The cancer cell’s “power plants” as promising therapeutic targets: An overview.pdf

J Bioenerg Biomembr (2007) 39:1–12

DOI 10.1007/s10863-007-9070-5

INTRODUCTION

The cancer cell’s “power plants” as promising therapeutic

targets: An overview

Peter L. Pedersen

Published online: 3 April 2007

Abstract This introductory article to the review series

entitled “The Cancer Cell’s Power Plants as Promising

Therapeutic Targets” is written while more than 20 million

people suffer from cancer. It summarizes strategies to de-

stroy or prevent cancers by targeting their energy production

factories, i.e., “power plants.” All nucleated animal/human

cells have two types of power plants, i.e., systems that make

the “high energy” compound ATP from ADP and P i .One

type is “glycolysis,” the other the “mitochondria.” In contrast

to most normal cells where the mitochondria are the major

ATP producers (

much effort is being focused on identifying agents that

induce “necrotic,” “apoptotic” or apoptotic plus necrotic cell

death only in cancer cells. Regardless how death is inﬂicted,

every cancer cell must die, be it fast or slow.

Keywords Bioenergetics

Warburg

Warburg effect

Cancer

Anti-cancer agents

Cancer therapy

3-bromopyruvate

3-BrPA, Cell death

Necrosis

Apoptosis

Energy metabolism

Power plants

Glycolysis

Mitochondria

Cytochrome c

90%) in fueling growth, human cancers

detected via Positron Emission Tomography (PET) rely on

both types of power plants. In such cancers, glycolysis may

contribute nearly half the ATP even in the presence of oxygen

(“Warburg effect”). Based solely on cell energetics, this

presents a challenge to identify curative agents that destroy

only cancer cells as they must destroy both of their power

plants causing “necrotic cell death” and leave normal cells

alone. One such agent, 3-bromopyruvate (3-BrPA), a lactic

acid analog, has been shown to inhibit both glycolytic

and mitochondrial ATP production in rapidly growing

cancers (Ko et al., Cancer Letts., 173, 83–91, 2001), leave

normal cells alone, and eradicate advanced cancers (19 of

19) in a rodent model (Ko et al., Biochem. Biophys. Res.

Commun., 324, 269–275, 2004). A second approach is to

induce only cancer cells to undergo “apoptotic cell death.”

Here, mitochondria release cell death inducing factors (e.g.,

cytochrome c). In a third approach, cancer cells are induced

to die by both apoptotic and necrotic events. In summary,

Introduction

Despite the enormous amount of funding targeted for cancer

research during the past half century by funding agencies

throughout the world and by private donors, particularly in

the U.S.A., a major victory in our ongoing war against this

frequently fatal disease does not appear imminent. Thus,

some reports predict even today that one in two men and

one in three women are likely to die of cancer. Consid-

ering there are about 6.5 billion people in the world to-

day (World Fact Book, Central Intelligence Agency, Of-

ﬁce of Public Affairs Washington, D.C., July 2006), and

about half are men and half are women, it can be estimated

that if anti-cancer agents with long-term curative effects are

not found soon, particularly for advanced “metastatic” can-

cers, that the cause of death for about 2.7 billion people,

i.e., about 42% of the world’s current population, will be

cancer.

Although during the past century, a number of diseases

have been seriously curtailed including those related to

bacterial infection, polio, smallpox, and even heart disease,

cancer is rapidly becoming “public enemy number one.”

Despite this, there is historical solace in the fact that bacterial

)

Department of Biological Chemistry, Johns Hopkins University,

School of Medicine, 725 North Wolfe Street, Baltimore,

Maryland 21205-2185, USA

e-mail: ppederse@jhmi.edu

Springer

Springer Science + Business Media, LLC 2007

P. L . P e d e r s e n (

J Bioenerg Biomembr (2007) 39:1–12

infections that had posed a constant death threat to humans

for at least 5,000 years were ﬁnally curtailed following the

discovery in 1928 of penicillin in a laboratory mold by a

single scientist, Sir Alexander Fleming (Fleming, 1929).

This led to the isolation/manufacture of penicillin on a

large scale and the discovery of other anti-bacterial agents

that collectively have greatly beneﬁted the whole world.

