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Advances in Brief |
Department of Immunology [J. H. P., T. W., D. G. S., Z. E.] and The Experimental Animal Center [A. H.], The Weizmann Institute of Science, Rehovot 76100, Israel; Department of Urology [J. H. P., J. R.], Sheba Medical Center, Tel-Hashomer 52621 Israel; and Departments of Pathology [J. W. S.] and Urology [A. B.], UCLA School of Medicine, Los Angeles, California 90095
| ABSTRACT |
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| Introduction |
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The optimal treatment for SCCP has not been determined. The tumor often appears mixed with adenocarcinoma of the prostate, and it is usually treated with chemotherapeutic regimens designed for small cell carcinoma of the lung (1) . The possible advantage of hormonal agents combined with chemotherapy remains unproven. Indeed, accelerated proliferation of the NE cells may represent an important step in the development of androgen-independent growth of PC driven by alternative growth signals. di Santagnese (4) described the NE cells (also known as the endocrine-paracrine cells of the prostate) as intraepithelial regulatory cells displaying hybrid epithelial, neural, and endocrine characteristics. Although devoid of AR (5) , the cells are capable of secreting alternative growth factors such as bombesin, serotonin, somatostatin, calcitonin, and parathyroid hormone-related protein (4, 5, 6) . The prostatic NE cells express the c-erbB-2 growth factor receptors (7 , 8) . It was suggested that SCCP is composed of an enriched population of androgen-independent cells whose growth is sustained through alternate paracrine and autocrine pathways (6) .
Only one model of human SCCP was reported thus far (9 , 10) . The WISH-PC2 line described here was derived from a PC patient, and expresses novel PC-specific molecular markers. This model should be extremely useful in studies aimed at the elucidation of critical aspects of the NE differentiation of PC, such as the regulatory mechanisms displayed by the NE cells and the interactions between the disseminated tumor and its various metastatic sites. In addition, the effect of various therapies on the primary tumor and on its disseminated form can be further evaluated.
| Materials and Methods |
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Injection of Tumors to Various Organs. Orthotopic Injection.
Tumor cell suspensions (20 µl) were injected into the dorsal
prostatic lobes through a midline lower abdominal incision as described
by Stephenson et al. (12)
. A well-localized
bleb within the injection site was considered to indicate technically
satisfactory injection.
Intraoseos Injection.
Through a skin incision in the medial aspect of the hind limb, the
femur and tibia were exposed. A hole was made in the cortical bone of
the distal femur or proximal tibia with a 27-gauge needle. Tumor cell
suspensions (10 µl) were injected through the duct using a 30-gauge
needle.
Intrahepatic Injection.
A midline abdominal incision was performed to expose the liver. Tumor
cell suspensions (1020 µl) were injected into both hepatic lobes
using a 27-gauge needle.
s.c. Injection.
Mixtures of tumor cell suspensions in various concentrations with 100
µl of Matrigel basement membrane matrix solution (Becton Dickinson)
were injected s.c. using a 27-gauge needle.
Preparation of Single-Cell Suspension.
The xenografted tumor tissue was harvested under sterile conditions and
placed immediately in cooled HBSS, (Sigma-Aldrich Co., Ltd.). Cells
were dissociated under sterile conditions, first by mincing the tissue
with scissors to small fragments and then by gentle mechanical
homogenization through a stainless still mesh. Viable cells were
separated from debris by layering over Ficoll-Paque 400 (Pharmacia
Biotech AB, Uppsala, Sweden) and centrifugation at 500 x g for 20 min. Viable cells at the interface were
collected, counted, and resuspended in cooled HBSS to the desired
concentrations.
FACS Analysis.
Single-cell suspensions made from the xenograft were washed and
incubated with primary antibody [anti-CD19, anti-CD20, anti-CD22 (a
generous gift from Dr. M. Little, DKFZ, Heidelberg, Germany), PA2.6
anti-HLA-ABC (ATCC HB-118), N29 anti-ErbB-2, and L96 anti-ErbB2, no.
