[CANCER RESEARCH 34, 328-336, February 1974]

Influence of Transitory, as Compared with Permanent, High-AltitudeExposure on the Pathogenesis of Spontaneous and X-ray-inducedNeoplasms in RF/Un Mice1

Pablo Mori-Chavez, Arthur C. Upton,2 Maximo Salazar J., and J. W. Conklin2

Laboratorio de Oncologia, Instituto de Investigaciones de la Altura, Universidad Peruana Cayetano Heredia, Lima, Peru [P. M. C., M. S. J.],and Biology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 [A. C. U., J. W. C.]

SUMMARY

This study was undertaken to investigate the mechanismsof effects of high altitude on spontaneous and irradiation-induced neoplasia, with particular reference to the influenceof the time and duration of high-altitude exposure on thepathogenesis of such growths. Female RF/Un mice exposedto 150 R whole-body X- or sham irradiation at 10 weeks ofage were subsequently housed at high (4540 m) or low (150m) altitude for various lengths of time. In such animals, theoverall incidence of neoplasms was increased and thelife-span was decreased by irradiation, irrespective of altitude; animals residing at high altitude into old age showedfurther life shortening with a decrease in the overallincidence of neoplasms, in addition to polycythemia, impairment of weight gain, lymphoid atrophy, decrease in theincidence of lymphoreticular tumors, and increase in theincidence of pulmonary neoplasms, angiomatoid, telangiec-tatic, and thrombotic lesions, myocarditis, and pneumonia.In animals residing at high altitude for only 3 months afterirradiation, the effects of high altitude were drasticallyreduced; however, the induction of granulocytic leukemia insuch animals was enhanced, perhaps through hypoxia-induced stimulation of hemopoietic stem cells which had notyet recovered from irradiation. Since the long-term effectsof high-altitude exposure prolonged into old age were notreproduced by transitory exposure early in adult life, theeffects may have been mediated largely through influenceson late stages in the pathogensis of neoplasms and otherage-related lesions, as opposed to early stages in theirdevelopment.

INTRODUCTION

In addition to characteristic adaptational changes thatoccur early in response to high altitude ( 1, 2, 5, 6, 17, 18, 25,26), mice residing at high altitude also subsequently show

' This research was sponsored by the National Cancer Institute-Atomic

Energy Commission Carcinogenesis Program, Biology Division, OakRidge National Laboratory, Oak Ridge, Tenn., and was supported in partby Research Grant ROI CA 08264 from the National Cancer Institute,NIH, USPHS.

2Present address: Health Sciences Center, State University of New

York at Stony Brook, Stony Brook, N. Y. 11790.Received April 9, 1973; accepted October 26, 1973.

changes in the incidence of neoplasms, other age-relatedlesions, and overall longevity (27-31). In X- and sham-irradiated RF/Un mice (32), as in other strains of mice,these changes include impairment of growth, shortening ofthe life-span, decrease in the incidence of lymphoid neoplasms, increase in the incidence, size, and malignancy ofpulmonary tumors, increase in the frequency of muralthrombosis of the endocardium and of myocarditis, anddecrease in the incidence of nephrosclerosis.

These studies were intended to explore the extent to whichthe effects of high altitude on the pathogenesis of neoplasiaand other age-dependent lesions might be elicited bytransitory as opposed to permanent exposure and to studythe sequence of events involved in the evolution of sucheffects.

MATERIALS AND METHODS

Life-Span Studies. A total of 920 female RF/Un micewas exposed at 10 weeks of age to 150 R whole-bodyX-rays or to sham irradiation. The irradiation was carriedout at the Oak Ridge National Laboratory, with theradiological factors reported previously (32). After irradiation, the mice were allowed 2 weeks to recover from theacute effects of X-rays and were then shipped by air toLima, Peru. After arrival in Lima, the X-irradiated miceand the nonirradiated controls were divided into 4 subgroups (Table 1): (a) mice maintained in Lima (i.e., at therelatively low altitude of 150 m) for a period of 3 monthsand then transferred to high altitude (at Morococha,altitude 4540 m) for the remainder of their lives ("low-high"

group); (b) mice maintained temporarily at high altitude forthe same period of 3 months and then transferred to thelower altitude ("high-low" group); (c) mice maintained athigh altitude throughout the entire period ("high" group);

and (d) mice maintained at the lower altitude throughout("low" group). At both high and low altitudes, the mice

were housed in groups of 4 to 5 to a cage, allowed access tofood and water from the same source ad libitum, andmaintained at constant average room temperature (24°)andrelative humidity (60%). All animals were weighed once aweek, and 20 mice from each group were subjected periodically to examination of peripheral blood, for erythrocytecount, hemoglobin, hematocrit, reticulocyte count, meancorpuscular volume, and total and differential white blood

