Oestrogen receptor positive breast cancer metastasis to bone: inhibition by targeting the bone microenvironment in vivo

Clinical trials have shown that adjuvant Zoledronic acid (ZOL) reduces the development of bone metastases irrespective of ER status. However, post-menopausal patients show anti-tumour benefit with ZOL whereas pre-menopausal patients do not. Here we have developed in vivo models of spontaneous ER+ve breast cancer metastasis to bone and investigated the effects of ZOL and oestrogen on tumour cell dissemination and growth. ER+ve (MCF7, T47D) or ER-ve (MDA-MB-231) cells were administered by inter-mammary or inter-cardiac injection into female nude mice ± estradiol. Mice were administered saline or 100 μg/kg ZOL weekly. Tumour growth, dissemination of tumour cells in blood, bone and bone turnover were monitored by luciferase imaging, histology, flow cytometry, two-photon microscopy, micro-CT and TRAP/P1NP ELISA. Estradiol induced metastasis of ER+ve cells to bone in 80-100 % of animals whereas bone metastases from ER-ve cells were unaffected. Administration of ZOL had no effect on tumour growth in the fat pad but significantly inhibited dissemination of ER+ve tumour cells to bone and frequency of bone metastasis. Estradiol and ZOL increased bone volume via different mechanisms: Estradiol increased activity of bone forming osteoblasts whereas administration of ZOL to estradiol supplemented mice decreased osteoclast activity and returned osteoblast activity to levels comparable to that of saline treated mice. ER-ve cells require increased osteoclast activity to grow in bone whereas ER+ve cells do not. Zol does not affect ER+ve tumour growth in soft tissue, however, inhibition of bone turnover by ZOL reduced dissemination and growth of ER+ve breast cancer cells in bone.

Imaging the effects of castration on bone turnover and hormone-independent prostate cancer colonization of bone

INTRODUCTION

Tumor populations may selectively colonize bone that is being actively remodeled. In prostate cancer patients, androgen deprivation directly inhibits tumor growth initially, whilst induced bone loss may facilitate tumor colonization of bone by androgen-insensitive cells. We have tested this hypothesis using a xenograft model of early growth of prostate cancer in bone.

METHODS

PC3 cells transfected with Green fluorescent protein (GFP) were injected into castrated and non-castrated athymic mice via intrabial and intracardiac routes. In vivo tumor growth was monitored daily and animals sacrificed 6-9 days following initial GFP-based detection of tumors. Tumor bearing and contra-lateral non-tumor bearing tibias were analyzed extensively by micro-CT and histology/immunohistochemistry for the presence of tumor cells and the effects of tumor and/or castration on bone cells and bone structure evaluated.

RESULTS

GFP-positive tumors in bone were visible from 12 days post-injection following intratibial injection, allowing tumors <1 mm diameter to be monitored in live animals. Castration did not affect tumor frequency, tumor volume, or time to initial appearance of tumors injected via intratibial or intracardiac routes. Castration decreased trabecular bone volume in all mice. Significant tumor-induced suppression of numbers of osteoblasts, coupled with increased numbers of activated osteoclasts, was evident in both intact animals and castrated animals.

CONCLUSIONS

In vivo GFP imaging allows the detection of early tumor growth at intra-osseous sites. Castration induces bone loss, but PC3-GFP cells are also capable of inducing bone remodeling in intact animals at early time points, independently of pre-existing castration-induced alterations to bone.

Bone marrow stromal cells promote growth and survival of prostate cancer cells

Prostate cancers frequently metastasize to the skeleton, and it has been hypothesized that this environment selectively supports the growth of these tumours. Specifically there is strong evidence that interactions between tumour cells and BMSCs (bone marrow stromal cells) play a major role in supporting prostate cancer growth and survival in bone. Here, we examine factors shown to be secreted by BMSCs, such as IGFs (insulin-like growth factors) and IL-6 (interleukin 6), shown to promote prostate cancer cell proliferation and to potentially replace the requirement for androgens. In addition we discuss another factor produced by BMSCs, osteoprotegerin, which may promote tumour cell survival by suppressing the biological activity of the pro-apoptotic ligand TRAIL (tumour-necrosis-factor-related apoptosis-inducing ligand).

