Some Pathogenetic Aspects

Dea,i F. DaVies, ‘¿�4.1).,Ph.D.

Reported Research

More thana yearand a halfago theMinisterof Healthof Great Britainsummarized in the House of Commons theconsidered judgment of the Standing Advisory Committee on Cancer and Radiotherapy of the question of smoking andlung cancer.' The Committee was of theopinion that:

1. There is a relationship betweensmoking and cancer of the lung.―

Although saying that there is strongpresumptive evidence that the relationship

,.—iii oaisitai, the Committee felt obliged to

‘¿�@ concede that the presence of a carcinogen

for lung in tobacco smoke is not yet cer@ tam; thatthereisstatisticalevidencethat@ the increased incidence of cancer of the

lung isnot entirelydue to increasesinsmoking; and that atmospheric pollutionand occupational risks might be operating.

These statements are in accord with thethinking of numerous active investigatorsin this country. They, too, want to knowthe relative importance of atmosphericpollution, smoking, and occupational risksin the causation of lung cancer. They alsowant to know specificallywhat the responsible agents are and the intermediatesteps in lung carcinogenesis.

To this end, activity in research onmechanisms of action of agents involvedin the pathogenesis of lung cancer hasrapidly accelerated in recent years. TheAmerican Cancer Society has implemented many studies that have beenundertaken in this direction.

Pathogenetic Factors

Some etiologicalfactorsintheproduction of lung cancer start outside the body.Others may be inherent in the cell. Theepidemiologicaland biologicalevidence

againsta number of agentsthathave beenincriminated as etiological factors is discussed in the accompanying article byHammond. For purposes of this discussion, pathogenetic factors will be arbitrarily defined as the interaction of two ormore factors contributing to the development of lung cancer. They will be calledcofactors. The substances involved in sucha cofactorrelationshiphave been considered to have either additive or synergistic, augmenting or inhibiting, initiatingor promoting action. They may be specificor nonspecific, direct or indirect, of intrinsic or extrinsic origin. While this problem is common to other forms of cancer,thereis insufficientevidenceto assumethatwhat appliestoone form would alsoapply to another. Therefore cofactor relationships specifically in lung cancer warrant examination.

External Factor(s). In a given case oflung cancer, it is not known whether oneor more than one external agent is etiologically responsible. Numerous potentialcofactors have been considered. Any orseveralofa hostofirritantscouldpreparethe epithelial soil for subsequent actionof a carcinogen, whether in tobaccosmoke, the inhaled atmosphere, or as anatural metabolite of the organism. Acombined effect of two such substancesmight fit much of the observed epidemiological data. Alternatively, carcinogensmay initiate,and cocarcinogenscomplete.the process.

For example, the compounds that havebeen reported to be present in tobaccosmoke include aldehydes, ketones, acids,alcohols, alkaloids, other nitrogen compounds, hydrocarbons, inorganic compounds, and others.27 Any one or severalof the numerous compounds representedin these chemical classes could act as acofactor.The substancesrangefrom those

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Current Status of Lung-Cancer Research

soluble only in water to those soluble onlyin organic solvents. Many carcinogenshave greatest solubility in organic solvents.The idea has been entertained that carcinogenic components of tobacco-smokecondensate may be carried into cells byvirtue of their ability to dissolve in otherconstituents that, in turn, are soluble inbody fluids. The substances acting assolvents then would serve in a cofactorcapacity.

The suggestion has been made that nosingle substance in tobacco smoke is present in sufficient concentration to be carcinogenic. A combination of such a carcinogen and a cofactor might be sufficientfor carcinogenesis. Only the most cursorystudies along these lines have been carriedout.

