The finite proliferation of mammalian cells is considered to be the result of a reduction of telomere length [1, 2]. The telomere contains repeated sequences of six nucleotide bases, TTAGGG, located at the termini of individual chromosomes, and has been shown to be shortened by 33–120 bp at each cell division in human fibroblastic cells and lymphocytes, thus causing aging and finite mitotic capability [3, 4]. Telomere length is maintained by telomerase, a ribonuclear protein complex consisting of an integral RNA (hTR), which serves as the telomeric template; a catalytic subunit (hTERT), which has reverse transcriptase activity; and associated protein components [5–11]. In the absence of hTERT, telomeres shorten during cell division because the DNA replication complex cannot completely copy telomeric DNA. Cellular senescence and growth arrest are proposed to occur when telomere lengths in germ cells and most cancer cells are decreased. However, ectopic expression of hTERT leads to telomere elongation and extended lifespan in several cell types [1, 12–14].
Possible mechanisms of age-dependent bone loss may be attributed, at least in part, to a deficiency of osteoblast function or a decrease in the number of osteogenic progenitor cells rather than to an increase in bone resorption by osteoclasts [15, 16]. It has been suggested that telomere-associated cellular senescence may contribute to various age-related disorders. Recent studies reported that the introduction of hTERT into osteoblasts isolated from human trabeculae induced telomerase activity and extended the lifespan of these cells [5, 6]. However, the role of telomerase in bone formation, particularly with respect to maintenance of the osteogenic precursor cell population, is largely unknown. Pluripotent human bone marrow stromal cells (BMSCs) were originally described as progenitors of osteoblasts because of their capacity to form normal bone in vivo [5, 6, 13]. Mesenchymal stem cells, including BMSCs and adipose stromal cell lines (ATSCs), are being analyzed as new therapeutic agents for repairing large bone defects that cannot undergo spontaneous healing [17–19].
The regeneration of diseased or damaged tissue is the principle goal of the emerging discipline of tissue engineering. A key requirement in tissue regeneration is the availability of the constituent cells. Adipose tissue stromal cells have been defined as multipotential adult stem cells, capable of differentiating into a variety of cell types such as osteoblasts, chondrocytes, adipocytes, muscle cells, and neural cells [20–23]. Recently, our group and others reported that human and nonhuman primate-derived ATSCs and BMSCs can propagate in vitro and contain detectable levels of telomerase activity. Forced division of ATSCs in vitro may cause excessive telomere shortening in the descendent lineages, although ATSCs themselves possess telomerase activity. Indeed, recent studies have demonstrated that the telomerase activity of mesenchymal stem cells is not sufficient to completely compensate for the reduction of telomere length during continuous in vitro subculture. To extend the proliferative lifespan of ATSCs, supplementation with transduced exogenous hTERT may be necessary, because the self-renewal and replicative potential of these cells may depend on sufficient telomerase activity to maintain stable telomeres. Reconstitution of telomerase activity through expression of exogenous hTERT enables normal human fibro-blasts, as well as retinal epithelial, myometrial, and endothelial cells, to avoid senescence [24–30]. After ectopic expression of telomerase, the lifespan of BMSCs was significantly increased, and proliferative capacity was extended in vitro. The cells were demonstrated to have an enhanced capacity for bone formation in vitro and in vivo [5, 6, 31–33]. The enhanced formation and normal morphology of the ectopically formed bone strongly suggest that ATSC-TERT cells represent a highly useful candidate cell source for bone tissue regeneration and engineering.