Hormetic Modulation of Aging in Human Cells

An experimental model system that has been used in testing and applying hormesis as a modulator of aging is the so-called Hayflick system of cellular aging in vitro. In modern biogerontology, the terms “cellular aging”, “cell senescence” or “replicative senescence” imply the study of normal diploid cells in culture, which during serial subcultivation undergo a multitude of changes culminating in the irreversible cessation of cell division. This process of cellular aging or replicative senescence in vitro is commonly known as the Hayflick phenomenon, and the limited division potential of normal cells is called the Hayflick limit, in recognition of the observations first reported by Leonard Hayflick in 1961. In many organisms, several cell types retain the capacity to divide during most of the adult lifespan, and are required to divide repeatedly or infrequently in carrying out various functions of the body. These functions include the immune response, blood formation, bone formation, and repair and regeneration of various tissues. Epithelial cells, epidermal basal cells (keratinocytes), fibroblasts, osteoblasts, myoblasts, glial cells and lymphocytes constitute major differentiated and proliferating cell types of an organism, and are distinct from the pluripotent stem cells. It is not only their differentiated and specialized functions that are critical for the organism, their capacity to divide is an integral part of their role in organismic growth, development, maintenance and survival (for details on the aging in vitro of various cell types (see Kaul and Wadhwa 2003).

The study of age-related changes in the physiology, biochemistry and molecular biology of isolated cell populations has greatly expanded our understanding of the fundamental aspects of aging. In addition to the normal diploid fibroblasts which have been the most frequently used cells for studies on cellular ageing in vitro, a variety of other cell types including epithelial cells, endothelial cells, keratinocytes, glial cells, lymphocytes and osteoblasts have also been used. Although the exact culturing conditions (such as the type of the culture medium, the source of growth factors, the use of antibiotics, and the incubation temperature, humidity and gaseous composition) may vary for different cell types, serial subcultivation or serial passaging of normal diploid differentiated cells can be performed only a limited number of time. The cumulative number of cell proliferations, measured as the cumulative population doubling level achieved in vitro, depends upon several biological factors, such as the maximum lifespan of the species, the age of the donor of the tissue biopsy, and the site of the biopsy. This is in contrast to the high proliferative capacity of transformed, cancerous and immortalized cells whose cultures can be subcultivated and maintained indefinitely.

Serial subcultivation of normal cells is accompanied by a progressive accumulation of a wide variety of changes before the final cessation of cell replication occurs. The progressively emerging senescent phenotype of serially passaged normal diploid cells can be categorized into the structural, physiological, and biochemical and molecular phenotypes, which can be used as biomarkers of cellular aging in vitro. Table 1 gives a summary of the major changes occurring during serial passaging and replicative senescence. For specific details for different cell types (see Kaul and Wadhwa 2003).