Cell, Vol. 100, 57–70, January 7, 2000, Copyright ª 2000 by Cell Press
`
`The Hallmarks of Cancer
`
`Review
`
`Douglas Hanahan* and Robert A. Weinberg†
`* Department of Biochemistry and Biophysics and
`Hormone Research Institute
`University of California at San Francisco
`San Francisco, California 94143
`† Whitehead Institute for Biomedical Research and
`Department of Biology
`Massachusetts Institute of Technology
`Cambridge, Massachusetts 02142
`
`After a quarter century of rapid advances, cancer re-
`search has generated a rich and complex body of knowl-
`edge, revealing cancer to be a disease involving dy-
`namic changes in the genome. The foundation has been
`set in the discovery of mutations that produce onco-
`genes with dominant gain of function and tumor sup-
`pressor genes with recessive loss of function; both
`classes of cancer genes have been identified through
`their alteration in human and animal cancer cells and
`by their elicitation of cancer phenotypes in experimental
`models (Bishop and Weinberg, 1996).
`Some would argue that the search for the origin and
`treatment of this disease will continue over the next
`quarter century in much the same manner as it has in
`the recent past, by adding further layers of complexity
`to a scientific literature that is already complex almost
`beyond measure. But we anticipate otherwise: those
`researching the cancer problem will be practicing a dra-
`matically different type of science than we have experi-
`enced over the past 25 years. Surely much of this change
`will be apparent at the technical level. But ultimately,
`the more fundamental change will be conceptual.
`We foresee cancer research developing into a logical
`science, where the complexities of the disease, de-
`scribed in the laboratory and clinic, will become under-
`standable in terms of a small number of underlying prin-
`ciples. Some of these principles are even now in the
`midst of being codified. We discuss one set of them in
`the present essay: rules that govern the transformation
`of normal human cells into malignant cancers. We sug-
`gest that research over the past decades has revealed
`a small number of molecular, biochemical, and cellular
`traits—acquired capabilities—shared by most and per-
`haps all types of human cancer. Our faith in such simplifi-
`cation derives directly from the teachings of cell biology
`that virtually all mammalian cells carry a similar molecu-
`lar machinery regulating their proliferation, differentia-
`tion, and death.
`Several lines of evidence indicate that tumorigenesis
`in humans is a multistep process and that these steps
`reflect genetic alterations that drive the progressive
`transformation of normal human cells into highly malig-
`nant derivatives. Many types of cancers are diagnosed
`in the human population with an age-dependent inci-
`dence implicating four to seven rate-limiting, stochastic
`events (Renan, 1993). Pathological analyses of a number
`of organ sites reveal lesions that appear to represent
`the intermediate steps in a process through which cells
`
`evolve progressively from normalcy via a series of pre-
`malignant states into invasive cancers (Foulds, 1954).
`These observations have been rendered more con-
`crete by a large body of work indicating that the ge-
`nomes of tumor cells are invariably altered at multiple
`sites, having suffered disruption through lesions as sub-
`tle as point mutations and as obvious as changes in
`chromosome complement (e.g., Kinzler and Vogelstein,
`1996). Transformation of cultured cells is itself a
`multistep process: rodent cells require at least two intro-
`duced genetic changes before they acquire tumorigenic
`competence, while their human counterparts are more
`difficult to transform (Hahn et al., 1999). Transgenic
`models of tumorigenesis have repeatedly supported the
`conclusion that tumorigenesis in mice involves multiple
`rate-limiting steps (Bergers et al., 1998; see Oncogene,
`1999, R. DePinho and T. E. Jacks, volume 18[38], pp.
`5248–5362). Taken together, observations of human
`cancers and animal models argue that tumor develop-
`ment proceeds via a process formally analogous to Dar-
`winian evolution,
`in which a succession of genetic
`changes, each conferring one or another type of growth
`advantage, leads to the progressive conversion of nor-
`mal human cells into cancer cells (Foulds, 1954; Nowell,
`1976).
`
`An Enumeration of the Traits
`The barriers to development of cancer are embodied
`in a teleology: cancer cells have defects in regulatory
`circuits that govern normal cell proliferation and homeo-
`stasis. There are more than 100 distinct types of cancer,
`and subtypes of tumors can be found within specific
`organs. This complexity provokes a number of ques-
`tions. How many distinct regulatory circuits within each
`type of target cell must be disrupted in order for such
`a cell to become cancerous? Does the same set of
`cellular regulatory circuits suffer disruption in the cells
`of the disparate neoplasms arising in the human body?
