The Prostate 6:49-59 (1985)

Metabolic Action of Prolactin in Regressing Prostate: Independent of Androgen Action Cornelius Smith, Dean Assimos, Chung Lee, and John T. Grayhack

Department of Urology, Northwestern University Medical School, Chicago

The mechanism of the observed synergistic effect of prolactin and androgen on the lateral lobe of the rat prostate is not established. The observation that prolactin alone delayed the rate of loss of weight, protein, and DNA of the lateral lobe in castrated rats has led us to question the assumption that the effect of prolactin is produced by a modification of recognized androgen-induced intracellular changes. The present study was conducted to explore whether or not the sites of prolactin action in the rat prostate coincided with those recognized as the androgen effect. Two anterior pituitaries from female donors were grafted under the right renal capsule of adult male Sprague-Dawley rats. Seven days later, bilateral orchiectomy and unilateral nephrectomy were performed in these rats. In one half of the animals, the kidney bearing the pituitary grafts was removed. In the other half, the contralateral kidney was removed. Seven days following the orchiectomy-nephrectorny, animals bearing the pituitary grafts had a higher level of serum prolactin (93 f 7 nglml, mean f SE) than in those without the graft (26 f 3 ng/ml). This condition of hyperprolac- tinemia was associated with the delay of castration-induced regression in the lateral prostate. The rate of protein degradation, as judged by the amount of radioactivity remaining in the tissue following a single i.v. pulse of 3H-leucine 24 hr before orchiectomy-nephrectomy, was significantly slower in the lateral prostate in graft-bearing animals than in those without grafts. However, the rate of protein synthesis, as judged by both in vitro and in vivo incorporation of 3H-leucine into the protein fractions of the prostate, was not significantly different between the two groups. Nor was the total activity of cathepsin D in the lateral prostate different. These results were not consistent with parameters known as the effect mediated through androgen-controlled mechanisms. We therefore conclude that the action of prolactin in the regressing rat prostate is probably not mediated through the action of androgen.

Key words: prostate, prolactin, protein metabolism, cathepsin D, androgen, castration

INTRODUCTION

Hyperprolactinemia, produced by transplanting pituitary grafts under the renal capsule, retards the rate of loss of weight and protein following castration in the prostate of rats [ 13. This effect of prolactin is most pronounced in the lateral lobe of the prostate. Although the mechanism of this prolactin action in the prostate remains unclear, the probability that modifications of androgen-dependent anabolic or cata- bolic events are responsible seems likely to us and to other investigators [ 2 4 . The possibility that a minute amount of residual endogenous androgen present in the prostate at the time of castration acts synergistically with the elevated levels of prolactin to cause a delay in prostatic regression seems to warrant primary consider- ation. However, a previous study indicating that a potent nonsteriodal antiandrogen,

Presented at the Workshop on Prostate Cancer: A Decade of Progress and New Horizons held in Bethesda, Maryland, January 9-11, 1984.

Address reprint requests to Dr. Chung Lee, Department of Urology, Northwestern University Medical School, Chicago, IL 60611.

0 1985 Alan R. Liss, Inc.

50 Smith et al

Flutamide, failed to inhibit the effect of prolactin on this delay of castration-induced regression in the rat prostate [5] led us to consider the possibility that prolactin may act independent of testosterone or, at least, at a site distal to the binding of dihydro- testosterone (DHT) with androgen receptors. Further investigation to delineate the effect of prolactin on events following the formation of DHT-receptor complex in the rat prostate seems desirable.

If the action of prolactin in the regressing prostate is to enhance the effect of the minute amount of residual androgen in the tissue, many metabolic features that are characteristic of androgen stimulation in the rat prostate should be evident. These androgen-induced features in protein metabolism include (a) an increase in the rate of protein synthesis [6-81, (b) a decrease in the rate of protein degradation [8], and (c) a decrease in the activity of proteolytic enzyme, cathepsin D [9]. The present study was conducted to determine the effect of prolactin on these androgen-controlled parame- ters in the rat prostate.

MATERIALS AND METHODS Animals and Surgical Procedures

Sprague-Dawley rats purchased from Harlan Industries (Indianapolis, IN) were kept in large plastic cages with five animals per cage. They were housed in an air- conditioned room (23°C) with 14 hours of illumination per day (05:OO-19:OO) and fed Purina Rat Chow and tap water ad libitum. The basic surgical scheme was camed out according to the procedure described by Kolbusz and associates [ 11. Briefly, two pituitaries from female rats weightng 175-200 g were transplanted under the right renal capsule of male rats weighing 300-325 g. Seven days later, all animals were castrated and a unilateral nephrectomy was done. In half of the animals the left kidney (non-graft-bearing) was removed; in the other half the right kidney (graft-bearing) was removed. Since animals in the former group retained the pituitary grafts, they should have higher levels of circulating prolactin and their prostates should regress at a slower rate than those in the latter group [l].

