
[Cancer Research 60, 4767-4770, September 1, 2000]
© 2000 American Association for Cancer Research
Astrocytes Contribute to the Brain-metastatic Specificity of Melanoma Cells by Producing Heparanase1
Dario Marchetti2,
John Li and
Ruijun Shen
Department of Neurosurgery, The University of Texas-Houston Health Science Center [D. M., R. S.], and Laboratory Medicine, M. D. Anderson Cancer Center [J. L.], Houston, Texas 77030
 |
ABSTRACT
|
|---|
Neurotrophins (NTs) modulate the brain invasion of melanoma cells and
the activity of an extracellular matrix degradative enzyme, heparanase,
that has been recently cloned. Heparanase degrades the heparan sulfate
proteoglycans (HSPGs) and is a critical mediator of tumor metastasis
and angiogenesis. Because astrocytes are among the first brain cells
encountered by extravasating melanoma cells, they may play important
roles in the development of brain metastases. To test this hypothesis,
we used purified in vitro astrocyte cultures and found
that they express heparanase transcript and functional enzyme that were
up-regulated by the prototypic NT, nerve growth factor. Coincubation of
astrocytes (or their conditioned medium) with brain-metastatic cells
resulted in a superadditive effect on heparanase activity and up to an
8-fold increase of in vitro chemoinvasion using purified
HSPGs. These observations indicate that astrocytes significantly
contribute to the brain colonization of melanoma cells via
heparanase-driven modalities.
 |
Introduction
|
|---|
Mechanisms responsible for the progression of malignant melanoma
to highly aggressive brain-metastatic disease remain largely unknown
(1)
. We have reported
NT3
effects on melanoma cell invasion and production of heparanase
(2
, 3)
. This enzyme is an
endo-ß-D-glucuronidase (4)
that degrades the
heparan sulfate chains of HSPGs, essential and ubiquitous
macromolecules associated with the cell surface and ECM of a wide range
of cells and tissues (5)
. Of relevance, human heparanase
has been cloned recently as a single gene family (4
, 6
, 7)
and found to be a potential target for antimetastasis drugs because of
its relevant roles in angiogenic and invasive processes
(8)
.
Astrocytes are among the brain cells encountered by extravasating
melanoma cells that have breached the first two layers of the BBB, the
brain microvessel endothelium, and the thick HSPG-enriched ECM.
Injury-reacting astrocytes are frequently found in areas surrounding
melanotic lesions and able to produce NGF, the prototypic NT
(9)
. Furthermore, they are capable of binding NT because
they express members of the Trk receptor family and the p75
low-affinity NT receptor (p75NTR; Ref.
9
). In this study, we have examined interactions between
astrocytes and selected melanoma clones capable of brain colonization
in vivo (10
, 11)
. We report that astrocytes
produce heparanase and potentiate melanoma-invasive properties as a
result from the concerted action of heparanases of neoplastic and
astrocytic origin.
 |
Materials and Methods
|
|---|
Cell Culture.
The human melanoma MeWo cellular system was obtained from Dr. Robert S.
Kerbel (Sunnybrook Health Science Center, Toronto, Ontario,
Canada). The MeWo parental line was propagated from a lymph node
metastasis and subjected to wheat germ agglutinin selection in tissue
culture (10)
. The MeWo brain-metastatic variant, named
70W, arose after multiple tissue culture passages in graded
concentrations of wheat germ agglutinin (10)
. 70W cells
are highly aggressive, with the capacity to colonize various organs and
to form brain colonies in nude mice (10)
. As such, target
organ site colonization by the 70W line is similar to the clinical
presentation of human melanoma metastasis in advanced disease stages.
Murine brain-metastatic melanoma B16B15b and parental B16F1 cells
(11)
were received from Dr. Garth L. Nicolson (Institute
for Molecular Medicine, Huntington Beach, CA). Purified astrocytes were
prepared from newborn rat or mouse cerebral cortices using a
modification of the method of McCarthy and deVellis (12)
.
