Status of mTOR activity may phenotypically differentiate senescence and quiescence

Gerosuppression in confluent cells

The most physiological type of cell cycle arrest - namely, contact inhibition in dense culture - is the least densely studied. Despite cell cycle arrest, confluent cells do not become senescent. We recently described that mTOR (target of rapamycin) is inactive in contact-inhibited cells. Therefore, conversion from reversible arrest to senescence (geroconversion) is suppressed. I this Perspective, we further extended the gerosuppression model. While causing senescence in regular cell density, etoposide failed to cause senescence in contact-inhibited cells. A transient reactivation of mTOR favored geroconversion in etoposide-treated confluent cells. Like p21, p16 did not cause senescence in high cell density. We discuss that suppression of geroconversion in confluent and contact-inhibited cultures mimics gerosuppression in the organism. We confirmed that levels of p-S6 were low in murine tissues in the organism compared with mouse embryonic fibroblasts in cell culture, whereas p-Akt was reciprocally high in the organism.

Fasting levels of hepatic p-S6 are increased in old mice

TOR is involved in aging in a wide range of species from yeast to mammals. Here we show that, after overnight fasting, mTOR activity is higher in the livers of 28 months old female mice compared with middle-aged mice. Taken together with previous reports, our data predict that the life-extending effect of calorie restriction (CR) may be diminished, if CR is started in very old age. In contrast, rapamycin is known to be effective, even when started late in life.

Long-term quiescent fibroblast cells transit into senescence

Cellular senescence is described to be a consequence of telomere erosion during the replicative life span of primary human cells. Quiescence should therefore not contribute to cellular aging but rather extend lifespan. Here we tested this hypothesis and demonstrate that cultured long-term quiescent human fibroblasts transit into senescence due to similar cellular mechanisms with similar dynamics and with a similar maximum life span as proliferating controls, even under physiological oxygen conditions. Both, long-term quiescent and senescent fibroblasts almost completely fail to undergo apoptosis. The transition of long-term quiescent fibroblasts into senescence is also independent of HES1 which protects short-term quiescent cells from becoming senescent. Most significantly, DNA damage accumulates during senescence as well as during long-term quiescence at physiological oxygen levels. We suggest that telomere-independent, potentially maintenance driven gradual induction of cellular senescence during quiescence is a counterbalance to tumor development.

Contact inhibition and high cell density deactivate the mammalian target of rapamycin pathway, thus suppressing the senescence program

During cell cycle arrest caused by contact inhibition (CI), cells do not undergo senescence, thus resuming proliferation after replating. The mechanism of senescence avoidance during CI is unknown. Recently, it was demonstrated that the senescence program, namely conversion from cell cycle arrest to senescence (i.e., geroconversion), requires mammalian target of rapamycin (mTOR). Geroconversion can be suppressed by serum starvation, rapamycin, and hypoxia, which all inhibit mTOR. Here we demonstrate that CI, as evidenced by p27 induction in normal cells, was associated with inhibition of the mTOR pathway. Furthermore, CI antagonized senescence caused by CDK inhibitors. Stimulation of mTOR in contact-inhibited cells favored senescence. In cancer cells lacking p27 induction and CI, mTOR was still inhibited in confluent culture as a result of conditioning of the medium. This inhibition of mTOR suppressed p21-induced senescence. Also, trapping of malignant cells among contact-inhibited normal cells antagonized p21-induced senescence. Thus, we identified two nonmutually exclusive mechanisms of mTOR inhibition in high cell density: (i) CI associated with p27 induction in normal cells and (ii) conditioning of the medium, especially in cancer cells. Both mechanisms can coincide in various proportions in various cells. Our work explains why CI is reversible and, most importantly, why cells avoid senescence in vivo, given that cells are contact-inhibited in the organism.

Rapamycin extends life- and health span because it slows aging

Making headlines, a thought-provocative paper by Neff, Ehninger and coworkers claims that rapamycin extends life span but has limited effects on aging. How is that possibly possible? And what is aging if not an increase of the probability of death with age. I discuss that the JCI paper actually shows that rapamycin slows aging and also extends lifespan regardless of its direct anti-cancer activities. Aging is, in part, MTOR-driven: a purposeless continuation of developmental growth. Rapamycin affects the same processes in young and old animals: young animals' traits and phenotypes, which continuations become hyperfunctional, harmful and lethal later in life.