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Influence of Altitude on Neoplasms in RF/Un Mice

Table 1Mean survival time and overall incidence of neoplasms as influenced by irradiation and by altitude

X-rayexposure(R)0015015000150150Altitude"LowHighLowHighHigh-lowLow-highHigh-lowLow-highTotalno.ofmice"101105121124108110115115Survivaltime(wk)90.4

±2.6C70.

0±2.272.8±1.962.9±1.990.1

±2.375.6±2.176.7±1.967.7±1.9No.856111397897211098Mice

with%84.258.193.478.282.465.595.785.2neoplasmsSurvivaltime(wk)93.0

±2.670.9±2.774.1±2.264.7±2.093.5

±2.278.1±2.477.8±1.970.7±1.9Mice

withoutneoplasmsNo.16448271938517Survivaltime(wk)76.9

±7.465.5±4.156.1±9.256.5±4.573.9

±7.171.1±4.251.6±10.550.3±5.1

' Permanent exposure, low and high; temporary exposure, high-low and low-high.' Excluding mice dying during or shortly after transportation from Oak Ridge, Tenn., to Lima, Peru.

Mean ±S.E.

cells counts. The blood examinations were performed by themethods of Wintrobe, as in our previous studies (32). Micefound dead or killed in extremis were necropsied, and theweights of their lymph nodes, tumors, and various organswere recorded as noted below. Samples of tissue were fixedin Helly's fluid, embedded in paraffin, sectioned, and

stained routinely with hematoxylin and eosin.Serial Sacrifice Studies. In addition to the animals

described above, a total of 304 X-irradiated and nonir-radiated mice, equally distributed between high and lowaltitudes, were sacrificed in groups of 8 mice each atintervals of 1, 2, 3, 5, 7, 10, 14, 18, 23, 31, and 45 days and 2,3, 4, 5, 6, 8, 10, and 12 months following distribution to therespective altitudes. At sacrifice, each mouse was necropsied and the body weight was recorded, as were the totalcombined weights of the cervical, axillary, inguinal, andmesenteric lymph nodes (all together 11 nodes). Theweights of the thymus, spleen, liver, kidneys, adrenalglands, uterus, and ovaries were also recorded. Specimensof tissue were then fixed in Helly's fluid, embedded in

paraffin, sectioned, and stained with hematoxylin andeosin.

RESULTS

Blood Changes

After ascent to high altitude, the erythrocyte count increased rapidly, from a little over 10 million per cu mm atsea level in the nonirradiated controls (or 9 million in theirradiated mice) to 12 million at the end of the 1st week, 13million at the end of the 1st month, and 14 million at the endof the 2nd month (Chart 1). Parallel changes in hematocritand hemoglobin occurred concomitantly. Blood countstaken 1 week after mice were returned to sea level showed arapid decline, with values temporarily falling below basallevels before returning to the normal range.

With advancing age, erythrocyte levels continued toincrease in mice kept permanently at high atltitude, whilethey tended to decline in mice at sea level.

16 n NONIRRADIATED X - IRRADIATED

K) 14 I 3 10 14

TIME AFTER START OF EXPERIMENTS (MONTHS)Chart I. Erythrocyte count and reticulocyte count of RF/Un mice as

influenced by age, altitude, and irradiation. •¿�,high altitude throughout(high); •¿�,low altitude throughout (low); D, high altitude to low altitude(high-low); O, low altitude to high altitude (low-high).

Trends in the irradiated mice were similar to those incorresponding nonirradiated mice, aside from irradiation-induced anemia at the outset (Chart 1).

The reticulocyte count showed a rapid increase within the1st week after ascent to high altitude, then declined duringthe remainder of the month and returned to its original levelby the end of the 2nd month. In mice returned to sea levelafter transitory exposure to high altitude, the reticulocytecount declined to subnormal levels within 1 week.

In nearly all groups of aging mice, the reticulocyte counttended to rise after the 8th month. In the irradiated group,reticulocyte values differed from those in the nonirradiatedgroup only at the outset, showing a transitory reticulocyto-sis associated with irradiation-induced anemia.