Therefore, optimistically, if we view cancerous cells as

sophisticated types of “infectious-like” cells that have

developed and gone awry in our own bodies, i.e., exhibit the

capacity to multiply, mutate, invade, spread (metastasize),

and kill, we can believe also that natural agents may already

exist, or synthetic agents can be made, that will selectively

and repeatedly kill the major types of cancer regardless of

stage.

Although in the U.S. alone, such success may impact

negatively on some major pharmaceutical companies, reduce

the size of the National Cancer Institute, and make the newly

built, or to be built, Comprehensive Cancer Centers in almost

all 50 states in the United States available for other health

purposes, it would most importantly reduce greatly human

suffering, increase life expectancy and the quality of that

increased life, and hopefully also reduce income taxes on

the general population who are supporting our never ending

losing war on cancer.

Despite everyone’s desire for one or more proven cancer

cures/preventions in the immediate future, our continuing

36 year war with cancer in the United States, formally de-

clared by President Richard Nixon upon signing the National

Cancer Act (U.S. Govt. Print, 1971) shows no signs of end-

ing soon. When applying this prognosis to the rest of the

world, the situation is no better and in fact in many cases

much worse. Thus, in the United States and in the rest of

the world, we are still looking for our ﬁrst “Magic Bul-

let” that will consistently destroy in different patients both

primary and metastatic cancers regardless of their tissue of

origin while exhibiting minimal toxicity to the human host.

The major challenge is not to ﬁnd agents that kill cancer

cells. Rather, the real challenge is to ﬁnd agents that kill

cancer cells while leaving normal cells alone. To do this,

one must target one or more phenotypes unique to cancer

cells.

synthesis that will be up-regulated or subjected to different

patterns of regulation than in normal cells. Likewise, it is ex-

pected in cancer cells that the two energy (ATP) producing

power plants “glycolysis” and the oxygen dependent “mito-

chondria” will be altered such that tumors comprised of such

cells will be able to survive when oxygen is either plentiful

or limiting.

In fact, the normal to cancer cell metabolic shifts sug-

gested above do occur (Warburg, 1930; Weber and Lea,

1966; Weber, 2001), and are most evident in the more

rapidly growing (more malignant types of cancers). Thus,

enzymes involved in nucleic acid synthesis have been up-

regulated whereas those involved in nucleic acid degrada-

tion have been down-regulated. This helps assure more rapid

cell division. Likewise, in the most malignant, rapidly grow-

ing cancer cells the mitochondria have been down-regulated

frequently resulting in fewer of these organelles (Schreiber

et al., 1970; Reviewed in Pedersen, 1978), whereas enzymes

involved in glycolysis have been up-regulated (Weber, 1968;

Weinhouse, 1972; Bustamante and Pedersen, 1977). This

“shift” allows cancer cells to assume an energetic advantage

over normal cells in the sense that cancer cells can remain

alive either when ample oxygen is present or when oxygen

becomes limiting. Thus, cancer cells within tumors that have

a limited blood supply (therefore less oxygen) or cancer cells

close to a tumor’s inner core, i.e., where blood vessels may

not reach will remain viable for a longer period of time by

relying more on ATP produced by glycolysis. It should be

noted also that glycolysis (via intermediates like glucose-

6-phosphate) plays a major role in biosynthesis of the cell’s

building blocks (i.e., proteins, phospholipids, fat, and nucleic

acids). Thus, when a cancer cell’s glycolysis is enhanced, it

not only gains an advantage over normal cells in being able

to produce ATP should oxygen become limiting, but also

provides increased numbers of building blocks to help make

more cancer cells (Arora and Pedersen, 1988).

Although the molecular biological and bioenergetic-

related phenotypes in cancers seemed to be the most obvious

two phenotypes to target in the middle of the last century, it

will be noted below that the former was almost exclusively

championed, and the latter almost completely ignored. Even

when it was clear near the end of the last century that the

almost exclusive focus on potential anticancer agents that tar-

get molecular biological phenotypes was not winning the war

on cancer, the shift in focus was to signal transduction targets

(Reviewed in Dancy and Sausville, 2003) rather than targets

related to bioenergetics. Only very recently, after it has been

learned that cell death pathways (particularly those involv-

ing apoptosis), have a close relationship to the cell’s bioen-

ergetic machinery (Reviewed in Jiang and Wang, 2004) has

there been an overwhelming interest in seriously considering

bioenergetic or bioenergetic-related phenotypes as potential

anticancer drug targets.

