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Novel applications of growth factors in solid tumors
Westermann, A.
Publication date
1999
Link to publication
Citation for published version (APA):
Westermann, A. (1999). Novel applications of growth factors in solid tumors.
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Anneke M. Westermann
1, Jos H. Beijnen
1, Wouter H. Moolenaar
2,
Sjoerd Rodenhuis
1From
1the Department of Medical Oncology and
2the Division of
Cellular Biochemistry, The Netherlands Cancer Institute,
Amsterdam, The Netherlands.
n
• o
CD
Growth factors in human ovarian NJ
cancer
Abstract
Background Ovarian cancer remains confined to the peritoneal cavity, even in the late stages of the disease. This particular growth pattern might be the result of the activity of growth factors present in malignant ascites, a common feature of advanced disease. However, presently identified growth factors cannot fully account for the mechanics of ovarian cancer.
Methods Papers on growth of ovarian cancer cell lines in vitro and the clonogenic assay of human ovarian cancers, and the factors influencing them are summarized. Presently known peptide growth factors were reviewed for their association with human ovarian cancer, as were cytokines. The information on the growth-promoting properties of ascites was surveyed, and the available evidence for possible novel growth factors was collected.
Results Growth of ovarian cancer cells, both in cell lines and in the clonogenic assay, is stimulated by peptide growth factors, although no combination of peptide growth factors can replace serum or cell free ascites. The growth-promoting characteristics of ascites seem to be completely explained by the presence of lysophosphatidic acid (LPA), a bioactive phospholipid with mitogenic features.
Conclusion The bioactive lipid LPA seems to be an important growth promoter present in ascites of ovarian cancer patients. LPA inhibition may be a novel target for therapy.
Introduction
Human ovarian cancer is the leading cause of death from gynecologic malignancy. In most cases, the initial diagnosis is established at an advanced stage, when current therapy can only cure a small proportion of patients. Even in the presence of advanced disease, there is rarely evidence of tumor cells outside the peritoneal cavity.1 Local growth factors may play a part in
promoting intraperitoneal spread and growth, and it is conceivable that these could represent unique targets for therapy.
Although many studies suggest that malignant ascites may contain one or more specific growth factors required for the proliferation of ovarian cancer cells, so far no specific ascites-borne growth factors have been isolated and identified that are candidates for such a role. A group of bioactive lipids, the lysophosphatidic acids, are present in both serum and in malignant effusions.2 It is possible that the elusive ascitic growth factor may belong to this novel group of
established mitogens. The present review summarizes current insights into the function of well-characterized growth factors in ovarian cancer growth, and discusses the potential influence of the abovementioned ascites-borne lipid growth promoters.
Growth of ovarian carcinoma cell lines in vitro
A large number of ovarian cancer cell lines have been described since the early seventies.323
Media used to maintain cell lines were routinely supplemented with antibiotics, vitamins and fetal calf serum (FCS) or horse serum. Although insulin, transferrin, gonadotropins, estradiol, dexamethasone, L-thyroxin or epidermal growth factor (EGF) were added in some instances, human ovarian cancer cell lines almost universally required the addition of serum. In Table 1, a selection of human ovarian carcinoma cell lines with their requirements for growth in vitro is listed.
Establishment of ovarian cancer cell lines in serum-free medium is apparently difficult. In the early 1980s, however, investigators succeeded in growing rat ovarian cancer cells in serum-free medium by adding EGF and transferrin.24 The serum requirement to sustain human ovarian
cancer cell lines could be reduced from 20 down to 0.5% when suitable growth factors were employed (table 1). In the complete absence of serum, serum components such as fetuin or bovine serum albumin were usually necessary to maintain the cell lines.10'2022'23 Rare exceptions
to this rule include certain subcultures of established ovarian cancer cell lines, as reported by Jozan ef a/.22 and Golombick etal.25 The latter described a cell line grown in a solution of amino acids, vitamins and simple salts. Berchuck etal. could sustain growth of ovarian cancer cell lines in serum-free medium for 48 hours, but not in permanent culture.26
It has been suggested that cell lines requiring little or no serum for in vitro growth are less well differentated than those that do not grow in chemically defined media,10 but this has not
Table 1. Reports of permanent human ovarian cancer cell lines in vitro.