EZH2 promotes proliferation and invasiveness of prostate cancer cells

BACKGROUND

The transcriptional repressor EZH2 is implicated in control of cell proliferation in embryonic, immortalized and transformed cells. EZH2 expression in prostate cancer correlates with progression to hormone-refractory and metastatic disease, but it is unknown whether EZH2 plays a specific role in the acquisition of an advanced prostate cancer phenotype.

METHODS

Using siRNA knockdown, we investigated the role of EZH2 in maintenance of prostate cancer cell proliferation and invasiveness. Using LNCaP cells with inducible EZH2 overexpression, we investigated whether EZH2 upregulation promotes an aggressive phenotype.

RESULTS

Knockdown of endogenous EZH2 reduced proliferation of androgen-responsive and androgen-independent prostate cancer cells. EZH2 knockdown also inhibited prostate cancer cell invasion. However, overexpression of EZH2 in androgen-responsive cancer cells did not appreciably affect either proliferation or invasiveness.

CONCLUSIONS

EZH2 promotes proliferation and invasion of prostate cancer cells, which can account for the correlation between EZH2 expression levels and an adverse prostate cancer prognosis.

The expression and regulation of ADAMTS-1, -4, -5, -9, and -15, and TIMP-3 by TGFbeta1 in prostate cells: relevance to the accumulation of versican

BACKGROUND

Benign prostatic hyperplasia (BPH) is characterized by a proportional increase in the size of the stromal compartment of the gland, involving alterations to extracellular matrix (ECM) components. Some of these changes have been associated with the activity and expression of transforming growth factor beta1 (TGFbeta1). Versican (chondroitin sulphate proteoglycan-2) is overexpressed in BPH and prostate cancer and potentially contributes to disease pathology. A sub-group of the ADAMTS lineage of metalloproteases possess versican-degrading properties and are potential regulators of proteoglycan accumulation associated with BPH. These enzymes have one major inhibitor in the ECM, tissue inhibitor of metalloproteinases (TIMP)-3.

METHODS

The effect of TGFbeta on mRNA expression in prostatic stromal cells was determined by real-time qRT-PCR using primers to ADAMTS-1, -4, -5, -9, -15, versican, and TIMP-3. MMP-inhibitory potential (TIMP activity) of conditioned medium was measured using a fluorometric peptide substrate.

RESULTS

Prostatic stromal cell cultures consistently expressed ADAMTS-1, -4, -5, -9, -15 and TIMP-3, in contrast to PC3, DU145, and LNCaP cells which failed to express at least two ADAMTS transcripts. In stromal cells, TGFbeta1 decreased ADAMTS-1, -5, -9, and -15 transcripts and increased ADAMTS-4, versican, and TIMP-3. TGFbeta also increased TIMP activity in conditioned medium.

CONCLUSIONS

The induction of versican expression by TGFbeta in BPH stromal cells is in agreement with histological studies. The negative effect of TGFbeta1 on ADAMTS-1, -5, -9, and -15 coupled with increases in their inhibitor, TIMP-3 may aid the accumulation of versican in the stromal compartment of the prostate in BPH and prostate cancer.

Osteoprotegerin (OPG) Produced by Bone Marrow Stromal Cells Protects Breast Cancer Cells from TRAIL-Induced Apoptosis

Advanced breast cancer is often associated with metastatic bone disease, causing a number of serious complications for the patients such as hypercalceamia, pain, nerve compression and fractures. The formation of bone metastases depends on complex interactions between tumour cells and the cells of the bone microenvironment, but the precise molecular mechanisms involved in the development of tumour-induced bone disease have not been identified. We have investigated the ability of bone marrow stromal cells (BMSC) isolated from breast cancer patients to generate osteoprotegerin (OPG), a molecule involved both in bone turnover and cell survival. The potential survival effects of OPG are mediated through binding to a member of the TNF super family, TNF-related Apoptosis Inducing Ligand (TRAIL), preventing association between TRAIL and its death-inducing receptors present on a number of tumour cell types. In the present report we show that bone marrow stromal cells isolated from breast cancer patients produce OPG when grown in culture. The levels of OPG present in BMSC conditioned medium is sufficient to protect breast cancer cells from undergoing TRAIL induced apoptosis. Our data suggest that bone-derived OPG may increase survival of breast cancer cells that reach the bone microenvironment as part of the metastatic process.

