Effects of telomerase and telomere length on epidermal stem cell behavior

Transforming growth factor beta suppresses human telomerase reverse transcriptase (hTERT) by Smad3 interactions with c-Myc and the hTERT gene

Telomerase underpins stem cell renewal and proliferation and is required for most neoplasia. Recent studies suggest that hormones and growth factors play physiological roles in regulating telomerase activity. In this report we show a rapid repression of the telomerase reverse transcriptase (TERT) gene by transforming growth factor beta (TGF-beta) in normal and neoplastic cells by a mechanism depending on the intracellular signaling protein Smad3. In human breast cancer cells TGF-beta induces rapid entry of Smad3 into the nucleus where it binds to the TERT gene promoter and represses TERT gene transcription. Silencing Smad3 gene expression or genetically deleting the Smad3 gene disrupts TGF-beta repression of TERT gene expression. Expression of the Smad3 antagonist, Smad7, also interrupts TGF-beta-mediated Smad3-induced repression of the TERT gene. Mutational analysis identified the Smad3 site on the TERT gene promoter, mediating TERT repression. In response to TGF-beta, Smad3 binds to c-Myc; knocking down c-Myc, Smad3 does not bind to the TERT gene, suggesting that c-Myc recruits Smad3 to the TERT promoter. Thus, TGF-beta negatively regulates telomerase activity via Smad3 interactions with c-Myc and the TERT gene promoter. Modifying the interaction between Smad3 and TERT gene may, thus, lead to novel strategies to regulate telomerase.

Telomerase regulation and stem cell behaviour

Telomerase expression is restricted to a few cell types of the adult organism, most notably germ cells and stem/progenitor cells. Telomerase activity in germ cells is sufficient to prevent telomere shortening with age. Stem cells, however, do not have sufficient telomerase to prevent telomere shortening associated with continuous tissue renewal with increasing age. Indeed, telomerase levels in the adult organism are thought to be rate-limiting for longevity. This is supported by rare human syndromes caused by mutations in telomerase components, which are characterized by premature loss of tissue renewal and premature death. More recently, the role of telomerase and telomere length in stem cells is starting to be elucidated.

Roots and stems: stem cells in cancer

Cancer develops from normal tissues through the accumulation of genetic alterations that act in concert to confer malignant phenotypes. Although we have now identified some of the genes that when mutated initiate tumor formation and drive cancer progression, the identity of the cell population(s) susceptible to such transforming events remains undefined for the majority of human cancers. Recent work indicates that a small population of cells endowed with unique self-renewal properties and tumorigenic potential is present in some, and perhaps all, tumors. Although our understanding of the biology of these putative cancer stem cells remains rudimentary, the existence of such cells has implications for current conceptualizations of malignant transformation and therapeutic approaches to cancer.

Telomerase flies the coop: the telomerase RNA component as a viral-encoded oncogene

Telomerase, the enzyme that elongates our telomeres, is crucial for cancer development based on extensive analyses of human cells, human cancers, and mouse models. New data now suggest that a viral telomerase RNA gene encoded by Marek's disease virus (MDV), an oncogenic herpesvirus of chickens, promotes tumor formation. These findings highlight the importance of telomerase in cancer and raise new questions regarding the mechanisms by which the telomerase RNA component supports tumorigenesis.

Stem cells and their niches

A constellation of intrinsic and extrinsic cellular mechanisms regulates the balance of self-renewal and differentiation in all stem cells. Stem cells, their progeny, and elements of their microenvironment make up an anatomical structure that coordinates normal homeostatic production of functional mature cells. Here we discuss the stem cell niche concept, highlight recent progress, and identify important unanswered questions. We focus on three mammalian stem cell systems where large numbers of mature cells must be continuously produced throughout adult life: intestinal epithelium, epidermal structures, and bone marrow.

Telomerase: not just for the elongation of telomeres

Telomerase RNA component (TERC) and telomerase reverse transcriptase (TERT) function together to elongate telomeres and to protect chromosomal ends. Recent studies have discovered that overexpression of telomerase's TERT subunit promoted epidermal stem-cell mobilization, hair growth and stem-cell proliferation without changes in length of telomeres.1,2 This telomerase functional characteristic is TERC independent and is operated through a mechanism other than telomere elongation. These findings open new doors for future explorations to understand telomerase function and its interaction with other cell components in the regulation of cell senescence and tumorigenesis.

