prolactin receptors in mammary carcinoma cells1prolactin receptors in mammary carcinoma cells...

[CANCER RESEARCH 34, 758-763, April 1974]

SUMMARY

Three types of experimental mammary carcinomas, the7 , 12-dimethylbenzanthracene-induced and R3230AC carcinomas in the rat and the C3HBA carcinoma ofthe mouse,were assayed for prolactin receptor activity. The prolactinâ€˜25I-bindingactivities of particles derived from the 7, 12-dimethylbenzanthracene-induced and R3230AC carcinomas exhibited the properties of high affinity (Kd = 7.1 xl0@'Cand 6.0 x l0'C M, respectively), low capacity, saturability at levels for physiological saturation, and hormonalspecificity. However, two other hormones, placental lactogen and growth hormone, exhibited significant competitivedisplacement of bound prolactin-'251 in proportion to theirknown lactogenic activities. The specific prolactin-125 I-binding activities of these carcinoma cells were found to reside inthe plasma membrane and were inactivated by treatmentwith trypsin or by heating. These properties ofthe prolactinâ€˜251-bindingactivities of these carcinomas are identical tothose of the prolactin receptor of the mammary alveolarcells of rats and mice.

Quantitative assays of receptor activities in these carcinomas revealed three individual patterns. The 7, 12-dimethylbenzanthracene-induced carcinoma, which is known to becharacteristically dependent upon prolactin for growth,exhibited a range of specific activities which varied between30 and 80% of the lactational mammary gland. TheR3230AC carcinoma, which has been shown to be prolactinresponsive for milk protein synthesis but not for growth,had a tissue concentration of prolactin receptor activity thatwas approximately 15% of that of lactational mammarygland. The C3HBA carcinoma, a relatively autonomoustumor, had no detectable prolactin receptor activity. Theresults provide evidence for the existence of prolactinreceptors in certain mammary carcinoma cells and suggestthat the degreeof prolactin dependenceof suchcarcinomasmay be characterized by the relative numbers of prolactinreceptors present.

INTRODUCTION

Environmental influences such as hormones, growthfactors, and sensitized lymphocytes may be important

L This work was supported by USPHS Grant CA 12904 from the

National Cancer Institute and by Grant VC-62B from the AmericanCancer Society, Inc.

Received September 24, 1973; accepted December 20, 1973.

factors that determine the rate of growth of breast carcinomas. A complete characterization of the hormonal influences that regulate a given cancer cell type would seemrequisite to a final interpretation of the roles of specifichormones in the regulation of its growth. Previous studieshave shown the importance ofestrogenic hormones (1, 9, 18,19, 23, 29), the mammary growth factor (5, 30), andprolactin (4, 6, 10) in the induction or growth of certainexperimental breast cancers.

The recognition of hormone receptors in target cells haspermitted analysis of hormonal dependence of cells in termsof intrinsic molecular properties of the receptor, a charactenistic that emphasizes the regulatory role of the hormoneon intracellular processes and distinguishes it from otherhost or systemic factors. Recently, the prolactin receptor ofmammary alveolar cells has been identified and partiallycharacterized (4â€”6,8, 3 1). The properties of the prolactinbinding activity prepared from normal mammary tissue arethose anticipated on theoretical grounds for a hormonereceptor: (a) it binds prolactin with high affinity; (b) itexhibits a requirement for a high degree of structuralspecificity in the lactogenic hormones that it binds; (c) itexists in small numbers in the target organ and thusrepresents a low binding capacity for the number ofprolactin molecules that are bound; (6) it binds prolactinrapidly and the population of receptors is easily saturatedwith this hormone, but binding is not irreversible; (e) it has arestricted distribution among the various cell types of theanimal, being present in highest concentration in knowntarget organs, particularly the mammary gland; (I) it ismacromolecular in nature and protein in large part; (g)binding of prolactin and activation of mammary alveolarcell processes increase in parallel over the physiologicalrange of prolactin concentrations, with saturation of bothactivities occurring at the same concentration level. Thesecharacteristics of the prolactin â€œreceptorâ€•support theproposition that it mediates the graded biological responsesof mammary cells to changing concentrations of prolactin.

In the present study several experimental mammarycarcinomas have been characterized with respect to thepresence of prolactin receptor activity. The results provideadditional information with which these â€œmodelâ€•carcinomas may be characterized, with respect to both the extent towhich they deviate from nonneoplastic mammary cells anda range of heterogeneity over which individual tumors of thesame presumed origin may vary. The results also suggestthat the degree of prolactin dependence of a given type ofmammary carcinoma may be predicted from the tissueconcentration of prolactin receptor activity present.

