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Vol. 1, 1089-1094, October 1995 Clinical Cancer Research 1089

Advances in Brief

Prostate-specific Antigen, a Serine Protease, Facilitates Human

Prostate Cancer Cell Invasion1

Mukta M. Webber,2 Anuradha Waghray,

and Diana Bello

Departments of Medicine and Zoology, Michigan State University,

East Lansing, Michigan 48824-i3i2

Abstract

Human prostatic epithelial cells constitutively secrete

prostate-specific antigen (PSA), a kallikrein-like serine pro-

tease, which is a normal component of the seminal plasma.

PSA is currently used as a specific diagnostic marker for the

early detection of prostate cancer. We demonstrate that PSA

degrades extracellular matrix glycoproteins fibronectin and

laminin and, thus, may facilitate invasion by prostate cancer

cells. Blocking PSA proteolytic activity with PSA-specific

mAb results in a dose-dependent decrease in vitro in the

invasion of the reconstituted basement membrane Matrigel

by LNCaP human prostate carcinoma cells which secrete

high levels of PSA. A novel PSA-SDS-PAGE zymography

method for the detection of matrix degrading ability of PSA

is also described. We propose that: (a) because of the dys-

plastic cellular disorganization in early neoplastic lesions

called prostatic intraepithelial neoplasia (PIN), PSA may be

secreted not only at the luminal end but also, abnormally, at

the cell-basement membrane interface, causing matrix deg-

radation and facilitating invasion; and (b) PSA, along with

urokinase, another serine protease secreted by prostatic

epithelium, may be involved in the proteolytic cascade dur-

ing prostate cancer invasion and metastasis. The discovery

of the extracellular matrix degrading ability of PSA not only

makes it a marker for early detection but also a target for

prevention and intervention in prostate cancer.

Introduction

Prostate cancer is the most common cancer in adult men in

the United States. An estimated 244,000 new cases and 40,400

deaths from prostate cancer will occur in the United States in

1995 (1). The incidence increases with age, and about 80% of

prostate cancers are diagnosed after the age of 65 years (2).

Because of the increasing life span and an aging American male

population, prostate cancer is a major health concern. The most

threatening and primary cause of death from prostate cancer is

invasion and metastasis. One of the first events in progression to

malignancy is the degradation of the BM3 and ECM, followed

by invasion, a critical early step in the metastatic cascade.

Proteases intervene at the transition from in situ to invasive

carcinoma where local dissolution of the BM occurs. A corre-

lation between an increase in the secretion of matrix-degrading

serine proteases and metalloproteases, and invasion, metastasis,

and aggressiveness of cancer has been demonstrated (3). We

have shown a direct relationship between the level of secreted

urokinase (u-PA) and the ability of the DU145 human prostate

carcinoma cell line, not producing PSA, to degrade and invade

ECM (4, 5). We now show that PSA, which is abundantly

secreted by prostatic epithelial cells, may also play a role in

prostate cancer invasion.

PSA is a single-chain, 240-amino acid glycoprotein with

Mr �33,000 and a primary structure showing considerable ho-

mology to kallikrein (6). The human PSA gene on chromosome

19 has been cloned (7). PSA has the His-Asp-Ser triad in its

catalytic domain, a characteristic of serine proteases. It has

chymotrypsin-like activity, does not hydrolyze synthetic sub-

strates for plasmin, and displays a weak interaction with apro-

tinin, a plasmin inhibitor (8). This suggests that PSA primarily

acts independently as a protease in protein degradation, and not

via plasmin, as does u-PA. PSA is organ-specific, is character-

istically expressed in prostatic epithelial cells, and its expression

is regulated by androgens (9). The androgen-responsive LNCaP

human prostatic carcinoma cells (10) respond by increased PSA

expression (1 1). Following surgical or hormonal castration for

prostate cancer, serum PSA levels in patients decline. A subse-

quent increase in serum PSA level indicates cancer recurrence

(12). Although elevated levels of PSA in the serum of prostate

cancer patients were observed over 25 years ago, PSA measure-

ment has come into wide use only recently for early detection of

prostate cancer, for monitoring patients following radical pros-

tatectomy, and for identifying metastatic tumors of unknown

origin (13). Serum PSA measurement in combination with dig-

ital rectal examination and transrectal ultrasonography has

greatly increased the ability to detect prostate-confined cancer

(12, 13).