Cell line Supplements References
154, 163 20% fetal calf serum (FCS), antibiotics DiSaia 3
#2774 20% FCS, antibiotics Freedman 4
163 20% FCS, antibiotics Kanabus 5
HeW 20% FCS Pattillo 6
A7, A10 10% FCS, antibiotics Abu Sinna 7
COLO 110, 316, 319, 330 10-20% FCS, antibiotics Woods 8
NIH:OVCAR-3 2% FCS, insulin 10 lU/ml, antibiotics Hamilton 9
EFO-3, EFO-21, EFO-27, 10% FCS, fetuin 5 |ig/ml, transferrin 2.5 ng/ml, insulin Simon 10 EFO-47, EFM-63 0.08 lU/ml, human menopausal gonadotropin
0.04 lU/ml, L-thyroxine 10 nM, antibiotics
EFO-21, EFO-47 fetuin 5 u.g/ml, 0 . 1 % bovine serum albumin, transferrin 2.5 ng/ml, insulin 0.08 lU/ml, L-thyroxine 10 nM, antibiotics
OAW42 10% FCS Wilson 11
CAOV-3, SKOV-3, HOC-1, 10-15 % FCS Buick 12
HOC-7, HEY
la 288 10% horse serum, 10% FCS, 1 \i/\ transferrin, T3 6.5 ng/ml, dexamethasone 14 ng/ml, 108M
estradiol, insulin 20 |ig/ml, antibiotics
Clamon 13
IGROV1 10% FCS Benard 14
JA-1.TR175, TR170 10% FCS Hill 15
HTOA 15% FCS Ishiwata 16
PE01, PE04, PE06, PE014, 10% FCS Langdon 17
PE016, PE023, PEA1, PEA2, T014
DO-s 2-15% FCS Briers 18
A1, A69, A90, A121 20% FCS Crickard 19
UWOV1, UWOV2 2-5% FCS, antibiotics Golombick 20
UWOV2 (Sf) insulin 1 mg/l, transferrin 1 mg/l, bovine serum albumin 500 mg/l
OMC-3 10% FCS Yamada 21
OVCCR1/sf insulin, transferrin, epidermal growth factor (EGF) in varying concentrations3
Jozan 22 SCHM-1, RIC-2, MAC-2, 3% FCS, antibiotics, EGF 5 ng/ml, insulin 5 ng/ml,
SIB-1 transferrin 10 u,g/ml
MAC-2, SIB-1 Bovine serum albumin 3 mg/ml, antibiotics, EGF 5 ng/ml, insulin 5 ng/ml, transferrin 10 ng/ml
Hirte 23
UWOV2 (Pf) vitamins Golombick 25
a0.5% serum was required for each passage to allow cell adhesion to the plastic.
been confirmed in later publications.23
The scarcity of serum-free cell lines strongly suggests the presence of as yet unknown growth promoting factors in serum. A large number of potential candidates for this serum-derived growth factor have been proposed and will be discussed below.
Clonogenic assay of human ovarian carcinoma cells
In the late 1970s, interest arose in the isolation of human tumor cells directly from patients for drug sensitivity testing and other studies. Successful direct cultures of human cancer cells were rarely described27 until Hamburger and Salmon developed a clonogenic assay providingconditions for cell growth of most tumor types.28 In their original bioassay, tumor cells were
cultured in a two-layer system, with the upper layer containing tumor cells suspended in agar supplemented with horse serum, antibiotics, glutamin, CaCI2, insulin, asparagine, dextran and
2-mercaptoethanol. The lower agar layer contained medium conditioned by adherent spleen cells of mineral oil-primed BALB/c mice. The majority (85 to 100%) of cells obtained from either biopsies or effusions of ovarian cancer patients could form colonies using this method.29 32 Selected reports on conditions for the clonogenic assay of human ovarian cancer cells are
summarized in Table 2.
It later turned out that feeder layers were not the only method to allow clonogenic in vitro growth, although it proved impossible to clone ovarian cancer cells in serum-free conditions with the addition of wellknown growth factors alone.33 For in vitro growth, ovarian cancer cells
may require a specific and individual set of growth factors, as has been suggested for other tumors grown in serum-free conditions, such as small cell lung cancer.34
Peptide growth factors
Peptide growth factors play a pivotal role in the control of proliferation of normal and tumor cells.35 They exert their action through binding of a specific cell surface receptor after secretion
into the extracellular environment. The capacity of tumor cells to produce and react to their own growth factors via functional external receptors is a central principle that has been termed 'autocrine secretion' or 'autocrine loop'. This paradigm includes the notion that growth factors can cause malignant transformation through both overproduction of positive autocrine growth factors and the loss of negative growth control present in normal cell growth.36
The role of peptide growth factors in human ovarian cancer cell lines and in direct cultures is summarized in Table 3.