Human cytomegalovirus (HCMV) US2 protein interacts with human CD1d (hCD1d) and down-regulates invariant NKT (iNKT) cell activity

To avoid host immune surveillance, human cytomegalovirus (HCMV) encoded endoplasmic reticulum (ER)-membrane glycoprotein US2, which interferes with antigen presenting mechanism of Major histocompatibility complex (MHC) class Ia and class II molecules. However, not many attempts have been made to study the effect of HCMV US2 on the expression of MHC class Ib molecules. In this study, we examined the effect of HCMV US2 on the expression and function of human CD1d (hCD1d), which presents glycolipid antigens to invariant NKT (iNKT) cells. Our results clearly showed that the physiological interaction between ER lumenal domain of HCMV US2 and α3 domain of hCD1d was observed within ER. Compared with mature form of hCD1d, immature form of hCD1d is more susceptible to ubiquitin-dependent proteasomal degradation mediated by HCMV US2. Moreover, the ectopic expression of HCMV US2 leads to the down-modulation of iNKT cell activity without significant change of hCD1d expression. These results will advance our understanding of the function of HCMV US2 in immune evasive mechanisms against anti-viral immunity of iNKT cells.

Recent progress in genetics of aging, senescence and longevity: focusing on cancer-related genes

It is widely believed that aging results from the accumulation of molecular damage, including damage of DNA and mitochondria and accumulation of molecular garbage both inside and outside of the cell. Recently, this paradigm is being replaced by the “hyperfunction theory”, which postulates that aging is caused by activation of signal transduction pathways such as TOR (Target of Rapamycin). These pathways consist of different enzymes, mostly kinases, but also phosphatases, deacetylases, GTPases, and some other molecules that cause overactivation of normal cellular functions. Overactivation of these sensory signal transduction pathways can cause cellular senescence, age-related diseases, including cancer, and shorten life span. Here we review some of the numerous very recent publications on the role of signal transduction molecules in aging and age-related diseases. As was emphasized by the author of the “hyperfunction model”, many (or actually all) of them also play roles in cancer. So these “participants” in pro-aging signaling pathways are actually very well acquainted to cancer researchers. A cancer-related journal such as Oncotarget is the perfect place for publication of such experimental studies, reviews and perspectives, as it can bridge the gap between cancer and aging researchers.

MTOR-driven quasi-programmed aging as a disposable soma theory: blind watchmaker vs. intelligent designer

If life were created by intelligent design, we would indeed age from accumulation of molecular damage. Repair is costly and limited by energetic resources, and we would allocate resources rationally. But, albeit elegant, this design is fictional. Instead, nature blindly selects for short-term benefits of robust developmental growth. "Quasi-programmed" by the blind watchmaker, aging is a wasteful and aimless continuation of developmental growth, driven by nutrient-sensing, growth-promoting signaling pathways such as MTOR (mechanistic target of rapamycin). A continuous post-developmental activity of such gerogenic pathways leads to hyperfunctions (aging), loss of homeostasis, age-related diseases, non-random organ damage and death. This model is consistent with a view that (1) soma is disposable, (2) aging and menopause are not programmed and (3) accumulation of random molecular damage is not a cause of aging as we know it.

Curcumin triggers p16-dependent senescence in active breast cancer-associated fibroblasts and suppresses their paracrine procarcinogenic effects

Activated cancer-associated fibroblasts (CAFs) or myofibroblasts not only facilitate tumor growth and spread but also affect tumor response to therapeutic agents. Therefore, it became clear that efficient therapeutic regimens should also take into account the presence of these supportive cells and inhibit their paracrine effects. To this end, we tested the effect of low concentrations of curcumin, a pharmacologically safe natural product, on patient-derived primary breast CAF cells. We have shown that curcumin treatment upregulates p16(INK4A) and other tumor suppressor proteins while inactivates the JAK2/STAT3 pathway. This reduced the level of alpha-smooth muscle actin (α-SMA) and the migration/invasion abilities of these cells. Furthermore, curcumin suppressed the expression/secretion of stromal cell-derived factor-1 (SDF-1), interleukin-6 (IL-6), matrix metalloproteinase-2 (MMP-2), MMP-9, and transforming growth factor-β, which impeded their paracrine procarcinogenic potential. Intriguingly, these effects were sustained even after curcumin withdrawal and cell splitting. Therefore, using different markers of senescence [senescence-associated β-galactosidase (SA-β-gal) activity, Ki-67 and Lamin B1 levels, and bromodeoxyuridine incorporation], we have shown that curcumin markedly suppresses Lamin B1 and triggers DNA damage-independent senescence in proliferating but not quiescent breast stromal fibroblasts. Importantly, this curcumin-related senescence was p16(INK4A)-dependent and occurred with no associated inflammatory secretory phenotype. These results indicate the possible inactivation of cancer-associated myofibroblasts and present the first indication that curcumin can trigger DNA damage-independent and safe senescence in stromal fibroblasts.

