Achieving lasting remissions in patients suffering from nonlocalized malignancies remains the central problem of clinical oncology.
Although the decades-old arsenal of classic anticancer treatment modalities such as surgery, chemotherapy, irradiation, and hormone ablation has been augmented by strategies including immunotherapy, gene therapy, inhibition of angiogenesis, hyperthermia, and a number of novel lesion-based approaches, the goal to eradicate all cancer cells in a metastasized condition is rarely within reach.
Anticancer treatment strategies may be insufficient for many reasons: potentially efficient therapies might not always find their way to virtually inaccessible tumor sites.
Moreover, the multitudes of tumor entities are known to differ remarkably in their susceptibilities to conventional DNA-damaging anticancer agents – particularly solid tumors, which often are largely refractory to chemotherapy or rapidly, re-emerge from a remission.
In addition, primarily susceptible tumors select for genetic defects during the course of therapy, which may render them resistant over time.
While dose escalation can overcome the problem of insufficient chemosensitivity in some entities, its clinical applicability is limited by the severe toxicity codelivered to the normal cell compartment.
Traditional cytotoxic treatment strategies are driven by the assumption that quantitative execution of cell death is required to eliminate the malignant cell population.
Interestingly, strategies to blunt properties of malignant growth by disabling proliferation without primarily targeting cancer viability have been less recognized, although forcing cells to exit the cell cycle by an irreversible arrest should terminate their contribution to disease progression just as effectively.
In fact, recent evidence underscores the theory that premature senescence may act as an acute, drug-inducible arrest program that may contribute to the outcome of cancer therapy.
Chemotherapy remains the mainstay in the treatment of systemic or metastasized malignancies.
Although these highly toxic agents interfere with a plethora of cellular functions and may damage a variety of cellular structures, their pivotal cellular target is genomic DNA.
It is now a well-accepted concept that drug-mediated DNA damage is not invariably lethal per se but provokes genetically encoded cellular responses.
Hence, unrelated chemotherapeutic anticancer agents – in spite of their different pharmacological features and their individual target molecules participating in DNA replication and integrity – initiate common downstream mechanisms.
Upon sensing DNA damage, cellular transducers activate pathways that either temporarily halt the cell cycle to allow the DNA repair machinery to fix the damage, or execute lethal programs such as apoptosis or mitotic catastrophe to restrain the damaged cell from further expansion.
Ultimate, i.e., irreversible responses to DNA damage do not always determine the fate of cancer cells by programmed forms of cell death but may blunt their uncontrolled proliferative capacity by provoking a terminal cell-cycle arrest termed premature senescence.
Moreover, recent reports demonstrated that tumor cell senescence is detectable following DNA-damaging treatment in vivo and significantly increases overall survival of the host.
In turn, the fact that different anticancer agents share genetic effector cascades renders the genetically encoded programs of apoptosis and senescence highly susceptible to inactivating mutations as a potential cause of chemoresistance.
Hence, thorough dissection and mutational analysis of the pathways leading to cell death or cellular senescence are expected to identify specific genetic lesions that may be utilized by novel targeting therapies.
Premature senescence recapitulates cellular and molecular features of replicative senescence, which is safeguard program that limits the growth potential, but not necessarily the viability, of a dividing cell as consequence of the progressive shortening of its telomeres.
Senescent cells, arrested in the G1 phase of the cell cycle, typically appear flattened and enlarged with increased cytoplasmic granularity.
In addition to the characteristic morphology, senescent cells display enhanced activity of senescence-associated β-galactosidase (SA-β-Gal) when assessed at an acidic pH.
While refractory to mitogenic stimuli, senescent cells remain viable and metabolically active and possess typical transcriptional profile that distinguishes them from quiescent cells.
At the protein level, numerous regulators of cell-cycle progression, checkpoint control, and cellular integrity such as p53 or p16INK4a have been found to be induced in response to various pro-senescent stimuli.
Extrinsic factors such as anticancer agents, γ-irradiation, or UV light have been shown to induce premature senescence as a DNA damage-mediated cellular stress response.