Epidermal growth factor family
Human epidermal growth factor (EGF) was among the first polypeptide growth factors isolated.37
It stimulates growth of normal human epithelial cells in culture as well as clonogenic growth of human tumor cell lines and freshly isolated tumor material, although inhibition of some cell lines
EGF-Table 2. , Reports on short term culture of human ovarian carcinoma from bi opsies or effusions.
Mat.a Medium Supplements Growth col/5x10 5 cells Ref.
T/E 78 Eagle's modified essential medium (MEM) E 8 two-layer system (TLS) of
0.3% agar in CMRL 1066, on medium conditioned by adherent spleen cells of mineral oil-primed BALB/c mice T/E 31 -TLS with medium conditioned
by adherent spleen cells of mineral oil-primed BALB/c mice -TLS with type-0 human
red blood cells
-TLS with medium conditioned by CD-1 or DBA/2 or non-oil primed BALB/c mice -TLS with medium conditioned
by MA-184, Wi-38 cell lines -TLS with cell-free autologous effusion 1:4
E 27 standard TLS T 51 enriched Eagle's MEM
E 13
T6 enriched Eagle's MEM in 0.3% agar
15%FCS, 10% human cord serum
15% PCS, 20% horse serum, antibiotics, 2-ME
15% horse serum, antibiotics, +/- 2-ME
10%FCS, 10% human cord serum, antibiotics, irradiated xenogeneic feeder layer of adherent peritoneal cells
10%FCS, 10% human cord serum, antbiotics
10%FCS, 10% human cord serum, antibiotics, irradiated xenogeneic feeder layer of adherent peritoneal cells 10% PCS, 10% human cord serum, antbiotics
69% 100% 10% PCS, 15% horse serum 81 % 4 1 % 3 1 % 100-800 85% 99-2549 320-833 109-320 0 29 20-620 100% 299(26-1464) 100% 507(50-2528) 27 28 29,30 32 31 T/E 16 T35 standard TLS 15% FCS, 20% horse 53% standard TLS, with 0-75% enriched CMRL 1066 in top layer serum 15% FCS, 20% horse serum, substitution of CMRL with 25-100% cell free ascites 71%
standard TLS 2.5-15% horse serumb
EGF 50 ng/ml, insulin 5 p.g/ml, transferrin 5 u,g/ml, selenium 5 ng/ml, 17B-estradiol W M , hydrocortisone lO^M 66% 0 113 33
Table 3. Overview of growth factors in ovarian cancer
Substance Effect on ovarian
cancer cell growth
References
Epidermal growth factor (EGF)
Amphiregulin (AR)
Transforming growth factor ß (TGF-ß) Platelet derived growth factor (PDGF) Basic fibroblast growth factor (bFGF) Insulin-like growth factors (IGF) Interleukin 1 (IL-1)
Interleukin 6 (IL-6)
Tumor necrosis factor a (TNF-oc)
Macrophage-colony stimulating factor (M-CSF) Cell free ascites (CFA)
Lysophosphatidic acid (LPA)
dual 26,39
stimulating 22,38,40,41 (cell lines),33,43-46 (direct cultures)
dual 65
inhibiting 39,42,70-72 (cell lines),73 (direct cultures) no effect 26 stimulating 19 stimulating 85 stimulating 93 no effect 95 stimulating 93 no effect 97 stimulating 11,38,58,116,117 (celllines),29,30 105, 113,118 (direct cultures) stimulating 128, Jalink 1995 (unpublished
observations)
receptor commonly present on many different cell types, including ovarian cancer cells. This general g r o w t h stimulating effect has been reported in most ovarian cancer cell lines26-3842 and
also in direct clonogenic assays of ovarian carcinoma.33'43-46 Some cell lines, however, showed no
response to EGF,26 or were inhibited by EGF. Varying responses of cell lines to peptide g r o w t h
factors are common. They are not usually dependent on concentration, but they are possibly due to different post-receptor events in the various cell lines.39
EGF was later shown to be part of a family of structurally homologous peptides also including transforming growth factor a (TGF-a)47 and amphiregulin (AR).48 The members of this family bind
to the EGF-receptor (EGFR), causing receptors to dimerize and initiate intracellular signalling. The EGFR has marked homology with the human c-erbB-2 gene (which is identical to HER-2 or the rat neu oncogene) product, a 185 kDa protein (p185e*B2) with tyrosine kinase activity.49 Although
TGF-a and EGF do not bind to p185e*B2, the not yet fully characterized 30 kDa glycoprotein ligand
of the erbB-2 oncogene product has been shown to interact directly with the EGFR.50 Other members
of this receptor family, c-enbB-3 and c-erbB-4, have recently been identified, and the dose similarity in structure of the four receptors allows extensive heterodimerization of these different receptor
a T denotes solid tumor, E effusion, and T/E both as a source of malignant cells, followed by the
number of patients tested.