Briefly, cerebral cortices were isolated, meningeal tissue was removed,
and tissue was incubated in calcium-free buffer (Hanks CMF)
containing 0.025% trypsin for 30 min at 37°C. Cells were dissociated
by trituration through a fire-polished Pasteur pipette, spun at
1000 x g for 5 min, and resuspended in 5 ml
of DMEM (Life Technologies, Inc., Grand Island, NY) containing 20%
fetal bovine serum (Life Technologies) and 2 mM
L-glutamine. After centrifugation, cells were
filtered through a 140- and 20-µm nylon filters, plated onto
polylysine-coated (0.1 mg/ml) 75-cm2 tissue
culture flasks at a density of 2.0 x 107 cells/flask, and incubated at 37°C with 5%
CO2 overnight. Primary cultures were then
enriched in astrocytes by removing the less adherent oligodendrocytes,
oligodendrocyte-type 2 astrocyte progenitor cells, and microglia (which
grow on top of astrocyte monolayers) by shaking at 250 rpm for 18 h at 37°C. The detached cells were removed and discarded. Cellular
confluence was reached 10 days after plating the cultures, showing a
polygonal flat cell morphology. The purity of the primary astrocyte
cultures was confirmed by immunostaining. The following antibodies were
used: a rabbit MAb against GFAP (Boehringer Mannheim, Indianapolis, IN)
that is a specific reactive astrocyte marker; a rabbit polyclonal
antibody against myelin basic protein (Dakopatts, Copenhagen, Denmark)
that recognizes oligodendrocytes; the antibody A2B5 (Chemicon
International, Temecula, CA), which binds to gangliosides and
recognizes oligodendrocyte-type 2 astrocytes; and a MAb against
vimentin (Santa Cruz Biotechnology, Santa Cruz, CA) that identifies
fibroblast contamination (13)
. Experiments were performed
with a nonspecific immune stimulation of the astrocytes in cross
species versus same species cocultures of astrocytes and
melanoma cells and as controls for possible effects attributable to
xenogeneic differences. Syngeneic astrocytes were used in experiments
involving B16 murine cell lines. To obtain ACM, astrocytes from
subconfluent primary cultures were seeded at 5 x 105 cells/well into 24-well plates (Costar,
Cambridge, MA) in complete medium. After 48 h, cells were washed
four times with serum-free medium and subsequently cultured for 48 h in 1.5 ml of serum-free medium supplemented with 5 µg/ml bovine
insulin, 5 µg/ml human transferrin, and 5 ng/ml sodium selenate
(ITS). The ACM was centrifuged at 5000 x g to remove any detached cells and cellular debris and
concentrated using Centricon-30 microconcentrator units (Millipore,
Bedford, MA).
Astrocyte Visualization by Epifluorescence Microscopy.
Confluent astrocytes monolayers were fixed in 4% paraformaldehyde in
0.15 M phosphate buffer, and coverslips were incubated in
rabbit anti-GFAP MAb (1:200 dilution), followed by a
rhodamine-conjugated secondary antibody (1:150 dilution; Zymed
Laboratories, Inc., South San Francisco, CA). Control experiments
included deletion or substitution of the primary antibody in the
labeling procedure. Astrocytes were then examined and photographed
using an inverted epifluorescence microscope (Diaphot-TMD, Nikon,
Japan).
Isolation of RNA, Heparanase Primers, and Semiquantitative
RT-PCR.
Astrocyte preparations were divided in two aliquots and incubated (or
not) with pure and biologically active recombinant rat NGF (R & D
Systems, Minneapolis, MN). Poly(A)+ RNA was
obtained from subconfluent cells using a mRNA purification kit (Qiagen,
Valencia, CA) according to the manufacturers instructions and
quantitated by ultraviolet absorption. RNA was reverse-transcribed with
avian myeloblastosis virus reverse transcriptase (Life
Technologies) and used for PCR with sense
(5'CTTCTAAGAAAGTCCACCTTC-3') and antisense
(5'-AAACTATATGAGAAAGCTGGC-3') oligonucleotide primers (Integrated DNA
Technologies, Coralville, IA) designed to amplify a 535-bp PCR product
specific for human heparanase (4)
. DNA sequence analysis
of this PCR product confirmed identity with the heparanase sequence.