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Mori-Chavez, Upton, Solazar J., and Conklin

Body Weight

Mice exposed to high altitude, even temporarily, gainedless weight than did mice at sea level (Chart 2). In the micemaintained permanently at low altitude, the body weightincreased steadily, to reach an average of 37 g at the age of60 weeks, remaining constant thereafter until late in life. Inthe high-altitude mice, on the other hand, the body weightreached a maximum of 32.5 g at 50 weeks, decliningafterwards to a final average of 29.5 g at 90 weeks of age.No significant differences in weight gain were attributableto irradiation.

Survival Time

Both irradiation and prolonged exposure to high altitudedecreased the survival time, the combined effects of the 2factors exceeding the effect of either alone; however,transitory exposure to high altitude did not significantlyaffect survival time (Table 1; Chart 3). In mice withneoplasms, the mean survival time exceeded that in othermice (Table 1).

Neoplasms

The overall incidence of neoplasms was increased byirradiation and decreased by prolonged exposure to high

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I20 80 10040 60

AGE (WEEKS)Chart 2. Body weight in relation to age, altitude, and irradiation; D, •¿�

values for nonirradiated mice; O, •¿�,values for irradiated mice; O, D,low or high-to-low altitude exposure;titude exposure.

high or low-to-high al-

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Chart 3. Survival in relation to age, altitude, and irradiation. •¿�,highaltitude throughout (high); •¿�low altitude throughout (low); D, highaltitude to low altitude (high-low); O, low altitude to high altitude(low-high). OR, Oak Ridge.

altitude (Table 1). It was not significantly affected bytransitory exposure to high altitude.

Leukemias and Lymphomas. The combined incidence ofall types of leukemias and lymphomas was decreased bylong-term exposure to high altitude (Table 2), averagingabout 34% in those maintained permanently at high altitude, as compared with about 1Q%in those at sea level. Inthe groups maintained temporarily at high altitude, thecombined incidence in the low-to-high mice was 36%,compared with 64.5% in the high-to-low mice (Table 2).

The incidence of granulocytic leukemia was increased byradiation and its latency correspondingly decreased (Table2); however, a lower incidence and a shorter latency werefound in mice aging at high altitude than in the corresponding groups aging at sea level. The incidence of the diseasewas highest in irradiated high- to low-altitude mice (Chart4).

The incidence of thymic lymphoma was increased, and itslatency decreased, by irradiation in all groups (Table 2).Both incidence and latency were decreased by exposure tohigh altitude, the magnitude of these effects increasing withthe duration of exposure.

Nonthymic lymphomas and reticulum cell sarcomas werereduced in incidence and latency by irradiation andhigh altitude, the effects of high altitude varying with theduration of exposure (Table 2).

Lung Tumors. Lung tumors occurred in 30 to 50%of micein all groups (Table 3). Although the final incidence oftumors did not vary significantly with irradiation or withaltitude (Table 3), the latency was reduced (Table 3) and the

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Influence of Altitude on Neoplasms in RF/Vn Mice

Table 2Incidence of leukemias and lymphomas in relation to irradiation exposure and altitude

Lymphoid

ThymicX-ray

exposure(R)01500

150AltitudeLowHigh

LowHighHigh-low

Low-highHigh-lowLow-highIncidence

(%)5.03.8

14.910.51.92.7

11.310.4Meanage

atdeath(wk)50.432.342.831.953.0

54.355.841.1NonthymicIncidence25.711.420.710.522.2

16.420.015.6Meanage

atdeath(wk)96.886.4

78.175.587.6

81.375.061. 8Reticulum

cellIncidence26.79.518.24.025.0

9.18.76.1Meanage

atdeath(wk)93.275.0

85.764.695.8

78.390.174.0Granulocytic

leukemiaIncidence10.9

±3.1°5.7

±2.316.5 ±3.412.9

±3.012.0

±3.14.5±2.0

27.8 ±4.27.0 ±2.4Meanage

atdeath(wk)92.565.8

75.856.3103.0

72.467.561.5All

typescombinedIncidence(%)68.3

±4.330.5±4.5

71.1±4.137.9±4.461.1

±4.732.7 ±4.567.8 ±5.039.1 ±4.5Age

atdeath

(wk)91.8

±2.973.4±3.9

72.4 ±2.656.6±3.093.9

±2.676.4 ±2.874.4 ±2.460.8 ±3.0

'Mean ±S.E.