PSA is an important component and one of the most

abundant serine proteases in the seminal plasma, where it is

found at an average concentration of about 1.0 mg/ml (9).

Immediately after ejaculation, the seminal plasma coagulates

into a viscous gel which liquefies within about 20 mm. PSA

mediates this liquefaction (14). An understanding of the lique-

faction process provides clues to the mechanism of matrix

degradation by PSA in prostate cancer invasion. The seminal

coagulum is formed by the two most abundant, large molecular

Received 3/23/95; revised 5/16/95; accepted 5/30/95.

I This work was supported by the Biotechnology Research Center and

Research Initiation Grant (Michigan State University).

2 To whom requests for reprints should be addressed, at 5-350 Plant

Biology Building, Michigan State University, East Lansing, MI 48824-

1312.

3 The abbreviations used are: BM, basement membrane; ECM, extra-cellular matrix; PIN, prostatic intraepithelial neoplasia; PSA, prostate-

specific antigen; PSA-Ab, antibody to PSA; u-PA, urokinase-type plas-

minogen activator; mAb, monoclonal antibody.

Research. on June 15, 2020. © 1995 American Association for Cancerclincancerres.aacrjournals.org Downloaded from

http://clincancerres.aacrjournals.org/

1090 PSA Facilitates Human Prostate Cancer Cell Invasion

weight proteins in semen, semenogelin and fibronectin, both of

which are contributed by the seminal vesicles. During liquefac-

tion, PSA-mediated degradation of semenogelin and fibronectin

runs parallel with the dissolution of the seminal gel (14). Lami-

nm, type IV collagen, and fibronectin are major components of

the BM and ECM, respectively, and their degradation is the

initial step in the metastatic cascade. Our objective was to

determine whether PSA was involved in ECM degradation and

invasion.

Materials and Methods

Materials. PSA, Calbiochem 539832, mAb to PSA,

DAKO M750; polyclonal, anticatalytic antibody to u-PA,

American Diagnostica 389; #{128}-aminocaproic acid, A7824, mono-

clonal mouse antihuman antibody against fibronectin, F-7387,

monoclonal mouse antihuman antibody against laminin,

L-827!, human plasminogen 5661, and BSA, A-2153 (Sigma);

gelatin, Bio-Rad 170-6537; fetal bovine serum, Intergen 1020-

75, fibronectin, 40008 laminin, 40232 (Collaborative Research);

Immobilon-P 1PVH 304FO, Millipore; Vector ABC kit, AK-

5001 for Western blots; Vector peroxidase Elite ABC kit PK-

6102, and 3,3’-diaminobenzidine (DAB) nickel kit SK-4100 for

immunostaining (Vector Laboratories); RPM! 1640 medium,

GIBCO 320-1875AJ; Nuclepore filters, 8-p.m pore size, Costar

150446, and HEMA-3, Curtin Matheson 122-911 were used.

SDS-PAGE Zymography for Detection of Urokinase

Activity. PSA samples were analyzed using SDS-PAGE zy-

mography (4) to confirm the absence of u-PA and plasmin

activity. Substrates for u-PA and plasmin were incorporated at a

concentration of 12 �i.g of a solid plasminogen preparation

(activity, 0.44 units/mg solid) and 0.9 mg gelatin, respectively,

per ml of acrylamide in a 10% separating gel. Six p.g PSMane

were run in minigels (200 V, 4#{176}C,45 mm). The gels were further

processed to renature the enzymes and stained with Coomassie

blue. The presence of enzyme activity is indicated by bands of lysis

against a dark background (4). This u-PA activity could be

blocked by antibody to u-PA. Six p.g PSA were mixed with 20

�i.g u-PA antibody and 50 p.g #{128}-aminocaproic acid and incubated

for 2 h at 37#{176}Cbefore SDS-PAGE zymography.