b The serum requirement could be reduced by the growth factors and hormones specified in the
serum-free experiments.
subtypes.51-52 Immunohistochemically, increased expression of these receptor proteins has been
demonstrated in malignant ovarian tumors compared to benign ones.53
TGF-a differs from EGF in that it can induce anchorage-independent growth not only in cancer cells but also in untransformed, non-neoplastic, indicator cells.54 TGF-a promotes ovarian cancer
cell line growth in vitro and enhances direct clonogenic growth of ovarian tumor cells, thereby mimicking EGF,3M1 while the amphiregulin effects are similar but less pronounced.48
Evidence to support the autocrine loop concept in ovarian cancer was presented by Bauknecht
et al., who documented elevated levels of EGF and of EGF-like factors in crude cellular extracts
of ovarian tumors compared to non-malignant tissue.55 In contrast, neither Berchuck etal.26 nor Ottensmeier et a I.56 found EGF activity in ovarian cancer cell-conditioned media. Production of EGF and TGF-a by ovarian cancer cell lines was, however, demonstrated by Kurachi etal.40 and Stromberg et al.57 Hanauske et al. showed a higher frequency and level of TGF-a in malignant effusions in a variety of tumors including ovarian cancer, as compared to benign effusions.58
Further support for a TGF-a/EGFR autocrine loop in ovarian cancer was provided by experiments with primary tumor cultures in which TGF-a but not EGF was detected in EGFR-positive ovarian tumors.59 Gordon etal. reported mRNA expression of EGF, TGF-a, AR, EGFR and erbB-2 in three
ovarian cancer cell lines, both in serum-supplemented and serum-reduced conditions.60 Other
authors found that in the presence of exogenous EGF the amount of serum needed to sustain in
vitro proliferation of cell lines42 and direct clonogenic assay33 could be lowered, though it could
not compensate completely for the serum requirement.
EGFR-positive ovarian cancer cells appear indeed to require the presence of a growth stimulating EGFR ligand for growth in vivo, as shown by experiments in nude mice who were EGF depleted by sialoadenectomy. Subcutaneous injection of EGFR-positive ovarian cancer cells led to tumors in animals receiving exogenous EGF, but not in those not receiving the growth factor.61
In ovarian cancer cell lines overexpression of erbB-2 is frequently encountered.64 erbB-2
expression can be detected by immunohistochemistry in 30 to 70% of human ovarian cancers,62'63
but in only 5 to 10% of normal ovarian cells.64
Both normal and ovarian cancer cells express and respond to amphiregulin. At low concentrations (picomolar range), amphiregulin inhibits growth of certain normal and malignant ovarian cells. At higher concentrations (nanomolar range) growth stimulation can be observed. These effects may vary between cell lines,65 as described above for EGF effects.
Transforming growth factor ß
The hallmark of transforming growth factors (TGF) is their ability to confer a transformed phenotype on untransformed indicator cells. In the early 1980s the TGFs were found to include two groups: TGF-a mentioned previously, which is structurally and functionally homologous
with EGF, and TGF-ß.66 TGF-ß does not bind to the EGFR, but a synergistic effect was observed
with EGF orTGF-a, leading to a potent transforming action on untransformed indicator cells.67
Later research indicated that the predominant effect of TGF-ß on normal cells in vitro is inhibition of proliferation. This dual effect makes TGF-ß a central mediator of cell growth regulation. TGF-ß is considered a principal negative control element in cell growth, and it is present in most body fluids and tissues.68 Loss of responsiveness to the growth inhibitory effects of TGF-ß may be a
general characteristic of malignant transformation. The TGF-ß family can be divided into at least three different subtypes in humans with similar in vitro functions and with multiple receptor subtypes.69
Studies on the effects of TGF-ß on ovarian tissue showed inhibition of growth of normal epithelial ovarian cells,70 ovarian carcinoma cell lines,39'427072 and directly cultured ovarian cancer cells,73
However, some cell lines in these studies showed no response to exogenous TGF-ß at all.26'70 or
only to one or two particular subtypes of TGF-ß.72
Production of TGF-ß was demonstrated in normal ovarian tissues,7074 ovarian cancer cell
lines,4270-7275 and directly cultured ovarian cancer cells.73-74 Though some tumors seemed to
have lost the capacity to produce TGF-ß,73 others were found to have increased expression of
certain TGF-ß isoforms.74 TGF-ß is secreted by most cells in a biologically inactive form and
regulation of bioactivation may be of physiologic importance. As a result, immunohistochemical demonstration of the peptide in tumors or cells may not directly reflect its biological significance.
