Human aging stands as one of the most enigmatic scientific topics. It is a highly complex process, resulting from a number of small changes that vary from tissue to tissue. Several major obstacles have blocked the study of aging at the level of whole organisms, including the genetic heterogeneity among individuals and the difficulty in distinguishing the consequences of normal aging from the effects of diseases that occur throughout life. As a result, human cells grown in culture offer a simplified and attractive model for studying the cellular processes of aging. More than forty years ago, Hayflick and Moorhead first reported that diploid fibroblasts undergo a finite number of cell divisions, after which they stop replicating their DNA and proliferating. This phenomenon was termed as “replicative or cellular senescence”. In addition to human fibroblasts, this phenomenon has also been observed in a variety of other human cell types as well as cells from other animal species.
Phenotypes of Senescent cells
Senescent cells do not die but remain viable for long periods of time if fresh cell culture media are added regularly. Senescent cells exhibit two major categories of phenotypes.
First, senescent cells are unable to resume DNA replication. Once arrested in the G1 stage of the cell cycle, physiological mitogens fail to induce re-entry into S phase. In other words, senescent cells remain in a permanent, irreversibly growth-arrested state. A number of mitogen-inducible genes are repressed in senescent cells, including genes encoding proteins required for DNA replication. For instance, essential growth stimulatory transcription factors such as E2F and the Id1 and Id2 inhibitors of basic helix-loophelix (bHLH) transcription factor are repressed. The reason for Id repression is still unclear. E2F repression, however, is likely to occur via the p53-p21 pathway and the pl6-Rb pathway.
Second, senescence results in specific changes in cellular differentiation. Normally, senescent cells exhibit an enlarged, irregular morphology and decreased rate of protein synthesis and degradation. Moreover, senescence induces selected changes in differential functions, including secretion of degradative proteases, cytokines and growth factors. For instance, interleukin-la (IL-1) is overexpressed in senescent human fibroblasts and endothelial cells; ex
An important breakthrough in the field was the discovery of a biomarker for cellular senescence in culture and in vivo. Senescent cells can be distinguished from quiescent and terminally differentiated cells by the presence of senescence-associated-β-galactosidase (SA-β-gal) activity. After histochemical staining at pH 6 using X-gal, the majority of senescent cells turn blue while quiescent cells, terminally differentiated cells, and immortal cells remain unstained.
Physiological functions of cellular senescence
Currently, two significant physiological functions of cellular senescence have been proposed. One suggests that cellular senescence contributes to age-related dysfunction, or aging, while the other proposes it acts as a tumor suppression mechanism.
Several observations suggest the importance of cellular senescence to aging: 1) an inverse correlation between the replicative lifespan of cultured cells and donor age; 2) a correlation between the replicative lifespan of cultured cells and donor species lifespan; 3) more directly, shorter replicative lifespan of cultured cells from humans with premature aging syndromes such as the Werner’s syndrome (WS). Furthermore, it has been found that senescent cells accumulate with age in human tissues. For instance, a higher fraction of cells display positive SA-β-gal activity in both the dermis and epidermis of human skin from older donors. How this accumulation contributes to age-related pathology is still unclear, although the altered function of senescent cells may play an important role. For instance, senescent cells secrete molecules such as ECM degrading enzymes and inflammatory cytokines, which would have deleterious effects on tissue microenvironment and/or tissue function. However, most such evidence is empirical. The contribution of cellular senescence to aging is in demand of critical examination at the molecular level.
Several lines of evidence also support the model that cellular senescence can suppress tumorigenesis: 1) Extensive cell division is critical for cells to acquire many genetic mutations that are the hallmark of malignant tumors. Cells with a fixed replicative life span are much less likely to form tumors than immortal cells, given the fact that most tumor cells are either immortal or have a significant extended life-span. 2) Some human viral oncogenes are able to overcome senescence and extend the life-span of target cells. These include certain viral oncoproteins such as the Simian Virus 40 (SV40) large T antigen, and the Human Papilloma Virus (HPV) E6 and E7. 3) Most directly, two major tumor suppressing pathways, the p53 pathway and the Rb pathway, have been shown playing an important role in establishing and/or maintaining cellular senescence. In fact, p53 and/or Rb inactivation is the primary means by which viral oncoproteins extend the replicative life-span of target cells. One of the goals of this thesis study is to clearly address the role of the Rb pathway in human cell senescence by directly knocking out the Rb gene in human fibroblast. 4) Overex