150 R X-RAYS

8 80-

60-

40-

20-

0 R CONTROLS

o-m'¿I/

20 40 60 80 100 120 140

AGE (WEEKS)Chart 4. Age-specific incidence of myeloid leukemia in relation to age,

irradiation, and altitude, adjusted for intercurrent mortality by life-tabletechnique of Sachs (36). •¿�,high altitude throughout (high); •¿�,low altitudethroughout (low); D, high altitude to low altitude (high-low); O, lowaltitude to high altitude (low-high). OR, Oak Ridge.

age-specific incidence was increased (Chart 5) by bothfactors, the effect of high altitude varying with the durationof exposure. The neoplasms were predominantly pulmonaryadenomas, such as have been described previously (32). Inmice maintained at high altitude into tumor-bearing age,the neoplasms tended to be larger and to include a greaterpercentage of adenocarcinomas than in mice of the othergroups (Table 3).

Ovarian Tumors. Tumors of the ovary were rare innonirradiated mice but common in all irradiated groups(Table 4). Although the incidence in irradiated mice decreased in those groups maintained at high altitude intotumor-bearing age, this difference disappeared when thevalues were adjusted for variations in survival time (Chart6). Tubular adenoma was the predominant type of tumor,with luteomas and granulosa cell tumors next in relativefrequency. Admixtures of these types were also commonlyencountered.

Other Neoplasms. Miscellaneous neoplasms of typesother than those already mentioned were found less commonly in mice aging at high altitude than in those aging atsea level, totaling 11 and 27 cases, respectively (Table 5).The most common types were pituitary and adrenocorticaladenomas. A few mammary gland carcinomas, endometrialcarcinomas, and sarcomas were also found. No consistentvariations in the types of neoplasms were attributable toirradiation or altitude, but there was a wider variety ofneoplasms in the low and high-to-low altitude groups thanin the other groups.

Nonneoplastic Lesions

Mural thrombosis of the endocardium, a lesion describedpreviously in mice of the same strain (32), was moreprevalent in animals aging at high altitude than in thoseaging at low altitude, as was myocarditis (Table 6).Similarly, pneumonia occurred more frequently and at anearlier age in mice aging at high altitude than in those agingat sea level (Table 6); the pulmonary lesions included typicalbronchopneumonia and interstitial pneumonitis. Nephro-sclerosis (13) was decreased in frequency and severity athigh altitude.

Serial Sacrifice Observations

Lymphatic Tissues. Histológica!changes attributable tohigh altitude included congestion and edema during the 1st

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Mori-Chavez, Upton, Solazar J., and Conklin

Table 3Lung tumors in relation to irradiation and altitude

All types of lung tumors Adenocarcinoma

X-rayexposure

(R)0ISO0ISOAltitude"LowHighLowHighHigh-lowLow-highHigh-lowLow-highNo.of

mice42

(27)"37(10)49

(37)48(9)54

(33)45(10)53(33)47(13)Age

at death%(wk)41.635.240.538.750.040.946.140.9±

4.9'±

4.6±4.5±4.4±

4.8±4.7±4.6±

4.699.2

±375.5±285.0±269.6±297

.6 ±280.1±387.

0±274.6±227286123Body

<g>30.5

±26.6±32.3±26.7±31.2

±26.8±30.7±26.3

±Av.

no.wt of tumors/mouse0.90.70.80.70.80.80.80.6.24.05.41.37.54.33.32.45Av.size(mm)3.44.83.13.33.03.83.03.3Incidence2.01.90.82.42.83.60.92.6

' Permanent exposure, low and high; temporary exposure, high-low and low-high.1Numbers in parentheses, number of mice with concurrent leukemia or lymphoma.

Mean ±S.E.

80 100 120 140

AGE (WEEKS)

Chart 5. Age-specific incidence of lung tumors in relation to age,irradiation, and altitude, adjusted for intercurrent mortality by life-tabletechnique of Sach (36). •¿�,high altitude throughout (high); •¿�low altitudethroughout (low); D, high altitude to low altitude (high-low); O, lowaltitude to high altitude (low-high). OR, Oak Ridge.

few days (particularly in the lymph nodes and spleen inirradiated mice), accentuation of extramedullary hemopoie-sis in the spleen, and an increase in the number ofhemosiderin-laden splenic macrophages.