Platelet derived growth factor
Platelet derived growth factor (PDGF) is a polypeptide growth factor first described in the 1980s as a megakaryocyte-derived mitogen for mesenchymal cells, but later it was also shown to be synthesized by a plethora of other normal and transformed cells. It stimulates both epithelial and endothelial cells in an autocrine fashion.76 The growth factor has three isoforms and binds
to two defined receptors (termed PDGFRa and PDGFRß).77
Normal ovarian tissue or benign ovarian tumors do not show PDGF expression, although PDGF-expression is common (70-100%) in ovarian cancer tissue and in cell lines,78 as detected
by immunohistochemistry. PDGFRa is less frequently expressed than PDGF in human tumors (0-35%).79 PDGF was found to have an autocrine role in a cell line growing in a defined
serum-free medium.75 Berchuck et al. found no stimulation of ovarian cancer cell lines by exogenous
PDGF,26 consistent with the lack of expression of PDGF receptors in other studies. It has therefore
been suggested that PDGF acts in a paracrine fashion in ovarian tumors and mediates supportive connective tissue stroma formation.71
Basic fibroblast growth factor
Basic fibroblast growth factor (bFGF) is part of a family of heparin-binding growth factors for many mesodermal and ectodermal cells, though originally described as a mitogen for cultured fibroblasts. The family consists of at least seven distinct proteins and four receptors. (For review see 80 ,81)
bFGF as well as the FGFR1 receptor subtype have been detected in ovarian cancer cell lines, while exogenous bFGF stimulated ovarian cancer cell line proliferation.82 An earlier study indicated
growth stimulation by FGF of only one of four ovarian carcinoma cell lines.26
Insulin-like growth factors
The insulin-like growth factors (IGF) IGF-1 and IGF-2 are polypeptides with endocrine, paracrine and autocrine effects on cell proliferation. Their mitogenic effects in vitro are suggestive of a role in growth regulation and even in the oncogenesis of malignant tumors. The IGFs are present in body fluids, bound to specific binding proteins that not only carry IGF, but also regulate access to the IGF receptors, thereby mitigating IGF's mitogenic action, (reviewed in 83) Both IGF-1, its binding proteins and its receptor are expressed in ovarian cancer cell lines and ovarian tumors.84 In a small study, Karasik et al. found that levels of IGF-1 and one of its binding
proteins could discriminate between malignant and benign ovarian tumors.85 IGF-1 stimulated
the growth of two ovarian cancer cell lines, whereas specific IGF-1 receptor RNA anti-sense oligodeoxynucleotides markedly stilted the growth of those cell lines. This hints at a role in the autocrine growth of ovarian cancer cell lines for IGF and its receptor,86 though more extensive
data are required to corroborate these findings.
Cytokines
Cytokines are peptide molecules initially thought to modulate the proliferation and bioactivity of cells of the immune system. It is presently clear that cytokines are peptide factors that mediate a host of different reactions in different cell populations and in the extracellular environment, and the distinction between cytokines and other peptide growth regulatory factors has lost most of its significance. The cytokine family was originally divided into interleukins (thought to be produced by leukocytes), lymphokines (originating from lymphocytes), monokines (released by monocytes), interferons (IFN) and colony-stimulating factors (CSF). These names stem from historical concepts about function and origin of these proteins, that are largely structurally unrelated, (reviewed in 87)
type, dose and circumstances.88 A large body of evidence has been accumulated that cytokines
are produced by ovarian tumors, but growth modulatory effects of cytokines have not been decisively documented, (for a review of the role of cytokines in ovarian cancer see ref 89).