105 anti-ErbB-3 and no. 77 anti-ErbB-4, a generous gift from Prof. Y.
Yarden (Ref. 13
and references therein), or an irrelevant
U76.6, antidi-nitrophenol monoclonal antibody]. The samples were
then washed and incubated with the secondary FITC-labeled antimouse
antibody. Stained cells were resuspended in propidium iodide to
identify and exclude dead cells. Analysis was performed on the FACScan
flow cytometer (Becton Dickinson).
Immunohistochemistry.
Immunohistochemistry was performed on sections from formalin-fixed,
paraffin-embedded blocks as described previously (14)
.
Antibodies used included mouse monoclonal antibodies to PAP, PSA, NSE, chromogranin and synaptophysin, obtained from DAKO Corp., Carpinteria, CA. Antibodies to human AR were purchased from Innovex Biosciences, USA. Antibody to PSCA was a gift from Dr. Robert Reiter (University of California at Los Angeles). After incubation with monoclonal antibodies, slides were incubated sequentially with peroxidase-conjugated rabbit antimouse immunoglobulins, and peroxidase-conjugated swine antirabbit immunoglobulins. Antibody localization was visualized with the diaminobenzidene reaction. Negative controls consisted of substitution of the primary antibody with an isotype matched non-cross-reacting antibody of irrelevant specificity.
Immunohistochemical techniques combined with image analysis were used to detect the presence of bcl-2, P glycoprotein (MDR1), and p53. An experienced commercial laboratory performed these pathological evaluations as well as determination of DNA ploidity and proliferation index of the tumor samples (Quantitative Diagnostic Laboratories).
Serum PSA levels were determined by Immulite Third generation PSA kit (Diagnostic Products Corp., Los Angeles, CA).
Plasma chromogranin-A levels were quantified using an ELISA kit as recommended by the manufacturer (DAKO Denmark).
Western Blot Analysis.
WISH-PC2 cell lysate was resolved by 12% SDS-PAGE, transferred to
nitrocellulose paper and blotted with polyclonal rabbit
anti-PCTA-1/galectin 8 antibodies (a generous gift from Prof. Y. Zick).
RT-PCR.
RNA was isolated from WISH-PC2 tissue samples using the TRI
Reagent kit (Molecular Research Center, Inc.), and RT-PCR was performed
using the Reverse Transcription System (Promega). The quality of all
cDNA samples was confirmed by PCR using primers for ß-actin. PCR was
performed using primers for AR, PCTA-1, PSA, PSMA, PSCA, STEAP, and
cytokeratin 8 and 18.
| Results and Discussion |
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Growth Pattern of the WISH-PC2 Xenograft.
The patient from whom the WISH-PC2 xenograft was established was
initially diagnosed with conventional high-grade adenocarcinoma of the
prostate (Fig. 1)
. The tumor converted to SCCP in parallel to an
expeditious clinical course of progression, emphasizing the linkage
between the two subtypes of the prostatic tumor (1
, 2
, 16)
. Indeed, in the first generation, 20% of the mice into
which the tumor pieces were implanted had elevated serum PSA levels.
The WISH-PC2 xenograft was established from a tumor-bearing mouse that
did not exhibit elevated serum PSA levels. We determined that the
WISH-PC2 cells are of human origin and did not result from an
overgrowth of the explants by murine cells (17)
. The
xenograft is stained with antiHLA-A,B,C antibodies (data not
shown). In addition, the tumor does not express the B-cell CD19, CD20,
and CD22 differentiation antigens (data not shown), excluding the
possibility of being an overgrowth of dormant EBV-transformed human B
cells (18)
. The xenografted carcinoma is highly cognate in
its gross histological appearance to the donors surgical specimen,
and it shares the expression of NE tumor markers (Fig. 1)
: chromogranin
A, NSE, and synaptophysin. Notably, chromagranin A is also secreted
into the plasma of WISH-PC2-bearing mice, and the plasma concentration
of chromogranin A is correlated to the size of the xenograft (data not
shown). Hence, chromogranin A can serve as a secreted tumor marker to
monitor the growth of this NE SCCP xenograft.
