ACTA
UNI VER SIT ATI S L O D Z I E N S I S
F O L IA B IO C H IM IC A E T B IO P H Y S IC A 14, 1999
Janusz Błasiak, B eata Smolarz, Dagmara Piestrzeniewicz
UROKINASE PLASMINOGEN ACTIVATION SYSTEM
AND ITS ROLE IN CANCER INVASION AND METASTASIS
Invasion and m etastasis are hallm arks o f tu m o r progression and depend o n the
ability o f tu m o r cells b o th to degrade and to m igrate th ro u g h connective tissue
barriers. These processes require com plex interactions betw een tu m o r cells and the
su rro u n d in g extracellular m atrix. T hese interactio n s are m odified by pro teo ly tic
enzym es and their receptors. T h e u rokinase plasm inogen a ctiv ato r system is well
kn o w n to play a n im p o rta n t role in cancer invasion and m etastasis. It co n ta in s the
uro k in ase type plasm inogen activ ato r (uPA ), the uro k in ase recep to r (u P A R ) and
specific plasm inogen activ ato r inhibitors, PAI-1 and PA I-2. T he re ce p to r for uPA ,
u P A R m ay reside on the surface o f tu m o r cells. uPA which is secreted by no rm al
and tu m o r cells binds w ith high affinity and specificity to uP A R . T h e u P A -u P A R
com plex focuses proteolytic activity to the tu m o r cell surface by converting the
p lasm a p ro tein plasm inogen in to the serine protease plasm in. Plasm in degrades
c om ponents o f th e extracellular m atrix in the vicinity o f the tu m o r cells th u s
facilitating tu m o r cell invasion and m etastasis. PAI-1 m ay be needed for the optim al
fu n c tio n o f the uP A system in these processes, by being necessary for the dynam ic
state o f the system , by regulating cell adhesion and for selective p ro tec tio n o f
vitronectin against proteolysis. PAI-1 m ay co u n te rac t m igration and invasion. uPA ,
u P A R a n d PAI-1 can be used as strong independent m ark e rs o f p ro g n o sis in m an y
types o f cancers - high levels o f these p roteins are associated w ith b o th sh o rter
disease-free interval and shorter overall survival o f the subjects w ith cancer.
Key words: cancer, m etastasis, plasm inogen activation system , urokinase, urokinase
receptor, plasm inogen a ctiv ato r inhibitor-1 (PA I-1)
IN T R O D U C T IO N
T he processes o f cancer invasion and m etastasis require the rem odelling
o f the extracellular m atrix (E C M ) (see [10] for review). R em odelling o f the
E C M m ay consist o f its degradation by m eans o f excision and deletion of
E C M com ponents, regeneration o f the EC M com ponents and their spatial
and tem poral reorganization. Several enzymes are know n to be integrated
co m ponents o f the E C M , others are secreted and m ay be associated with
the cell m em brane. T he proteolytic function o f the enzymes are c o u n ter­
balanced and regulated by ap p ro p riate protease inhibitors. T hese proteases
and their inhibitors actively participate in determ ining the com plex structure
and biochem ical characteristics o f the E C M , which lead to the expression
o f specific form s o f biochem ical behavior. In order to establish m etastatic
deposits, the adhesive ability o f cancer cells m ust be regulated. In the initial
stage o f dissem ination, the release o f cells from the prim ary tu m o rs requires
a decrease in intracellular adhesion. T he cells th at arc released have to
acquire the ability to adhere to, and penetrate the barriers presented in the
form o f th e E C M , host tissue stro m a and the vascular en d o th eliu m .
R em odelling o f the EC M [75, 76] provides the adaptability o f the cell to
enter into these interactions with the ECM and other cellular elem ents.
T his process is assisted by the proteolytic enzymes synthesized by the
tu m o r cells.