Funnelling into the larger bronchi offoreign material caught in outdriftingmucus may act as a mechanical cofactor.32Here the efficiency of ciliary action maybe critical. Components of smoke thatinhibit ciliary action may play an ancillaryrole in slowing the removal of exogenousmaterial from the tracheobronchial tree.Ciliary action is also affected by particlesize and density. The distribution throughthe lung and potential carcinogenicity ofinhaled particulates are also affected bytheir characteristics. If carcinogens areadsorbed on soot, carcinogenicity is decreased. 14

Additive vs. Synergistic Co/actors.Aside from the additive or synergisticcarcinogenic effects of two or more agentsin tobacco smoke, the idea of synergismbetween tobacco smoke and atmosphericor other pollutants has caused speculation.Some believe that, regardless of the presence of small amounts of carcinogen intobacco-smoke condensate, its major roleis as a cofactor. Gellhorn's studies―' onskin carcinogens suggest such a role. Withsmoking a cofactor essential for theaction of carcinogens of smog, lung cancer would be expected to occur earlier orwith more than additive frequency in thepresence of both smoking and exposureto atmospheric pollution. The study ofStocks and Campbell47 suggests that (1)lung cancer occurs among smokers with

negligibleexposuretoairpollution,(2) itoccursamong nonsmoking residentsof aheavily air-polluted urban area, and (3)its incidence is roughly additive for thoseexposed to both. For example. among men65 to 74 years of age, the death rate was336 in 100,000 for nonsmokers in Liverpool, and for heavy smokers in the cleanair of a rural part of Wales it was 506 per100,000. The heavy smokers in Liverpoolhad a rate of 588 or 254 per 100,000 lessthan the sum of these rates, which was842. The same held true for the combinedgroups, 45 to 74 years. The figures areconsistent with the theory that cigarettesmoking and exposure to atmospheric pollutants are not synergistic.

Specificity vs. Nonspecificity, Whilecarcinogenshaveoftenbeenfoundtobemore or lessspecificforcertaintissuesaswell as for certain strains or species of animals, the property of carcinogenicity ofboth tobacco tar3' 16.43.31.32.@ @“¿�and smog3. 13. 28 for systems other than human lung

is of more than casual interest to severalinvestigators. The criticism has frequentlybeen made that such data may not be applicable to man.4' Similar arguments havebeen advanced for carcinogenicity inorgan systems other than the lung. Whendemonstrated for two or three otherspecies, carcinogenicity for man becomesmore likely. The same reasoning is usedwith respect to specificity of a carcinogenfor different organ systems.

Although tobacco-smoke condensateshave been shown to be carcinogenic forlung,43 skin,5' @l@ and bladder24 in atleast three strains of mice5 and rabbits,'7to date these findings have been achievedwith highly complex mixtures; there is noproof that the active agent is the samecompound in each instance.

Direct or Indirect Co/actor. By indirectcofactor is meant one that acts through anintermediate step rather than by enhancing the mode of action of a carcinogen. Its action may therefore be that of aprecursor. It may either increase or decrease the susceptibility of cells to a carcinogen. For example, under such a cofactor relationship would he included ahistory of any exposure or disease leading

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to proliferativechanges in the bronchialepithelium.Aftertheiriliuenzapandemicof 1917—

1918,Winternitz@―calledattentiontoproliferative alveolar epithelium and suggested that this might predispose to thedevelopment of cancer. Subsequently Niskanen'8 saw metaplastic bronchial epithehum frequently in cases of chronic bronchitis, bronchiectasis, and tuberculosis butnot with malignant transformation. In adiscussion of the pathogenesis of bronchialcarcinoma, Rennie42 discounted any association with tuberculosis, silicosis, bronchiectasis, bronchitis, emphysema, or anychroniclungdiseasesresultingfrom exposure to war gases.

On the other hand, an extensive studyof 1267 World War I prisonerspoisonedby mustard gas was made by Case andLea°todeterminetheincidenceof lungcancer. A significantly increased numberof deathsfrom cancer of the lung wasfoundintheexposedgroup,aswellasina bronchitis group that had not been exposed. Unfortunately, no smoking or residential history was obtained for the men,and the control group was composed ofsingle-leg amputees. Their smoking habitsmay well have differed from the others.Palmer'hadpreviouslyreportedthat,byhistory, clinical evidence, and radiologicalevidence, bronchitis is more common inheavy smokers than in light smokers orin nonsmokers.