PSA-SDS-PAGE Zymography for Detection of PSA

Proteolytic Activity and the Anticatalytic Ability of Anti-

body to PSA. To determine whether PSA alone has protease

activity and can degrade fibronectin, PSA samples were ana-

lyzed in our new and novel SDS-PAGE zymography method

where fibronectin was incorporated into a 12% acrylamide gel

to serve as a substrate for PSA. Gels were run as described

above, but stained using a silver stain (15). 1� determine

whether the antibody to PSA (PSA-Ab) has anticatalytic ability,

10-pig PSA samples were preincubated with 0.75 pg PSA-Ab

(IgG) for 17 h at 37#{176}Cprior to SDS-PAGE zymography. It

should be noted that not only does the anticatalytic ability of

various PSA antibodies but also the protease activity of different

PSA samples needs to be established because PSA can lose this

activity during the purification process.

SDS-PAGE and Western Blot Analysis. Affinity-pun-

fied PSA derived from human seminal plasma was used for

Western blots and for experiments to further examine fibronec-

tin and laminin degradation by PSA. To determine whether PSA

can degrade fibronectin and laminin, it was necessary to first

establish whether the PSA sample had any contaminating u-PA

activity. Therefore, PSA samples were subjected to Western blot

analysis as well as zymography. Twelve �i.g PSA/lane were

analyzed using SDS-PAGE on 4-15% gradient minigels (200

V, 4#{176}C,45 mm) and transferred to Immobilon membrane. Blots

for PSA were stained with mAb to PSA (1 : 100), and for uroki-

nase, with polyclonal antibody to urokinase (1:500), followed

by avidin-biotin alkaline phosphatase using a Vector ABC kit.

Fibronectin and Laminin Degradation. Degradation of

fibronectin and laminin by PSA is demonstrated by immunoblot

analysis. Twelve pg PSA were incubated with 20 pg antibody

to u-PA and 40 pg E-aminocaproic acid for 2 h at 37#{176}Cto block

any trace of u-PA and plasmin activity. A 2.5-p.g sample of

fibronectin only was run as a control. For fibronectin degrada-

tion, 2.5 pig fibronectin were incubated (17 h, 37#{176}C)with 12 jig

PSA prepared as above, run on a 4-15% gradient gel, trans-

ferred to Immobilon, and stained with mouse antihuman primary

antibody against fibronectin (1 :500), biotinylated goat anti-

mouse secondary antibody (1: 10,000), and avidin-biotin alka-

line phosphatase. For a laminin control, 2.5 jig of reduced

laminin only, boiled with �3-mercaptoethanol for 5 mm, were

run. For laminin degradation, 2.5 jig laminin were incubated

with 12 jig PSA prepared as described above, resolved by

SDS-PAGE, and stained with mouse antihuman primary anti-

body against laminin (1:500). Laminin sample was reduced

before loading into the gel. These experiments were repeated

four times.

Immunocytochemical Staining for PSA in LNCaP

Cells. LNCaP cells were plated on glass coverslips in RPMI

1640 medium and 15% fetal bovine serum at a density of

20,000/400 p.1 medium/well in 24-well culture plates. The fol-

lowing steps were performed at room temperature. Cells were

rinsed with PBS (10 mm) between all successive steps after

primary antibody application. Cells were fixed in 50:50 meth-

anol:acetone, blocked with horse serum (Vector kit) for 1 h, and

incubated for 24 h with mAb to PSA diluted 1:20 in horse

serum. Coverslips lacking primary antibody served as controls.