`Which of these circuits operate on a cell-autonomous
`basis, and which are coupled to the signals that cells
`receive from their surrounding microenvironment within
`a tissue? Can the large and diverse collection of cancer-
`associated genes be tied to the operations of a small
`group of regulatory circuits?
`We suggest that the vast catalog of cancer cell geno-
`types is a manifestation of six essential alterations in cell
`physiology that collectively dictate malignant growth
`(Figure 1): self-sufficiency in growth signals, insensitivity
`to growth-inhibitory (antigrowth) signals, evasion of pro-
`grammed cell death (apoptosis),
`limitless replicative
`potential, sustained angiogenesis, and tissue invasion
`and metastasis. Each of these physiologic changes—
`novel capabilities acquired during tumor development—
`represents the successful breaching of an anticancer
`defense mechanism hardwired into cells and tissues.
`We propose that these six capabilities are shared in
`common by most and perhaps all types of human tu-
`mors. This multiplicity of defenses may explain why can-
`cer is relatively rare during an average human lifetime.
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`Figure 1. Acquired Capabilities of Cancer
`We suggest that most if not all cancers have acquired the same set
`of functional capabilities during their development, albeit through
`various mechanistic strategies.
`
`We describe each capability in turn below, illustrate with
`a few examples its functional importance, and indicate
`strategies by which it is acquired in human cancers.
`
`Acquired Capability: Self-Sufficiency
`in Growth Signals
`Normal cells require mitogenic growth signals (GS) be-
`fore they can move from a quiescent state into an active
`proliferative state. These signals are transmitted into the
`cell by transmembrane receptors that bind distinctive
`classes of signaling molecules: diffusible growth fac-
`tors, extracellular matrix components, and cell-to-cell
`adhesion/interaction molecules. To our knowledge, no
`type of normal cell can proliferate in the absence of
`such stimulatory signals. Many of the oncogenes in the
`cancer catalog act by mimicking normal growth signal-
`ing in one way or another.
`Dependence on growth signaling is apparent when
`propagating normal cells in culture, which typically pro-
`liferate only when supplied with appropriate diffusible
`mitogenic factors and a proper substratum for their inte-
`grins. Such behavior contrasts strongly with that of tu-
`mor cells, which invariably show a greatly reduced
`dependence on exogenous growth stimulation. The con-
`clusion is that tumor cells generate many of their own
`growth signals, thereby reducing their dependence on
`stimulation from their normal tissue microenvironment.
`This liberation from dependence on exogenously de-
`rived signals disrupts a critically important homeostatic
`mechanism that normally operates to ensure a proper
`behavior of the various cell types within a tissue.
`
`Acquired GS autonomy was the first of the six capabili-
`ties to be clearly defined by cancer researchers, in large
`part because of the prevalence of dominant oncogenes
`that have been found to modulate it. Three common
`molecular strategies for achieving autonomy are evi-
`dent, involving alteration of extracellular growth signals,
`of transcellular transducers of those signals, or of intra-
`cellular circuits that translate those signals into action.
`While most soluble mitogenic growth factors (GFs) are
`made by one cell type in order to stimulate proliferation
`of another—the process of heterotypic signaling—many
`cancer cells acquire the ability to synthesize GFs to
`which they are responsive, creating a positive feedback
`signaling loop often termed autocrine stimulation (Fedi
`et al., 1997). Clearly, the manufacture of a GF by a cancer
`cell obviates dependence on GFs from other cells within
`the tissue. The production of PDGF (platelet-derived
`growth factor) and TGFa (tumor growth factor a) by
`glioblastomas and sarcomas, respectively, are two illus-
`trative examples (Fedi et al., 1997).
`The cell surface receptors that transduce growth-
`stimulatory signals into the cell interior are themselves
`targets of deregulation during tumor pathogenesis. GF
`receptors, often carrying tyrosine kinase activities in
`their cytoplasmic domains, are overexpressed in many
`cancers. Receptor overexpression may enable the can-
`cer cell to become hyperresponsive to ambient levels
`of GF that normally would not trigger proliferation (Fedi
`et al., 1997). For example, the epidermal GF receptor
`(EGF-R/erbB)
`is upregulated in stomach, brain, and
`breast tumors, while the HER2/neu receptor is overex-
`pressed in stomach and mammary carcinomas (Slamon
`et al., 1987; Yarden and Ullrich, 1988). Additionally, gross
`overexpression of GF receptors can elicit ligand-inde-
`pendent signaling (DiFiore et al., 1987). Ligand-indepen-
`dent signaling can also be achieved through structural
`alteration of receptors; for example, truncated versions
`of the EGF receptor lacking much of its cytoplasmic
`domain fire constitutively (Fedi et al., 1997).