Metaplasia and Precancer. The question of whether metaplastic changes areprecursors of carcinomas in situ is a critical one.Several studiescorrelatingtissue

changes with cancer of the respiratorytract are available. Moore and others'u!'served a large amount of leukoplakia int'iabitual tobacco chewers, which they feltwould have progressed with further stimulation into “¿�frankmalignancies.― Theyfound the association between the use ofsnuff and chewing tobacco and cancers ofthe mouth was statistically significant.Friedell and Rosenthal5 felt that the patients with the greatest leukoplakic reaction were the ones who most often developed cancer at that site.

Mellors― was able to localize fluorescent material from cigarette smoke withinsquamous epithelial cells scraped fromsmokers' buccal mucosa. Nonsmokers hadno fluorescent material in their smears. Healsofound thatthefluorescentmaterialhasa predilectionforkeratohyalingranules commonly seen in leukoplakia.

Ryan and others43 made a study of thepathologicaleffectsof smoking on thelarynx and found significantly greaterthickness of epithelium in smokers thaninnonsmokers.

Recently, Auerbach2' :@ and his coworkers have examined sections fromtracheobronchial trees obtained at autopsyby step sectioning and preparing 180 to200 blocksof each tree.One or moreslidesfromeveryblockwereexaminedbytwo pathologists.Basal-cellhyperplasia,stratification, squamous metaplasia, andcarcinomainsituwererecordedwithoutaccess to clinical information. Each parameter was found to increase in frequencyof occurrence from those who had neversmoked throughthosewho smoked oneor more packsof cigarettesperday.Inthosewithlungcancer,allcharacteristicswere more common than in the heavysmokers. Cancers were never seen in theabsenceof hyperplasia,metaplasia,andatypism.

With a different approach, Chang andCowdry8 and Chang7 studiedwholemounts of bronchial epithelium obtainedat autopsy from a major bronchus ofsmokersand nonsmokers.Inseventy-onecases, smokers, on the average, had shortercilia,more numerousgobletcells,thickerepithelium, more metaplasia, more intraepithelial inflammatory cell infiltration,and more frequent carcinoma in situ.

While the actual transition between themetaplastic epithelium and neoplasticcells is difficult to demonstrate, Black andAckerman4believetheinitialstageintheformation of carcinoma in situ of the lungis progressive metaplasia of the surfaceepithelium with cellular hyperactivity inthebasallayer.Kuschner2m'hasnotseenexperimental

epidermoid cancer of the lung withoutneighboring metaplastic changes. Black

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and Ackerman point out, however, thatthese associations may be coincidental,since similar metaplastic changes takeplace in bronchiectasis, hipoid pneumonia,tuberculosis, and chronic lung abscess,none of which is particularly associatedwith carcinoma of the lung. It is not yetknown which, if any, of these acquiredalterations in cytological structure of bronchial epithelium are necessary in the production of lung cancer.

Endogenous Factors. The question hasoften been asked why so many heavysmokers do not get lung cancer if cigarette smoke is an etiological factor. Fewattempts to answer this have been made.Nonspecific susceptibility may be manifestby the metaplastic changes already discussed. In this case, an individual wouldnot get lung cancer unless he had beenexposed previously to agents giving rise toproliferative epithelial changes. There isinsufficient evidence to support or denythis concept. On the other hand, specificsusceptibility or resistance would dependon variations in the reaction of the host toeither carcinogens or their cofactors.Maurer and Spain,4 Harkavy,2° andothers feel that tobacco smoke produceshypersensitive reactions more commonlythan is generally realized. Lowell3' concedes that hypersensitivity to tobaccoexists as manifest by dermatitis and respiratory allergies. Whether such sensitivityhas import in carcinogenesis is, of course,not known.