The possibility of alternative outcomes in response to drug-induced DNA damage – apoptosis, mitotic catastrophe, cellular senescence, or simply necrosis – raises the question of whether additional stimuli or specific contextual scenarios determine the ultimate cell fate.
Xenotransplant and transgenic mouse models have been used to visualize premature senescence as a quantitative response to anticancer agents in vivo.
Importantly, lymphomas generated in the E μ-myc transgenic mouse model were prone to massive apoptosis as a default response following therapy with the alkylating agent cyclophosphamide, but they uniformly displayed premature senescence when apoptosis was blocked by overexpression of the strictly antiapoptotic mediator Bcl2.
Although mice harboring senescent lymphomas ultimately succumbed to their disease, they lived much longer than those bearing lymphomas with a defect in both apoptosis and senescence because of p53 loss.
Given the rapid induction of apoptosis and the rather delayed detectability of an SA-β-Gal-positive long-term arrest response to anticancer therapy in vivo, the data suggested that the senescence machinery may act as a back-up program to substitute for or to reinforce an insufficient apoptotic response.
According to many naturally occurring mutations in apoptosis-related genes, disruption of apoptosis is, at least in conjunction with certain oncogenic scenarios, a pivotal step in tumor development.
Given the complex overlap between apoptosis and senescence as cellular fail-safe systems on one hand and tumor-suppressor mechanisms and drug-effector programs on the other hand, the capability to execute senescence might be disabled in cancer cells for numerous reasons.
In fact, mutations in genes that control cellular senescence may not only be selected for during therapy but might have been acquired already during tumor development.
Oncogenes such as activated ras that are known to provoke premature senescence as the primary fail-safe mechanism may rely on defects in this program as a prerequisite to a fully transformed phenotype, possibly inactivating senescence as a drug response as well.
Nevertheless, tumors that preserve both an intact apoptotic and a functional senescence program may display a particularly robust drug response consisting of acutely inducible cell death in a first phase, corroborated by delayed apoptosis out of senescence at a later point (Figure 1).
Although apoptosis is a fast-acting response mode, little is known about the possibility that apoptosis-competent cells might be sent into senescence following DNA-damaging therapy – and whether a senescent tumor could ever undergo apoptosis upon an additional proapoptotic signal.
Finally, senescence could be recruited as an amplifier mechanism to lock temporarily arrested tumor cells – with their reduced susceptibility to checkpoint-licensed apoptosis – into irreversible cytostasis.
The fate of apoptotic cells is determined by their acute disruption of metabolic processes, rapid disintegration, and engulfment by attracted macrophages.
In stark contrast, induction of cellular senescence as a formally irreversible growth arrest results in the preservation of a potentially malignant cell population locked into a nondividing state, yet possessing at least some metabolic activity.
Although apoptotic cells provoke rather little inflammatory reaction, tumor-infiltrating immune cells reportedly can recognize altered autoantigens presented by apoptotic cancer cells.
To what extent altered senescent tumor cells that previously managed to escape immunosurveillance can now challenge an antitumor immune response requires further investigation (Figure 1).
Correlative evidence points toward a link between dermal autoimmunity in the elder population and an age-dependent increase of SA-β-Gal-positive keratinocytes in human skin samples.
Although it is likely that senescent cells will ultimately be cleared by phagocytosis, no "eat-me" signals, as recently described for apoptotic cells, have been identified yet for the senescent state.
While senescent neutrophils, like apoptotic granulocytes, might ultimately face phagocytosis through a yet unknown recognition mechanism, focal enrichment of lysosome-related β-galactosidase activity at autodigestive vacuoles indicated that aged human fibroblasts arrested in replicative senescence may eventually eliminate themselves by autophagy.
As a side effect of anticancer therapy, DNA damage can also force susceptible normal cells to enter an acute SA-β-Gal-positive arrest in vivo.
Drug-inducible senescence: friend or foe?
In response to DNA-damaging agents, cancer cells can rapidly undergo apoptosis or may enter premature senescence as a potential back-up mechanism.
Whether cells re-enter the cycle or execute apoptosis out of drug-mediated senescence remains unclear.