`Cancer cells can also switch the types of extracellular
`matrix receptors (integrins) they express, favoring ones
`that transmit progrowth signals (Lukashev and Werb,
`1998; Giancotti and Ruoslahti, 1999). These bifunctional,
`heterodimeric cell surface receptors physically link cells
`to extracellular superstructures known as the extracellu-
`lar matrix (ECM). Successful binding to specific moieties
`of the ECM enables the integrin receptors to transduce
`signals into the cytoplasm that influence cell behavior,
`ranging from quiescence in normal tissue to motility,
`resistance to apoptosis, and entrance into the active
`cell cycle. Conversely, the failure of integrins to forge
`these extracellular links can impair cell motility, induce
`apoptosis, or cause cell cycle arrest (Giancotti and Ru-
`oslahti, 1999). Both ligand-activated GF receptors and
`progrowth integrins engaged to extracellular matrix
`components can activate the SOS-Ras-Raf-MAP kinase
`pathway (Aplin et al., 1998; Giancotti and Ruoslahti,
`1999).
`The most complex mechanisms of acquired GS auton-
`omy derive from alterations in components of the down-
`stream cytoplasmic circuitry that receives and pro-
`cesses the signals emitted by ligand-activated GF
`receptors and integrins. The SOS-Ras-Raf-MAPK cas-
`cade plays a central role here. In about 25% of human
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`Figure 2. The Emergent Integrated Circuit of the Cell
`Progress in dissecting signaling pathways has begun to lay out a circuitry that will likely mimic electronic integrated circuits in complexity
`and finesse, where transistors are replaced by proteins (e.g., kinases and phosphatases) and the electrons by phosphates and lipids, among
`others. In addition to the prototypical growth signaling circuit centered around Ras and coupled to a spectrum of extracellular cues, other
`component circuits transmit antigrowth and differentiation signals or mediate commands to live or die by apoptosis. As for the genetic
`reprogramming of this integrated circuit in cancer cells, some of the genes known to be functionally altered are highlighted in red.
`
`tumors, Ras proteins are present in structurally altered
`forms that enable them to release a flux of mitogenic
`signals into cells, without ongoing stimulation by their
`normal upstream regulators (Medema and Bos, 1993).
`We suspect that growth signaling pathways suffer
`deregulation in all human tumors. Although this point
`is hard to prove rigorously at present, the clues are
`abundant (Hunter, 1997). For example, in the best stud-
`ied of tumors—human colon carcinomas—about half
`of the tumors bear mutant ras oncogenes (Kinzler and
`Vogelstein, 1996). We suggest that the remaining colonic
`tumors carry defects in other components of the growth
`signaling pathways that phenocopy ras oncogene acti-
`vation. The nature of these alternative, growth-stimulat-
`ing mechanisms remains elusive.
`Under intensive study for two decades, the wiring
`diagram of the growth signaling circuitry of the mamma-
`lian cell is coming into focus (Figure 2). New downstream
`effector pathways that radiate from the central SOS-
`Ras-Raf-MAP kinase mitogenic cascade are being dis-
`covered with some regularity (Hunter, 1997; Rommel
`and Hafen, 1998). This cascade is also linked via a variety
`of cross-talking connections with other pathways; these
`cross connections enable extracellular signals to elicit
`
`multiple cell biological effects. For example, the direct
`interaction of the Ras protein with the survival-promot-
`ing PI3 kinase enables growth signals to concurrently
`evoke survival signals within the cell (Downward, 1998).
`While acquisition of growth signaling autonomy by
`cancer cells is conceptually satisfying, it is also too
`simplistic. We have traditionally explored tumor growth
`by focusing our experimental attentions on the geneti-
`cally deranged cancer cells (Figure 3, left panel). It is,
`however, increasingly apparent that the growth deregu-
`lation within a tumor can only be explained once we
`understand the contributions of the ancillary cells pres-
`ent in a tumor—the apparently normal bystanders such
`as fibroblasts and endothelial cells—which must play
`key roles in driving tumor cell proliferation (Figure 3,
`right panel). Within normal tissue, cells are largely in-
`structed to grow by their neighbors (paracrine signals)
`or via systemic (endocrine) signals. Cell-to-cell growth
`signaling is likely to operate in the vast majority of human
`tumors as well; virtually all are composed of several
`distinct cell types that appear to communicate via het-
`erotypic signaling.