Role of endogenous transforming growth factor beta (TGFbeta)1 in prostatic stromal cells

BACKGROUND

In this study, defined culture conditions were used to examine the effects of recombinant TGFbeta1 on prostatic stromal cells and to determine the role of endogenous TGFbeta produced by these cells.

METHODS

Cells were grown +/- recombinant TGFbeta1 and cell population sizes in replicate cultures determined. In other experiments, TGFbeta1 production by prostatic stromal cells was examined and the effects of neutralization of this activity on cell population sizes and apoptosis evaluated.

RESULTS

At > 1 ng/ml, TGFbeta1 reduced cell population sizes while at 0.01 ng/ml cell numbers were increased cf. controls. Stromal cells produced up to 10 ng/ml/48 hr of latent TGFbeta1 of which < 0.2% was biologically active. When cells were treated with anti-TGFbeta1 antibodies, cell numbers decreased cf. controls and the proportion of apoptotic cells increased.

CONCLUSIONS

These observations suggest that TGFbeta1 is an autocrine factor made by prostatic stromal cells in which it inhibits apoptosis at the activity levels produced.

Regulation of prostatic stromal cell growth and function by transforming growth factor beta (TGFbeta)

BACKGROUND

The majority of the overgrowth in benign prostatic hyperplasia (BPH) specimens is comprised of connective tissue. Factors that control stromal growth in the prostate are poorly understood; however, members of the transforming growth factor beta (TGFbeta) family may be of particular importance in the etiology of BPH.

METHODS

Thirty-two low-passage stromal cultures were generated from human prostatectomy specimens. Their stromal origin was confirmed and expression of TGFbetas analyzed by duplex reverse transcription-polymerase chain reaction (RT-PCR). Challenge experiments were designed to study the effects of exogenous TGFbeta1 on stromal cell growth and synthesis of extracellular matrix components.

RESULTS

The expression of TGFbetas 1, 2, and 3 was demonstrated in all 32 cell strains. The stromal origin of the cell lines was confirmed. Exogenous TGFbeta1 added to stromal cultures resulted in inhibition of cell growth and increased production of type I collagen.

CONCLUSIONS

The prostatic stromal cell strains we have developed are a reliable mod- el for investigating prostatic connective tissue biology. The challenge experiments with TGFbeta1 provide further evidence for the involvement of TGFbetas in prostatic enlargement, as modulators of the extracellular matrix in the absence of growth stimulation.

Fibroblast growth factors and their receptors in parathyroid disease

Fibroblast growth factors (FGFs) comprise a family of polypeptide growth factors implicated in the control of proliferation of glandular tissues. The aim of this study was to determine whether FGFs are produced in normal and abnormal parathyroid glands and if these tissues have the potential to respond to this growth factor family. We have examined the expression of FGF receptor (FGF-R) types 1 and 2 and of FGFs 1, 2, 3, and 7 in a series of human parathyroid tissues using the reverse transcription polymerase chain reaction (RT-PCR). Samples of 10 parathyroid adenomas and 10 hyperplastic glands from patients with renal hyperparathyroidism (HPT) were compared with samples from normal parathyroid glands from patients with primary HPT in the study. FGF-R1 was expressed in all samples and FGF-R2 in most. FGF1 and FGF2 were expressed in all samples at variable levels, with no correlation between disease type and amplified RT-PCR product levels. FGF7 was expressed in normal parathyroid tissue and in all adenomas but was absent in all but one of the hyperplastic parathyroid glands from renal failure patients. FGF3 was expressed at low levels in normal tissue and variably expressed in diseased tissue, in some instances at high levels. These findings suggest that parathyroid tissue is potentially responsive to FGFs. The absence of FGF7 expression in all but one of the renal parathyroid samples compared with normal and adenomatous tissue requires further investigation. The presence of elevated levels of FGF3 expression in abnormal parathyroid tissue may be significant, as the FGF3 gene (int-2 proto-oncogene) is located on chromosome 11q13.3, a region already identified as being susceptible to rearrangement/mutation in parathyroid disease.