Experiment 1: In vivo retention of 'H-leucine from the rat prostate: Estimating the apparent rate of protein degradation based on the rate of in vivo release of 3H-leucine by the single pulse technique has been characterized previously for the rat prostate [S]. Following a single pulse of 3H-leucine in male rat, the radioactivity present in the protein fraction of the prostate could be accounted for by 3H-leucine. Reutilization of incorporated 3H-leucine was found to be insignificant in the rat prostate. The same approach was used in the present study. A total of 35 male rats were used in this experiment. Six days after the pituitary grafting, each rat received a single intravenous injection of a sterile saline solution containing 3H-leucine with a concentration of 100 pCi per ml at a dose of 15 pCi per 100 g body weight (3H4,5- L-leucine, 56.5 Ci per m o l e , New England Nuclear, Boston).

Twenty-four hours later, bilateral orchiectomy and unilateral nephrectomy were camed out. At the same time, a group of 13 3H-leucine-injected animals were sacrificed to establish the initial activity in the tissue. The three lobes of the prostate (ventral, lateral, and dorsal) were dissected separately, according to the procedure of Jesik et a1 [lo]. The prostates were weighed and placed in 10% trichloroacetic acid (TCA) for protein purification. Eight days after the 3H-leucine pulse (or 7 days after orchiectomy and unilateral nephrectomy), the remaining 22 animals were sacrificed

Prolactin Action in Regressing Prostate 51

and three lobes of the prostate were collected for protein extraction. The amount of radioactivity remaining in the protein fraction of the prostate, expressed as cpm/ prostatic lobe, served as an indication of the rate of protein degradation. The greater amount of radioactivity remaining in the prostate the slower being the rate of degradation.

Experiment 2: In vitro incorporation of 3H-leucine by the rat prostate: The technique of in vitro incorporation of 3H-leucine by prostatic tissue pieces was employed as a means to assess apparent protein-synthesizing ability [8]. One group of six animals were sacrificed by decapitation on the day of castration (or 7 days after pituitary grafting) and two groups of four each were sacrificed 7 days postcastration. Three lobes of the prostate were dissected separately. Each lobe was placed in medium 199 (GIBCO, Grand Island, NY), which was prewarmed to 37"C, and was cut with a scalpel into 2 mm3 pieces. The tissue pieces were then incubated in 4 ml of Medium 199 containing 3H-leucine (0.5 pCi per ml) for 2 h under 95 % oxygen and 5 % carbon dioxide at 37°C in a shaker water bath as described by Lee [ 111 and Lee et al [ 121. The reaction was stopped by placing the tissue in 10% TCA. The tissue was processed for protein extraction and the amount of radioactivity present in the protein fraction, expressed as cpm/mg protein/hr, served as an indication of the rate of protein synthesis.

Experiment 3: In vivo incorporation of 3H-leucine by the rat pros- tate: Results of our preliminary study indicated that, following a single i.v. pulse of 3H-leucine in rats, the amount of radioactivity incorporated into the protein fraction of the prostate reached a peak within 24 h. The radioactivity thus incorporated by the prostate during this 24-h pulse period was used as another reference to indicate the protein-synthesizing ability of the tissue. The 13 animals used at 1 day before castration were the same animals used at the initial stage of Experiment 1. Six days following castration two additional groups of 13 rats each received a single i.v.injection of the same sterile saline solution containing 3H-leucine as described earlier. They were sacrificed 24 h after the 3H-leucine pulse and their prostates were removed for process as described. The amount of radioactivity present in the protein fraction was expressed as c p d m g protein.