Primary tumor
Invasion <=0- Extravasation «=£> Metastasis
directed tumor celt invasion
Fig. 1. T u m o r invasion and m etastasis o f solid m alig n an t tu m o rs
(reproduced w ith perm ission from S c h m i t t et al., 1997)
T u m o r invasion and m etastasis o f solid m alignant tum ors involves
invasion and extravasation o f tu m o r cells into and ou t o f vessels and into
the E C M (Fig. 1). T hree steps control tum or spreading: attach m en t to and
in teractio n o f tu m o r cells with com ponents o f the basem ent m em brane and
the E C M , local proteolysis and tu m o r cell m igration [73], F o u r different
classes o f proteases are know n to be correlated with local proteolysis:
m atrix m etallproteases including collagenases, gelatinases and strom elysins
[46, 47, 53, 83], cysteine proteases including cathepsin B and L [5, 51, 77,
78, 79], the aspartyl protease cathepsin D [52, 69, 71] and serine proteases
including com ponents o f the plasm inogen activation system [8, 19, 48-50].
T h e suggestion th at the com ponents o f the urokinase plasm inogen activation
system m ay play an im p o rtan t role in the processes o f cancer invasion and
m etastasis goes back several decades [18].
U R O K IN A S E PA TH W A Y OK P L A S M IN O G E N A C T IV A T IO N
T he m ain com ponents o f the urokinase plasm inogen activation system
are displayed in Fig. 2.
plasminogen------------------------ ► plasmin
pro-uPA — ------------------------- ► uPA
/
PAI-1
/
/
f
\
\
\
\
PAI-2
Fig. 2. Sim plified scheme o f th e urokinase plasm inogen activ atio n system
U rokinase plasm inogen activ ato r (uPA ) is a serine protease, which is
released from cells as a single polypeptide-chain, virtually inactive proenzym e
(p ro-uP A ) [61]. pro-uP A is proteolytically converted into a tw o chain,
active form o f uPA , which in tu rn activates plasm inogen into plasm in.
p ro -u P A itself is efficiently activated by plasm in, leading to am plification
o f the overall reaction. H ow ever, the m echanism o f the initial activation
o f p ro -u P A under physiological conditions is not yet clarified. P ro-uP A
an d uPA can bind to a specific cell surface receptor, uP A R [19, 70, 85]
and co n co m itan t binding o f pro-uP A to uP A R and o f plasm inogen to as
yet unidentified cell surface binding sites strongly enhances plasm in generation
[19, 27], uP A activity is regulated by two inhibitors, plasm inogen activator
inhibitors type 1 (P A M ) and type 2 (PAI-2) [2], The complexes uPA -inhibitor
are internalized in a process dependent on both uP A R an d the a-m acroglobulin receptor [17, 57].
Fig. 3. T u m o r cell-associated proteolysis
(reproduced w ith perm ission from S c h m i t t e t a!., 1997)
In m odel system s, uP A an d u P A R are required for invasion and
m etastasis [15, 42, 44], T he recep to r for uPA , uP A R resides on the
surface o f tu m o r cells (Fig. 3). uPA which is secreted by norm al and
tu m o r cells binds with high affinity and specificity to uP A R . T he uPA-uP A R com plex focusses proteolytic activity to the tum or cell surface by
converting the plasm a protein plasm inogen into the serine protease plasm in. Plasm in degrades com ponents o f the EC M (tu m o r strom a) in the
vicinity o f the tum or cells thus facilitating tum or cell invasion and m e ta ­
stasis [73],
U R O K IN A S E -T Y P E P L A S M IN O G E N A C T IV A T O R
The protein and the gene
U rokinase-type plasm inogen activator, uPA , is a serine protease o f
m o lecu lar w eight o f approxim ately 50 000. T he m olecule consists o f
2 disulfide bridge-linked polypeptide chains, a C -term inal serine proteinase
d o m ain , and a N -term inal chain which contains a kringle dom ain and
a grow th factor-like dom ain. uPA converts the inactive zym ogen plasm inogen
into plasm in th at has tripsine-like activity. Plasm in is involved in degradation
o f ex tracellular m atrix p ro tein s such as fibronectin, type IV collagen,