Considerable variation in the architecture, size, andnumber of germinal centers was observed, such as ischaracteristic for the mouse and "considered as a reflectionof the natural background immune response of the animal"

(3, 4). The variations bore no discernible relation to altitudeor to irradiation.

The thymus/body weight ratio decreased at high altitudeand with aging (Table 7). The lymph node/body weightratio increased at high altitude during the 1- to 31-dayinterval, but decreased thereafter, particularly in the irradiated groups. The spleen/body weight ratio also increased athigh altitude, except in nonirradiated mice, during the 1.5-to 12-month interval.

Lungs

The earliest response to high altitude consisted of capillaryand arteriolar dilation, evident within 24 to 48 hr. Thesevascular changes, which persisted for months, were paralleled by dilation of alveoli and alveolar ducts. In their earlierstages, the changes were detectable only by microscopicobservation but, in later stages, the lungs appeared enlargedand reddened at autopsy. Similar changes, which supposedly denote an increase in the capillary bed by elongationand sprouting, have also been observed in the lungs ofguinea pigs exposed to high altitude (26).

The combined weights of the lungs of mice sacrificed at 1to 31 days showed no definite changes attributable toaltitude or irradiation, averaging 0.24 ±0.004 g at highaltitude and 0.21 ±0.003 g at sea level. In later sacrifices,from 1.5 to 12 months, the combined weights averaged0.030 ±0.008 g at high altitude and 0.23 ±0.005 g at sealevel. No histológica!change in the lungs was detected thatcould be designated as preneoplastic. No mice were observed to die of acute pulmonary edema.

The Ovaries and the Endometrium

Retrogressive changes in the ovary were observed as earlyas 4 to 6 weeks after irradiation, irrespective of altitude.These included degeneration of ova, followed by necrosis offollicular cells and eventual disappearance of ovarian follicles. This process was sometimes accompanied by reductionin the size of the ovary and wrinkling of its surface. Within 3

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Influence of Altitude on Neoplasms in RF/Un Mice

Table 4Ovarian tumors in retalian to irradiation and altitude

All types of ovariantumorX-rayexposure(R)0ISO0ISOAltitude0LowHighLowHighHigh-lowLow-highHigh-lowLow-highNo.

ofmice"1

(1)3(2)65

(43)46(11)2(1)270

(46)55(17)%1.0

±l.O12.9

±1.651.2±4.537.1±4.31.91.860.9

±4.647.8±4.6Meanage

atdeath(wk)14273.383.9

±1.975.7±2.398.577.586.6

±1.880.3±1.4Body

wt(g)26.531.8

±0.826.3±0.631.525.730.6

±0.626.3±0.5No.

of micewithTubularadenoma2362813238Granulosacell

tumor1137144Luteoma116II122413

' Permanent exposure, low and high; temporary exposure, high-low and low-high.' Numbers in parentheses, number of mice with concurrent leukemia or lymphoma.

Mean ±S.E.

100-

a8 80-

60

£ 40,

aOT

IIIV

150 R X-RAYS

20 40 60 80 100 120 140

AGE (WEEKS)Chart 6. Age-specific incidence of ovarian tumors in irradiated females,

in relation to age and altitude, adjusted for intercurrent mortality bylife-table technique of Sach (36). •¿�,high altitude throughout (high): •¿�lowaltitude throughout (low); D, high altitude to low altitude (high-low); O,low altitude to high altitude (low-high).

months, hyperplasia of stremai elements was observed,which included granulosa cells, fibrous elements, luteincells, and downgrowths of germinal epithelium formingtubular structures penetrating into the stroma of the ovary.The proliferative changes, ranging from hyperplasia toneoplastic growth, frequently included mixtures of thesecomponents. Less commonly, proliferation of vascularelements of the ovary gave rise to hemangiomas or, morerarely, hemangioendotheliomas. Also observed were telan-giectases or blood-filled cysts, which filled the ovary almostcompletely, leaving only a narrow margin of stromalelements. The vascular changes were observed much morefrequently at high altitude than at sea level.

Paralleling the changes in the ovaries were changes in theendometrium, characterized by cystic and polypoid glandular hyperplasia.

Pituitary Gland and Adrenal Glands

In the pituitary gland, dilation of capillaries and sinusoidswas sometimes prominent at high altitude, but no otherchanges were detected.