Novel roles for telomerase in aging

Aging is a multifactorial and highly complex process. Telomeres, repetitive DNA elements at the end of linear chromosomes, and telomerase, the reverse transcriptase responsible for the synthesis and elongation of telomeres, are implicated in mammalian aging. Intact telomeres are essential for genome stability and chromosomal integrity, as well as for extended proliferative life span of cells. Lack of telomerase activity in human somatic tissues and concomitant telomere erosion correlate with age-related pathologies. Mouse models either lacking or overexpressing telomerase support the notion that short telomeres cause premature aging. Recent evidence suggests that telomerase might have other functions besides maintaining telomere length. Here, we propose a possible role for telomerase in delaying the aging process, which is independent of telomere length. The positive effects of telomerase on aging seem to come of the price of tumour promotion. These antagonistic roles of telomerase in aging and cancer may have important implications for putative telomerase based therapies.

Expression of mTert in primary murine cells links the growth-promoting effects of telomerase to transforming growth factor-β signaling

Here, we show that ectopic expression of the catalytic subunit of mouse telomerase (mTert) confers a growth advantage to primary murine embryonic fibroblasts (MEFs), which have very long telomeres, as well as facilitates their spontaneous immortalization and increases their colony-forming capacity upon activation of oncogenes. We demonstrate that these telomere length-independent growth-promoting effects of mTert overexpression require catalytically active mTert, as well as the formation of mTert/Terc complexes. The gene expression profile of mTert-overexpressing MEFs indicates that telomerase enhances growth in these cells through the repression of growth-inhibiting genes of the transforming growth factor-beta (TGF-beta) signaling network. We functionally validate this result by showing that mTert abrogates the growth-inhibitory effect of TGF-beta in MEFs, thus demonstrating that telomerase increments the proliferative potential of primary mouse embryonic fibroblasts by targeting the TGF-beta pathway.

Epithelial skin stem cells

Major progress in understanding epithelial skin stem cells has been accomplished. This has been possible by developing new methods for labeling, tracking, isolating, and characterizing enriched populations of stem cells. This chapter summarizes in vivo and in vitro assays that are currently employed to analyze skin epithelial stem cells. Despite progress, the definition of a stem cell is currently a functional one. Unambiguous identification of a stem cell in intact tissue is still not possible. These limitations hamper molecular studies aimed at unraveling the cellular mechanisms operating in the stem cell compartment. This chapter emphasizes current methods for analyzing hair follicle stem cells, as opposed to other epithelial compartments, because the hair follicle has been most intensively studied up to date.

Molecular signaling and genetic pathways of senescence: Its role in tumorigenesis and aging

In response to progressive telomere shortening in successive cell divisions, normal somatic cells enter senescence, during which they cease to proliferate irreversibly and undergo dramatic changes in gene expression. Senescence can also be activated by various types of stressful stimuli, including aberrant oncogenic signaling, oxidative stress, and DNA damage. Because of the limited proliferative capacity imposed by senescence, as well as the ability of senescent cells to influence neighboring non-senescent cells, senescence has been proposed to play an important role in tumorigenesis and to contribute to aging. Considerable effort has been put into elucidating the molecular mechanisms of senescence, including the signals that trigger senescence, the molecular pathways by which cells enter senescence, and evidence that supports its role in tumorigenesis and aging.

The mitochondrial contribution to stem cell biology

The distribution and functions of mitochondria in stem cells have not been examined, yet the contributions of these organelles to stem cell viability and differentiation must be vitally important in view of their critical roles in all other cell types. A key role for mitochondria in stem cells is indicated by reports that they translocate in the oocyte during fertilisation to cluster around the pronuclei and can remain in a perinuclear pattern during embryo development. This clustering appears to be essential for normal embryonic development. Because embryonic stem cells are derived from fertilised oocytes, and eventually can differentiate into ‘adult’ stem cells, it was hypothesised that mitochondrial perinuclear clustering persists through preimplantation embryo development into the stem cells, and that this localisation is indicative of stem cell pluripotency. Further, it was predicted that mitochondrial activity, as measured by respiration and adenosine triphosphate (ATP) content, would correlate with the degree of perinuclear clustering. It was also predicted that these morphological and metabolic measurements could serve as indicators of ‘stemness.’ This article reviews the distribution and metabolism of mitochondria in a model stem cell line and how this information is related to passage number, differentiation and/or senescence. In addition, it describes mitochondrial DNA deletions in oocytes and embryos that could adversely affect stem cell performance.