lam inin and fibrin [18, 48] and activates latent collagenase to p o ten tate
th eir lytic activity [74, 85]. uPA is produced and released by n orm al cclls
like pneum ocytes, fibroblasts, keratinocytes or phagotic cells and from
tu m o r cells as a single-chain zym ogen (pro-uPA ) with very low o r w ithout
any activity. P ro-uP A is converted to active-high m olecular-w eight tw o
chains form o f uPA (IIM W ) [19] by cleavage o f the peptide bond. T his
reactio n m ay be catalyzed by plasm in [18] and o th e r m olecules, like
kallikrein and blood coagulation factor X lla [18] therm olysin, nerve grow th
factor-)’ [20], cathepsin B [41], cathepsin L [18], p ro state specific antigen
[33], T-cell associated serine proteinase [5], hum an m ast cell tryptase [81]
o r tripsine-like proteinases isolated from hum an ovarian tu m o r [54]. Pro-uPA m ay be converted to inactive IIM W uPA by th rom bin and elastase
from granulocytes [74], IIM W uPA can be fu rther degraded by plasm in
and o th er proteinases such as unknow n proteinase [63] existing in the urine
o r m atrix m etalloproteinase Pum p I [34], These reactions produce low-m olecular-w eight tw o-chain form o f uPA (L M W ) and am ino-term inal-fragm ent containing kringlc dom ain and the grow th factor-like dom ain
[73], C o n cen tratio n o f uPA in the blood is ab o u t 20 pM . M ost o f it is
com plexed with its type 1 inhibitor, the rest is in the p ro -u P A form [3],
P ro -u P A and uPA can be bound, through its grow th factor dom ain, to
a specific cell surface receptor uPA R with the sam e affinity. A ctivation o f
p ro -u P A can occur on binding to uPA R . T he binding strongly accelerates
activation o f pro -u P A and increases the enzym atic activity o f uP A itself
[27], C onversion o f pro-uP A into uPA is inhibited by specific plasm inogen
ac tiv ato r in h ib ito r type 1 and 2 or protein C and nexin proteinase (PN -1)
b u t reactions with these factors are m uch slower [81]. uPA has m itogenic
activity for som e cell lines [52]. U rokinase with plasm in are involved in
proteolytic activation o f grow th factors: hepatocyte grow th facto r/scatter
facto r (H G F /S F ), m acrophage stim ulating protein, transform ing grow th
facto r fi o r basic fibroblast grow th factor but these are devoid o f p ro te o ­
lytic activity [3].
T h e hu m an uPA gene is located on the 10-th chrom osom e, organized in
11 exons and is 6.4 kb long. A t the pro m o ter region there are repeated
A LU -type sequences, the enhancer and negative-regulation regions. A fragm ent
w hich show s p ro m o te r activ ity co n ta in s the h ex a n u cleo tid e sequence
G G C G G G th a t is repeated three times between the C A A T -box and the
T A T A -b o x and m ay play im p o rtan t role for pro m o ter activity [65]. Tw o
polym orphism s o f the uPA gene are know n: m ore frequent one near the
beginning o f exon 8 and less frequent resulted in the replacem ent o f leucine
residue by proline in the kringle dom ain [14],
uPA and cancer
uPA is involved in the cell m igration by plasm in generation on a cell
surface by uP A R bound to uPA over ECM substrate. D uring this process
uPA and its receptor are found at the leading cell edge. It was reported
th a t inactive variants o f uPA , bound to uP A R , m ay inhibit cell m igration
[3]. T h e cell m igration m ay be stim ulated by at least two m echanism s:
a proteolytic one and non-proteolytic. Proteolytic m echanism m ay involve
plasm in generation at focal adhesion sites, catalysed by uPA R bound uPA
and m ay initiate ECM degradation. Second m echanism m ay stim ulate cell
m ig ratio n by changes in the adhesion at the leading edge. T u m o r cells
secret large am ounts o f plasm inogen activators and evoke increased cell
m igration and developm ent o f secondary tum or focus. M oreover, uPA/plasm in
system m ay play a role o f the m ediator in tu m o r invasion and p ro pagation