Cancers’ major phenotypes as rational drug targets

Although cancer cells have many phenotypic differences

from normal cells, two general categories stand out, i.e.,

those related directly to enhanced cell division ( molecular

biological phenotypes ) and those related to fueling this en-

hanced cell division ( bioenergetic phenotypes ). Because can-

cer cells divide more rapidly than normal cells, it is expected

that there will be certain enzymes involved in DNA and RNA

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J Bioenerg Biomembr (2007) 39:1–12

Therapy for cancer, part I: An initial and persistent

half century focus on targeting molecular

biological phenotypes

mechanism of hormone action discovered cyclic AMP (Rall

and Sutherland, 1958; Sutherland and Rall, 1958). Subse-

quently, many signal transduction pathways were discovered

that commenced via either hormone binding or other types

of induction. This led eventually to the additional discovery

that the human genome encodes over 500 protein kinases

that can be grouped into approximately 20 known families

(Manning et al., 2002), thus revealing the extent to which

signal transduction pathways are involved in modulating key

events occurring in human cells.

Now, over 30 clinical trials are under way to defeat sev-

eral types of cancers with either small molecule inhibitors

of selected protein kinases or monoclonal antibodies to one

or more of their domains (Fischer et al., 2003). Of these

novel anticancer agents, Gleevec (Imatinib mesylate) and

Herceptin (Trastuzumab) have received the greatest atten-

tion (Fischer et al., 2003). Gleevec is a small molecule

ATP analogue that targets protein kinase domains in the

proteins named BCR-Abl, ckit, and the platelet-derived

growth factor receptor while Herceptin is a monoclonal

antibody that targets the receptor tyrosine kinase named

HER2. Remarkably, Gleevec has been approved by the FDA

(http://www.fda.gov/cder/cancer/druglistframe.htm) on 9

different occasions in this new century for different cancers.

Among these are chronic myelogenous leukemia (CML),

malignant gastrointestinal stromal tumors (GIST), and newly

diagnosed Ph

Following World War II, the thrust of cancer research focused

primarily on the development of new anticancer agents that

targeted molecular biological phenotypes that were accentu-

ated in cancer cells. Among these were derivatives of folic

acid, a member of the vitamin B complex that is required

for DNA synthesis via an enzyme named thymidylate syn-

thase (Reviewed in McGuire, 2003). Such derivatives were

ﬁrst used by Sidney Farber and colleagues (Farber, 1950)

at Harvard to treat acute lymphoblastic leukemia in chil-

dren. This initial thrust into targeting DNA related processes

was likely accelerated immensely after 1962 by the award-

ing of the Nobel Prize to James Watson, Francis Crick, and

Maurice Wilkins for their work on the “double helix.” Thus,

many other agents were later either synthesized or isolated

from natural sources to target nucleic acid synthesis/function

in cancers. The NCI website (http://dtp.nci.nih.gov/docs/

cancer/searches/standard mechanism.html) list these agents

under 6 different categories: Alkylating agents (e.g., cis-

platinum, mitomycin c), Antimitotic Agents (e.g., taxol

(paclitaxel), colchicines, vincristine), DNA Antimetabolites

(e.g., ara c, hydroxyurea, and thioguanine), RNA/DNA

Antimetabolites (e.g., 5-ﬂuorouracil, methotrexate), Topoi-

somerase I inhibitors (e.g., campothecin and deriva-

tives), and Topoisomerase II inhibitors (e.g., doxorubincin,

daunorubincin).

Although a detailed analysis of the literature will certainly

reveal many success stories with such agents either alone or

in combination, cancer has continued to be a major health

problem worldwide, the U.S. being no exception. This is

despite other notable advances that include earlier detection

methods, improved treatment centers, and more knowledge

about the disease. Therefore, there has been a continued

search for newer, more powerful, and speciﬁc agents, and

hopefully one or more agents that might be called “the peni-

cillin for cancer,” or at the minimum “the penicillin” for one

or more cancer types.

chronic myelogenous leukemia. Herceptin,

approved earlier by the FDA in 1998, is used in the treatment

of metastatic breast cancer. Other signal transduction based

drugs in clinical development are targeting pancreatic, lung,

colorectal, stomach, ovarian, and prostate cancer (Fisher

et al., 2003).