It is of passing interest to know thatRapport41 has found antigenic differencesbetween normal tissue and tumor in specimens from human beings, most notably inlung. It would indeed be fortuitous ifantigens found in lung cancers could betraced to substances that had been inhaled.

Antigens, being frequently formed bycombination of substances of exogenousorigin with body proteins, are receiving increasing attention by oncologists. Heidelberger2' has demonstrated a good correlation between the protein-binding capacityof seven C'4-labelled hydrocarbons andtheir carcinogenic potency. Whether thisisa causaloronlya secondarycorrelation

isnot known, but evidencethatproteinbinding is necessary for chemical carcinogenesisisgrowing.2'ExperimentallyShubik4―has recently achieved protein bindingof hydrocarboninthelung.

Sex as a Co/actor. From epidemiological data― about 30 per cent of the difference between lung-cancer incidence inmen and women remains unexplainedafter correction according to age and differences in smoking habits. Lilienfeld'°has suggested that, since the lungs arisefrom a laryngealanlage that developssecondary sex characteristics in adult life,such developmental differences may extend into the tracheobronchial tree andaccount for part of the sex difference insusceptibility to lung cancer. In supportof the hypothesis that the sex differenceof susceptibilityto carcinogenesisis ofendogenous origin, Lilienfeld found thatabortions had occurred more frequently inwomen having cancers with a high malefemale—incidence ratio (including those oflung and bronchus) than among womenwith other types of cancer. While thecorrelation could be secondary to a thirdexogenous factor, the possibility of anendocrine-determined predisposition ofmen to lung cancer can not be entirelydismissed.

Age in Pat/iogene.sis of Lung Cancer.To say thatage isa significantfactorinsusceptibility to lung cancer is of littlehelp in explaining pathogenesis. Little isknown of the morphological or cellularphysiological changes of the lung occurring in adult life. MackIm, in a discussionof aging of the respiratory system, statesthere is a decline of efficiency of bronchoeliminative,nervous,and otherfunctionsrelated to respiration.33 Although definiteinterstitial age changes occur, no metaplastic changes could be attributed solelyto age. Woglom3° in reviewing experimental tar cancer and Hueper23 reviewingthe evidence on chimney sweeps' cancerconcluded that age per se is an unimportant factor. Others―'@ have found that, ifanything. younger animals were moreprone to develop induced skin and subcutaneous neoplasms than older animals.This is in contrast to incidence rates in

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man and remains unexplained. Whetherthese findings apply in human lung cancer is a matter of speculation.

Dose and Latent Period. When all thefacts are known a formula could theoretically be derived to indicate the relativecontribution of the concentration of doseand time. One such formula states thatthe product of concentration, C, and time,T, must equal a constant for a given system before malignant transformation willtake place. While this serves a useful purpose, it does not take into account thecofactors of endogenous and exogenousorigin already discussed. Convincing datarelating the age at onset of exposure toboth concentration and time of exposureare not available. Discussions of latentperiodhave dealtchieflywithintervalsbetween beginning of exposure and diagnosis of cancer. How much of the intervalis true latent period and how much is dueto cumulative dosage effect is difficult toascertain. The question is of more thanacademic interest. Latent period, used inthe strict sense, connotes the irreversibilityof a process that has already received itseffective dose of carcinogen. On the otherhand, cumulative exposures to subthreshold concentrations of carcinogens neednot imply irreversibility, even though thetotal interval from beginning of exposuremay exceed the latent period for a singledose in the same system.

Evidence of reversibility of cumulativedoses on skin has been obtained for tobacco-smoke condensate by Graham andCroninger.'7 Epidemiological evidencefrom England and the United States is notcompletely consistent on this point. Doll'2interprets his data as evidence that cessation of smoking among men who havesmoked for fifteen or more years wouldnot appreciably decrease their mortalityrate from lung cancer. Hammond andHorn.― on the other hand, presented data,derived from their prospective study onthe incidence of lung cancer, showing thatmen who had been regular cigarettesmokers and stopped smoking at least ayear prior to the study had lower lungcancer—mortality rates than those whocontinued to smoke.

