How telomerase reaches its end: mechanism of telomerase regulation by the telomeric complex

The telomerase enzyme, which synthesizes telomeric DNA repeats, is regulated in cis at individual chromosome ends by the telomeric protein/DNA complex in a manner dependent on telomere repeat-array length. A dynamic interplay between telomerase-inhibiting factors bound at duplex DNA repeats and telomerase-promoting ones bound at single-stranded terminal DNA overhangs appears to modulate telomerase activity and to be directly related to the transient deprotection of telomeres. We discuss recent advances on the mechanism of telomerase regulation at chromosome ends in both yeast and mammalian systems.     

Human telomerase inhibition by substituted acridine derivatives.

A series of 3,6-disubstituted acridine derivatives have been rationally designed as telomerase inhibitors. They have been designed on the basis that inhibition of telomerase occurs by stabilising G-quadruplex structures formed by the folding of telomeric DNA. The most potent inhibitors have IC50 values against telomerase of between 1.3 and 8 microM, comparable to their cytotoxicity in ovarian cancer cell lines.     

Human protein metabolism: its measurement and regulation

The body's protein mass not only provides architectural support for cells but also serves vital roles in maintaining their function and survival. The whole body protein pool, as well as that of individual tissues, is determined by the balance between the processes of protein synthesis and degradation. These in turn are regulated by interactions among hormonal, nutritional, neural, inflammatory, and other influences. Prolonged changes in either the synthetic or degradative processes (or both) that cause protein wasting increase morbidity and mortality. The application of tracer kinetic methods, combined with measurements of the activity of components of the cellular signaling pathways involved in protein synthesis and degradation, affords new insights into the regulation of both protein synthesis and breakdown in vivo. These insights, including those from studies of insulin, insulin-like growth factor I, growth hormone, and amino acid-mediated regulation of muscle and whole body protein turnover, provide opportunities to develop and test therapeutic approaches with promise to minimize or prevent these adverse health consequences.     

Multiple levels of telomerase regulation

Normally, cell division leads to shortening of telomeres, the nucleoprotein complexes located at the ends of linear chromosomes. When telomeres reach a critically short length, cells cease to divide. However, immortal tumor cells display stable telomere lengths and are able to maintain their proliferative state. Wong and colleagues have found that telomerase is sequestered by nucleoli during certain stages of the cell cycle, decreasing the likelihood of telomerase access to chromatin until the late S phase. Additionally, they demonstrate that ionizing radiation tends to keep telomerase sequestered in nucleoli, whereas cell transformation leads to telomerase translocation into the nucleoplasm, where, presumably, it can catalyze the lengthening of telomeres at appropriate and inappropriate sites. The sequestration of telomerase thus imposes a newly identified level of regulation on telomerase activity, implicating telomerase localization as a potentially useful target for pharmacotherapy.     

RNase III-dependent regulation of yeast telomerase

In bakers' yeast, in vivo telomerase activity requires a ribonucleoprotein (RNP) complex with at least four associated proteins (Est2p, Est1p, Est3p, and Cdc13p) and one RNA species (Tlc1). The function of telomerase in maintaining chromosome ends, called telomeres, is tightly regulated and linked to the cell cycle. However, the mechanisms that regulate the expression of individual components of telomerase are poorly understood. Here we report that yeast RNase III (Rnt1p), a double-stranded RNA-specific endoribonuclease, regulates the expression of telomerase subunits and is required for maintaining normal telomere length. Deletion or inactivation of RNT1 induced the expression of Est1, Est2, Est3, and Tlc1 RNAs and increased telomerase activity, leading to elongation of telomeric repeat tracts. In silico analysis of the different RNAs coding for the telomerase subunits revealed a canonical Rnt1p cleavage site near the 3' end of Est1 mRNA. This predicted structure was cleaved by Rnt1p and its disruption abolished cleavage in vitro. Mutation of the Rnt1p cleavage signal in vivo impaired the cell cycle-dependent degradation of Est1 mRNA without affecting its steady-state level. These results reveal a new mechanism that influences telomeres length by controlling the expression of the telomerase subunits.     

Human caspase-7 activity and regulation by its N-terminal peptide

Central to the execution phase of apoptosis are the two closely related caspase-3 and -7. They share common substrate specificity and structure, but differ completely in the sequence of their respective N-terminal regions including their N-peptides, a 23-28 residue segment that are removed during zymogen activation. We show that the N-peptide of caspase-7 plays no role in the fundamental activation or properties of the active protease in vitro. However, the N-peptide modifies the properties of caspase-7 in vivo. In ectopic expression experiments, caspase-7 constructs with no N-peptide are far more lethal than constructs that have an uncleavable peptide. Moreover, the N-peptide of caspase-7 must be removed before efficient activation of the zymogen can occur in vivo. These disparate requirements for the N-peptide argue that it serves to physically sequester the caspase-7 zymogen in a cytosolic location that prevents access by upstream activators (caspase-8, -9, and -10). The N-peptide must first be removed, probably by caspase-3, before efficient conversion and activation of the zymogen can occur in vivo.