All PCR reactions (25 µl) were performed with an annealing
temperature of 68°C, standardized over a wide range of cycles
(2040) to avoid plateau effects and to allow a semiquantitative
comparison. Aliquots of 10 µl of amplification products were
separated by 1% TBE (Tris-borate-EDTA) agarose gel electrophoresis and
bands visualized by ethidium bromide staining. PCR primers
(5'-TGAAGGTCGGAGTCAACGGATTTGGT-3', forward;
5'-CATGTGGGCCATGAGGTCCACCAC-3', reverse) designed to amplify a 983-bp
fragment of glyceraldehyde-3-phosphate dehydrogenase were used to
normalize the cDNA preparation. Quantitation of signal expression was
performed by densitometric analysis and computer integration of
normalized data.
Chemoinvasion and Heparanase Assays.
Melanoma cell invasion was assayed by cell culture chambers (Costar) as
reported (2
, 3)
. ACM (100 µl) was applied to
high-density (2.5 x 104
cells/filter) 70W and B16B15b cultures in chemoinvasion assays, the
Transwell filters (6.5-mm diameter; 8-µm pore size) of which were
coated previously with purified HSPGs (50 µg/filter). Invasion was
monitored by fluorescence using a multiwell plate reader (CytoFluor
Series 4000; PE Biosystems, Foster City, CA; Refs. 2
, 3
),
taking into consideration both background and map reading area of the
instrument. ECM-derived HSPG preparations were pure, as determined by
420% gradient SDS-PAGE and Western blotting using HSPG MAb.
Heparanase activity was determined by degradation of purified ECM
[35S]HS using high-speed gel permeation
chromatography or by agarose gel electrophoretic analyses. We have
demonstrated previously that: (a) B16B15b and 70W
brain-metastatic clones possess higher heparanase content than their
respective parental counterparts (B16F1 and MeWo; Ref. 2
);
and (b) the heparanase activity is indistinguishable in
these two cellular sources by the two heparanase assays used (3
, 14)
. Only data within the linear range for relative activity
measurements were taken into account, with relative degradation
activity determined by the amount of radiolabeled HS degraded/min/µg
of protein.
 |
Results
|
|---|
Heparanase Expression and Its Functionality in Astrocytes.
We investigated heparanase presence in astrocyte cultures. Primary
glial cells were obtained from newborn rat or mouse cerebra, and their
identification as astrocytes was established by positive
immunoreactivity with an antibody against the astrocyte-specific
intermediate filament GFAP (Fig. 1A)
. Neither vimentin, myelin basic protein, nor A2B5 was
expressed by the astrocyte cultures, confirming their purity (data not
shown). A specific heparanase transcript (5.0 kb) amplification was
detected by semiquantitative RT-PCR and up-regulated in astrocytes
incubated previously with purified and biologically active NGF (Fig. 1B)
. Similar results were obtained in human brain-metastatic
70W cells (Fig. 1B)
that are known to possess NGF-regulated
heparanase activity (2)
. Densitometric analyses from four
independent experiments revealed that NGF caused a 1.52.3-fold
increase of heparanase RT-PCR product in astrocytes, whereas up to
almost a 4-fold increase was obtained after NGF treatment of 70W cells.
Heparanase is a hydrolase that differs from heparinases or HS-specific
elimination enzymes, cleaving HS into characteristic large molecular
weight fragments, approximately one-third of the original HS size
(15)
. Heparanase activity was detectable and NGF
regulated in cellular extracts from purified astrocytes. Distinct HS
degradation products were detected by gel shift assays (Fig. 2)
or by HPLC analysis (Fig. 3)
.

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Fig. 1. A, fluorescence micrography of isolated
cerebral cortical astrocytes immunostained for the astrocyte-specific
intermediate filament GFAP. B, semiquantitative PCR
analysis for heparanase expression from rat cortical astrocytes and
human brain-metastatic 70W melanoma. Independent cortical astrocytes
preparations from rat cerebra were not exposed (Lanes 1
and 2) or exposed (Lanes 3 and
4) to NGF (4 nM for 6 h at 37°C).
Similarly, the presence and up-regulation of the human heparanase
transcript were analyzed in brain-metastatic 70W not exposed
(Lane 5) or exposed to NGF (Lane
6). See "Materials and Methods" for
experimental details.