by changes o f cell surface and by hydrolysis o f fibrin deposition around
the tum ors. uPA m ay break norm al cell barriers and decrease cellular
attach m en t to EC M by affecting E C M and cytoskeleton proteins. P roteinase
secretion and tissue d egradation are elem ents o f the invasive and m etastatic
ph en otype and correlate directly with tum or cells invasiveness. T he level o f
uP A activity is significantly higher in the m alignant tum ors in com parison
to n o rm al tissues or in benign tum ors o f the same tissue [22, 23]. uPA
was the first proteinase which was used as a prognostic m ark e r in hum an
m alignancy. T he results o f studies have shown th a t patients with tum ors
displaying high level o f uPA activity have shorter disease-free survival than
p atients w ith low level. T he level o f the uPA antigen ap p ear to be one of
the strongest prognostic m arkers in the m ultivare analysis com prising node
statu s, tu m o r size, estrogen receptor status and cathepsin D levels [24, 25,
74]. Increased level o f uPA have been proposed as a prognostic m ark er in
m an y cancers including cancers o f breast [8, 39], lung [58, 59], bladder [34],
stom ach [55], colorectum [53], cervix [42], ovary [45], kidney [36], brain [7],
p ro state [1] and m any soft tissue sarcom as [14]. Elevated level o f uPA and
PAI-1 m ay be prognostic m ark er for patients survival such as in the
cancers o f ovary like endom etrial carcinom a [43], gastric [13], colorectal
and dedifferentiated chondrosarcom a [33]. F o r o th er types o f cancer level
o f uP A is independent prognostic facto r in a case o f breast, p ro state and
kidney cancer. In only a few studies o f colon [67], breast [21], ovarian [72]
and p an creatic carcinom as [30] uPA was show n to be uP A R -dependent
pro g n o stic m arker.
P L A S M IN O G E N A C T IV A T O R IN H IB IT O R T Y P E 1
The protein and the gene
Plasm inogen activator inhibitor type 1, PA I-1, is a 50 k D a glycop­
ro tein , which belongs to the serine protease inhibitor (Serpin) superlam ily
[81], It contains 379 am ino acids and is a fast-acting inhibitor o f both
tissue-type and urokinase-type plasm inogen activators. T h e deduced am ino
acid sequence o f PAI-1 reveals a signal peptide o f 23 am ino acid residues
and a m a tu re p ro tein co n tain in g 379 am ino acid residues w ith three
p otential sites o f N-linked glycosylation [9], PAI-1 is produced by platelets,
endothelial cells, granulosa cells, hepatocytes and sm ooth m uscle cells [9].
Insulin and triglyceride rich lipoproteins stim ulate PAI-1 p ro d u ctio n by
cultured endothelial cells or hepatocytes [40]. Insulin resistance syndrom e,
w hich predisposes to diabetes and ischemic h eart disease, m ay be a m ajo r
reg u lato r o f PAI-1 expression [40]. T here are tw o distinct pools o f PAI-1:
one in the a-granules o f blood platelets and the o ther in plasm a. PAI-1
is ra th e r unstable but the form ation o f com plex w ith vitronectin stabilizes
PAI-1 activity [68], T he expression o f PAI-1 is enhanced in several situ a­
tions. E levated level o f PAI-1 was observed after m ajo r surgery, tra u m a
and m yocardial infarction. Its activity is correlated w ith body m ass index
(B M I) and serum trigliceride levels [87]. Plasm a PAI-1 level is positively
correlated with plasm a insulin level. Insulin and trigliceride rich lipo­
p roteins stim ulate PAI-1 production by cultured endothelial cells or h ep a­
to cy tes [40], Its biosynthesis is regulated by a n u m b er o f h o rm o n es,
cytokines, glucocorticoids, throm bin [20]. PAI-1 activity in plasm a is also
induced by agents such as interleukin-1 (IL-1), tu m o r necrosis fa cto r
(T N F ), tran sfo rm in g grow th factor (T G F). T he level o f antigens against
PAI-1 o f hepatocytes was stim ulated by cytokines (IL-6, IL-1, T G F /f) [20].
It is likely, th a t cytokines can effect in regulation o f the PAI-1 gene
tran scrip tio n .