CANCER RESEARCH VOL. 34758

Prolactin Receptors in Mammary Carcinoma Cells1

Roger W. Turkington

St. Luke's Hospital Research Foundation, Milwaukee, Wisconsin 53215

on April 12, 2020. © 1974 American Association for Cancer Research. cancerres.aacrjournals.org Downloaded from

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Prolactin Receptors in Mammary Carcinoma Cells

MATERIALS AND METHODS

Animals. Mammary carcinomas were induced by theinjection of a lipid emulsion of DMBA2 (0.5%, w/w) into acaudal vein of female Sprague-Dawley rats. The dosage ofDMBA was 35 mg/kg, and this amount was given in 6weekly injections beginning at 50 days of age. R3230ACmammary carcinomas were maintained by serial transplantation in 90-g female Fischer rats, and C3HBA mammarycarcinomas were serially transplanted in female C3H/HeJmice. Normal mammary tissue was derived from theabdominal, inguinal, and thoracic mammary glands ofSprague-Dawley or Fischer rats or C3H/HeJ mice afterapproximately 10 days of lactation.

Chemicals and Hormones. For iodination of prolactin,carrier-free sodium-'25I was obtained from New EnglandNuclear, Boston, Mass., lactoperoxidase was obtained fromCalbiochem, Los Angeles, Calif.; and diluted hydrogenperoxide was obtained from Mallinckrodt Chemical Works,St. Louis, Mo. The following hormones were used: ovineprolactin (NIH-Ps9, 30.1 units/mg); rat prolactin(NIAMD-RP-l, 30.0 units/mg); human growth hormone(NIH-GH-HS1394); human placental lactogen (NIAMD);ovine follicle-stimulating hormone (NIH-FSH-S9); ovineluteinizing hormone (NIH-LH-Sl8); bovine thy roidstimulating hormone (Sigma Chemical Co., St. Louis,Mo.); bovine zinc insulin (Eli Lilly and Co., Indianapolis,Ind., 23.6 units/mg); synthetic adrenoconticotropic hormone (Ferning AG, Stockholm, Sweden); and oxytocin(Sandoz Pharmaceuticals, Hanover, N. J.). The ovine andrat prolactin preparations were each found to contain asingle protein on polyacrylamide gel electrophoresis (28), aspreviously described (2, 3, 7, 21).

Trypsin, RNase, and DNase (pancreatic, 5 times crystallized) were obtained from Worthington Biochemical Corp.,Freehold, N. J.

Iodination of Prolactin. lodination of ovine prolactin wascarried out by the lactoperoxidase method, as previouslydescribed (5, 27). Prolactin-'251 was purified by gel filtrationthrough Sephadex G-75 and by chromatography on DEAEcellulose (5), which separated it completely from a smallamount of iodinated lactoperoxidase formed during theradioiodination reaction. The final prolactin-'25I obtainedwas found to have high biological activity (>20 units/mg)and was calculated to have specific activities rangingbetween 100 and 175 @tCi/@tg.Each preparation was obtamed as a single, sharp peak by chromatography onDEAE-cellulose (5) and constituted a single radioactiveprotein on polyacrylamide gel electrophoresis (28).

Preparation of Tissue Particles. Animals were killed bycervical dislocation, and tumors or normal mammaryglands were quickly excised and chilled. For the preparationof a crude, low-speed pellet, the tissues were minced withscissors for 10 mm in iced 0.001 M sodium bicarbonate (pH7.0, 1 g of tissues per 10 ml), an equal volume of 10%sodium citrate (pH 7.4) was added to the suspension, and

2 The abbreviation used is: DM BA, 7 , 12-dimethylbenzanthracene.

the mixture was homogenized for 30 sec in an all-glasshomogenizer. The homogenate was then strained throughcheesecloth and centrifuged at 3000 x gay for 10 mm. Thepellet (derived from approximately I g of tissue) wasresuspended with a Vortex mixer in 10 ml of mediumcontaining 0.25 M sucrose, 10 m@iTnis-HCI (pH 7.4), 3mM MgCl2 , and 0. 1 M KCI. The particulate preparationwas centrifuged twice at 3000 x g5@for 10 mm, and thefinal pellet was resuspended in 2.0 ml of the same medium.Highly purified plasma membranes, nuclei, and nibosomeswere prepared by previously described methods (1 5, 34).The protein concentrations of these particulate preparationswere determined by the method of Lowry et al. (13).