Discussion

In thescientificliterature,controversyover the importance of cigarette smokingand atmospheric pollution in the causation of lung cancer has been limited. Thisis in contrast to statements emanating inthe lay press in this country and abroad, aswell as in correspondence and in a feweditorials in medical journals. The existingdifferences of opinion hinge on what constitutes reasonable proof. No agreementhas been achieved, and perhaps properlyso, on what is required for reasonableproofofa causativefactorinlungcancer.The question pertains as much to exposure to atmospheric pollutants as tocigarette smoking. If it is assumed thateach may play a role, estimates of theirrelative importance in the total lungcancer—death rate vary greatly.

To get at the question for any suspectedetiological factor, two possible approachesareused.The pathogeneticapproachtakestwo epidemiologicallyassociatedstatistics and attempts to fill in the steps leadingfrom one to the other, on the hypothesisthat they are etiologically related. Some ofthe data bearing on this point have beencovered in this review.The other,orthedirectmethod,entails

a comparison of two matched populations,one exposed, and the other not exposed,to the suspected agent. In the case ofhuman lung cancer, such a study wouldrequire a minimum of twenty years evenif practicable. An alternative to this directapproach might be described as the withdrawal-type experiment in which a random half of a group of exposed individuals is withdrawn from exposure to theagent. It is noteworthy in this compromisemeasure that negative results would notbe conclusive evidence that no etiological relationship exists. Withdrawal fromfurther exposure during the latent periodafter a full carcinogenic dose would haveno effect on mortality rates. As recentlyshown,―' 2@ithe effective initiating dosemay be extremely small provided there issubsequent exposure to cocarcinogens. Inthis case, removal of a population fromcarcinogens after chronic exposure with

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outwithdrawingitfrompromotingagentsmay, forallpracticalpurposes,be againalteringtheexternalrelationshipsAFTERthe pathogenetic process has become irreversible. However, as stated before, thereis some epidemiological evidence, thoughpreliminary,thatwithdrawal,atleastfromcigarettesmoking, may decreasethemortalityrateoflungcancer.In the meantime,thephysicianmust

continuallymake clinicaldecisionson thebest available, though often inadequate,information. Carl V. Weller, in the final

Correction

chapter of his recent book on Causal Factors in Cancer of the Lung4―summarizesthe change in his own attitude as follows:“¿�Ihave been resistant to a ready acceptance of the association between cigarettesmoking and bronchogenic carcinoma. Ihave searched the literature for other reasonable explanations or for recognizablefallacies. I have found none of importance. As of today, I must agree withmany of the specialists in statistical analysis and in the endemiology of cancer, thatthis association has been established.―

References to articles mentioned are available upon request. Address:

Mr. Russell Gray SmithDistribution ManagerAmerican Cancer Society, Inc.521 West 57 StreetNew York 19. New York

In the abstract, Radiotherapeutic Progress on page 38 of the Marchissue of CA:For tumors, in the fourth line, read humans.For improvements . . ., lines 10-14, read With equal care and effort ex

pended on treating tile patients with conventional methods and lessspectacular radiation equipment, results can be obtained comparablewith those from the Big Berthas and atomic cannons.

For (7) . . . Wilms's tumor, in the second column, read (7) preoperative!)'in certain cancers such as breast with fixed axillary involvement, rectumwith fixation, corpus uteri, bone, and some fixed kidney tumnors, amidpostoperatively in stage II breast, thyroid, ovary, testis, Wi/ms's tumor ofthe kidney, carcinoma of the kidney with renal vein involvement or permeation through tile capsule, and accessory nasal sinuses.

For me/anosarcoma,inthefollowingsentence,readmnelanocarcinomna.

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