PAI-1 gene is localized to 7q21.3-q22 ad its 12,3 kb D N A contains
n in e exons and eight in tro n s [9]. E xons range from 84 to 1823 bp,
in tro n s from 119 to 1764 bp in length. 12 A lu elem ents and 5 repeats
o f a long polypurine elem ent are present in PAI-1 gene. 8 polym orphism s
o f PAI-1 are currently know n: tw o G /A substitutions at positions -844
and + 9 7 8 5 , 4 G /5 G insertion/deletion at position -675, tw o polym orphism
in the 3’ untran slated region - substitution T /G at position + 1 1 053
and a 9-nucleotide insertion/deletion located between nucleotides + 1 1 320
an d + 1 1 345, tw o (CA)„ rep eat polym orphism s, one in th e p ro m o te r
and one in in tro n 4, and HindlW restriction fragm ent length polym orphism
[35]. Several polym orphic sites in the PAI-1 gene can be associated with
circulating levels o f the inhibitor, including a 3’ Hin&Ul restriction site and
an in tronic dinucleotidc (CA ) repeat [12]. It has also been show n th at the
polym orphism 4G /5G found in the prom oter region o f the PAI-1 gene is
o f functional im portance in regulating PAI-1 gene expression [35], This
polym orphism can be associated with elevated level o f the protein. PAI-1
m ay be an im p o rtan t com ponent in the regulation o f a variety o f both
physiological and pathological processes.
The role in cancer invasion and metastasis
C onsiderable evidences su p p o rt the view th a t elevated plasm a PAI-1
activity is often observed in individuals with angina pectoris, deep vein
throm bosis, diabetes and in survivors o f m yocardial infarction [13, 87],
T he increase o f plasm a PAI-1 activity is associated with early recurrence
o f m yocardial infarction [87], PAI-1 m ay play an im p o rtan t role in the
developm ent o f ath ero tro m b o tic diseases. Patients with o th er th ro m b o tic
disorders such as deep vein throm bosis and stroke have higher PAI-1
levels th a n controls. Increased level o f PAI-1 is com m only found in
patients w ith coronary heart disease [28]. M oreover, PAI-1 m ay have
functions o th er th an intravascular control o f plasm inogen activators ac­
tivity. Epidem iological studies have revealed th at plasm a PAI-1 activity
is positively co rrelated w ith body m ass index and serum triglyceride
levels [87],
PAI-1 is involved in tu m o r invasion and m etastasis. M oreover, invasion
and m etastasis o f solid tu m o rs require the concerted action o f various
proteolytic enzyme systems, which degrade the basem ent m em branes [84],
As m entioned above uPA m ay play a key role am ong the proteases and
the activity o f uP A during m etastasis is regulated by PAI-1 th at inhibits
uPA th ro u g h the form ation o f a covalent inhibitor-enzym e com plex. PAI-1
is an in h ib itor o f bo th free and receptor bound uPA [54], T he role o f
PAI-1 in tu m o r biology has no t been defined clearly. Several studies have
suggested th at PAI-1 protects the tum or tissue against the proteolytic
d eg rad atio n th a t the tum or im poses upon the surrounding n orm al tissue
[54], PAI-1 m ay bind to the cell surface associated u P A R /u P A com plex
form ing an enzymatically inactive trim eric receptor-protease-inhibitor com plex
which is internalized by the tu m o r cell [73], In addition, PAI-1 m ay be
involved in the m odulation o f u P A R binding to EC M com ponents and
interfere with cell attachm ent to the m atrix. PAI-1 has strong effect o n the
m alig n an t phenotype o f tu m o r cells [74], H igh level o f PAI-1 in tu m o rs
predicts p o o r prognosis for the patient. Increased levels PAI-1 m ay be
a cause for the developm ent o r the progression o f cancer [82]. H igh level
o f PAI-1 is observed in a variety o f m alignancies including cancer o f the
breast, cervix uteri, ovary, stom ach, colon, lung, brain, kidney [73]. C ancer
p atients m ay be defined as low- or high-risk patients depending on the
am o u n t o f PAI-1 in prim ary tu m o r [74]. R ecently, it has been proposed
th a t PAI-1 is a potentially im p o rtan t prognostic factor in breast carcinom a
[74], In tu m o r tissues from patients with cancer elevated levels o f PAI-1
are observed when com pared with norm al tissues.