In the adrenal gland, no changes in weight or morphologywere attributable to irradiation or altitude; however, atrophy of the glomerular zone, occasionally with focal nodularhyperplasia, was commonly noted in aging mice.

DISCUSSION

In general, the long-term changes in mice exposedtemporarily to high or low altitude tended to resemble thosecharacteristic for the altitude to which the mice wereultimately transferred and in which they spent the greaterpart of their lives; however, groups subjected to transitoryexposure at either altitude tended to be intermediate in theirlate effects between those maintained permanently at onealtitude or the other. These findings call to mind observations on flight personnel exposed intermittently to highaltitude in nonpressurized cabins, in whom the erythropoi-etic changes were intermediate between those typical forhigh-level subjects and those typical of low-level subjects(16, 17).

One exception to the above generalization was theenhancing influence of an initial transitory exposure to highaltitude on the incidence of myeloid leukemia, especially inX-irradiated mice. Such effects may be tentatively attributed to hypoxia-induced proliferative stimulation of thebone marrow, since other hemopoietic stimuli, such as theinjection of turpentine (40) and microbial contamination(42), have also been observed to enhance the induction ofmyeloid leukemia in X-irradiated mice of the same strain. Ifthis interpretation is correct, the failure of more sustainedhigh-altitude exposure to elicit the same effect is paradoxical; however, it is conceivable that the depression oferythropoiesis following descent of the mice to low altitudeenabled the shunting of pluripotent stem cells towardmyelopoiesis (44), thereby permitting expression of anotherwise latent leukemogenic transformation.

In mice returned to sea level after exposure to highaltitude, the decline in erythrocyte and reticulocyte valuesbelow the original basal levels may be tentatively ascribed tothe erythropoiesis-inhibitory factor that has been found in

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Table 5

Miscellaneous solid neoplasms in relation lo irradiation and altitude exposure

Permanent exposure at

High altitude Sea level

X-rayexposure

(R)0ISOSite and type ofneoplasmEndometrial

carcinomaEpidermoidcarcinoma(skin)Adrenocortical

adenomaAdrenalchromaffinomaFibrosarcoma(uterus)EndometrialcarcinomaOsteogenic

sarcomaAge

atdeath(wk)101535470519469Siteand type ofneoplasmEpidermoid

carcinoma(skin)Epidermoidcarcinoma(stomach)Epidermoidcarcinoma(cervix)Pituitary

adenomaPituitaryadenomaPituitaryadenomaLeiomyosarcoma(uterus)Mammary

glandcarcinomaMammary

glandcarcinomaAdrenocorticaladenomaAdrenocorticaladenomaEndometrial

carcinomaAge

atdeath(wk)93901099587659790389097101

Temporary exposure at

Low-to-high altitude High-to-low altitude

X-rayexposure(R)0Site

and type ofneoplasmMammary

gland carcinomaAge

atdeath(wk)81Site

and type ofneoplasmPituitary

adenomaPituitaryadenomaAge

atdeath(wk)141113

150 Pituitary adenomaPituitary adenomaMammary gland carcinoma

1029151

Pituitary adenomaOsteoma (spine)Endometrial carcinomaHemangioma (liver)Leiomyoma (uterus)

Adrenocortical adenomaHepatomaHemangioma (liver)Mammary gland carcinomaPituitary adenomaPituitary adenomaLeiomyosarcoma (uterus)Fibrosarcoma (ovary)Fibroblastic meningioma

98

1177895

7910985707486716375

the plasma of animals made polycythemic (21, 43) and inthe plasma of high-altitude natives brought down to sealevel (33, 34).

The enhancing influence of high altitude on the development of pulmonary neoplasms, reconfirmed in these experiments, occurred in spite of marked reduction of bodyweight. The effect remains to be explained, especially sincecaloric restriction per se has been shown to be anticarcino-genic (35, 39). The proliferative reaction observed in thepulmonary tissues of our mice at high altitude, associatedwith chronic inflammation, deserves investigation for itspossible contribution to the increased incidence of lungtumors.