Effect of n-6 polyunsaturated fatty acids on growth and lipid composition of neoplastic and non-neoplastic canine prostate epithelial cell cultures

BACKGROUND

Polyunsaturated fatty acids (n-6) are reported to selectively kill malignant cells. Most investigations, however, did not compare neoplastic with non-neoplastic cells from the same tissue type. Here we evaluate the effects of n-6 fatty acids on a non-neoplastic epithelium cell line (CAPE) and a spontaneous carcinoma cell line (CPA) derived from the canine prostate.

METHODS

Cell lines were cultured in DME in the presence of fatty acids and their effects on cell proliferation monitored by coulter counting. Lipids were extracted and quantitized by gas chromatography.

RESULTS

Cell proliferation was reduced more in CAPE. A neoplastic strain (CPA-GLA) tolerant to prolonged culture in 18:3n-6 was isolated. CPA grown in an 18:2n-6 or 18:3n-6 supplemented media accumulated 20:3n-6 and contained little 20:4n-6.

CONCLUSIONS

Polyenoic n-6 fatty acids are not specifically inhibitory to neoplastic cells which exhibited a marked alteration in the metabolism of 20:4n-6.

Comparative studies of the mitogenic effects of epidermal growth factor and transforming growth factor-alpha and the expression of various growth factors in neoplastic and non-neoplastic prostatic cell lines

BACKGROUND

The role of growth factors in prostate cell growth has been investigated as these peptides may be involved in the autonomous growth of hormone-independent prostate cancer.

METHODS

Responses of neoplastic (PC-3 and CPA) and non-neoplastic (CAPE) prostatic cell lines to epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-alpha) were determined using clonogenic and growth curve analysis. The constitutive expression of EGF, TGF-alpha, and TGF-beta 1-3 mRNA was examined using Northern blotting and EGF and TGF-alpha protein levels were determined immunohistochemically.

RESULTS

Growth curve and clonogenic analysis indicated that EGF and TGF-alpha were mitogenic in each cell line. The magnitude of the clonogenic response varied between the cell lines, with CPA cells showing the greatest growth increases. CPA cells also displayed the highest levels of EGF and TGF-alpha mRNA and protein. TGF-beta 1 mRNA was detected in the order of magnitude, PC-3 > CPA > CAPE. Furthermore, PC-3 and CPA cells expressed TGF-beta 3 and TGF-beta 2 transcripts respectively. In each cell line, the expression of any growth factor mRNA was not affected by exogenous EGF.

CONCLUSIONS

The growth responses of the cell lines to EGF and TGF-alpha did not correlate with their constitutive levels of EGF and TGF-alpha mRNA and protein, thus whilst growth factors may be important in malignant cell growth, other pathways may also be involved in the autocrine regulation of cell proliferation.

Response of cell growth and retinoic acid receptor expression to retinoic acid in neoplastic and non-neoplastic prostate cell lines

BACKGROUND

Retinoic acid (RA) is recognized as an inhibitor of tumorigenesis, but conversely, has also been shown to act as a tumor enhancer, therefore its role in prostate tumor cell growth was investigated.

METHODS

The response of two human prostate tumor cell lines (PC-3 and DU-145), and cell lines derived from a well-differentiated canine prostate adenocarcinoma (CPA) and normal canine prostate epithelium (CAPE) to all-trans RA was determined using growth curve analysis. Additionally, the constitutive expression and RA-challenged expression of retinoic acid receptors (RARs) -alpha, -beta, and -gamma mRNA was examined using Northern blotting techniques.