A n insertion/deletion polym orphism in the prom oter o f the PAI-1 gene
m ay have an influence on plasm a PAI-1 level. W e studied frequency o f
4 G /5 G g enotype in subjects with cancer. Blood was d raw n from 53
p atients (9 gastric cancers, 16 breast cancer, 9 m elanom as, 12 colorectal
cancers, 7 head and neck cancers) and 53 m atched controls [80]. O ur
study show ed th a t the genotype distribution differed significantly between
these tw o groups. T he 4 G /5 G genotype was observed m o re frequently in
p atien ts with cancer th a n in the controls. T hese results suggest th at
4 G /5 G genotype m ay be associated with elevated PAI-1 level in cancer,
w hat predicts p o o r prognosis. F u rth e r studies are needed to check w hether
the prevalence o f the 4 G /5 G genotype m ay influence an in d iv id u al’s
plasm in o g en activ ation system capacity and thereby co n trib u te to the
cancer risk profile.
It is n o t easy to understand why the elevated tu m o r tissue content o f
PAI-1 indicates a p o o r prognosis for the cancer patients. PAI-1 m ay be
involved in th e m o d u la tio n o f uP A R binding to extracellular m atrix
co m ponents and interfere with cell attachm ent to the extracellular m atrix.
PAI-1 has strong effect on the m alignant phenotype o f the tu m o r cells.
In cancer, the increase o f PAI-1 is associated with tu m o r progression
and w ith shortened disease-free an d /o r overall survival in patients afflicted
with m alig n an t solid tum ors. Strong correlation betw een elevated PAI-1
values in prim ary cancer tissues and the tum or invasion/m etastasis capacity
o f cancer cells, PAI-1 has been selected as target for anticancer therapy.
U R O K IN A S E -T Y P E P L A S M IN O G E N A C T IV A T O R R E C E P T O R
The protein and the gene
T he receptor for uP A R was first identified on m onocytes and the
m on o cy to id cell line U937 as a m em brane p rotein th at binds the am ino-term inal fragm ent o f uPA with high affinity (5 x 10 10 M ) [13, 56, 85].
uP A R consist o f a single polypeptide chain and behaves as a n am phiphilic
m em b ran e protein [29, 56].
T h e specific cellular receptor for uPA, uP A R binds its ligands th ro u g h
a high affinity interaction with the N -term inal epiderm al grow th factor
(E G F )-like dom ain o f uPA [27], uP A R is a glycolipid-anchored m em brane
glycoprotein encoded as a 335-residue polypeptide th a t is p osttranslationally
processed to approxim ately 283 residues by rem oval o f the N -term inal
signal sequence and a C -term inal sequence responsible for the ad d itio n of
the glycosylphosphatidylinositol (G P I) m oiety [64], It is established th at
u P A R is com posed o f three dom ains th a t are tho u g h t to share a com m on
stru ctu re and form a p art o f the Ly-6/uPA R superfam ily o f G P I-anchored
proteins, characterized by a highly conserved spacing and disulfide bonding
o f cysteine residues. T he solution stru ctu re o f the L y -6 /u P A R fam ily
m em ber C D 59 was solved by nuclear m agnetic resonance [30] and confirm s
a prediction based on a variety o f considerations th at these proteins share
a com m on structural fram ew ork w ith the large fanily o f structurally defined
snake venom a-neurotoxins [62, 63], D efinitive p ro o f o f this as a m odel
for uP A R aw aits its direct structural determ ination, but the m odel has
already been useful in tentatively identifying a hydrophobic binding site in
uP A R [62, 63],
T h e un ique m u ltid o m ain stru ctu re o f uP A R an d its p o ten tial role
in the binding o f uPA and other putative uPA R ligands are o f c o n ­
tin u in g interest, as this m ultidom ain structure has proved to be essential
for uPA binding function. It was originally considered th a t the N-term inal do m ain 1 o f uP A R contained all the determ inants necessary for
uPA binding [4]. T he direct involvem ent o f this dom ain in ligand bin ­
ding is also supported by o th er observations, including the recent iden­
tification o f T y r57 as p art o f the uPA binding site [63]. It has been
show n in com petitive binding studies with recom binant C -term inally tru n ­
cated soluble uP A R (term ed s-uPA R ), however, th at liberation o f d o ­
m ain 1 reduces its uP A binding affinity by at least 1500-fold, from
0.1 nM to greater than 150 nM [62]. This suggests th a t it ca n n o t be
excluded th a t the o th er do m ain s o f u P A R directly co n trib u te to the
in teraction with uPA.
