecule will be ∼180 kDa and likely has a longer circulation half-life, as renal clearance should be impeded at these higher molecular weights. The antibody targeting domain of the fusion protein will ensure better tumour accretion and retention (for schematic see Figure 4) [76–80]. Importantly, antibody fragment-mediated binding to a cell surface-expressed target antigen converts the sTRAIL into membrane-bound TRAIL that efficiently signals apoptosis via TRAIL-R1 but also TRAIL-R2 in a mono- and/or bi/multi-cellular manner [81,82].
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