Despite the success today with Gleevec as an anticancer

agent, particularly in patients with CML, it now seems clear

that this agent in some patients can cause cardiotoxicity, i.e.,

left ventricle dysfunction (Kerkela et al., 2006; Strebhardt

and Ullrich, 2006).

Therapy for cancer, part III: Targeting phenotypes

related to bioenergetics

Therapy for cancer, part II: A shift toward agents

that target signal transduction pathways

Death by necrosis

The end of the 20th century and our entry into the 21st cen-

tury has seen some signiﬁcant movement away from molec-

ular biological phenotypes as the almost exclusive targets for

anticancer drug development and some rather signiﬁcant fo-

cus on signal transduction phenotypes (Dancy and Sausville,

2003). This new direction had its origin more than 3 decades

earlier following the awarding of the Nobel Prize in 1971 to

Earl Sutherland who while working with T. W. Rall on the

Necrosis is a term that has been associated more in the lit-

erature to death of normal cells as a consequence of tissue

injury than to cancer therapy. Such injury to normal tissues

may result because of a lack of oxygen, e.g., to the heart re-

sulting in cardiac arrest or to the brain resulting in stroke. In

either case, the mitochondria become compromised, glycol-

ysis cannot fully compensate, cell ATP levels drop precip-

itously and death results. Death by necrosis is a quick way

to die. To understand the targeting of cancer via agents that

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J Bioenerg Biomembr (2007) 39:1–12

induce necrosis requires some understanding of the bioen-

ergetics/metabolism of cancers, particularly those that are

referred to as “PET positive.”

One of the problems with cancer as a disease in humans is

that it is frequently asymptomatic and not detected until an

advanced stage. At this stage such tumors are likely poorly

differentiated and have undergone many changes. The most

well known bioenergetic change (phenotype) of such can-

cers is their capacity to catabolize glucose at elevated rates

to pyruvic acid, a signiﬁcant portion of which is then con-

verted to lactic acid and transported out of the cell (Warburg,

1930). This metabolic property of many cancers occurs even

in the presence of oxygen and is referred to as the “ Wa r bu rg

effect ”or“ high glycolytic phenotype ,” the former des-

ignation after its discoverer, the German scientist Otto

Warburg. In fact, PET analysis for cancer (Reviewed in

Shields, 2006; Cherry, 2006) monitors the “Warburg effect”

and is one of the most common detection methods for can-

cer used clinically throughout the world. Warburg received

the Nobel Prize in 1931 for his pioneering studies on cell

respiration but never lived to fully appreciate the clinical

applications of his work.

It is important to note that in PET scan positive tumors

not all the pyruvate derived from glucose catabolism is con-

verted to lactic acid. Some enters the mitochondria and

becomes oxidized, at least in part, to carbon dioxide and

water, a process that involves the tricarboxylic acid cycle

and the electron transport chain. In fact, in those advanced

“highly glycolytic” cancer cells that have been examined

carefully, it is found that signiﬁcant energy (ATP) is derived

from both glycolytic and mitochondrial events (Aisenberg,

1961) . In one of the most extreme cases studied, mitochon-

drial ATP production was estimated at about 40% and gly-

colytic ATP production at about 60% (Nakashima et al.,

1984). This is in sharp contrast to most normal cells where

the mitochondria are responsible for over 90% of the ATP

production.

From the above, one can see clearly why the “high gly-

colytic phenotype” serves the advanced cancer cell well and

gives it an energetic advantage over those normal cells re-

siding in its tissue of origin. Thus, the advanced cancer

cell thrives when oxygen is either plentiful or reduced, and

because of its rapid division can “crowd out” surrounding

normal cells while subjecting them to a constant stream of

acid (“chemical warfare”). In addition, once a solid tumor is

formed within one human organ, some of its cancer cells can

separate, sometimes as clusters, travel (metastasize) through

the blood stream (Elshimali and Grody, 2006) where “food”

(glucose) is plentiful, and eventually settle into a comfortable

environment in one or more other body organs. Here, it will

undergo multiple divisions until a new tumor is developed

that will become vascularized and receive both oxygen and

glucose from the blood of the human host. Thus, the new

tumor in its new body organ will be using also both of its

power plants (glycolysis and mitochondria) to provide the

ATP essential for its survival and rapid growth (Fig. 1A).