T h e uP A R gene is com posed o f seven exons (101, 111, 144, 162, 135,
147 and 563 bp) separated by six introns (approxim ately 2.04, 2.62, 8.42,
0.906, 3.10 and 2,78 kb) [86], E xons 1-7 encode 19, 37, 48, 54, 45, 49 and
83 am ino acid residues, respectively. A C pG -rich islands an d sequences
related to the tran scrip tio n factors AP-1, A P-2 and c-Jun are present, but
no p otential T A T A o r C A A T boxes were found in the proxim al 5’ region
o f the uP A R gene.
Tumor invasion and metastasis
As m entioned above, E M C degradation in cancer is a result o f an
in teraction between the epithelial tum or cells and the infiltrating strom al
cells, so the cellular localization o f uPA R in cancer cells m ay give inform ation
on its potential role in the process o f cancer invasion. In 49 from 60
sam ples derived from p atien ts w ith invasive d u ctal b reast carcinom as,
uP A R reactivity was observed in m acrophages located in close vicinity to
the infiltrating epithelial cancer tissue [66], In an o th er study, the cellular
localization o f uP A R was studied in 59 invasive breast cancer sam ples, 12
n o rm al breast tissue cases and 4 fibroadenom as [6], By using an anti-uP A R
polyclonal antib o d y it was found 49/59 invasive breast carcinom a to express
uP A R im m unoreactivity. S trong surface staining o f tum or-associated m ac­
rophages was evident in m ost o f the cases. Staining o f tu m o r cells was
observed in 21/59 cases while no staining was seen in norm al breast tissue
o r in the fibroadenom as. T he expression o f uP A R in tum or-infiltrating
m acrophages in breast cancer is consistent with the p attern o f uP A R
m R N A expression in hum an colon adenocarcinom as [64],
uP A R also appears to be a prognostic m ark e r in certain tum ors. In
colorectal m alignancies, high levels o f uPA R have been reported to cons­
titu te an independent prognostic m arker [31]. In breast cancer also, high
uP A R levels are associated with both shorter disease-free interval and
sh o rter overall survival [26, 32], F u rth erm o re, uP A R was show n to be an
independent m ark e r o f disease outcom e in squam ous carcinom a o f the lung
b u t n o t in large-cell lung cancer [59], Finally, it was dem o n strated th a t
expression o f uP A R by dissem inated carcinom a cells in bone m arro w of
p atien ts w ith gastric cancer is a highly significant p redictor o f early disease
relapse [34],
C O N C L U D IN G R E M A R K S
T he urokinase plasm inogen activator system consist o f pro-enzym es,
active enzymes and binding protein th at interact in a highly organized way
w ith E C M proteins facilitating cancer invasion and m etastasis.
uP A enhances in vitro cell m ig ratio n and invasion, b u t th ere are
suggestions th a t the function o f the system does n o t rely on the unbrided
enzym e activity o f uPA . T he dynam ic state o f the system at the cell surface
allows spatial and tem poral rearrangem ents o f its com ponents d u rin g these
processes. R ecent w ork has suggested the existence o f n o n -p ro te o ly tic
function o f the system, consisting o f intracellular signal-transduction cascades
being initiated by binding o f uPA to uPA R , o f uP A R as a vitronectin
receptor and as a regulator o f integrin function and o f a function o f PAI-1
as a reg u lator o f binding o f uP A R and o f intcgrins to vitronectin. Some
observations sup p o rt the hypothesis th at PAI-1 is needed for the optim al
function o f the uPA system in these processes, by being necessary for the
d ynam ic state o f the system , by regulating cell adhesion and for selective
protection o f vitronectin against proteolysis. PAI-1 m ay counteract m igration
and invasion.
