Senescence-inducing 'mitogens' inhibit CDKs

In normal cells, mitogens (growth factors) initiate and maintain the transition from G1 phase to S phase in the cell cycle. Mitogen-activated protein kinase (MAPK) signalling pathways induce cyclin D1, which results in the activation of either CDK4 or CDK6 (henceforth referred to as CDK4/6), after which the cell no longer requires mitogens to complete the cell cycle. This point in late G1 phase is known as the 'restriction point' (Pardee, 1974). CDK4/6 phosphorylates the retinoblastoma protein (Rb), causing the release of E2F. This in turn transactivates cyclin E, which then activates CDK2. The upstream, mitogen-activated pathways thus stimulate the downstream cell-cycle machinery by inducing cyclins, which are the activators of CDKs. However, MAPK pathways can also induce CDKIs, offering the cell two possibilities: proliferation or growth arrest (Fig. 1). For example, the classical MAPK signalling cascade involves the sequential activation of Ras, Raf1, MAPK kinase (MEK) and extracellularsignal-regulated kinase (ERK), which stimulates activators (such as cyclin D) and inhibitors (such as p21, p16, p15 and p57) of CDKs (Marshall, 1995; Sewing et al., 1997; Woods et al., 1997; Chang et al., 2002). The same is true of MAPK pathways that act through the JNK (Jun kinase) and p38 kinases. Furthermore, both p21 and p27 can have opposing effects on CDK4/6 and CDK2 (Sherr & Roberts, 1999). What determines which of the two options the cell takes? It has been suggested that it is the strength or duration of the signal that is important; strong and/or sustained activation of the MAPK pathways arrests the cell cycle, whereas transient activation induces cell-cycle progression (Marshall, 1995). In support of this, low levels of Raf1 activity induce cyclin D1 and therefore proliferation, whereas high levels lead to p21 induction and growth arrest (Sewing et al., 1997; Woods et al., 1997). In addition, the simple explanation—that the proliferative status of the cell predetermines its response—should not be overlooked. Whereas a 'mitogen' may arrest a cycling cell or stimulate G0–G1 phase progression in a resting cell, it cannot possibly arrest a cell that is already resting. Thus, the overall cellular response may be predetermined by whether resting or cycling cells are targeted.

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Figure 1

Dual mitogenic signalling. Mitogens simultaneously induce activators (such as cyclin D1) and cyclin-dependent-kinase inhibitors (CDKIs, such as p21, p16 and p15) through mitogen-activated pathways (Raf1/mitogen-activated protein kinase kinase (MEK)/extracellular-signal-related kinase (ERK), p38 and JNK (Jun kinase)), leading ultimately to cellular proliferation or arrest.

Regardless of what determines the choice between growth arrest and proliferation, cell senescence occurs only when mitogenic stimuli lead to CDK inhibition. In primary cells, Ras and the downstream MAPK pathways can induce senescence due to the induction of CDKIs (Missero et al., 1996; Serrano et al., 1997; Lin et al., 1998; Zhu et al., 1998; Malumbres et al., 2000; Wang et al., 2002; Brookes et al., 2002). All CDKIs induce senescence when ectopically expressed in fibroblasts (McConnell et al., 1998). Either the overexpression of positive regulators acting downstream of cyclin D1 (for example, CDK4/6, E2F1 and c-Myc) or the inactivation of tumour suppressors (such as Rb, p53 and p16) can block Ras-induced senescence. Even spontaneous senescence can be delayed by the overexpression of CDK4/6 (Morris et al., 2002; Holland et al., 1998). We can therefore conclude that the inhibition of pathways, either at the level of CDK4/6 or downstream of CDK4/6, is essential for all forms of senescence.