Assay of Prolactin-binding Activity. The standard bindingreaction was made to contain 100 or 125 @.tgtissue protein,2.5 mg bovine serum albumin, 0.5 zmole NaCl, 5 .tmolesTnis-HCI (pH 7.4), and pnolactin-'25I (approximately 5 ng)in a total volume of 250 .tl. The reaction was carried out inpolyethylene microtubes, and the mixture minus tissueprotein was precounted to ensure equal amounts of prolactin-'251 in each of the tubes. Unless otherwise stated, thebinding reactions were initiated by the addition of the tissueprotein and the incubation was carried out at 4Â°for 30 mm.The binding particles were then separated by centrifugation,washed, and counted in an Autogamma counter at 30%efficiency, as previously described (4, 5).

RESULTS

Binding reactions of prolactin-'251 with rat mammarycarcinoma particles were carried out under conditions ofloading and washing that had been previously used toidentify prolactin-specific binding in mouse mammarygland particles (4). Following the binding step, prolactin1251 that was bound to low-affinity binding sites or merely

trapped in the sedimented particles was rapidly removedduring two 15-mm incubations in prolactin-free medium.The proportion of the total residual binding that wasdisplaceable during a subsequent incubation with nativeprolactin but not with luteinizing hormone was designatedas the prolactin-specifIc binding fraction. Binding of thisfraction was complete within 10 mm at 4Â°in all ofthe tissuepreparations studied.

Chart I shows the results of competitive displacementreactions at various concentrations of unlabeled competitorpolypeptide hormones. In these experiments prolactin-'25Iwas allowed to bind to particles that were subsequentlywashed free of unbound and loosely bound iodinatedhormone. Incubations with native prolactin in concentrations equivalent to the physiological range then resulted insignificant dissociation of the labeled prolactin from theDMBA-induced mammary carcinoma particles. The displacement reaction was sensitive to 25 ng or less of prolactinper ml. A saturation level for competitive displacement wasobserved between 0.5 and 1.0 sg/ml, and incubation withsupraphysiological concentration of prolactin (greater than1.0 @tg/ml)produced no further displacement of the labeledprolactin . Luteinizing hormone and thyroid-stimulating

APRIL 1974 759



Specific binding(cpm/ 100 @igprotein)DM

BA-inducedR3230ACPolypeptide(1000 ng/ml)carcinomacarcinomaNone9,870

3,100Prolactin(ovine)2,0002,100Prolactin(rat)2, 1502,000Growth

hormone5,2002,700Placentallactogen4,7002,210Follicle-stimulating

hormone9,4003,080Thyroid-stimulating-hormone9,9803,010Luteinizing

hormone9,4003,270Adrenocorticotropichormone10,0002,980Insulin10,900

3,020AngiotensinII9,7003,510Oxytocin9,820

3,040

Roger W. Turkington

hormone did not significantly displace the labeled pnolactinduring an equivalent incubation period. This specific effectof prolactin was very similar to that observed in parallelstudies with particles derived from rat mammary gland andwhich utilized the same range of hormone concentrations.

Table I shows the results of further studies on thehormonal specificity of the prolactin-binding activity of rat

zLuI-0a.

b

a.0

0zD0

H

z

-j0a.

Chart 1. Competitive displacement of prolactin-'25l from subcellularparticles by various native hormones. Subcellular particles were allowed tobind prolactin-'251 during an incubation for 30 mm at 4Â°.The particleswere separated by centrifugation and incubated in prolactin.free mediumduring 2 successive 15-mm incubation periods. The washed bindingparticles were then incubated with various concentrations of polypeptidehormones for an additional period of 180 mm, at which time they wereseparated by centrifugation and assayed for residual radioactivity. Eachpoint represents 2 or 3 observations, and each value is plotted as the meanÂ± S.D. TSH, thyroid-stimulating hormone; LII, luteinizing hormone.

Table I

Competitive displacement by various polypeptide hormones ofprolactin-I'2' bound to mammary carcinoma particles

The results are representative of 2 experiments with carcinomas ofeach type. Each value represents the mean of closely agreeing triplicatedeterminations.

mammary carcinomas. Approximately 80% of the totalbinding of prolactin-'25I to these DMBA-induced carcinoma particles was displaced specifically by native prolactin. Only placental lactogen and growth hormone, polypeptides with intrinsic lactogenic activities, also displacedbound prolactin. The relative binding activities of prolactin,placental lactogen, and growth hormone were similar to therelative binding activities of these hormones observedpreviously with the prolactin receptors of mouse mammarygland. In contrast to the DMBA-induced carcinomas,subcellular particles of the R3230AC carcinoma exhibitedrelatively low prolactin-binding activities. In this expeniment, only approximately 35% ofthis binding activity couldbe shown to be specific. Significant competitive displacement was detected with placental lactogen and possiblygrowth hormone, but not with other hormones tested.