RESULTS

In response to all-trans RA, the PC-3 and DU-145 cell lines showed considerable growth promotion, while CAPE and CPA cell growth was dramatically inhibited. Each cell line expressed RAR alpha and RAR gamma, with either negligible or no RAR beta transcripts being detected. RAR alpha and -gamma mRNAs detected in the four cell lines were variably regulated in response to RA, and no distinct patterns of RAR regulation that could be related to cell growth responses were observed.

CONCLUSIONS

The data indicates that no simple association exists between the expression or regulation of RAR subtype mRNAs and the divergent growth responses to RA displayed by the prostate cell lines.

Transforming growth factor beta 1 expression in benign and malignant prostatic tumors

The expression of transforming growth factor beta 1 (TGF-beta 1) in prostate specimens obtained from patients with benign prostatic hyperplasia (BPH, n = 32) and prostate carcinoma (n = 66) was investigated using Northern blot analysis and immunohistochemistry. Northern blot analysis revealed TGF-beta 1 message (2.5 kb) in virtually all of the samples examined, reflecting the ubiquitous nature of this growth factor. No statistical difference was found between the levels of mRNA detected in benign and malignant tissues due, in part, to the inherent heterogeneity of prostate tissue. Immunohistochemical methods using an antibody to native TGF-beta 1 revealed a novel pattern of immunoreactivity. Staining observed only in certain epithelial cells of benign glands was associated with areas of infection rather than tumorigenesis. Interestingly, intense staining was also seen in polymorphonuclear leukocytes. No correlation was found with the mRNA results, suggesting that this antibody is binding to TGF-beta 1 activated in response to infection rather than detecting sites of synthesis of latent TGF-beta 1.

Relationship between EGF-R, c-erbB-2 protein expression and Ki67 immunostaining in breast cancer and hormone sensitivity

The expression of the epidermal growth factor receptor (EGF-R), c-erbB-2 protein product and Ki67 have been evaluated in 105 breast cancers of known responsiveness to endocrine therapy using immunohistochemistry. EGF-R staining was observed in 62 of the tumours and was significantly associated with elevated rates of cell proliferation (%Ki67 positive cells) and loss of hormone sensitivity. In contrast, c-erbB-2 expression was not correlated with cell proliferation rates and was less strongly related to hormone insensitivity. Subdivision of the EGF-R data according to c-erbB-2 measurements revealed an association between c-erbB-2 immunostaining and worsened patient outlook and hormone insensitivity in moderately EGF-R-positive tumours. c-erbB-2 immunostaining in highly EGF-R-positive tumours did not further contribute to the already poor prognosis of these patients. These data confirm the prognostic importance of EGF-R measurements in breast cancer and may infer a functional interaction between this protein and the c-erbB-2 protein product in the aberrant growth of a subset of breast tumours.

Steroid hormones and the pathogenesis of benign prostatic hyperplasia

The pathogenesis of benign prostatic hyperplasia (BPH) is still poorly understood: there is, however, general acceptance that the condition is not premalignant and that it has an etiology distinct from that of cancer. Interest now focuses on the biochemistry of the target prostate cells and the propensity of the gland for uncontrolled growth. Dihydrotestosterone (DHT) is the active intracellular androgen formed from testosterone by 5 alpha-reductase. DHT concentrations appear a little higher in BPH tissue than in normal tissue, and there is no doubt that DHT-receptor complex modulates gene expression. Current studies suggest that DHT is essential but not sufficient for proliferation, and that other regulatory factors, including peptide growth factors, are prerequisite. The growth responsiveness of prostate tissue to androgens may be dependent on the balance between epithelial and stromal tissues, with biologic processes in the epithelium indirectly controlled by androgen-dependent mediators of stromal origin.