To quickly arrest the growth of a tumor characterized by

the “high glycolytic” phenotype where it is in fact fueled

by signiﬁcant amounts of ATP derived from both glycolysis

and the mitochondria (See above discussion), it is essen-

tial to have an agent that will “selectively” destroy both

“power plants” of the tumor while leaving the power plants

in the surrounding normal tissue alone. That is, to subject

the tumor to a quick death it is important to induce necro-

sis in each of its cells while doing minimal harm to normal

cells.

One approach (“Trojan Horse”) is to screen for and iden-

tify an agent that will preferentially enter tumor cells, and

once inside destroy both its “power plants,” i.e., glycoly-

sis and mitochondria. This would cause a rapid decline in

cell ATP levels inducing death mainly by “necrosis” (Fig.

1A). The second approach (“Backdoor Block”) is to iden-

tify an agent that will selectively block the exit of lactic

acid from the tumor cells (i.e., inhibit the lactate transporter)

without doing the same to normal cells. By blocking the

exit of lactic acid, the tumor cells’ internal pH will be low-

ered and this increased acidity will have a deleterious effect

on both power plants resulting in death predominantly by

necrosis.

The ﬁrst approach (“Trojan Horse”) noted above, i.e., “to

identify an agent that will preferentially enter tumor cells but

not normal cells, and once inside the tumor cells destroy both

power plants (Fig. 1A),” has been accomplished. Thus, fol-

lowing the discovery by Ko et al. (2001) that the lactic acid

analog 3-bromopyruvic acid (3-BrPA) inhibits hepatocellu-

lar carcinoma cells in tissue culture, advanced cancers (hepa-

tocellular carcinomas) were completely eradicated (Fig. 1B)

with this agent in 19 out of 19 animals without harm to the

animals (Ko et al., 2004; Also see Supplement to Ko et al.,

2004). Moreover, all animals lived thereafter a normal life

without tumor recurrence. [A unique aspect of this study that

is rarely presented in published work to verify therapeutic

success over a given type of cancer is that photographs of

animals bearing healthy growing tumors to be treated are

presented followed by photographs of the same animal at

different stages of treatment. In addition, photographs of the

tumor free animals are shown after the tumors have disap-

peared. Finally, photographs revealing the results of PET, the

analysis that monitors the high glycolytic cancer phenotype,

are shown before and after treatment. Signiﬁcantly, these

two types of visual data verify tumor eradication. What is

more common to see in many animal studies with a poten-

tial anticancer agent is the absence of such photographs/data

during the treatment process. Rather, a Kaplan-Meier sur-

vival curve (Allison, 1995) is shown that reveals, relative

to untreated controls, how many of the animals’ lives have

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J Bioenerg Biomembr (2007) 39:1–12

been extended and for how long. Unfortunately, this survival

curve also reveals for most, if not all cases, that the agent

being tested rarely “cured” the animal of its cancer, unlike

3-BrPA in the study noted above by Ko et al., 2004]. Fi-

nally, it should be noted also that an earlier study involving

the use of 3-BrPA to treat liver implanted dermatoid tumors

in rabbits also showed impressive results, although animal

survival was not monitored (Geschwind et al., 2002).

The potent anticancer agent 3-BrPA has been called an

“energy blocker” (Foubister, 2002) for highly malignant

cancer cells as it inhibits both of their “power plants”

(glycolysis and mitochondria) while leaving normal cells

alone. Like a “Trojan horse” 3-BrPA likely enters cancer

cells through the same “gates,” i.e., lactic acid transporters,

that lactic acid goes out (Fig. 1A). Normal cells are spared

as they have a much lower number of such transporters.

(The transporter for lactic acid is in the “mono-carboxylic

acid transporter family”).