A terminal arrest of the entire cancer cell population, possibly augmented through increased immunogenicity of senescent cells, is beneficial for the host.
In contrast, feeder-like growth that reflects paracrine activity of senescent cells on their non-senescent neighbors, or escape from senescence based on acquired or preexisting mutations, is considered a detrimental outcome.
Hence, it is conceivable that senescent cancer and normal bystander cells reside for some time next to their non-senescent malignant neighbors.
In line with the role of irradiated fibroblasts as feeder cells, interspersed senescent – i.e., metabolically active – cells may support survival and growth of tumor cells in their vicinity.
Senescent human fibroblasts stimulated proliferation of epithelial cell lines in vitro.
Moreover, coimplantation of senescent fibroblasts together with preneoplastic epithelial cells in nude mice accelerated tumor formation in vivo, mainly via soluble factors secreted by the senescence-activated fibroblasts.
Thus, the outcome of anticancer therapy is not only determined by a quantitative effect on cancer cells forced to irreversibly exit the cell cycle but may also depend on novel capabilities acquired by senescent cells that can impact their malignant neighbors in different ways (Figure 1).
It is quite possible that, because of functionally compromised cell-cell interactions, senescent tumor cells can even exert a tumor-suppressive effect on their bystander cells.
The uncertainties regarding control and irreversibility of drug-induced senescence raise concerns as to what extent this effector mechanism reflects a desirable outcome of cancer therapy, particularly in light of therapy-inducible apoptosis as the alternate and possibly safer outcome in response to DNA damage.
However, intact apoptotic machinery is often unavailable in established malignancies.
Since anticancer agents kill mainly by apoptotic cell death and, in turn, achieve little clinical efficacy in the presence of apoptotic defects, promising treatment alternatives must use effector mechanisms that do not rely on intact apoptotic machinery.
Importantly, in vivo analyses of treatment responses in primary lymphomas harboring defined genetic lesions demonstrated that induction of senescence despite the presence of an apoptotic block improved the outcome of anticancer therapy – regardless of a potential reversal or possible emergence of preexisting escape mutants at a later time.
Appropriate test systems are critical to elucidate the complex implications of drug-inducible senescence.
Not surprisingly, drug sensitivity assays performed on primary tumor material in culture unveiled that adaptation to the nonphysiological culture conditions selected for apoptotic defects and chemoresistance.
Paradoxically, many cell lines retain the ability to enter senescence following drug exposure in vitro, although inactivation of the terminal-arrest program appears to be a key prerequisite for any primary tumor during successful establishment as a continuous cell line.
Irrespective of the technical inducibility of a senescence-like phenotype in culture-adapted cells, a Petri dish setting cannot mimic the complexity and interactivity of a natural tumor environment in vivo.
Hence, many of the questions rose about the role of drug-induced senescence need to be addressed in vivo using physiological model systems.
Tractable mouse models of cancer, such as the transgenic E μ-myc lymphoma model, in alliance with sophisticated genetic tools will allow researchers to dissect the pathways and the impact of drug-inducible senescence in vivo.
Moreover, large-scale analyses of the transcriptome and proteome of primary human tumor samples will expand our understanding of the molecular mechanisms that underlie drug responses in sensitive and resistant conditions.
Given the impact of apoptotic defects on tumor biology and treatment outcome, it is a research priority to invent small compound- or gene therapy-based approaches that may resensitize cancer cells to death signals.
Likewise, one can envision lesion-based strategies to restore a defective senescence response.
Ultimately, direct activation of pro-senescent pathways without induction of deleterious DNA damage seems to be a particularly appealing concept Cellular senescence and its potential use as a drug-effector program remains a complex biological phenomenon with unknown significance in cancer therapy.
Whether cellular senescence is rather friend or foe most likely depends on accompanying lesions, first in apoptotic response programs, and on the cellular context.
In further preclinical investigations, it will be of particular interest to explore therapies that do not deliver devastating DNA damage to the cell, that do not rely on functional DNA-damage transducer systems, and that do not target pathways already mutated to cancel apoptosis, but that directly prompt a senescence response.