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Fig. 2. Purified [35S]HS from brain subendothelial
ECM HSPGs was incubated with astrocyte cell extracts at indicated
times: A: 0 (Lane 1), 1 h
(Lane 2), 3 h (Lane 3), 6 h
(Lane 4), 12 h (Lane 5), 24 h
(Lane 6); extract treated with the heparanase inhibitor
suramin (100 µM; Ref. 14
; Lane
7). B: 0 (Lane 1); 6 h,
extract from NGF-treated astrocytes (Lane 2). Isolation
and characterization of [35S]HS and heparanase assays
were performed as reported (Ref. 3
; see also "Materials
and Methods"). Estimation of molecular sizes is based on elution
profiles from Superose 6 column calibrated with standard glycans
(14)
. The resulting products were separated by agarose gel
electrophoresis and visualized by autoradiography.
|
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Cooperativity Between Melanoma and Astrocytic Cell Heparanases.
We analyzed heparanase activity for brain-metastatic and astrocytic
cell populations in logarithmic growth. We obtained HPLC-derived
elution profiles of HS-digested products by heparanase assays performed
at various incubation times (2)
. Highly brain-metastatic
70W cells produced a gradual and time-dependent increase of heparanase
activity for up to 72 h. The same was found when astrocytes were
used (Fig. 3)
. However, coincubation of 70W and astrocytes in
equicellular amounts, followed by heparanase assays at defined time
intervals, resulted in a superadditive increase of enzymatic activity
(Fig. 3)
. These results were confirmed coincubating the murine B16B15b
cell line, a brain-metastatic B16F1-variant (11)
, with
syngeneic astrocytes. Heparanase levels were as much as 65% lower at
confluency in melanoma and astrocytic cells, alone or together (data
not shown).
ACM Enhances Invasion via Heparanase-driven Mechanisms.
We wanted to determine the relevance of melanoma and astrocytic cell
heparanases in invasion. We incubated ACM with 70W and B16B15b and
examined ACM effects on their invasive behavior. Consistent increases
in invasive values after ACM exposure to 70W and B16B15b were found.
Invasion was most pronounced using CM from NGF-treated astrocytes and
completely abrogated in presence of an antibody against human
heparanase in its activated form (HEP; Ref. 16
; Table 1
). The invasion enhancement caused by this NGF treatment was also
abolished in presence of a neutralizing NGF MAb (17)
.
Controls for chemoinvasion specificity consisted in seeding the
invasive but non-brain-metastatic parental cells (human MeWo and murine
B16F1). In both cases, invasion rates were significantly less
pronounced when compared with brain-metastatic 70W and B16B15b cells
(data not shown; Ref. 2
).
 |
Discussion
|
|---|
Because the brain microvasculature represents an exceedingly small
fraction of the total microvasculature, much smaller than the
proportion of tumors that metastasize to the brain, the process of
brain metastasis is thought to be selective and nonrandom
(1)
. Neoplastic cells with the potential to colonize the
brain may express unique molecular determinants and may also respond to
brain-derived factors important for their invasion in the brain
(18)
. Malignant melanoma is the cancer type with the
highest increase of cases among young adults and one with the highest
frequencies of brain metastases formation (1)
. Melanoma
cells that have successfully penetrated the first two zones of the BBB,
the brain microvessel endothelium and the thick basement membrane, must
then establish interactions with, and respond to, astroglial cells.
Among the different cytokines that are produced by astrocytes, NGF play
a specific role on brain-metastatic cells. Recent studies have
indicated that melanoma cell-surface receptor
p75NTR and its ligand, NGF, are excellent
candidates as determinants of melanoma brain metastasis (2
, 17
, 20)
. For example, p75NTR strongly
correlates with brain colonization potential and is capable of
signaling in response to NGF independently of its high-affinity
counterpart TrkA (19)
. Moreover, NGF is produced by normal
brain tissue at the invasion front of melanoma lesions growing in the
brain (20)
. Exogenously supplied NGF, and possibly other
NTs, may stimulate melanoma cells to invade further into the brain
parenchyma by augmenting the production of heparanase, which degrades
the glycosaminoglycan side chains of HSPGs (3
, 4)
and play
important roles in tumor metastasis (15)
. Interestingly,
brain-metastatic melanoma cells express transforming growth
factor-ß1, interleukin 1ß, and basic fibroblast growth factor,
factors that have been shown to stimulate NGF synthesis by astrocytes
(18
, 21) . Therefore, brain metastasis outcome can depend
on interactions with, and responsiveness to, astrocyte-released NGF
that aid in the survival, growth, and invasion of
p75NTR-expressing melanoma cells in the central
nervous system.