Senescence-inducing cytostatic stress

Ionizing radiation, DNA-damaging drugs, the p53 tumour suppressor, microtubule-active drugs (such as Taxol), oxidative stress and hypoxia-mimicking iron chelators, inhibitors of histone acetylase, transforming growth factor-β (TGF-β) and retinoids are all able to trigger premature cell senescence (McConnell et al., 1998; Chang et al., 1999; Roninson et al., 2001; Terao et al., 2001; Itahana et al., 2001). Most of these agents (especially in high doses) can also induce apoptosis, particularly in cells with 'ready-to-run' apoptotic machinery (that is, caspases). In such cells, the inhibition of apoptosis may allow cytostatic stress to induce senescence instead. For example, by inhibiting apoptosis, Bcl2 can promote p53-dependent senescence (Rincheval et al., 2002). All anti-mitogens directly or indirectly inhibit CDKs, causing G1-phase arrest and/or G2-phase arrest. For example, DNA-damaging agents cause the induction of CDKIs (such as p21) and changes in the phosphorylation states of CDKs and, thereby, their inhibition. Nevertheless, MAPKs are not inhibited, and these agents activate mitogenic signalling. Radiation stimulates Ras and numerous mitogenic kinases, including Raf1 and Akt (Kasid et al., 1996; Liu et al., 1996; Shaulian et al., 2000; Lee et al., 2000; Fang et al., 2001). Furthermore, p53 can induce growth factors that activate MAPK and Akt signalling, hypoxia causes autocrine secretion of mitogens and activation of mitogenic kinases, and DNA damage and hypoxia may, paradoxically, induce proliferation (Ishii et al., 1995; Adelman et al., 2000; Fang et al., 2001; Das et al., 2001; Salnikow et al., 2002). This suggests that, while inhibiting CDKs, senescence-inducing cytostatic stress may actually activate mitogenic pathways.

As is the case for premature senescence, spontaneous senescence occurs in the presence of mitogens. In the latter case, senescence is caused by the stress due to telomere shortening as the cells proliferate. In mitogen-induced senescence, hyperactive MAPK pathways inhibit CDKs. The elegance of cell senescence is that one stimulus can both activate upstream mitogenic signalling and arrest the cell cycle downstream.

Upstream and downstream signalling

It is assumed that cellular senescence evolved to suppress tumorigenesis (Lin et al., 1998; Campisi, 2001); that is, normal cells can respond to potentially oncogenic stimuli by entering the senescent state. This is reminiscent of the conflicting signal model, in which 'unscheduled' activation of potentially oncogenic stimuli leads to apoptosis, “ensuring that cell growth is restricted to the correct paracrine environment” (Raff, 1992; Evan & Littlewood, 1998). Does this mean that potentially oncogenic stimuli can cause both apoptosis and senescence? If so, how are the two outcomes determined? Here, we view them in the light of the upstream and downstream signalling pathways. During normal proliferation, the upstream mitogen-activated pathways stimulate downstream cell-cycle signalling (Fig. 2A). In the absence of upstream mitogenic signalling, the downstream cell-cycle pathways are inhibited (Fig. 2B), and the cells are in G0 arrest. Conversely, forced, isolated overexpression of downstream oncogenes (for example, viral E1A, c-Myc and E2F) can induce apoptosis (Fig. 2C). However, cell-cycle progression and apoptosis are not permanently connected, and can be separated; for example, apoptosis can be suppressed by mitogens (Evan & Littlewood, 1998). Finally, in the presence of mitogenic signalling, 'unscheduled' inhibition of the cell cycle, such as by CDKIs, results in senescence (Fig. 2D). Therefore, whereas oncogene-induced apoptosis is associated with 'forced' proliferation, senescence is associated with 'forced' arrest (Fig. 2C versus ​versus2D).2D). In this view, oncogene-induced apoptosis and senescence are mirror images, each involving the isolated activation of either upstream or downstream mitogenic pathways.

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Figure 2

Sequential and reciprocal activation of upstream and downstream signalling pathways. (A) Normal mitogenic signalling involves the sequential activation of both upstream and downstream pathways. Note that, due to feedback loops, the distinctions between 'upstream' and 'downstream' are not always maintained, and the molecules indicated in the boxes are merely examples. (B) Classical growth arrest in G0 occurs when there is no activation of either upstream or downstream pathways. (C) The conflicting signal model of apoptosis involves the isolated overactivation of downstream pathways. (D) The conflicting signal model of senescence involves the isolated overactivation of upstream pathways.