`Heterotypic signaling between the diverse cell types
`within a tumor may ultimately prove to be as important
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`Figure 3. Tumors as Complex Tissues
`The field of cancer research has largely been
`guided by a reductionist focus on cancer cells
`and the genes within them (left panel)—a fo-
`cus that has produced an extraordinary body
`of knowledge. Looking forward in time, we
`believe that important new inroads will come
`from regarding tumors as complex tissues in
`which mutant cancer cells have conscripted
`and subverted normal cell types to serve as
`active collaborators in their neoplastic agenda
`(right panel). The interactions between the
`genetically altered malignant cells and these
`supporting coconspirators will prove critical
`to understanding cancer pathogenesis and to
`the development of novel, effective therapies.
`
`in explaining tumor cell proliferation as the cancer cell-
`autonomous mechanisms enumerated above. For ex-
`ample, we suspect that many of the growth signals driv-
`ing the proliferation of carcinoma cells originate from
`the stromal cell components of the tumor mass. While
`difficult to validate at present, such thinking recasts the
`logic of acquired GS autonomy: successful tumor cells
`are those that have acquired the ability to co-opt their
`normal neighbors by inducing them to release abundant
`fluxes of growth-stimulating signals (Skobe and Fu-
`senig, 1998). Indeed, in some tumors, these cooperating
`cells may eventually depart from normalcy, coevolving
`with their malignant neighbors in order to sustain the
`growth of the latter (Kinzler and Vogelstein, 1998; Olumi
`et al., 1999). Further, inflammatory cells attracted to sites
`of neoplasia may promote (rather than eliminate) cancer
`cells (Cordon-Cardo and Prives, 1999; Coussens et al.,
`1999; Hudson et al., 1999), another example of normal
`cells conscripted to enhance tumor growth potential,
`another means to acquire necessary capabilities.
`
`Acquired Capability: Insensitivity
`to Antigrowth Signals
`Within a normal tissue, multiple antiproliferative signals
`operate to maintain cellular quiescence and tissue ho-
`meostasis; these signals include both soluble growth
`inhibitors and immobilized inhibitors embedded in the
`extracellular matrix and on the surfaces of nearby cells.
`These growth-inhibitory signals, like their positively act-
`ing counterparts, are received by transmembrane cell
`surface receptors coupled to intracellular signaling cir-
`cuits.
`Antigrowth signals can block proliferation by two dis-
`tinct mechanisms. Cells may be forced out of the active
`proliferative cycle into the quiescent (G0) state from
`which they may reemerge on some future occasion
`when extracellular signals permit. Alternatively, cells
`may be induced to permanently relinquish their prolifera-
`tive potential by being induced to enter into postmitotic
`states, usually associated with acquisition of specific
`differentiation-associated traits.
`Incipient cancer cells must evade these antiprolifera-
`tive signals if they are to prosper. Much of the circuitry
`that enables normal cells to respond to antigrowth sig-
`nals is associated with the cell cycle clock, specifically
`
`the components governing the transit of the cell through
`the G1 phase of its growth cycle. Cells monitor their
`external environment during this period and, on the ba-
`sis of sensed signals, decide whether to proliferate, to
`be quiescent, or to enter into a postmitotic state. At the
`molecular level, many and perhaps all antiproliferative
`signals are funneled through the retinoblastoma protein
`(pRb) and its two relatives, p107 and p130. When in a
`hypophosphorylated state, pRb blocks proliferation by
`sequestering and altering the function of E2F transcrip-
`tion factors that control the expression of banks of genes
`essential for progression from G1 into S phase (Wein-
`berg, 1995).
`Disruption of the pRb pathway liberates E2Fs and
`thus allows cell proliferation, rendering cells insensitive
`to antigrowth factors that normally operate along this
`pathway to block advance through the G1 phase of the
`cell cycle. The effects of the soluble signaling molecule
`TGFb are the best documented, but we envision other
`antigrowth factors will be found to signal through this
`pathway as well. TGFb acts in a number of ways, most
`still elusive, to prevent the phosphorylation that inacti-
`vates pRb; in this fashion, TGFb blocks advance through
`G1. In some cell types, TGFb suppresses expression
`of the c-myc gene, which regulates the G1 cell cycle
`machinery in still unknown ways (Moses et al., 1990).