We report that astrocytes contribute to the invasive capabilities of
brain-colonizing melanoma cells. Importantly, our laboratory is the
first to provide evidence that astrocytes are per se a
source of heparanase, able to produce the active enzyme and modulate
cell invasion. We suggest that astrocytes contribute to the
microenvironment that selectively supports the growth of metastatic
cells once they have crossed the BBB, thus being directly involved in
the development of brain metastases. Our data support the concept that
melanoma brain invasion results from establishing reciprocal circuits
between the tumor cells and the normal glial cells present in the
central nervous system. After mechanical/chemical brain insults,
increased NGF presence is imperative for regeneration events of injured
areas (18)
. These changes may be paralleled by brain
invasive melanoma cells, the colonization of which can trigger NGF and
NGF-regulated heparanase secretion by brain cells as a response to the
invasion event. Melanoma cells, known to overexpress
p75NTR (19)
, can benefit from such a
synergistic microenvironment and further invade into the brain
parenchyma.
Questions related to the benefit of therapeutic suppression of the
NGF/heparanase axis in brain-metastatic melanoma remain unanswered.
Further studies will be useful to address these questions.
 |
ACKNOWLEDGMENTS
|
|---|
We would like to thank Dr. Philip Tofilon, M. D. Anderson
Cancer Center, for comments and help in the initial stages of our
studies, Drs. Motono Nakajima and Jian Dong, Novartis Pharma K. K.
Takarazuka, Japan for helpful discussion, and Dr. Robert L. Heinrikson,
Research Operations, Pharmacia and Upjohn, Inc., Kalamazoo, MI, for
making available antibodies against human active heparanase.
 |
FOOTNOTES
|
|---|
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1 Supported by Grant NIH R-29 CA64178 (to D. M.)
and by the Vivian L. Smith Foundation for Neurological Research. 
2 To whom requests for reprints should be
addressed, at Department of Neurosurgery, University of Texas-Houston
Health Science Center, MSB 7.136, 6431 Fannin Street, Houston, TX
77030. Phone: (713) 500-6137; Fax: (713) 500-7787; E-mail: Dario.Marchetti{at}uth.tmc.edu 
3 The abbreviations used are: NT, neurotrophin;
NGF, nerve growth factor; HSPG, heparan sulfate proteoglycan; BBB,
blood brain barrier; ECM, extracellular matrix; ACM, astrocyte
conditioned medium; GFAP, glial fibrillary acidic protein; HPLC,
high-pressure liquid chromatography; MAb, monoclonal antibody; CM,
conditioned medium. 
Received 4/27/00.
Accepted 7/20/00.
 |
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F. Uno, T. Fujiwara, Y. Takata, S. Ohtani, K. Katsuda, M. Takaoka, T. Ohkawa, Y. Naomoto, M. Nakajima, and N. Tanaka
Antisense-mediated Suppression of Human Heparanase Gene Expression Inhibits Pleural Dissemination of Human Cancer Cells
Cancer Res.,
November 1, 2001;
61(21):
7855 - 7860.
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A. Koliopanos, H. Friess, J. Kleeff, X. Shi, Q. Liao, I. Pecker, I. Vlodavsky, A. Zimmermann, and M. W. Buchler
Heparanase Expression in Primary and Metastatic Pancreatic Cancer
Cancer Res.,
June 1, 2001;
61(12):
4655 - 4659.
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O. Goldshmidt, E. Zcharia, H. Aingorn, Z. Guatta-Rangini, R. Atzmon, I. Michal, I. Pecker, E. Mitrani, and I. Vlodavsky
Expression Pattern and Secretion of Human and Chicken Heparanase Are Determined by Their Signal Peptide Sequence
J. Biol. Chem.,
July 27, 2001;
276(31):
29178 - 29187.
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