The affinity of the prolactin-'25I-binding activity in theDM BA-induced carcinoma was estimated by Scatchardanalysis (22). In relating the amount of prolactin-'25I boundto that amount that remained free, 2 assumptions weremade. First, because of the relative lability of the bindingactivities, the incubation times werenot extendedto a pointof absolute equilibrium. Rather, it was assumed thatequilibrium was approached at a point after which nofurther increment of binding occurred, and the reactionswere terminated at this time. Second, binding affinity wasmeasured only at â€œphysiologicalâ€•concentrations of prolactin. As in previous studies, rechromatography of the boundprolactin-'25I and analysis of the free fraction provided noevidence of degradation or loss of biological activity(bindability) during the reaction. Chart 2 compares dataobtained with a DMBA-induced carcinoma and with normal rat mammary gland by these procedures. The dissociation constant, Kd , calculated from these results was 7. 1 xl0@ M. This value, indicating a relatively high order ofbinding affinity in the neoplastic tissue, was identical to thatobserved in the normal rat mammary gland. The intercepton the abscissa is an estimate of the number of binding sites.Assuming that I molecule of binder binds I molecule ofprolactin, the number of binding sites in the DMBAinduced carcinoma preparation is 14.5 x 10 13 mole/mgprotein, while the mammary gland preparation has 15.5 xl0 â€˜@mole/mg protein. A similar analysis ofthe R3230ACcarcinoma binding activity yielded a Kd value of 6.0 x l0@M and approximately 2.0 x l0@ mole of binding sites permg protein.

The nature of the receptor-like binding activity in theDMBA-induced carcinoma and in the R3230AC carcinomawas further characterized in several additional experiments.As shown in Table 2, the prolactin-specific binding activitiesof these carcinoma particles were markedly reduced bypretreatment with trypsin but not by pretreatment withDNase or RNase. Heating at 90Â°for 10 mm completelydestroyed this activity in both preparations.

The cytological localization of the prolactin-bindingactivities of these carcinomas was determined by assayinghighly purified organelle preparations. As shown in theresults in Table 3, prolactin-specific binding was detected inthe plasma membrane preparations from both carcinomasbut not in purified nuclei or ribosomes.

0@ 0.01 0.025 0.05 0.1 0.25 0.5 .0@@ 0

CONCENTRATIONOF UNLABELED HORMONE Cpg I ml)

760 CANCER RESEARCH VOL. 34



Binding (cpm/100 @gprotein)DM

BA-inducedcarcinomaR3230ACcarcinomaPreparationTotal

SpecificTotalSpecificPlasma

membranesNucleiRibosomes16,800

13,9002,100 04,100 03,700

2,4002,800 03,400 0

AnimalTissueSpecific

binding(cpm/ 100 ;zgprotein)Crude

PlasmaparticlesmembranesI

234

5

6789

1011Carcinoma

CarcinomaCarcinomaCarcinomaCarcinomaCarcinomaCarcinomaCarcinomaCarcinomaCarcinomaGlandGlandGlandl

23456789

108,740

12, 1204,3002,9608, 100

9, 10020,00017,00025.00022,000

10,2009,700 31,000

10,500 29,700

Specific binding (cpm/I00 @gprotein)DMBA-inducedR3230ACTreatmentcarcinomacarcinomaNone63901200Trypsin1930610RNase64001170DNase

Heating, 900 for 10 mm6520 N.S.Â°1010 N.S.