`More directly, TGFb causes synthesis of the p15INK4B and
`p21 proteins, which block the cyclin:CDK complexes
`responsible for pRb phosphorylation (Hannon and
`Beach, 1994; Datto et al., 1997).
`The pRb signaling circuit, as governed by TGFb and
`other extrinsic factors, can be disrupted in a variety of
`ways in different types of human tumors (Fynan and
`Reiss, 1993). Some lose TGFb responsiveness through
`downregulation of their TGFb receptors, while others
`display mutant, dysfunctional receptors (Fynan and
`Reiss, 1993; Markowitz et al., 1995). The cytoplasmic
`Smad4 protein, which transduces signals from ligand-
`activated TGFb receptors to downstream targets, may
`be eliminated through mutation of its encoding gene
`(Schutte et al., 1996). The locus encoding p15INK4B may be
`deleted (Chin et al., 1998). Alternatively, the immediate
`downstream target of its actions, CDK4, may become
`unresponsive to the inhibitory actions of p15INK4B be-
`cause of mutations that create amino acid substitutions
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`in its INK4A/B-interacting domain; the resulting cyclin
`D:CDK4 complexes are then given a free hand to inacti-
`vate pRb by hyperphosphorylation (Zuo et al., 1996).
`Finally, functional pRb, the end target of this pathway,
`may be lost through mutation of its gene. Alternatively,
`in certain DNA virus-induced tumors, notably cervical
`carcinomas, pRb function is eliminated through seques-
`tration by viral oncoproteins, such as the E7 oncoprotein
`of human papillomavirus (Dyson et al., 1989). In addition,
`cancer cells can also turn off expression of integrins and
`other cell adhesion molecules that send antigrowth sig-
`nals, favoring instead those that convey progrowth sig-
`nals; these adherence-based antigrowth signals likely
`impinge on the pRb circuit as well. The bottom line is
`that the antigrowth circuit converging onto Rb and the
`cell division cycle is, one way or another, disrupted in
`a majority of human cancers, defining the concept and
`a purpose of tumor suppressor loss in cancer.
`Cell proliferation depends on more than an avoidance
`of cytostatic antigrowth signals. Our tissues also con-
`strain cell multiplication by instructing cells to enter irre-
`versibly into postmitotic, differentiated states, using di-
`verse mechanisms that are incompletely understood; it
`is apparent that tumor cells use various strategies to
`avoid this terminal differentiation. One strategy for
`avoiding differentiation directly involves the c-myc on-
`cogene, which encodes a transcription factor. During
`normal development, the growth-stimulating action of
`Myc, in association with another factor, Max, can be
`supplanted by alternative complexes of Max with a
`group of Mad transcription factors; the Mad–Max com-
`plexes elicit differentiation-inducing signals (Foley and
`Eisenman, 1999). However, overexpression of the c-Myc
`oncoprotein, as is seen in many tumors, can reverse this
`process, shifting the balance back to favor Myc–Max
`complexes, thereby impairing differentiation and pro-
`moting growth. During human colon carcinogenesis, in-
`activation of the APC/b-catenin pathway serves to block
`the egress of enterocytes in the colonic crypts into a
`differentiated, postmitotic state (Kinzler and Vogelstein,
`1996). Analogously, during the generation of avian eryth-
`roblastosis, the erbA oncogene acts to prevent irrevers-
`ible erythrocyte differentiation (Kahn et al., 1986).
`While the components and interconnections between
`the various antigrowth and differentiation-inducing sig-
`nals and the core cell cycle machinery are still being
`delineated, the existence of an antigrowth signaling cir-
`cuitry is clear (Figure 2), as is the necessity for its circum-
`vention by developing cancers.
`
`Acquired Capability: Evading Apoptosis
`The ability of tumor cell populations to expand in number
`is determined not only by the rate of cell proliferation
`but also by the rate of cell attrition. Programmed cell
`death—apoptosis—represents a major source of this
`attrition. The evidence is mounting, principally from
`studies in mouse models and cultured cells, as well as
`from descriptive analyses of biopsied stages in human
`carcinogenesis, that acquired resistance toward apo-
`ptosis is a hallmark of most and perhaps all types of
`cancer.