T h e c o rrelatio n betw een uP A and uP A R c o n c en tratio n s in hum an
tu m o rs and p o o r prognosis is in agreem ent with the basic idea o f uP A R -bo u n d uP A at the surface o f cancer cells being necessary for cancer
invasion and m etastasis. uPA and uPA R engaged in tissue rem odelling
events in the tissues surrounding the cancer cells m ay also contribute to
the levels o f these com ponents and p o o r patient prognosis. T he role of
PAI-1 as a strong prognostic m arker in cancer m ay be related to a possible
role o f PAI-1 in cancer cell m igration and invasion. D ifferential expression
o f uPA and uP A R , on one side, and o f PA I-1, on the other, by different
cell types and in different tissue areas m ay contribute to proteolysis being
directional at the level o f tissue areas. In this situation, PAI-1 w ould be
playing a p art in tissue rem odelling events and strom a rem odelling by
protecting newly deposited E C M in the center o f islands o f cancer cells
and in the norm al tissue surrounding them.
F u rth e r studies are needed to elucidate the role o f the u ro k in ase
plasm inogen activator system in cancer invasion and m etastasis. A n exact
identification o f the processes in tum ors in which various com ponents o f
the uPA system are im plicated is a challenge for the future research. T he
no n -p ro teo lytic interactions o f the com ponents o f the system w ith ECM
proteins should be taken into account as possible m echanism involved in
tu m o r progression.
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W płynęło d o R edakcji
F o lia biochim ica et biophysica
24.04.1998
D e p a rtm e n t o f M olecular
G enetics
U niversity o f Ł ó d ź
Janusz Błasiak, Beata Sm olarz, Dagm ara Piestrzeniewicz
U R O K IN A Z O W Y U K ŁA D A K T Y W A C JI P L A Z M IN O G E N U I J E G O R O L A
W IN W A Z Y JN O Ś C I N O W O T W O R Ó W I IC II Z D O IJV O Ś C I D O P R Z E R Z U T O W A N IA
Inw azyjność n o w o tw o ró w i ich zd o ln o ść d o p rz erz u to w an ia są klinicznym i cecham i
progresji now otw orów i w ym agają degradacji i m igracji przez tk an k ę łączną przez kom órki
rakow e. Procesy te zależą od złożonego oddziaływ ania pom iędzy k o m ó rk am i no w o tw o ru
i otaczającą m acierzą zew nątrzkom órkow ą. O ddziaływ ania te są m odyfikow ane przez enzym y
p roteolityczne i ich receptory. U rokinazow y uk ład aktyw acji plazm inogenu odgryw a w ażną
rolę w inw azyjności now otw orów i ich zdolności d o p rzerzutow ania. U kład len zaw iera
u ro k in azo w y a k ty w ato r plazm inogenu (uPA ), receptor urokinazow y (uP A R ) i specyficzne
in h ib ito ry a k ty w ato ró w plazm inogenu, PAI-1 i PA I-2. R ecep to r uPA m oże znajdow ać się na
pow ierzchni k o m órek rakow ych. uPA m oże być uw alniany zarów no przez k o m ó rk i no rm aln e
ja k i zm ienione now otw orow o. K om pleks uP A -uP A R skupia aktyw ność p ro teo lity czn ą na
pow ierzchni k o m órek rakow ych przez konw ersję osoczow ego białka plazm inogenu d o pro teazy
serynow ej plazm iny. P lazm ina d egraduje m acierz zew nątrzkom órkow ą w sąsiedztw ie k o m órek
rakow ych ułatw iając w ten sposób inw azję now otw orów . uPA , u P A R i PAI-1 m ogą być
stosow ane ja k o m ark ery prognostyczne w wielu typach rak a.