Table 4 represents a quantitative comparison of theprolactin-specific binding activities determined in a series ofDMBA-induced carcinomas with those observed in nonneoplastic mammary tissues prepared and assayed underidentical conditions. Two types of procedureswere usedtoprepare the binding activity. Preparations of the crudeparticulate fraction appeared to offer less opportunity forloss of total binding activity as a consequenceof receptordegradation. Preparations of the plasma membranes represented a much purer system and provided a comparisonbetween specific cell organdIes. In general, these types ofpreparations yielded similar results. The DMBA-inducedcarcinomas exhibited a range of activities varying betweenapproximately 30 and 80% of the values observed in normaltissues. In 1 case the neoplastic tissue actually exceeded thelactational mammary gland in prolactin receptor activityconcentration. Significant variability was also observedbetween carcinomas arising in the same animal. Similarassays of the specific activity of prolactin receptor activitywere also performed on the R3230AC and C3HBA carcinomas. Table 5 compares these results with values obtainedwith the corresponding nonneoplastic mammary tissuesprepared and assayed under identical conditions. Because ofthe relatively low levels of specific binding activity in thepreparations derived from these carcinomas, the totalbinding activity is also listed for each tissue. The R3230AC

Table 3

Distribution 1251-binding activity in preparations of variouscell organelles from mammary carcinomas

LuLuU.

0z0

Chart 2. Scatchard plot ofthe binding ofprolactin-'25I (specific activity,135 zCi/@.ig) to subcellular particles derived from rat mammary gland(closedsymbols) or DM BA-induced mammary carcinoma (open symbols).The binding reaction was carried out at 4Â°for 30 mm. Bound and freefractions were then separated after sedimentation of the binding particles at10,000 x g@ for 5 mm. Each assay tube contained 125 @gprotein. Theprolactin-125I used was at least 75% bindable, as determined underconditions of tissue protein excess. The bound/free values are the ratio ofmoles of prolactin bound per mg protein binder per molar concentration offree prolactin. The bound values are expressed as moles of prolactin. Theapparent dissociation constant Kd was determined for each preparationfrom the Scatchard equation: bound/free = I/K,, (binding sites) â€”(bound).The experimental points shown are extrapolated to the x intercept.

Table 2

Effects of various treatments on the prolactin-'261-spec:jlc bindingactivity ofparticles prepared from rat mammary carcinomas

Tissue particles (I mg/mI) were incubated at 37Â°for 10 mm in 50 mMTris-HCI (pH 7.4) with or without addition of I of the above enzymes(final concentration, 200 @g/ml)or at 90Â°for 10 mm. The particles werethen washed 3 times and assayed for prolactin-'25I-specific binding activity. The bindability of the unbound prolactin-125I in the initial bindingreaction with enzyme-treated particles was equal to that of the prolactin1251incubated with control particles, as indicated in a subsequentassay.Furthermore, the unbound prolactin-1251 from the incubation with trypsin-treated particles yielded specific binding levels of 6600 cpm/ 100 @zgfor DMBA carcinoma protein and 1000 cpm/@.tgfor R3230AC carcinomaprotein in subsequent assays with untreated particles.

2 3 4 5 6 7

PROLACTIN - l251 BOUND(IO@ M)

Table 4

Specific binding ofprolactin- 125Jto crude pellet particles or purifiedplasma membranes prepared from DMBA -induced mammary

carcinomas or Sprague-Dawley rat mammary glands

a N.S., not significant.

APRIL 1974 761



AnimalTissueProlactin-

@ binding(cpm/ 100 @gprotein)Total

SpecificI

23456789

10I I12Rat

mammary glandRatmammaryglandR3230AC carcinoma IR3230ACcarcinoma2R3230AC carcinoma 3R3230ACcarcinoma4C3H mouse mammary glandC3H mouse mammary glandC3H BA carcinoma IC3H BA carcinoma 2C3H BA carcinoma 3C3H BA carcinoma 49,300

8,32010,100 9,0003,700 1,7003,100 1,2001,900 1, 1002,900 1,400

11,000 8,37012,200 9,7001, 160 N.S.1, 200 N.S.

900 N.S.2,010 N.S.

Roger W. Turkingion

carcinomas exhibited a relatively low level of specificbinding, at approximately I5% of the values obtained for thenonneoplastic tissue. In contrast to the DMBA-inducedcarcinomas, this neoplasm exhibited a relatively uniformlevel of activity among individual tumors. No significantprolactin-specific binding was detected in the C3HBAcarcinomas tested. To rule out the possibility of an inhibitorof binding or of a degradative activity in any of thesepreparations, admixtures were made ofthe binding preparations derived from the normal and neoplastic tissues.In allcases the binding activities of such admixtures representedadditive effects of the individual preparations.