Cell-cycle arrest: hypomitogenic and hypermitogenic

The model of mitogen-induced senescence presented here leads to an unexpected prediction: there are two opposing types of growth arrest. Classical growth arrest, or G0 arrest, which is caused by growth-factor withdrawal, is essentially an exit from the cell cycle (Fig. 3), and is characterized by low levels of cyclin D1. After mitogenic stimulation, cells in the G0 state enter G1, during which they induce cyclin D1 and progress through the restriction point (Blagosklonny & Pardee, 2002). It is generally assumed that all resting cells ultimately exit the cell cycle. However, when cells arrest in response to mitogens, the consequences are different. In this state of hypermitogenic arrest, cells arrest at or beyond the restriction point, in either G1 or G2 phase (Fig. 3), and numerous anticancer drugs arrest cells when the levels of cyclins D, E, A or B are elevated (Gong et al., 1995; Darzynkiewicz, 2002). The hypermitogenic arrest induced by phorbol esters is characterized by CDK inhibition in the presence of cyclin D1 (Blagosklonny, 1998), and iron chelators, contact inhibition and transforming growth factor-β induce a senescence-like late G1 arrest that is often associated with the induction of p27 (Kletsas et al., 1995; Yoon et al., 2002). DNA damage can induce permanent growth arrest in G2, which is dependent on Rb (Naderi et al., 2002). Also, hyperactivated Raf1 can arrest cells in G2 (Kerkhoff & Rapp, 1998). In cycling cells, Ras induces cyclin D1 in G2 phase (Sherr, 2002). Therefore, forced hypermitogenic arrest is associated with elevated cyclin D1.

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Figure 3

From proliferation to either G0 arrest or hypermitogenic arrest. 'Cycling cells' proliferate in the presence of growth factors (GFs). The withdrawal of GFs causes a reversible exit from the cell cycle, known as G0, producing a 'resting cell'. In normal cells, a complete lack of GFs may cause apoptosis. GF stimulation in the presence of cyclin-dependent-kinase inhibitors (+CDKI) can cause hypermitogenic arrest, and continuous stimulation can lead to senescence.

The existence of G0 has been questioned time after time, precisely because the idea of G0 arrest has been overgeneralized. The distinction between G0 (hypomitogenic arrest) and hypermitogenic arrest may explain much of this controversy.

Hypermitogenic arrest, cell morphology and function

Intriguingly, the model of hypermitogenic arrest predicts a 'large-cell' morphology, and high levels of autocrine and/or paracrine secretion by senescent cells. Hypermitogenic arrest must be accompanied by protein synthesis—as mitogens stimulate not only DNA synthesis, but also RNA and protein synthesis—and by metabolism. It is an arrest of the cell cycle, not of growth. In fact, high-intensity Raf signals are known to convert mitotic cell cycling into cellular growth (Kerkhoff & Rapp, 1998). Thus, hypermitogenic arrest can explain the large-cell morphology that has been associated with senescence.

It is also possible that a cell may attempt to compensate for the cell-cycle block by the overinduction of mitogenic signalling. Senescent cells are highly active in the autocrine and paracrine secretion of mitogens (Roninson et al., 2001). Unlike the hypomitogenic arrest at G0, hypermitogenic arrest is enhanced by stimulation by mitogens. Therefore, hypermitogenic arrest and cell senescence are stable conditions, which are often seen as irreversible. For example, consistent with the senescent phenotype, p38-induced G1 cell-cycle arrest is irreversible after four days (Haq et al., 2002). As another example, when p16 expression was induced for six days, DNA synthesis remained inhibited and the cells acquired the morphological features of senescence. If p16 induction was interrupted after six days, most cells retained these morphological features (Dai & Enders, 2000).

Overturning hypermitogenic arrest and senescence

The concept presented here predicts that, unlike G0 arrest, hypermitogenic arrest cannot be blocked by mitogenic stimuli and, indeed, senescent cells have been shown to be resistant to mitogens (Mathon & Lloyd, 2001). However, as illustrated by the following examples, it can be overcome by induction of downstream 'oncogenes'. Both c-Myc and E1A override the hypermitogenic arrest caused by PMA (phorbol-12-myristate-13-acetate) (Blagosklonny, 1998). Reactivation of cyclin E1, which acts downstream of p16, is sufficient to trigger escape from Ras-induced senescence (Peeper et al., 2002). In contrast to stimulation by mitogens, stimulation by viral proteins can reactivate terminally differentiated cells of the muscle system: for example, the large-T viral oncoprotein induces the cycling of terminally differentiated skeletal muscle myotubes (Ohkubo et al., 1994), E2F1 can override 'irreversible' arrest in post-mitotic ventricular myocytes (Agah et al., 1997), and the E1A oncogene can reactivate the cell cycle in terminally differentiated skeletal muscle cells (Tiainen et al., 1996). Also, the inactivation of either p53 or simian virus 40 (SV40) induces senescent cells to re-enter S phase and to revert to their 'young' morphology (Gire & Wynford-Thomas, 1998). Finally, oncogenes that act downstream of the restriction point (such as c-Myc and CDK4), and thus may overcome senescence, cooperate with hypermitogenic oncogenes (such as Ras), leading to malignant transformation (Land et al., 1983).

I apologize to those authors whose work was not cited due to page limitations.