Cellular senescence as a tumor-suppressor mechanism

Novel pathways associated with bypassing cellular senescence in human prostate epithelial cells

Cellular senescence forms a barrier that inhibits the acquisition of an immortal phenotype, a critical feature in tumorigenesis. The inactivation of multiple pathways that positively regulate senescence are required for immortalization. To identify these pathways in an unbiased manner, we performed DNA microarray analyses to assess the expression of 20,000 genes in human prostate epithelial cells (HPECs) passaged to senescence. These gene expression patterns were then compared with those of HPECs immortalized with the human Papillomavirus 16 E7 oncoprotein. Senescent cells display gene expression patterns that reflect their nonproliferative, differentiated phenotype and express secretory proteases and extracellular matrix components. A comparison of genes transcriptionally up-regulated in senescence to those in which expression is significantly down-regulated in immortalized HPECs identified three genes: the chemokine BRAK, DOC1, and a member of the insulin-like growth factor axis, IGFBP-3. Expression of these genes is found to be uniformly lost in human prostate cancer cell lines and xenografts, and previously, their inactivation was documented in tumor samples. Thus, these genes may function in novel pathways that regulate senescence and are inactivated during immortalization. These changes may be critical not only in allowing cells to bypass senescence in vitro but in the progression of prostate cancer in vivo.

Markers of senescence?

The specific identification of cellular senescence in clinical material has important implications for determining the role of senescence in age-related pathologies and in neoplasia in certain tumours. One suggested marker of senescence is the histochemical identification of a specific beta-galactosidase enzyme operative at pH 6.0. However, recent data indicate that this enzyme may not be specific for senescence in all tissues and probably represents the expression of endogenous lysosomal acid beta-galactosidase, which is expressed by a variety of differentiated cell types.

Stem cells from birth to death: The history and the future

The concept that adult stem cells, despite their impressive proliferative potential, are immortal has been challenged by experimental studies of hematopoietic stem cells. In this review, we discuss the properties that characterize a stem cell, the growing list of tissues in which stem cells are found, how they can be identified and isolated, how stem cells may transdifferentiate, and the findings that illustrate how age affects the hematopoietic stem cell population. We propose that an aging stem cell population affects tissue and organ homeostasis, particularly in response to environmental stresses, and we hypothesize that through this mechanism the functional status of stem cells affects the longevity of the organism.

Cancer and Aging: Yin, Yang, and p53

The author discusses a recent paper published in Nature that shows that a hyperactive allele of the tumor suppressor p53 leads to the development of some premature aging phenotypes in mice. This new discovery is discussed in light of previous findings related to mechanisms of aging.

Do sunscreens increase or decrease melanoma risk: an epidemiologic evaluation

Ultraviolet adiation is an important cause of melanoma, so the use of sunscreen lotions has been advocated for melanoma prevention. Several arguments have been raised in opposition to this inference. Sunscreen use may interfere with cutaneous vitamin D synthesis, which some have hypothesized may lower melanoma risk. Sunscreen users may compensate for their sunscreen use by staying out much longer in the sun, or may use sunscreen lotions inconsistantly. Published melanoma case-control studies have not consistantly demonstrated a protective effect of sunscreens; however, these studies do not provide strong evidence, ultraviolet radiation is a known cause of melanoma, and ultraviolet B may be particularly potent, so on balance the evidence supports continued advocacy of sunscreen lotion use as part of an overall sun-protection regimen. Uncertainty will remain, however, until the action spectrum of melanoma is convincingly demonstrated or the methodologic limitations of existing epidemiologic evidence are overcome. The latter may require another decade or more of experience with sunscreen use.