6.2. TRANSLATIONAL CHANGES DURING AGING
6.2.1. INITIATION OF TRANSLATION
Association of 40S and 60S ribosomal subunits, an initiating tRNA called methionyl (Met)-tRNAi and the initiation factors (eIFs), results in the formation of an active 80S initiation complex placed on the start codon of the mRNA. The initiation step is considered to be the main target for the regulation of protein synthesis during cell cycle, growth, development, hormonal response and under stress conditions, including heat shock, irradiation and starvation (10). With respect to aging, the rate of translation initiation appears to remain unaltered.
However, the amount of the precursor form of one of the initiation factors, eIF - 5A, is reduced by severalfold in senescent human fibroblasts (11). Recent developments in our understanding of the genomic organization, catalytic functioning and the post-translational modifications of various eIFs during cell growth, proliferation, tumorigenesis, stress and pathological conditions have made it imperative that detailed studies on eIFs also be undertaken in the context of aging. The question of the regulation of protein synthesis at the level of initiation also needs to be reinvestigated. As regards ribosomes, there is a reduction in the activity of ribosomal genes and in the levels of various ribosomal RNAs during aging (12, 13). Furthermore, there is a slight decrease in the number of active ribosomes during aging, although this may not be rate limiting for total protein synthesis because of the ribosomal abundance in the cell (14).
On the other hand, the translational capacity of ribosomes does show an age-associated decline, which, however, is highly variable in various parts of the body (14). Reasons for such variability are not clear at present but may be related to variable protein synthetic activity of different organs.
6.2.2. POLYPEPTIDE CHAIN ELONGATION
A repetitive cyclic event of peptide chain elongation, which is a series of reactions catalyzed by elongation factors, (EFs; also abbreviated now as eEFs for the eukaryotic species) follows the formation of the 80S initiation complex. The regulation of protein synthesis can also occur totally and differentially at the level of polypeptide chain elongation. The regulation of bulk protein synthesis at the level of elongation has been reported for normal and transformed cells during cell cycle transition, amino acid starvation, serum stimulation and phorbol ester treatment (15). Similarly, alterations in the rates of elongation have also been reported in full-term human placenta from diabetic mothers and in rat livers during fasting and refeeding (16-18).
Recently, significant advances have been made in our understanding of some of the structural and functional aspects of prokaryotic elongation factors, which are the prototypes of the superfamily of G proteins (19). In eukaryotes, the addition of an amino acid to the growing polypeptide chain involves at least two elongation factors, EF-1 and EF-2. EF-1 is composed of two distinct parts: a G-binding protein, EF-1α, and a nucleotide exchange protein complex, EF-lβγ. EF-1 usually occurs in multiple molecular forms, composed of varying amounts of EF-1α and EF-lβγ. EF-1α is a ubiquitous, highly expressed and very conserved translational factor. The human EF-1α gene family consists of two actively transcribed isoform genes EF-1α1 and EF-1α2 and more than 18 pseudogenes. EF-1α2 is expressed in a tissue specific manner, whereas EF-1α1 is expressed ubiquitously, and both of them can function in translation (20, 21).
Other interesting features of EF-1α include its abundance and several other functions in addition to its requirement in protein synthesis, including binding to cytoskeletal elements, binding to mitotic apparatus, promoting ubiquitin-dependent degradation of certain N-acetylated proteins and binding to calmodulin (22). Recently, it has been demonstrated that apoptosis rate can be accelerated or decelerated by overexpression or reduction of the level of EF-1α in mouse 3T3 fibroblasts (23). With regard to aging, the activity of EF-1 declines with age in rat livers and Drosophila, and the drop parallels the decrease in protein synthesis. A 35-45% decrease in the activity and amounts of active EF-1α in serially passaged senescent human fibroblasts (24), and in old mouse and rat livers and brains has been reported (25, 26). However, other studies have reported a lack of age-related decline in the expression of EF-1 genes in Drosophila (13), and in human skeletal muscle (27).
Experiments on the germ line insertion of an extra copy of EF-1α gene under the regulation of a heat shock promoter resulted in a better survival of transgenic Drosophila at high temperature as compared with their controls (28). However, this relative increase in the lifespan of transgenic insects at high temperature was not accompanied by any increase in the levels of mRNA, amount and activity of EF-1α (13, 29). Various other transgenic combinations of Drosophila having altered levels of EF-la expression did not show any relationship with lifespan, although large changes in other fitness components, including fecundity could be observed (30). Similarly, no increased expression of EF-1α genes was observed in Drosophila with extended longevity phenotype in a long-lived strain (31).
The other elongation factors, EF-lβγ and EF-2, are involved, respectively, in the post-hydrolytic exchange of GDP with GTP and in the translocation of peptidyl-tRNA on the ribosome. Of these, EF-2 has a unique characteristic in the form of a histidine residue at position 715 modified into diphthamide, as a result of which it can be ADP-ribosylated either endogenously or by bacterial toxins such as diphtheria toxin (32). A decline of more than 60% in the amount of active EF-2 during aging of human fibroblasts in culture has been reported (33, 34).
Future research on the genetic regulation of the structure and function of eukaryotic elongation factor genes and proteins will unravel their pluripotent roles in various biological processes, including aging.