`Observations accumulated over the past decade indi-
`cate that the apoptotic program is present in latent form
`
`in virtually all cell types throughout the body. Once trig-
`gered by a variety of physiologic signals, this program
`unfolds in a precisely choreographed series of steps.
`Cellular membranes are disrupted, the cytoplasmic and
`nuclear skeletons are broken down, the cytosol is ex-
`truded, the chromosomes are degraded, and the nu-
`cleus is fragmented, all in a span of 30–120 min. In the
`end, the shriveled cell corpse is engulfed by nearby cells
`in a tissue and disappears, typically within 24 hr (Wyllie
`et al., 1980).
`The apoptotic machinery can be broadly divided into
`two classes of components—sensors and effectors. The
`sensors are responsible for monitoring the extracellular
`and intracellular environment for conditions of normality
`or abnormality that influence whether a cell should live
`or die. These signals regulate the second class of com-
`ponents, which function as effectors of apoptotic death.
`The sentinels include cell surface receptors that bind
`survival or death factors. Examples of these ligand/
`receptor pairs include survival signals conveyed by IGF-
`1/IGF-2 through their receptor, IGF-1R, and by IL-3 and
`its cognate receptor, IL-3R (Lotem and Sachs, 1996;
`Butt et al., 1999). Death signals are conveyed by the
`FAS ligand binding the FAS receptor and by TNFa bind-
`ing TNF-R1 (Ashkenazi and Dixit, 1999). Intracellular
`sensors monitor the cell’s well-being and activate the
`death pathway in response to detecting abnormalities,
`including DNA damage, signaling imbalance provoked
`by oncogene action, survival factor insufficiency, or hyp-
`oxia (Evan and Littlewood, 1998). Further, the life of most
`cells is in part maintained by cell–matrix and cell–cell
`adherence-based survival signals whose abrogation
`elicits apoptosis (Ishizaki et al., 1995; Giancotti and Ru-
`oslahti, 1999). Both soluble and immobilized apoptotic
`regulatory signals likely reflect the needs of tissues to
`maintain their constituent cells in appropriate architec-
`tural configurations.
`Many of the signals that elicit apoptosis converge
`on the mitochondria, which respond to proapoptotic
`signals by releasing cytochrome C, a potent catalyst of
`apoptosis (Green and Reed, 1998). Members of the Bcl-2
`family of proteins, whose members have either pro-
`apoptotic (Bax, Bak, Bid, Bim) or antiapoptotic (Bcl-2,
`Bcl-XL, Bcl-W) function, act in part by governing mito-
`chondrial death signaling through cytochrome C re-
`lease. The p53 tumor suppressor protein can elicit apo-
`ptosis by upregulating expression of proapoptotic Bax
`in response to sensing DNA damage; Bax in turn stimu-
`lates mitochondria to release cytochrome C.
`The ultimate effectors of apoptosis include an array
`of intracellular proteases termed caspases (Thornberry
`and Lazebnik, 1998). Two “gatekeeper” caspases, 28
`and 29, are activated by death receptors such as FAS
`or by the cytochrome C released from mitochondria,
`respectively. These proximal caspases trigger the acti-
`vation of a dozen or more effector caspases that execute
`the death program, through selective destruction of sub-
`cellular structures and organelles, and of the genome.
`The possibility that apoptosis serves as a barrier to
`cancer was first raised in 1972, when Kerr, Wyllie, and
`Currie described massive apoptosis in the cells populat-
`ing rapidly growing, hormone-dependent tumors follow-
`ing hormone withdrawal (Kerr et al., 1972). The discovery
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`NOVARTIS EXHIBIT 2029
`Breckenridge v. Novartis, IPR 2017-01592
`Page 5 of 14
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`Cell
`62
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`of the bcl-2 oncogene by its upregulation via chromo-
`somal translocation in follicular lymphoma (reviewed in
`Korsmeyer, 1992) and its recognition as having anti-
`apoptotic activity (Vaux et al., 1988) opened up the in-
`vestigation of apoptosis in cancer at the molecular level.