Prior to staining, cells were treated with 3% hydrogen peroxide

for 3 mm to quench endogenous peroxidase activity, then

stained using a Vectorstain Elite ABC avidin-biotin peroxidase

complex for 30 mm and developed with diaminobenzidine-

nickel chloride substrate for 5 mm.

Invasion Assay. Nuclepore filters were coated with 500

jig/ml Matnigel (16). LNCaP cells were released from mother

flasks using 1 mM EDTA and suspended in RPM! 1640 medium

containing 0.1% BSA. Two hundred thousand or 400,000 cells/

200 jil medium were plated on the coated filter in Boyden

chambers containing 650 p1 medium. The lower chamber con-

tamed 220 p1 of the chemoattractant conditioned medium from

NIH/3T3 fibroblasts grown for 24 h in serum-free medium

containing 50 jig/ml ascorbic acid. Cells were allowed to mi-

grate for 6 to 48 h, fixed on the filter, and stained with HEMA-3.

Nuclear stain was extracted for 15 mm with 0.1 N HC1, and

absorbance was measured at 620 nm using a Titertek microplate

reader (17). Three replicate cultures were prepared per treat-

ment, and the mean values were plotted as percentage of control,

taking untreated invaded cells as 100%. In experiments to de-

termine the role of PSA in invasion, PSA activity was blocked



1

A

1 2

3lkDa-54kDa-

C

A12 1 2

Clinical Cancer Research 1091

B

D1 2

440 -

220

2 1 0

B

400-. _

220

-31kDa-i� .�

.� 8��#{149}”�

Fig. 1 A, an immunoblot of affinity-purified PSA derived from human

seminal plasma. Lanes I and 2, a PSA sample. Lane 1, stained with mAb

to PSA (1:100). A series of bands at Mr �33,0(X) represent PSA. Lane

2, stained with polyclonal antibody to urokinase (1:500). B, an SDS-

PAGE zymogram (10% gel) for the detection of trace urokinase activityin the PSA sample. Lanes I and 2, received a sample of PSA. Lane 1,

faint zone of lysis at Mr 54,(K)O represents urokinase. Lane 2, PSAsample was blocked with antibody to u-PA. C, SDS-PAGE zymogram(12% gel) to determine whether PSA has protease activity and candegrade fibronectin. Three mg fibronectin were incorporated into the gel

as a substrate for 6 jig PSA loaded into the lane. Lysis caused by PSA

is indicated (arrowhead) at a level lower than the Mr 31,00() marker. D,

SDS-PAGE zymogram (12% gel) to determine whether the antibody toPSA has anticatalytic ability. The gel contained 5 mg fibronectin sub-

strate for PSA. Lane I, 10 jig PSA. Two zones of lysis at Mr �31,000

and a lower molecular weight (arrowhead) are seen. Lane 2, 10 p.g PSAwere incubated with 0.75 jig antibody (lgG) to PSA for 17 h at 37#{176}C

prior to SDS-PAGE.

with PSA antibody (IgG) at 6.25, 12.5, 25, 50, and 100 ngjml in

the final cell suspension. A suspension of one or two million

LNCaP cells/ml of serum-free RPMI 1640 medium containing

0.1% BSA was incubated with the antibody for 2 h before

performing the assay. A nonspecific immunoglobulin (IgG) was

used as a control. Nine such separate experiments were con-

ducted.

Results and Discussion

To attribute degradation of fibronectin and laminin to PSA,

it was necessary to first ascertain whether any u-PA activity was

present in the PSA sample to be used to assess its potential for

matrix degradation. Both PSA and urokinase are normally se-

creted by the prostate and constitute the prostate’s contribution

to the seminal plasma, from which PSA was purified. As stated

earlier, urokinase is also involved in matrix degradation (4, 5).

