Genes, hormones, and aging

Nutritional interventions to promote post-exercise muscle protein synthesis

Resistance exercise is a powerful stimulus to augment muscle protein anabolism, as it can improve the balance between muscle protein synthesis and breakdown. However, the intake of food during post-exercise recovery is necessary for hypertrophy to occur. Therefore, athletes need to ingest protein following exercise to attain a positive protein balance and maximise their skeletal muscle adaptive response. The interaction between exercise and nutrition is not only important for athletes, but is also of important clinical relevance in the elderly. Exercise interventions combined with specific nutritional modulation provide an effective strategy to counteract or reduce the loss of skeletal muscle mass with aging.

Altered responses in skeletal muscle protein turnover during aging in anabolic and catabolic periods

One of the most important effects of aging is sarcopenia, which is associated with impaired locomotion and general weakness. In addition, there is increased susceptibility to illness in aging, which often results in muscle wasting episodes. In such instances, the mobilization of muscle proteins provides free amino acids that are used for energetic purpose, the synthesis of acute phase proteins, and the immune response. However, since muscle protein mass is already depleted, the ability of the aged organism to recover from stress is impaired. Therefore, elucidating the mechanisms that result in sarcopenia is of obvious importance. Age-related changes in protein synthesis and proteolysis are rather small and our current methodology does not enable one to establish unequivocally whether sarcopenia results from depressed protein synthesis, increased proteolysis or both. By contrast, in anabolic and catabolic periods, a number of dysregulations in muscle protein turnover became clearly apparent. The aim of this review is to provide an overview of such altered responses to nutrients and catabolic treatments, which may ultimately contribute to explain sarcopenia. This includes impaired recovery in catabolic states, impaired anabolic effects of nutrients, in particular leucine, and a lack of regulation of the ubiquitin-proteasome proteolytic system. These alterations are discussed with respect to modifications in the insulin/IGF-1 axis and glucocorticoid related effects.

Changes in gene expression during senescence of adrenocortical cells in culture

Bovine adrenocortical cells undergo a process in which expression of steroid hydroxylases is lost progressively as a function of population doubling level (PDL) in culture. Each cytochrome P450 shows a characteristic rate of loss of expression as a function of PDL (in order of rates of loss: CYP11B >CYP21 >CYP17 >CYP11A). CYP11B and CYP21 require insulin-like growth factor I as well as cyclic AMP; these are the only factors required for induction in the primary culture. Middle- and later passage cells do not express CYP11B and CYP21 under the same conditions, but will do so when cells are grown in extracellular matrix Matrigel. In late-passage cells neither CYP17, CYP21, nor CYP11B are expressed, even in the presence of Matrigel; only CYP11A is expressed in late-passage cultures. When the different environmental factors required for induction of CYP11B and CYP21 are taken into account, induction of these genes disappears with the same kinetics as previously shown for CYP17 as a function of PDL. The primary cause of the loss of expression of these genes is likely to be a phenotypic switching event similar to that previously demonstrated for CYP17 by in situ hybridization. The mechanism of phenotypic switching is unknown. However, one HpaII site at -2.3 kb of CYP17 was methylated in the bovine adrenal cortex in vivo but showed rapid and complete demethylation when adrenocortical cells were placed in culture. This indicates a unique, reproducible, environmentally determined change in methylation, with as yet undetermined consequences. However, data from reporter constructs suggest that phenotypic switching does not result from a simple loss of regulatory factors that act within 2.5 kb of the promoter. Previous data suggested that SV40 T antigen may affect phenotypic switching, and thus that SV40 may be useful for the derivation of functional adrenocortical cell lines. Adaptation of methods previously used for bovine cells to human adrenocortical cells to produce SV40 T antigen-transfected clones yielded data indicating preservation of essential aspects of the human adrenocortical cell differentiated phenotype.

Demethylation of satellite I DNA during senescence of bovine adrenocortical cells in culture

Over the finite proliferative life span of cultured bovine adrenocortical cells, satellite I DNA shows a progressive and extensive loss of methylation at CCGG sites. This was shown by Southern blotting after digestion with the methylation-sensitive enzyme HpaII alone, which provides a sensitive indicator of methylation loss, or digestion with the combination of EcoRI and HpaII, which provides a quantitative indication of loss of methylation. Bovine tissues, including adrenal cortex, all showed a much higher level of satellite methylation than cultured adrenocortical cells. After adrenocortical cells are placed in culture, some demethylation of satellite I is seen as early as 10 population doublings. By 80 population doublings, loss of satellite DNA methylation is extensive. The loss does not appear to prevent continued cell division, since an extended life span clone of bovine adrenocortical cells transfected with SV40 T antigen showed a similar pattern of extensive demethylation. Satellite demethylation has been reported in aging in vivo and the present cell culture system may provide an in vitro model for this form of genetic instability.

Changes in gene expression and DNA methylation in adrenocortical cells senescing in culture

Recent experiments in cultured bovine adrenocortical cells show that the previously observed phenotypic switching of CYP17 (steroid 17 alpha-hydroxylase) expression is preceded at a much earlier time by changes in methylation in the CYP17 5' flanking region. Two CpG sites that are methylated in the adrenal cortex in vivo were observed to undergo rapid demethylation when adrenocortical cells were placed in culture. Two adjacent CpG sites that are also methylated in vivo did not demethylate; these two sites are completely nonmethylated in fibroblasts. All CpG sites downstream, in the promoter or coding region, are always methylated in all tissues and in bovine adrenocortical cells even after many population doublings in culture. In contrast to the specific and rapid demethylation of sites in CYP17, satellite I shows a slower and apparently random loss of methylation that extends over the entire replicative life span. These changes in methylation provide examples of genetic instability in cells that undergo senescence in culture. Future experiments will focus on the relationship of these events to the phenotypic switching process.