Extraction of prostatic protein and determination of radioactivity: The prostatic tissue was homogenized in 10% TCA with a Polytron (model PT-10, Brinkman Instrument, Westbury, NY). The homogenate was heated to 90°C for 30 min and the resulting precipitate was washed twice with 5 ml of a mixture of ethano1:ether (2:l by volume) and thereafter once with 5 ml of ether. The washed precipitate was then dissolved in 2 ml of 0.4 N NaOH. Duplicated aliquots of 0.5 ml of this alkaline protein solution were counted with 10 ml of scintillation fluid (Dimil- ume, Packard Instrument Co, Downers Grove, IL) in a Beckman model 8100 liquid scintillation counter with a counting efficiency for tritium of 48% which was estab- lished by using a 3H-water standard under the above described conditions. Protein determination was performed on a portion of the remaining solution according to the procedure of Lowry et al [13]. Results were expressed either as cpm per mg protein or cpm per prostate lobe.

Experiment 4 Assay for cathepsin D activity in the prostate: One group of five rats was sacrificed on the day of castration-nephrectomy and two groups of ten rats each (one hyperprolactinemic group and one control group) were sacrificed 7 days later. Three lobes of the prostate were removed and stored in liquid nitrogen

52 Smithetal

freezer. Cathepsin D activity in the prostatic tissue was determined by the method of Anson [ 141 and of Ferguson and associates [ 151. The procedure was carried out with minor modifications [9,16]. Each prostatic lobe was allowed to thaw in the refrigera- tor and was homogenized in 3 ml of an ice-cold solution containing 1 % NaCl and 2% n-butanol using a glass-glass homogenizer. A volume of 10% Triton-X100 was then added to this suspension to bring the final concentration of Triton-X100 to 0.5%. This preparation was placed in the refrigerator overnight and was then centrifuged for 20 min at 10,OOO rpm (Sorvall RC-SB, with SE-12 Rotor). An aliquot of 0.2 ml of the Supernatant fraction was added to 2 ml of a hemoglobin buffer substrate which was prewarmed for 5 min in a 40°C water bath. The substrate consisted of a 4: 1 mixture of 0.83% bovine hemoglobin solution (twice crystallized and lyophilized, Sigma Chemical Co, St. Louis) and 1.35 M acetate buffer, pH 3.7. The incubation was carried out for 30 min at 40°C and the reaction was stopped by adding 2 ml of cold 10% TCA. For the zero-time tubes, the aliquot of 0.2 ml of the supernatant, instead of at the beginning of the incubation, was added to the hemoglobin substrate at the end of incubation after the reaction was stopped by 10% TCA. The incubated samples were placed in the refrigerator overnight and were then centrifuged at 3,000 rpm for 10 min. The clear supernatant was filtered through a 0.45-pm Millipore filter (type HA). The filtrate was measured in a spectrophotometer at the wavelength of 280 nm. A sample having its filtrate to exhibit an increase in absorbance of 0.01 above that of the zero-time tubes was considered of having one unit of cathepsin D. The protein content in the prostatic homogenate was measured following TCA precipitation and the activity of cathepsin D was expressed as units/mg protein.

Prolactin dererminurions: The levels of serum prolactin were measured by radioimmunoassay using the NIADDK rat prolactin kit, with NIADDK rat PRL-RP- 3 as the standard [ 171.

Data analysis: All numerical values were expresed as mean f standard error. The Student’s t-test was employed to detect differences in means of different param- eters between castrated animals bearing the pituitary grafts and those without the grafts. For the study of in vivo decay of the labeled protein in the prostate, the amount of radioactivity per prostate lobe (cpdprostate) was adjected for the prostatic wet weight in each group by a covariance analysis prior to statistical analysis [18].

RESULTS Serum Prolactin Levels

A total of 24 animals were sacrificed at 1 week following pituitary grafting. The average concentration of serum prolactin in these animals was 92.7 f 7.6 ngl ml. The remainder of the animals received orchiectomy and unilateral nephrectomy at that point. One week later, 38 animals bearing the pituitary grafts continued to have an elevated level of serum prolactin (93.8 f 6.8 ng/ml). The other 38 animals were without the grafts. The average serum prolactin at a level normally observed in nongrafted animals (26.2 f 3.4 ng/ml) was significantly lower (P < 0.01) than that of the graft-bearing animals.