`When coexpressed with a myc oncogene in transgenic
`mice, the bcl-2 gene was able to promote formation of
`B cell lymphomas by enhancing lymphocyte survival, not
`by further stimulating their myc-induced proliferation
`(Strasser et al., 1990); further, 50% of the infrequent
`lymphomas arising in bcl-2 single transgenic transgenic
`mice had somatic translocations activating c-myc, con-
`firming a selective pressure during lymphomagenesis
`to upregulate both Bcl-2 and c-Myc (McDonnell and
`Korsmeyer, 1991).
`Further insight into the myc-bcl-2 interaction emerged
`later from studying the effects of a myc oncogene on
`fibroblasts cultured in low serum. Widespread apoptosis
`was induced in myc-expressing cells lacking serum; the
`consequent apoptosis could be abrogated by exoge-
`nous survival factors (e.g., IGF-1), by forced overexpres-
`sion of Bcl-2 or the related Bcl-XL protein, or by disrup-
`tion of the FAS death signaling circuit (Hueber et al.,
`1997). Collectively, the data indicate that a cell’s apo-
`ptotic program can be triggered by an overexpressed
`oncogene. Indeed, elimination of cells bearing activated
`oncogenes by apoptosis may represent the primary
`means by which such mutant cells are continually culled
`from the body’s tissues.
`Other examples strengthen the consensus that apo-
`ptosis is a major barrier to cancer that must be circum-
`vented. Thus, in transgenic mice where the pRb tumor
`suppressor was functionally inactivated in the choroid
`plexus, slowly growing microscopic tumors arose, ex-
`hibiting high apoptotic rates; the additional inactivation
`of the p53 tumor suppressor protein, a component of
`the apoptotic signaling circuitry, led to rapidly growing
`tumors containing low numbers of apoptotic cells (Sy-
`monds et al., 1994). The role of extracellular survival
`factors is illustrated by disease progression in trans-
`genic mice prone to pancreatic islet tumors. If IGF-2
`gene expression, which is activated in this tumorigene-
`sis pathway, was abrogated using gene knockout mice,
`tumor growth and progression were impaired, as evi-
`denced by the appearance of comparatively small, be-
`nign tumors showing high rates of apoptosis (Christofori
`et al., 1994). In these cells, the absence of IGF-2 did not
`affect cell proliferation rates, clearly identifying it as an
`antiapoptotic survival factor. Collectively, these obser-
`vations argue that altering components of the apoptotic
`machinery can dramatically affect the dynamics of tu-
`mor progression, providing a rationale for the inactiva-
`tion of this machinery during tumor development.
`Resistance to apoptosis can be acquired by cancer
`cells through a variety of strategies. Surely, the most
`commonly occurring loss of a proapoptotic regulator
`through mutation involves the p53 tumor suppressor
`gene. The resulting functional inactivation of its product,
`the p53 protein, is seen in greater than 50% of human
`cancers and results in the removal of a key component
`of the DNA damage sensor that can induce the apoptotic
`effector cascade (Harris, 1996). Signals evoked by other
`
`abnormalities, including hypoxia and oncogene hyper-
`expression, are also funneled in part via p53 to the apo-
`ptotic machinery; these too are impaired at eliciting
`apoptosis when p53 function is lost (Levine, 1997). Addi-
`tionally, the PI3 kinase–AKT/PKB pathway, which trans-
`mits antiapoptotic survival signals, is likely involved in
`mitigating apoptosis in a substantial fraction of human
`tumors. This survival signaling circuit can be activated
`by extracellular factors such as IGF-1/2 or IL-3 (Evan
`and Littlewood, 1998), by intracellular signals emanating
`from Ras (Downward, 1998), or by loss of the pTEN
`tumor suppressor, a phospholipid phosphatase that
`normally attenuates the AKT survival signal (Cantley and
`Neel, 1999). Recently, a mechanism for abrogating the
`FAS death signal has been revealed in a high fraction
`of lung and colon carcinoma cell lines: a nonsignaling
`decoy receptor for FAS ligand is upregulated, titrating
`the death-inducing signal away from the FAS death re-
`ceptor (Pitti et al., 1998). We expect that virtually all
`cancer cells harbor alterations that enable evasion of
`apoptosis.
`It is now possible to lay out a provisional apoptotic
`signaling circuitry (Figure 2); while incomplete, it is evi-
`dent that most regulatory and effector components are
`present in redundant form. This redundancy holds im-
`portant implications for the development of novel types
`of antitumor therapy, since tumor cells that have lost
`proapoptotic components are likely to retain other simi-
`lar ones. We anticipate that new technologies will be
`able to display the apop