Tumor necrosis factor
TNF was one of the first cytokines to be identified.90 Although the subtypes TNF-a and TNF-ß
have a similar scale of biological activities, TNF-ß effects are generally weaker, and TNF has come to mean TNF-a unless specified otherwise in most publications. Both TNF and its encoding mRNA have been detected in uncultured ovarian cancer specimens,9193 although Wu eta/.found
no expression in ovarian cancer cell lines in vitro.93 Exogenous TNF as well as interleukin 1 (IL-1) could reinduce TNF expression in a clonogenic assay of ovarian cancer cells that had lost this ability after 7 days in culture.93
Interleukin 1
IL-1 is a cytokine produced primarily by monocytes and macrophages, with both inhibitory and stimulatory effects on human tumor cells. Two subtypes, IL-1 a and IL-1 ß, can be discerned, that bind to the same receptors. Promotion of tumor metastases by IL-1 has been described for some tumors,94 and in a proportion of ovarian tumors IL-1 can be detected.89 IL-1 can induce
TNF expression in a primary culture of ovarian cancer cells, as described above.93
Interleukin 6
II-6 is a pleiotropic cytokine with both growth- and differentiation-inducing activity, that can be produced by T lymphocytes, monocyte/macrophages, and a variety of tumor cells. It is also secreted by ovarian cancer cell lines and by ovarian tumor cells in the clonogenic assay, and it can be detected in malignant ascites.95 IL-6 mRNA expression has furthermore been demonstrated
in normal ovarian epithelial cells.96 However, exogenous IL-6 did not stimulate proliferation of
six different ovarian cancer cell lines.95
Culture supernatants from human monocytes contain significant amounts of IL-6, TNF-a and IL-1, and stimulate clonogenic growth of ovarian cancer cell lines in vitro.97 These growth factors could, however, not completely account for the growth enhancement observed when ovarian cancer cells were grown in peripheral blood monocyte-conditioned medium,97 suggesting that
as yet unidentified monocyte-derived factors also play a part.
Macrophage colony stimulating factor
Macrophage-colony stimulating factor (M-CSF, also called CSF-1) enhances monocytopoiesis, attracts macrophages and can stimulate production of TNF-a. In ovarian cancer cell lines and in
ovarian cancer in vivo, expression of M-CSF,98100 and its receptor101J02 could be demonstrated,
while normal ovarian epithelial cells do not express M-CSF. Addition of either one of the colony stimulating factors M-CSF, G-CSF or GM-CSF did not influence growth of ovarian tumor cell lines,97 although this has been reported for other CSF-receptor positive cancer cell lines.103
Cytokines in ascites
Increased levels of CSF,104'105 TNF-a,100'106<107 |L-1 ß95-100-106 and |L-6 95-100-l0S-108 have been detected
in effusions of ovarian cancer patients,100 while benign ovarian follicular fluid contained much
lower levels, especially of IL-6 and ll_-1ß.106 It has been suggested that high CSF levels in ascites
correlate with a poor prognosis.105 The cellular source of these cytokines is uncertain. Malignant
effusions are known to contain large numbers of macrophages. An increased number of cytokine-producing macrophages found at ovarian cancer sites may account for some of the biological characteristics of the tumor.
In conclusion, there is strong evidence that the peptide growth factor/cytokine network is inappropriately functioning in human ovarian cancer, and that this imbalance contributes to ovarian cancer cell growth. The growth promoting characteristics of serum can not, however, be fully substituted for or explained by the activities of the known peptide growth factors and cytokines.
Growth promoting properties of malignant effusions
As in many other solid tumors, no single group of growth factors or cytokines has been shown to be unequivocally responsible for ovarian tumor growth. Research into possibly specific growth substances has enhanced interest in novel growth substances present in ascites. In addition, cell lines grew more rapidly during establishment in the presence of cell-free ascites (CFA) compared to serum,17 suggesting the presence of adequate levels of one or more criticalgrowth promoters in ascites.