In Vivo Retention of 3H-Leucine in the Rat Prostate Table I shows that, at 7 days postcastration, rats with pituitary grafts had

significantly (P < 0.01) larger lateral lobes (both wet weight and protein content)

TABL

E I. Effect o

f Hyp

erpr

olac

tinem

ia on

Wet

Wei

ght,

Prot

ein

Con

tent

, and

the

Am

ount

of R

adio

activ

ity R

emai

ning

in T

hree

Lob

es of

the

Pros

tate

in Rats

Tha

t Rec

eive

d a

Sing

le P

ulse

of 3

H-L

euci

ne 24

Hou

rs B

efor

e Cas

trat

ion Pr

osta

te p

rote

in

Rad

ioac

tivity

N

umbe

r B

odyw

t Pr

osta

tic w

t (m

g)

(mgl

lobe

) (c

pm/lo

be)

7 Tr

eatm

ent g

roup

s of

rat

s (g

) V

entra

l La

tera

l D

orsa

l V

entra

l La

tera

l D

orsa

l V

entra

l La

tera

l D

orsa

l

Sacr

ifice

d 30

1 f

358 f

101 f

111 f

37.0

f

9.9 f

11.2

f

42,1

64 f

9,08

4 9,

401 *

at c

astra

tion

13

5 21

8

9 3.

5 1 .o

0.

8 1,

636

402

646

7-da

y po

stca

stra

tion

270 f

102 f

60 f

60 f

8.4 f

6.5 f

7.0 f

3,81

9 f

2,09

7 f

2,65

4 f

z * w

ithou

t pitu

itary

11

5

8 4a

6

0.7

0.6b

0.

7 18

Ib

110”

11

5 .,

7-day p

ostc

astra

tion

286 f

113 f

81 k

59 f

9.6 f

8.4

+ 6.

7 f

4,65

3 f

3,07

0 f

2,86

2 f

g w

ith p

ituita

ry g

raft

II

6 7

5 4

0.4

0.6

0.4

179

172

104

5 E 1 8

All

valu

es a

re m

ean f SE.

Th

e va

lue

is si

gnifi

cant

ly d

iffer

ent f

rom

the

corr

espo

ndin

g val

ue in

the

grou

p w

ith p

ituita

ry g

raft

(P <

0.0

1).

-he

valu

e is

sign

ifica

ntly

diff

eren

t fro

m th

e co

rres

pond

ing v

alue

in th

e gr

oup

with

pitu

itary

gra

ft (P

< 0

.05)

.

rp s:

54 Smithet al

than those without the grafts. The rate of decay of labeled protein as assessed by the radioactivity remaining in the lateral lobe following a single i.v. pulse of 3H-leucine given on the day before castration was slower in castrated rats bearing pituitary grafts than that of castrated rats without the grafts (P < 0.01; Table I). The radioactivity was significantly greater in the ventral lobes of rats with the pituitary grafts than in those without the grafts (P < 0.05), despite the absence of a significant difference in the wet weight and protein content of the ventral lobes of these two groups of rats. The wet weight, protein content, and radioactivity of the dorsal lobe were not significantly different in the graft-bearing and nongraft animals.

In Vitro and In Vivo Incorporation of 3H-Leucine by the Rat Prostate

Tables II and III show the in vitro and in vivo incorporation of 3H-leucine in the three lobes of the rat: prostate, respectively. Although the wet weight and protein content were significantly greater for the lateral lobe of the graft-bearing rats than those of the nongrafted rats, the amount of radioactivity incorporated by respective lobes was not significantly different (P > 0.05) in these two groups of animals. The parameters for the ventral and the dorsal lobes were not significantly different between the two groups (P > 0.05).

Cathepsin D Activity

Table IV indicates that, although the wet weight and the protein content in the lateral lobe in rats with pituitary graft were significantly greater (P < 0.01) than in those without the graft, no significant difference (P > 0.05) was noted in the activity of cathepsin D in these tissues between the two groups of rats. Parameters in the ventral and dorsal lobes were not significantly different (P > 0.05) between the two groups.

DISCUSSION

Results of the present study have confirmed and added to the observation that hyperprolactinemia achieved by pituitary grafts under the renal capsule causes a delay in the rate of castration-induced regression in the rat prostate, particularly in the lateral lobe [ 11. The demonstration that the delay in regression in the lateral prostate is characterized by a rate of protein synthesis normally observed in 7-day castrated rats and a lower than normal rate of protein degradation indicates that the catabolic phase of cellular metabolism is primarily affected by hyperprolactinemia. Since testosterone stimulation in the prostate is characterized both by an increase in protein synthesis and a decrease in protein degradation [8,19], the present observations demonstrate that the metabolic action of prolactin and testosterone in the rat prostate is different.