The WISH-PC2 xenograft grows relatively rapidly (Fig. 2)
and with a
high take rate (visible tumor growth is evident in 90100% of the
animals). Upon s.c. injection of tumor cells or implantation of tumor
tissue, growth can be detected within 23 weeks. In the presence of
Matrigel, the doubling time of the tumor after the s.c. implantation of
3 x 106 cells, is 11 or 13.5 days
for a tumor growing in the presence or absence of androgen,
respectively, and 15 and 18 days, respectively, for a tumor that arises
from s.c. implantation of tissue (70 mg) with and without androgen
supplementation. This pattern of androgen-responsive growth most
probably reflects an indirect effect of androgen, inasmuch as the tumor
cells do not express ARs as expected for SCCP (1
, 3)
.
Because the WISH-PCR xenograft has been originated from a mixed-type
tumor, there is a possibility that it is a mixture of SCCP with some
residual adenocarcinoma cells that are hormone-responsive and expand in
the presence of androgen. To test this, we stained the WISH-PC2 tissue
that grew for more than 80 days in the presence of a continuous supply
of testosterone with anti-PSA and anti-AR antibodies. The results did
not reveal any staining above background of these tumors. Moreover,
RT-PCR analysis of RNA derived from WISH-PC2 that grew in the absence
or presence of androgen was completely negative for PSA and AR (data
not shown). Similarly, no PSA could be detected in the sera of mice
bearing WISH-PC2, regardless of whether they were hormone-supplemented
or not. All these data argue against the possibility that the enhanced
growth observed in the presence of androgen (Fig. 2)
is attributable to
residual androgen-responsive adenocarcinoma cells. Apparently, the
serial transfer of WISH-PC2 in the absence of an external source of
testosterone in the first few generations provided a selective
advantage to the SCCP component over the adenocarcinoma. Androgen was
demonstrated to have indirect effects on PC via up-regulation of
surrounding stromal vascular endothelial growth factor
production and angiogenesis (19)
. The high growth rate is
also reflected by the high proliferation index, depicted by staining
with the Ki-67 antibody, and directed at a nuclear antigen expressed in
proliferating cells (data not shown).
Growth of WISH-PC2 in Various Organs.
To establish a valid model in which to test various therapeutic
strategies, we developed a xenograft model using WISH-PC2 cells that
would closely model human SCCP and its metastasis. The ability of the
WISH-PC2 xenograft to grow orthotopically within the murine prostate
(Fig. 3)
provides such a model. Interestingly, orthotopically transplanted
human small cell lung carcinoma displays a different chemosensitivity
pattern compared with the s.c. transplanted model (20)
.
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Malignant Phenotype of the WISH-PC2 Cells.
Next we tested WISH-PC2 cells for the expression of several prostate
specific markers. Table 1
lists various molecular markers whose expression was evaluated on
WISH-PC2. The xenograft does not express PSA, PSMA, PSCA, or PAP.
Nevertheless, its prostatic origin is supported by the following
molecular markers: (a) expression of cytokeratin 8 and 18
common to PC and prostatic secretory cells (3)
;
(b) expression of PCTA-1, which is a surface marker of PC
and its precursor, prostatic intraepithelial neoplasia, but is not
found on normal prostate or benign prostatic hyperplasia
(22)
; and (c) expression of the STEAP. This
recently described surface marker (23)
is highly expressed
at all stages of PC and does not seem to be modulated by androgen.
Although STEAP is also expressed in multiple cancer cell lines, its
expression in normal human tissues is restricted to the prostate and
bladder.