Macrophages
For clonogenic growth of ovarian cancer cells obtained from effusions, feeder layers were not required in the clonogenic assay. When tumor cells freshly obtained from patients were kept suspended in their native ascites, colony growth could be observed until 24 hours after collection.32
This was ascribed to the abundant macrophages present in malignant effusions, providing a feeder layer effect. This hypothesis was supported by the fact that removal of macrophages from cell populations in malignant effusions lowered the average number of colonies in the original clonogenic assay by Hamburger ef a/.2930 Buick ef a/, showed that this decrease in
plating efficiency was reversible in reconstitution experiments that used the removed autologous adherent cells as an underlayer in a two-layer agar system.109 Even xenogeneic peritoneal
macrophages (obtained from mice) were able to improve the cloning efficiency of primary ovarian cancer cells in culture.31
Mantovani etal. found an array of effects of macrophages isolated from human ascitic ovarian tumors on in vitro tumor growth of established murine and human cell lines, with both cytotoxic, cytostatic and stimulatory effects observed in different experiments.110
These data suggest that adherent, phagocytic cells - possibly macrophages - may release or produce one or more factors vital to clonogenic growth of human tumors.111
Cell-free ascites
Vaage and Agarwal have reported growth stimulation of several murine neoplastic cells not only by serum but even more so by cell-free ascites (CFA).112 Uitendaal etal. also demonstrated
the growth promoting properties of cell-free ascites on ovarian tumors in the clonogenic assay.113
Replacement of the medium supplements (including FCS) by CFA from ovarian cancer patients resulted in increased plating efficiency in the clonogenic assay of ovarian tumor specimens, and this effect was maintained if ascites of other ovarian cancer patients was used. In further experiments by Broxterman et al.,38 CFA consistently enhanced clonogenic growth of ovarian cancer cell lines more efficiently than a range of sera.
When Hamburger and Salmon first added CFA to the culture medium in the clonogenic assay, they found decreased plating efficiency as compared to BALB/c spleen cell-conditioned media.30 In
addition to growth-stimulating factors, ascites may thus also contain growth inhibiting molecules. The presence of growth inhibiting factors in ascites that co-purified with TGF-a was shown in experiments with rat ascites fluids that suppressed the growth of human colon carcinoma cell lines CBS and MOSER.114 The growth of other cell lines, HCT 116 and HCT C was not inhibited.
These observations are suggestive of a balance between growth promoting and inhibitory factors present in ascites, with a net effect of growth promotion in malignant ascites consistent with the familiar autocrine concept of the role of growth factors in tumorigenesis. The ascitic growth factors have not yet been fully characterized, and their cellular source remains undefined, with both peritoneal macrophages and tumor cells being possible candidates.
In a study of Wilson etal., a comparison was made between the growth promoting properties of ascitic fluids, cyst fluids and peritoneal washings from patients with benign or malignant gynecological conditions. Effects on a variety of tumor- and normal cell lines were studied. All fluids stimulated colony formation in soft agar, and this activity was not tumor or cancer-related. The 'colony stimulating activity' could be inhibited by the addition of heparin and thrombin.115
Evidence for a non-peptide growth factor
Mills et al. provided evidence for the presence of a novel growth factor in ascites. They reported that ascitic fluids from ovarian cancer patients but not from patients with other cancers or benign diseases induced proliferation of fresh ovarian cancer cells and of the ovarian cancer cell line HEY. The proliferative response was associated with rapid increases in phospholipid hydrolysis and free cytosolic calcium. None of a number of well characterized growth regulators such as EGF, PDGF, FGF, thrombin, vasopressin, angiotensin, IL-1, IL-2, interferons, TGF-ß, or TNF-oc, increased intracellular calcium concentrations. Pretreatment of the tested cell lines with these factors did not in any way alter the response to CFA, suggesting that CFA contains hitherto unknown growth promoters that stimulate cells through phospholipid hydrolysis. CFA could completely substitute for human plasma or FCS in these experiments. "6 Subsequent experiments
showed that the human ovarian cancer cell line HEY could grow intraperitoneal^ in immunodeficient nude mice in the presence, but not in the absence of ascitic fluid from ovarian cancer patients. Ascitic fluid from patients with benign disease could not support this growth. After the intraperitoneal tumors progressed, CFA was no longer needed for tumor growth. Eventually the xenografted tumors developed ascites with potent growth factor activity, suggesting an autocrine mechanism.117
These data suggest that CFA contains a growth promoting substance that is different from known polypeptide mediators. Xu and Mills termed the putative growth factor present in ascites from ovarian cancer patients OCAF (ovarian cancer ascites factor) and postulated that it might be a lipid, based on the observations that the growth promoting fraction in ascites is resistant to all known proteases, and stable under such drastic conditions as boiling, pH extremes and detergent treatment that should destroy proteins. It was furthermore soluble in 80% acetone and 100% methanol, though it could be purified by techniques used in protein chemistry such as reverse phase chromatography and isoelectric focusing, probably due to a tight association with albumin or other serum proteins.118 In their analysis of extracted purified OCAF, OCAF
appeared to be a phospholipid, with biochemical characteristics identical to lysophosphatidic acid (LPA), an established bioactive phospholipid with growth promoting properties.