Results of the present study also indicate that, unlike the action of testosterone stimulation which leads to a decrease in the activity of cathepsin D in the prostate [9, 191, the action of prolactin has failed to alter this enzyme activity from a level normally seen in the prostate of 7-day castrated rats. Since both hormones cause a reduction in degradation of prostatic protein, the difference in enzyme activity in the tissue may reflect different sites of action. Our previous findings have indicated that administering exogenous testosterone to castrated rats resulted in a dose-related decrease in prostatic cathepsin D activity which bears an inverse relationship with the

TABL

E 11

. Effe

ct o

f Hyp

erpr

olac

tinem

ia o

n In

Vitr

o 3H

-Leu

cine

Inco

rpor

atio

n by

Thr

ee Lo

bes

of th

e R

at P

rost

ate

3H-le

ucin

e inc

orpo

ratio

n N

umbe

r w

t Pr

osta

tic w

t (m

g)

Prot

ein

cont

ent (

mg/

lobe

) (c

pmlm

g pr

otei

nlhr

) Tr

eatm

ent g

roup

s of

rat

s (g

) V

entra

l La

tera

l D

orsa

l V

entra

l La

tera

l D

orsa

l V

entra

l La

tera

l D

orsa

l

Sacr

ifice

d 35

1 f

412 f

131

f 13

1 f

40.2

k

15.0

f

14.1

f

6,87

7 f

4,22

3 f

3,59

1 *

3

7-da

y-po

stca

stra

tion

329 f

186 f

97

83 f

15.7

f

11.0

f

10.3

f

3,03

5 k

3,23

9 f

2,55

5 f

8 at

cas

tratio

n 6

7 13

8

14

1.1

0.8

0.7

263

175

487

E- 'I-

day

-pos

tcas

tratio

n 32

8 rt

143 f

53 f

75 f

10.9

f

6.6 i-

9.4 f

3,40

9 f

3,17

7 +

2,18

6 f

E w

ithou

t pitu

itary

4

I1

10

3=

9 I .

6 0.

3a

1.1

80

1 I9

279

* w

ith p

ituita

ry g

raft

4 7

11

11

7 I .

3 1.

7 0.

6 14

3 68

23

1 1

All

valu

es a

re m

ean f S

E.

Th

e va

lue

is si

gnifi

cant

ly d

iffer

ent f

rom

the

corr

espo

ndin

g va

lue

in th

e gr

oup

with

pitu

itary

gra

ft (P

< 0

.05)

.

01

01

TA

BL

E 11

1. Ef

fect

of

Hyp

erpr

olac

tine

mia

on

In V

ivo

'H-L

euci

ne

Inco

rpor

atio

n by

Thr

ee L

obes

of

the

Rat

Pro

stat

e

'H-le

ucin

e in

corp

orat

ion

Num

ber

Bod

ywt

Pros

tatic

wt

(mg)

Pr

otei

n co

nten

t (rn

g/lo

be)

(cpm

lmg

prot

ein)

Tr

eatm

ent g

roup

s of

rats

(g

) V

entra

l La

tera

l D

orsa

l V

entra

l La

tera

l D

orsa

l V

entra

l La

tera

l D

orsa

l

Sacr

ifice

d 30

1 f

358 f

101 f

Ill f

37.0

f

9.9 f

11.2

f

1.19

4 f

963 f

863 f

at c

astra

tion

13

5 21

8

9 3.

5 I .o

0.

8 69

59

60

7-

day

-pos

tcas

tratio

n 28

7 f

95 f

54 f

60 f

7.4 f

5.3 f

6.1 f

369 f

490 f

358 f

with

out p

ituita

ry

13

6 4

2"

5 0.

5 0.

4"

0.5

15

15

18

with

oitu

itarv

era

ft

13

7 9

3 4

0.7

0.3

0.5

23

20

29

7-da

y-po

stca

stra

tion

277 f

114 f

81 f

63 f

8.5 f

8.0 f

6.5 f

364 f

534 f

394 *

All

valu

es a

re m

ean f S

E.

The

grou

p of

ani

mal

s sac

rific

ed a

t cas

trat

ion

are

the

sam

e an

imal

s lis

ted

in T

able

11.

"The

val

ue is

sig

nific

antly

diff

eren

t fr

om c

orre

spon

ding

val

ue in

the

grou

p w

ith p

ituita

ry g

raft

(P

< 0

.01)

.

TABL

E IV.