In the immunoblot in Fig. 1A, both lanes received a PSA

sample. Lane 1 was stained with human PSA-specific and Lane

2 with u-PA-specific antibody. In Lane 1, a series of bands at Mr

25,000-33,000 represent PSA. Variations in prostatic PSA mo-

lecular weight (20,000-35,000) due to glycosylation and alter-

native splicing have been reported (6, 18). Absence of staining

in Lane 2 indicates that u-PA was not detected in the PSA

Fig. 2 Degradation of fibronectin and laminin by PSA is demonstrated

by Western blot analysis. Samples were resolved on 4-15% gradientgels using SDS-PAGE and transferred, and blots were stained with the

respective antibodies. A, immunoblot to show fibronectin degradation

by PSA. Lane 1, 2.5-jig sample offibronectin only (control); Lane 2, 2.5

�i.g fibronectin were incubated with PSA prepared as described in

‘ ‘ Materials and Methods. ‘ ‘ fibronectin degradation products are shown.

Blot was stained with mouse antihuman primary antibody against fi-

bronectin (1:5(X)). B, immunoblot for laminin degradation by PSA. Lane1, 2.5 jig of reduced laminin only; Lane 2, 2.5 jig laminin were

incubated with PSA prepared as described above. Laminin sample was

reduced before loading into the gel. Blot was stained with mouse anti-human primary antibody against laminin (1:5(X)).

sample using immunoblot analysis. A more sensitive zymo-

graphic assay, which can detect as little as 10- “ mol u-PA (19),

was then used to detect trace u-PA activity in the PSA sample by

SDS-PAGE in which the gel also contained plasminogen, a

substrate for urokinase (4). The faint zone of lysis at Mr

-54,000 (Fig. 1B, Lane 1) represents the high molecular weight

u-PA, indicating the presence of trace u-PA activity in the PSA

sample. This activity could be completely blocked by urokinase-

specific antibody (Fig. 1B, Lane 2). Even traces of urokinase are

sufficient to generate plasmin in the presence of plasminogen.

Plasmin, also a serine protease, is formed by urokinase-medi-

ated catalysis of plasminogen to plasmin, and it can degrade

both fibronectin and laminin. The zone of lysis in the PSA

zymogram in Fig. 1C (6 jig PSA), which used fibronectin as a

substrate, shows that PSA has protease activity and it can

degrade fibronectin. In Fig. 11) (Lane 1), when a larger amount

(10 jig) of PSA was run in a similar gel, two zones of lysis, one

at Mr �3l,000 and the other at a lower molecular weight

(-25,000) were observed. This lysis could be blocked by pre-

incubation of PSA with PSA-Ab (Fig. 1D, Lane 2), indicating

that the antibody to PSA used has anticatalytic ability.

To further determine whether PSA alone can degrade fi-

bronectin and laminin, samples of PSA were incubated with

antibody to urokinase to block u-PA activity. Although the

absence of other lysis bands in the zymogram (Fig. !B, Lane 1)

excluded the presence of plasmin, #{128}-aminocaproic acid, a plas-

mm inhibitor, was also incubated with PSA concurrently to

block any trace plasmin activity. PSA samples prepared in this



PSA Ab

DIgG

C0(I)

>C

LNCaP 6.25 12.5 25 50 100

Antibody Concentration (ng/mI)

Fig. 4 Effects of antibody to PSA on the ability of LNCaP humanprostate carcinoma cells to invade the reconstituted BM Matrigel, using

the Boyden chamber assay. A nonspecific immunoglobulin (IgG) wasused as a control. Bars, SD.


4 Unpublished data.

b

Fig. 3 Indirect avidin-biotin immunoperoxidase staining of LNCaP cells using mAb to PSA. a, cells stained with PSA antibody; b, control. Bar, 20

p.m. X 532.

manner were then used to examine the ability of PSA to degrade

the matrix proteins. Samples of fibronectin or laminin were

incubated with PSA, and the degradation products were sepa-

rated by SDS-PAGE and detected using immunoblot analysis.