Phospholipid growth factors - lysophosphatidic acid (LPA)
Since the 1980s, it has become increasingly clear that phospholipids do not only have a structural function in biological systems, but are also active in intracellular signalling and can act as growth factors. The best known of these is platelet-activating factor (PAF), a phospholipid that induces a rapid increase in intracellular calcium levels after binding to a high affinityOH receptor.119120 It has a role in hypersensitivity and inflammatory
Q _ p Q u responses such as asthma and septic shock, but has so far not been ! implicated in cancer growth nor in the control of normal cell y proliferation.121
Q Q Q Lysophosphatidic acid (LPA; monoacylglycerol-3-phosphate) (See
I | fig. 1 for structure) is the simplest naturally occurring glycero-ls phospholipid, that has long been known to be a critical intermediate 0 = C in de novo phospholipid biosynthesis.122
Recently, the broad spectrum of biological activities of LPA has p
been found to include platelet activation, induction of neuronal shape
LPA
changes, smooth muscle contraction and stimulation of cell Figure 1. Structure of proliferation when added to fibroblasts and epithelial cells in lysophosphatidic acid (LPA).
culture.123 LPA acts on its cognate G-protein coupled cell surface
receptor, which is apparently present in many cell types.124
LPA is a normal constituent of serum, where it is present in albumin-bound form, but it is absent in platelet-poor plasma. Serum contains a factor that co-purifies with albumin and that causes neurite retraction in PC12 cells, inhibits the proliferation of certain tumor cells in vitro, and activates the phophatidylinositol/Ca2+ second messenger system in a variety of cells. There
is evidence that albumin-bound LPA is responsible for at least part of these activities. Synthetically prepared lysophosphatidates reproduce the biological activities of the natural serum factor.125
Kreps etal. demonstrated that serum effects on cyclic AMP accumulation in nine different cell lines were completely mimicked by LPA.126 Jalink etal. reported identical neuronal shape changes
in neuroblastoma cell lines after exposure to LPA or serum.127
LPA is formed and released by activated platelets, but this is probably not the only source of LPA, as there is evidence that LPA is also produced by growth factor stimulated fibroblasts and injured cells.128 It is of interest that earlier studies have demonstrated that platelet lysates can
enhance the growth of some, but not all epithelial and mesodermally-derived tumors in soft agar,129'130 and that a platelet-derived TGF-like compound present in serum, distinct from PDGF,
EGF or insulin can stimulate fibroblast growth in vitro.'13'
The mitogenic action of LPA on fibroblasts in culture is well-documented,132 and occurs in
the absence of synergizing peptide growth factors. The concentration of LPA required for the full mitogenic effect is micromolar as opposed to the nanomolar concentration necessary for neurite retraction and for the other responses mentioned previously. The mitogenic activity critically depends on the length of the fatty acid chain, with 1-oleoyl LPA being most potent and 1-decanoyl LPA showing hardly any mitogenic activity.133
LPA promotes invasion of hepatoma and carcinoma cells into monolayers of mesothelial cells. Serum produced a similar effect, but peptide growth factors could not substitute for serum or LPA in this study.134 Xu et al. found that LPA could stimulate proliferation of the human Jurkat T
cell line in the absence of serum.135 LPA activates breast and ovarian cancer cell lines SKOV-3
and OCC1 in completely serum-free medium,136 as well as OVCAR-3 and Lucy (Jalink K et al.
Unpublished observations.), suggesting that LPA might be the serum-derived growth factor that makes it so difficult to culture cells in serum-free media.
Since Xu ef a/, suggested that LPA might be the potent growth enhancing factor present in ascites,118 both their group and ours have shown the presence of an LPA-like mitogenic activity
in ascites of ovarian cancer patients, and found its levels to be substantially higher than those in
Conclusion
Known peptide growth factors and cytokines have a role in ovarian tumor growth, but they cannot fully account for the growth promoting properties of both serum and malignant effusions of ovarian cancer patients. We postulate that the so far elusive growth promoter might be the bioactive phospholipid LPA, based on the known properties of LPA and on the substitution experiments described above. More data are required to confirm the connection between LPA and ovarian cancer. Further studies should include quantitation of LPA-levels in malignant effusions of ovarian cancer patients compared to other patients, as well as identification of the cellular source of ascitic LPA.
Ovarian cancer remains confined to the peritoneal cavity until late in its clinical course. If LPA in ascites is indeed an important growth factor in this disorder, therapy might be targeted towards inhibiting its activity. The multifunctional growth factor inhibiting agent suramin currently is the only drug available to block LPA activity at the receptor level, but the development of more potent and more specific LPA antagonists may be a viable enterprise.
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