Although the mechanisms by which radiation and altitudemay influence the pathogenesis of neoplasia and the other

conditions of interest remain largely speculative (32), it isnoteworthy that the serial sacrifice examinations in thisstudy have confirmed the occurrence of prompt and persistent thymic and lymphoid atrophy induced by high altitude.These changes may help to explain the observed inhibitionof lymphoid neoplasia in our high-altitude mice, in view ofearlier observations on the role of thymic atrophy and ofendocrine factors in lymphopoiesis and lymphoid tumorformation (7, 9, 10, 19, 22, 24). Hypoxie stress has beenshown previously to cause adrenal hypertrophy (1, 2, 5, 6,8), with thymic involution (8, 9), and stress-induced hyper-corticism has been observed to inhibit'leukemogenesis (20,

41) depending on thyroid function (37). Hypothroidism,likewise, whether induced by antithyroid substances orradiothyroidectomy, has also been found to inhibit the

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Influence of Altitude on Neoplasms in RF/Un Mice

Table 6Nonneoplaslic lesions

Mural thrombosis ofendocardium Nephrosclerosis Pneumonia

X-rayexposure(R)0ISO0ISOAltitudeLowHighLowHighHigh-lowLow-highHigh-lowLow-highIncidence(%)7.924.84.132.32.836.43.527.8Meanageatdeath(wk)97.080.048.568.9105.177.882.272.5Incidenceofmyocarditis(%)1.01.902.40.97.31.70Incidence(%)78.258.168.645.283.366.473.057.4Meanageatdeath(wk)94.991.082.976.592.781.983.478.4Incidence(%)13.841.99.135.513.931.813.030.4Meanageatdeath(wk)71.669.770.164.683.470.084.210.6

Table 7Weights of body and lymphoid organs

X-ray

exposure(R)0

0ISO

ISOAltitudeLow

HighLowHighLow

HighLowHighTime131

days1-31 days

1.5-12mo.1.5

12mo.1-31

days1-31 days

1.5-12mo.1.5-12mo.Av.

body wt

(g)29.22

27.1431.931.3629.00

27.3732.5931.46Av.thymus

wt(g)0.098

0.0760.0860.0520.095

0.0750.0730.062Av.thymus/

body wtratio0.0034

0.00280.00270.00170.0033

0.00270.00220.0020Av.lymph

nodeswt

(g)0.156

0.1550.1790.1470.140

0.1520.1660.150Av.lymph

node/body wtratio0.0053

0.00570.00560.00470.0048

0.00550.00510.0048Av.spleen

wt

(g)0.187

0.2060.2410.1930.193

0.2160.2050.214Av.spleen/

body wtratio0.0064

0.00760.00750.00610.0066

0.00790.00630.0068

induction of lymphoid tumors by radiation (11, 12) and toinhibit the enlargement of the thymus and lymph nodesproduced by castration and adrenalectomy (22, 37). Hypo-thyroidism, which may be induced by hypophysectomy (14,15), has also been shown to increase altitude tolerance inrats (11, 38).

From the present results it may be inferred that the effectsof long-term high altitude exposure arise chiefly frominfluences on late stages in neoplasia and in the pathogenesisof other age-related lesions, as opposed to early stages. Thepossibility that the observed effects on neoplasia may bemediated in some measure through stress-induced impairment of the immune response deserves to be explored. Onthe other hand, effects on the cardiovascular and hemopoi-etic systems clearly may also be related to physiologicalchanges observed in these systems associated with theirlong-term adaptation to hypoxia.

The effects of long-term exposure to high altitude observed in this study confirmed those noted previously (32),consisting principally of polycythemia, lymphoid atrophy,impairment of weight gain, reduction in life-span, decreasein the cumulative incidence of spontaneous and irradiation-induced lymphoreticular neoplasms, and increase in theage-specific and cumulative incidence of pulmonary neoplasms, angiomatoid and telangiectatic lesions, endocardia!thrombosis, myocarditis, and pneumonia.

Similarly, the long-term effects of irradiation on life-span,body weight, blood picture, and incidence of neoplasmsresembled those noted previously (23, 32).

ACKNOWLEDGMENTS

We are grateful to the Cerro de Pasco Corporation, Cerro de Pasco.Peru, for facilities to establish the High Altitude Laboratory in Moroco-

cha, Peru, and to Purina Peru, S.A., for the generous food supply for theanimal colony.

We are also grateful to Francisca Cardenas and Olga Lopez fortechnical assistance.

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1974;34:328-336. Cancer Res   Pablo Mori-Chavez, Arthur C. Upton, Maximo Salazar J., et al.   and X-ray-induced Neoplasms in RF/Un MiceHigh-Altitude Exposure on the Pathogenesis of Spontaneous Influence of Transitory, as Compared with Permanent,

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