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FACS analysis using monoclonal antibodies against the epidermal growth factor receptor erb-B family revealed that WISH-PC2 expresses erb-B2, erb-B3, and erb-B4 on its surface. The expression of these growth factor receptors was stable through all of the passages of the tumor (data not shown). Iwamura et al. (7) demonstrated immunostaining of c-erb-B2 on prostatic NE cells using polyclonal antibodies. To the best of our knowledge, no data have been reported to date concerning the coexpression of erb-B3 and/or erb-B4 with erb-B2 on prostatic NE cells, a combination that is necessary for the binding of Neuregulin ligands to these receptors and for their activation (13) . The presence of the erb-B set of receptors may provide an alternative pathway of growth signaling for the androgen-independent proliferation of these cells, either directly or by regulating NE peptides that function in an autocrine or paracrine manner.
Potential Application of the WISH-PC2 in Therapeutic Models.
Prostatic small cell carcinoma is a notoriously aggressive
malignancy with a very poor prognosis (21)
. No effective
treatment for SCCP has been established, most probably because of the
limited patient population and the aggressiveness of the disease.
WISH-PC2 provides an in vivo model for the evaluation of
different possible therapeutic strategies for SCCP with an inherent
plasma tumor marker (chromogranin A).
The issue of whether castration is a justified treatment for SCCP is
still unresolved. The progression from adenocarcinoma of the prostate
to SCCP usually appears after castration (1)
. However,
based on the evidence that most SCCP are mixed with adenocarcinoma of
the prostate, the common practice is to combine hormonal and cytotoxic
therapy (21
, 25)
. The fact that androgen supplements
somewhat increase WISH-PC2 tumor growth (Fig. 2)
suggests that
androgens may enhance the growth of the AR-negative xenograft, probably
via an indirect effect on the surrounding stroma (19)
. It
is therefore possible that in the case of SCCP, especially in those
present as mixed histology (adenocarcinoma and small cell elements),
hormonal manipulation may slow tumor progression.
The WISH-PC2 model can serve as a useful model for testing established
and novel cytotoxic drugs. Targeted drug delivery to the various
anatomical sites of visceral distribution of SCCP, such as liver or
bones, may be readily tested in this model system. The WISH-PC2
xenograft, lacking the p-170 multi-drug efflux pump (MDR1; Table 1
),
that mediates the MDR phenotype, is therefore susceptible to
chemotherapy. It has still to be demonstrated however, whether the
WISH-PC2, expressing mutated p53 and bcl-2 are susceptible to
apoptosis-inducing drugs.
In conclusion, the WISH-PC2 SCCP xenograft is an excellent source for NE prostatic cells and their factors in studies of intercellular interactions that take place during PC progression. Moreover, this novel human xenograft can serve as a model for the exploitation of new therapeutic modalities on this aggressive variant of PC.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 This study was supported by CaP CURE, the
Israel Cancer Association, and the Department of the Army,
USAMRMC Grant No. DAMD17-98-1-8507. ![]()
2 To whom requests for reprints should be
addressed, at Department of Immunology, Weizmann Institute of Science,
Rehovot 76100 Israel. ![]()
3 The abbreviations used are: SCCP, small cell
carcinoma of the prostate; AR, androgen receptor; FACS,
fluorescence-activated cell sorter; NE, neuroendocrine; NSE,
neuron-specific enolase; PAP, prostate acid phosphatase; PC, prostate
cancer; PCTA-1, prostate carcinoma tumor antigen-1; PSA,
prostate-specific antigen; PSCA, prostate stem cell antigen; PSMA,
prostate-specific membrane antigen; RT-PCR, reverse transcription-PCR;
SCID, severe combined immunodeficient; STEAP, six transmembrane
epithelial antigen of the prostate; MDR, multidrug resistance. ![]()
Received 5/16/00. Accepted 10/12/00.
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