Studies involving the second approach (“Backdoor”)

noted above to selectively arrest the growth of highly gly-

colytic cancers had its roots three decades ago in studies by

Spencer and Lehninger (1976). These investigators showed

that lactic acid transport could be inhibited by alpha-cyano-4-

hydroxycinnamate (ACCA), alpha-cyano-3-hydroxycinna-

mate, or DL-p-hydroxyphenyl-lactate. Subsequent studies

pursued by Johnson et al. (1980) showed that lactic acid

transport could be inhibited also by isobutylcarbonyl lactayl

anhydride (iBCLA) resulting in a decrease in intracellular

pH. Although these early studies focused on identifying

inhibitors of lactate transport out of tumor cells, more recent

work has applied this approach to cancer therapy. In these

studies one ﬁrst inhibits lactic acid exit (“Backdoor”) from

highly glycolytic cancer cells and then allows time for the

acid concentration to rise inside the cell, lower the pH,

and induce cell death. Speciﬁc examples are the recent

studies of Mathupala et al. (2004) who were able to inhibit

dramatically (92%) the viability of malignant glioma cells

(U-87) using small interfering ribonucleic acids speciﬁc

for monocarboxylic acid transporters 1 and 2 (i.e., MCT1

and MCT2), and the studies of Fang et al. (2006) who

were able to diminish cell viability in neuroblastoma cells

using the lactic acid transport inhibitor (ACCA) noted

above.

of cell death from that which results from “necrosis.” In bio-

chemical terms the author of the current article has already

deﬁned necrotic cell death above as death that results when a

living cell is deprived of both its ATP sources (Fig. 1A), i.e.,

glycolysis and mitochondria. Apoptotic cell death is quite

different (Fig. 1A). In this case, the cell suicide program

of events commences with a death signal (i.e., “it’s time to

die”) that may originate either within the cell or from ex-

ternal signals acting on membrane receptors. Whatever the

case, i.e ., an intrinsically initiated pathway for cell death or

an extrinsically initiated pathway (Fig. 1A), central players

in the death events include mitochondrial proteins, in partic-

ular cytochrome c that is released, and a number of proteins

that reside outside the mitochondria that include proteases

referred to collectively as caspases (Reviewed in Jiang and

Wang, 2004).

During cell life cytochrome c is required in our mito-

chondria to participate in the ﬁnal stages of the biological

oxidation of food that we consume (Reviewed in Hosler

et al., 2006). Interestingly, it is loosely bound to the outer

surface of the inner membrane where it transfers electrons

from complex III (b-c 1 complex) of the mitochondrial elec-

tron transport chain to complex IV (cytochrome oxidase),

the terminal complex that reduces the oxygen that we con-

sume to water. This overall process of electron transport to

molecular oxygen via the mitochondrial electron transport

chain yields the free energy necessary to drive the synthesis

of ATP that is needed for cell growth and development. If

cytochrome c is not “in place” within the mitochondrial elec-

tron transport chain, the critical process of electron transport

to molecular oxygen will come to a screeching halt, and no

ATP will be synthesized by the ATP synthase. If glucose is

present, which it usually is in a living system, the cells lack-

ing the capacity to make ATP via their mitochondria may

still survive, at least temporarily from ATP that is produced

by glycolysis. However, to assure that the cell does not sur-

vive indeﬁnitely when cytochrome c is released, a number

of proteases are present called caspases, and with the help

of cytochrome c facilitate the cell death process via what is

referred to as the caspase cascade (Jiang and Wang, 2004).

Numerous publications (Jiang and Wang, 2004; Neuzil et al.,

2006; Cereghetti and Scorrano, 2006; Goodsell, 2004) have

dealt with how cytochrome c is released from mitochon-

dria in the initial stages of the cell death program, its in-

teraction with a complex called the apoptosome, and sub-

sequent activation of the caspase cascade leading to cell

death.

In contrast to normal cells which turnover, and in so do-

ing must undergo cell death on a programmed schedule,

cancer cells have become immortalized. Although there are

likely a number of factors involved in this process, one of

the most critical is Type II hexokinase (hexokinase II). This

crucial metabolic enzyme was ﬁrst shown in a collaborative

Death by apoptosis

“Apoptosis” is deﬁned as the process of programmed cell

death. Unlike necrosis, apoptosis is not a quick way to die.

The pioneers in the discovery of this process were Sydney

Brenner, H. Robert Horvitz, and John E. Sulston all of whom

shared a Nobel Prize in 2002. The term “apoptosis” was used

by Kerr et al. (1972) in order to distinguish this natural type

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