Fig. 14 (Lane 1) shows the undegraded, control fibronectin

bands at Mr �440,000 dimer and at Mr 220,000 and 210,000

monomeric forms. After incubation of fibronectin with PSA,

several low molecular weight fibronectin degradation bands

were observed, indicating that PSA can degrade fibronectin

(Fig. 2A, Lane 2). Similarly, in Fig. 2B, Lane 1 shows the

undegraded, control laminin bands at MrS �400,000 and

220,000, and in Fig. 2B, Lane 2 shows loss of the two high

molecular weight laminin bands and the appearance of several

low molecular weight degradation products, indicating that PSA

degrades laminin.

To demonstrate that PSA may play a role in invasion by

prostate cancer cells, the relationship between free, secreted

PSA and the invasive ability of LNCaP cells was examined.

LNCaP cells are a useful model for this study because they

produce only a small amount of u-PA as compared to a more

invasive DU145 human prostate carcinoma cell line, and they

also secrete very low levels of gelatinases.4 Nevertheless, they

are invasive, although much less than DU145 cells in an in vitro

invasion assay.4 LNCaP cells were derived by Horoszewicz et

a!. (10) from a lymph node metastasis of prostate carcinoma,

and they formed invasive tumors at the site of injection in nude

mice 8 weeks after injection, but distal metastases were not

apparent at that time. LNCaP cells express (Fig. 3) and secrete

high levels of PSA. The possible role of PSA in invasion is

further suggested by results which show (Fig. 4) that in the

presence of PSA-specific antibody, the ability of LNCaP cells to

invade a reconstituted BM Matrigel is markedly reduced in a

dose-dependent manner. However, a nonspecific immunoglob-

ulin did not inhibit invasion. PSA zymography results (Fig. 1D)

already showed that the PSA-Ab used has anticatalytic ability.

Therefore, a dose-dependent inhibition of invasion by this an-

tibody in the invasion assay suggests that PSA may be associ-

ated with invasion by LNCaP cells.

On the basis of these results, we propose that PSA, along

with u-PA, may be involved in the proteolytic cascade in pros-

tate cancer. Evidence suggests that a proteolytic cascade inde-

pendent of metalloproteases may exist and that degradation of

type IV collagen, an important component of the BM, can occur

via a plasmin-dependent but metalloprotease-independent path-

way (20). Furthermore, kallikreins such as PSA can activate

prourokinase to its active form and subsequently, both u-PA and

plasmin can activate procollagenases (5). Our results show that

ECM components can serve as substrates for PSA, and, thus,

PSA may be involved in localized proteolysis in tumor cell

invasion in prostate cancer.



Basementmembrane

Invading Capillarycells

Clinical Cancer Research 1093

If PSA plays a role in the proteolytic cascade in prostate

cancer, as suggested here, why are LNCaP cells, which secrete

PSA, less invasive than DU145 human prostatic carcinoma

cells, which do not secrete PSA? The answer may lie in the fact

that DU145 cells secrete high levels of urokinase (4). In the

presence of plasminogen, which is abundantly available in vivo

in blood and extracellular fluids, and in vitro, in serum and

pituitary extract, plasmin is generated by the activation of plas-

minogen by u-PA. Plasmin, a potent protease, has a wide sub-

strate specificity and is able to degrade not only fibrin, fibronec-

tin, and laminin but also type IV collagen (20). Therefore,

plasmin-mediated proteolysis is much more efficient than that

mediated by PSA. As a result, DU145 cells, which secrete high

levels of u-PA, are more invasive than LNCaP cells. If prostate

cancer cells are secreting urokinase, PSA would not be required

for invasion. However, PSA may contribute to the invasive

ability of prostate cancer cells. Our results suggest that in the

absence of high urokinase secretion, PSA could mediate inva-

sion, as in LNCaP cells. These proposed mechanisms do not,

however, exclude the involvement of other proteases in the

proteolytic cascade.