TFEB regulates lysosomal proteostasis

Loss-of-function diseases are often caused by destabilizing mutations that lead to protein misfolding and degradation. Modulating the innate protein homeostasis (proteostasis) capacity may lead to rescue of native folding of the mutated variants, thereby ameliorating the disease phenotype. In lysosomal storage disorders (LSDs), a number of highly prevalent alleles have missense mutations that do not impair the enzyme's catalytic activity but destabilize its native structure, resulting in the degradation of the misfolded protein. Enhancing the cellular folding capacity enables rescuing the native, biologically functional structure of these unstable mutated enzymes. However, proteostasis modulators specific for the lysosomal system are currently unknown. Here, we investigate the role of the transcription factor EB (TFEB), a master regulator of lysosomal biogenesis and function, in modulating lysosomal proteostasis in LSDs. We show that TFEB activation results in enhanced folding, trafficking and lysosomal activity of a severely destabilized glucocerebrosidase (GC) variant associated with the development of Gaucher disease (GD), the most common LSD. TFEB specifically induces the expression of GC and of key genes involved in folding and lysosomal trafficking, thereby enhancing both the pool of mutated enzyme and its processing through the secretory pathway. TFEB activation also rescues the activity of a β-hexosaminidase mutant associated with the development of another LSD, Tay-Sachs disease, thus suggesting general applicability of TFEB-mediated proteostasis modulation to rescue destabilizing mutations in LSDs. In summary, our findings identify TFEB as a specific regulator of lysosomal proteostasis and suggest that TFEB may be used as a therapeutic target to rescue enzyme homeostasis in LSDs.

Quantification of cell cycle-arresting proteins

Cellular senescence, which can be defined as a stress response preventing the propagation of cells that have accumulated potentially oncogenic alterations, is invariably associated with a permanent cell cycle arrest. Such an irreversible blockage is mainly mediated by the persistent upregulation of one or more cyclin-dependent kinase inhibitors (CKIs), including (though not limited to) p16( INK4A ) and p21( CIP1 ) and p27( KIP1 ). CKIs operate by binding to cyclin-dependent kinases (CDKs), de facto inhibiting their enzymatic activity. Here, we provide an immunoblotting-based method for the detection and quantification of CKIs in vitro and ex vivo, together with a set of guidelines for the interpretation of results.

Mechanistic or mammalian target of rapamycin (mTOR) may determine robustness in young male mice at the cost of accelerated aging

Males, who are bigger and stronger than females, live shorter in most species from flies to mammals including humans. Cellular mass growth is driven in part by mTOR (Target of Rapamycin). When developmental growth is completed, then, instead of growth, mTOR drives aging, manifested by increased cellular functions, such as hyper-secretion by fibroblasts, thus altering homeostasis, leading to age-related diseases and death. We hypothesize that MTOR activity is elevated in male mice compared with females. Noteworthy, 6 months old males were 28 % heavier than females. Also levels of phosphorylated S6 (pS6) and phospho-AKT (p-AKT, Ser 473), markers of the mTOR activity, were higher in male organs tested. Levels of pS6 were highly variable among mice and correlated with body weight and p-AKT. With age, the difference between levels of pS6 between sexes tended to minimize, albeit males still had hyperactive mTOR. Unlike fasting, the intraperitoneal (i.p.) administration of rapamycin eliminated pS6 in all organs of all females measured by immunoblotting and immunohistochemistry without affecting p-AKT and blood insulin. Although i.p. rapamycin dramatically decreased levels of pS6 in males too, it was still detectable by immunoblotting upon longer exposure. Our study demonstrated that both tissue p-AKT and pS6 were higher in young male mice and were associated with increased body weight and insulin. These data can explain bigger body size and faster aging in males. Our data suggest higher efficacy of rapamycin compared to fasting. Higher sensitivity of females to rapamycin may explain more pronounced life extension by rapamycin observed in females compared to males in several studies.