DISCUSSION

These studies demonstrate the presence in 2 experimentalrat mammary carcinomas of prolactin-binding activitieswith the characteristics of the prolactin receptor. Thebinding activity in both the DMBA-induced and R3230ACcarcinomas exhibited a high affinity for prolactin, as shownby the dissociation constants. Binding was highly specificfor prolactin. As found for the prolactin receptor of mousemammary gland (4, 5, 3 1), 2 other lactogenic hormones,placental lactogen and growth hormone, were also bound inthe competitive displacement reactions, but to a lesserdegree. The prolactin-specific binding activities displayedeasy saturability and low capacity, and binding was reversible. The sensitivity to heat and proteolytic treatmentindicate that the prolactin-binding activity is macromolecular in nature and protein in part. The prolactin-bindingactivity was found in the plasma membrane, as in the case ofmammary alveolar cells. The range of prolactin concentrations for competitive displacement was in the physiologicalrange (Chart I ), and prolactin-specific displacement paralleled that observed with rat mammary gland receptoractivity under the same conditions. Previous studies with theprolactin receptor of mouse mammary gland demonstratedthat binding of prolactin paralleled activation of mammaryalveolar cell processes, such as the induction of the milkproteins casein and N-acetyllactosamine synthetase (4, 28).This final criterion for identification ofa hormone receptor,i.e. , activation of cellular processes as a consequence ofbinding, has not been fulfilled in these studies on ratmammary carcinomas. However, studies relating prolactinbinding to regulation of carcinoma cell division are currently being conducted in this laboratory.

The prolactin receptor-like activities of the DMBAinduced and R3230AC carcinomas were quantitativelycompared to those of rat mammary gland by binding assayscarried out under conditions of saturation of binding sites.The relative results of these studies agreed well withdeterminations of binding site concentration in each of thetypes of carcinomas as measured by Scatchard analysis(Chart 2; unpublished data). Determinations of the numberof binding sites was complicated by the probable presence ofan unknown amount ofendogenous, bound prolactin in eachof the tissue preparations. However, it is likely that the levelof endogenous prolactin was highest in the lactationalmammary gland, a circumstance that would not precludethe general conclusion that the nonneoplastic tissuegener

Table 5

Comparison of total and specific prolactin- â€œI-bindingactivities ofcrude pellet particles prepared from normal or neoplastic

mammary tissuesof the Fischer rat or C3H mouse

ally contained higher concentrations of receptor activitythan did the neoplastic tissues tested. The highest concentration of prolactin receptors in the carcinomas tested wereconsistently observed in the DM BA-induced carcinomas.This result is consistent with the well-demonstrated dependence of the vast majority of these carcinomas on prolactin(20, 35). A marked heterogeneity in the tissue concentrationof receptor activity was characteristic of this group ofmammary carcinomas, a finding consistent with the significant heterogeneity of growth rates, morphological features,and biochemical responses in vitro that have been observedpreviously with this carcinogen-induced carcinoma (8, 10).The R3230AC carcinomas consistently exhibited a receptoractivity that was approximately 15% of the activity observedin lactational mammary gland. This degree of cell differentiation is similar to the relative rates of milk proteinsynthesis (32) and to the relative tissue concentration ofestrogen receptor (8) previously observed in comparisonswith lactational tissue. The R3230AC carcinoma has beenshown to be responsive to prolactin in terms of induced milkprotein synthesis both in vivo and in vitro (17, 32) but not tobe dependent upon pituitary hormones for growth (9). Incontrast, the C3HBA carcinoma, which appears to be alargely autonomous carcinoma without known prolactindependence (1 1, 12, 16, 24, 25, 33), had no detectableprolactin receptor activity in these studies.

These observations thus illustrate 3 classes of mammarycarcinomas: those with high tissue concentrations of prolactin receptor and demonstrated dependence upon prolactinfor growth response; those with relatively low (i.e., 10 to15% of normal) levels of prolactin receptor and which areprolactin responsive but not prolactin dependent in overallgrowth responses; and those without detectable levels ofprolactin receptor and which function as apparently autonomous carcinomas as far as prolactin is concerned. Furtherstudies are now in progress to test this potential correlationin terms of the tissue concentration of prolactin receptorsand growth rates in response to changing serum concentrations of prolactin in individual neoplasms. Further investigation will also be required to determine whether theprolactin receptor activity of normal or neoplastic cells is

762 CANCER RESEARCH VOL. 34




heterogeneous in terms of its structure and functionalintegration into the plasma membrane. Such studies mayprovide new insights into the functional constitution of themammary carcinoma cell membrane and its interactionwith environmental influences.

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APRIL 1974 763



1974;34:758-763. Cancer Res Roger W. Turkington Prolactin Receptors in Mammary Carcinoma Cells

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