The presence of low grade PINs has been observed in men

in their 20’s and 30’s, the frequency of preinvasive high-grade

PIN increases with age and PINs may precede cancer by 10 or

more years (21, 22). Focal proliferation, cellular disorganization

and heterogeneity, and disruption of the basal epithelial cell

layer, followed by disruption of the BM in high-grade PIN have

been observed (22, 23). In the normal prostate gland in viva,

polarized secretion of prostatic proteases, such as PSA and

u-PA, takes place at the apical, luminal end of glandular epi-

thelium. Disorganization of prostatic epithelial cell monolayers

in vitro can lead to a loss of polarized secretion so that cells

begin to secrete proteases at their apical as well as at their basal

end (24). Accepting the cellular disorganization as seen in

dysplasia in vivo, changes in the polarity of secretion by epi-

thelial cells could take place, leading to similar loss of polarized

secretion. This would result in the secretion of PSA and u-PA

not only into the gland lumen but also at the cell-BM interface,

causing localized proteolysis of the BM and ECM which cul-

minates in invasion and metastasis. Such protease secretion

would lead to their leakage and entry into capillaries (Fig. 5),

resulting in increased serum PSA levels in prostate cancer

patients. This would also explain why increased serum PSA

levels have been observed as early as 6 years before prostate

cancer is discovered by rectal examination (25). For example,

the mean serum PSA level in normal, high-grade PIN, and

carcinoma patients is 4, 7, and 17.9 ng/ml, respectively (23).

The increased serum PSA levels need not necessarily result

from an increased production by carcinoma cells but may be due

to its leakage into the blood vessels. Detection of PSA mRNA

by PCR in cells from peripheral blood of prostate cancer pa-

tients also suggests that dissemination of prostate cancer may be

a relatively early event (26).

We postulate that since prostatic epithelium secretes both

urokinase and PSA, cells in preinvasive prostatic lesions, such

as high-grade PIN, may have an inherent ability and predilection

to invade and metastasize. This may explain why a majority of

prostate cancer patients already have disseminated disease be-

yond the prostate at initial diagnosis by rectal examination. In

Fig. 5 Diagrammatic representation of a prostatic gland with early

invasive carcinoma, which suggests the role of PSA in invasion. Normal

pseudostratified epithelium rests on a continuous BM, whereas in the

area of invasion, it is interrupted. The focus of malignant cells shows

dysplasia so that now the cells are secreting PSA into the gland lumen

as well as at the basal end, causing matrix degradation. The diagram

depicts some of the secreted PSA entering the blood circulation via

capillaries, which would result in a rise in serum PSA levels.

invasion, the net extracellular proteolysis at the cell-BM inter-

face will depend on the ratio between the levels of extracellular

proteases and their natural inhibitors. The balance between the

protease and its inhibitors must be in favor of the active protease

in order for proteolysis to occur. Serine protease inhibitors

(serpins; Ref. 27) and their modulators may serve as useful

agents for blocking the progression of preinvasive to invasive

prostatic carcinoma. Work on the association between the se-

cretion of different proteases and the invasive ability of several

different immortalized and malignant human prostate cell lines

(28) is in progress. The discovery of the ECM-degrading ability

of PSA not only makes it a marker for early detection but also

a target for prevention and intervention in prostate cancer.

Acknowledgments

We thank Dr. Nikolay Dimitrov for helpful comments, Dr. Hynda

K. Kleinman for providing the Matrigel, Aaron I. Steele for preparing

the manuscript, and Kay Steele and Chris Fetters for graphics.

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1995;1:1089-1094. Clin Cancer Res M M Webber, A Waghray and D Bello prostate cancer cell invasion.Prostate-specific antigen, a serine protease, facilitates human

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