Effe

ct o

f Hyp

erpr

olac

tinem

ia on

the

Act

ivity

of C

athe

psin

D in

Thr

ee L

obes

of

the

Rat

Pro

stat

e

Cat

heps

in D

leve

ls

Num

ber

Body w

t Pr

osta

tic w

t (m

g)

Prot

ein

cont

ent (

mgl

lobe

) (u

nits

hg pr

otei

n)

7 Tr

eatm

ent g

roup

s of

rats

(g

) V

entra

l La

tera

l D

orsa

l V

entra

l La

tera

l D

orsa

l V

entra

l La

tera

l D

orsa

l

Sacr

ifice

d 32

6 f

411 f

140 f

125 f

39.8

f

15.8

f

14.1

f

1.3 f

1.7 f

23 f

* 7d

ay-p

ostc

astr

atio

n 32

1 +

145 f

62 f

74 f

12.4

f

7.0

8.6 f

7.6

+ 17

.2 k

27 k

2

7-da

y-po

stca

stra

tion

325 f

163 f

100 f

96 f

13.9

f

11.5

f

10.9

f

6.9

15.6

f

26 f

5’ ,

5’ A

ll va

lues

are

mea

n SE.

aThe

val

ue is

sig

nific

antly

dif

fere

nt fr

om th

e co

rres

pond

ing

valu

e in

the

grou

p w

ith p

ituita

ry g

raft

(P

< 0

.01)

. %

he v

alue

is s

igni

fican

tly d

iffe

rent

from

the

corr

espo

ndin

g va

lue

in th

e gr

oup

with

pitu

itary

gra

ft (P

< 0

.05)

.

e at

cas

tratio

n 5

10

10

5 10

0.

5 1 .o

0.

9 0.

4 0.

6 3.

0 2

with

out p

ituita

ry

10

9 8

3”

4b

I .o

f 0

.5“

0.5a

1.

3 1.

8 3.

5

with

pitu

itary

gra

ft

10

6 13

5

6 1 .o

I .o

0.

6 1.

3 2.

9 1.

9

x i! 5 aa

58 Smith et al

magnitude of prostatic growth [9]. Furthermore, an inhibitor to protein synthesis, actinomycin D, can reduce the rate of prostatic regression in castrated rats [20], while at the same time it blocks the castration-induced increase in cathepsin D activity in the prostate [9]. These observations provide support to indicate that the reduction in prostatic regression and protein degradation by testosterone is, at least in part, due to its action to reduce the synthesis of prostatic cathepsin D. Our present findings demonstrate that the activity of cathepsin D is stable in the presence of hyperprolac- tinemia. This important difference in proiactin and testosterone effects on the rat prostate suggests differing mechanisms are involved.

The reduction in protein degradation in the prostate in the hyperprolactinemic rats in the presence of an unsuppressed activity of this proteolytic enzyme seems paradoxal. A similar partial delay of prostatic regression and an elevated activity of cathepsin D has been noted in castrated rats treated with chloroquine [16]. We have hypothesized that, being a lysosomal stabilizing agent, chloroquine somehow inter- fered with the normal interaction between the proteolytic enzymes and their cellular substrates in the prostate [19]. Although the effect of prolactin does not necessarily duplicate that of chloroquine, the possibility that hyperprolactinemia alters the inter- action between cathepsin D (and other proteolytic enzymes) and cellular proteins in the prostate is high. Under this circumstance, the synthesis of the proteolytic enzyme in the prostate would be increased as usual in 7day castrated rats but the synthesized enzyme somehow could not interact with its cellular substrate. As a consequence, the rate of protein degradation would be decreased.

Based on the above consideration, we conclude that, the action of prolactin on protein metabolism in the regressing prostate is likely to be independent of the action of testosterone. Testosterone stimulates protein synthesis and reduces protein degra- dation. Prolactin does not stimulate protein synthesis; it only reduces the rate of protein degradation. Furthermore, the manner in which the two hormones act on the prostate to reduce the rate of protein degradation is different. The testosterone- stimulated reduction in protein degradation is due to a reduction in synthesis of proteolytic enzymes; while prolactin interferes with the interaction of the existing degradative enzymes with their respective substrates.

ACKNOWLEDGMENTS

The authors wish to thank Julia Sensibar and Anthony Emerson for technical assistance; Brigittee G. Mann from the laboratory of Dr Neena B. Schwartz, North- western University, Evanston, Illinois, for her help on radioimmunoassay for prolac- tin; and the National Institute of Arthritis, Diabetes, Digestive Disease and Kidney for the supply of the kit for rat prolactin radioimmunoassay (rat-rat: RIA-NIADDK- rPRL-W-3).

Supported by NM grant HD11611, Edwin and Lucy Kretschmer Fund of Northwestern University Medical School, and a grant from the Grainger Foundation.

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