Steroids and the prostate

Interelationships between steroid and growth factor regulation of cell proliferation has been examined in two androgen sensitive prostatic cell lines, grown in defined medium. The cell lines used were derived from normal (CAPE) and neoplastic (LNCaP) tissues. The growth of both cell lines was elevated by challenge with serum, androgens and epidermal growth factor (EGF) used as single agents. The effects of androgen in CAPE were small, but significant while the profound effects of these agents on the growth of LNCaP were confirmatory of other studies. Androgens upregulated EGF receptor expression in LNCaP measured by both ligand binding capacity and mRNA analysis. This was not observed in the CAPE cells. Addition of serum (whole or charcoal stripped) suppressed the observed androgenic stimulation of EGF receptor expression in LNCaP. This apparent anomaly is discussed in relation to the growth enhancing properties of serum in these cell lines and in the wider context of normal and neoplastic growth control in the prostate.

The role of endocrine therapy in prostatic cancer

When judged by randomized clinical trial, current endocrine therapies offer symptomatic relief to prostatic cancer patients for an average period of 1-2 years following initiation of therapy. Medical castration with LHRH analogues is a safe and effective way of achieving 'castrate' levels of circulating androgens without the undesirable aspects of surgery. While there is some evidence for the value of combined therapies using these agents in combination with anti-androgens for 'total androgen blockade' in some patients, overall this approach has not been shown to offer advantages over castration, either surgical or medical, alone in controlled trials. Secondary endocrine therapy does not offer convincing objective response rates, suggesting that disease progression is independent of androgens.

Binding of epidermal growth factor by human normal, hypertrophic, and carcinomatous prostate

Saturable binding sites for radioiodinated epidermal growth factor (EGF) have been quantified in surgical specimens of benign prostate hypertrophy (BPH), histologically normal (HN) prostate, and prostate cancer. Values for EGF binding did not differ significantly between HN prostate and prostate cancer, although dedifferentiated samples tended to higher levels. These were coincident with lower comparative levels of androgen receptors. Unfractionated BPH tissue contained lower levels of EGF binding than either HN or carcinomatous prostate, but in separated epithelial cells EGF binding fell into the same range. Saturation analyses showed two affinity classes of binding in all except dedifferentiated tissues.

Prostatic cancer: aetiology and endocrinology

Prostate cancer is common in the male population, worldwide. Several therapies are currently available, but only greater understanding of the complex processes that govern the growth of the cancer will produce real progress in treatment.

Growth factor involvement and oncogene expression in prostatic tumours

The effects of EGF, TGF alpha and 5 alpha-dihydrotestosterone on the growth of a prostatic epithelial cell line have been evaluated in clonal growth assays. Similar bioassay systems have been used to identify tumour-associated growth promoters derived from a human prostatic carcinoma cell line (PC3). Growth factor activity was associated with proteins of Mr 20-30 kDa. In a separate study, EGF receptor concentration and cellular proto-oncogene expression was assessed in prostatic tumour samples. In prostatic carcinoma samples, strong correlation was observed between EGF receptor concentration and c-myc expression. There were no significant correlations between EGF receptor concentration and tumour grade or androgen receptor content in carcinoma samples. EGF receptor concentration was significantly higher in prostatic carcinoma specimens than in BPH.

Growth of a spontaneous canine prostatic adenocarcinoma in vivo and in vitro: isolation and characterization of a neoplastic prostatic epithelial cell line, CPA 1

Neoplastic epithelium derived from a spontaneous canine prostatic adenocarcinoma has been maintained and grown in cell culture and as xenografts in athymic mice. An epithelial cell line (CPA 1) has been isolated from primary cultures and has been partially characterized in vitro. The growth of this cell line was not modified by either androgens or estrogens, and high-affinity receptors for these steroids could not be demonstrated in these cells. Xenografts were serially transplantable, with growth being similar in both sexes. Receptors for androgens and estrogens could not be detected in homogenates of xenografts or primary tumor. The histological appearances of serially transplanted tumors, and of xenografts generated by inoculation of the cell line (CPA 1) and several cloned substrains, were very similar to that of the primary tumor and were judged to be well differentiated. The characteristics of this neoplastic cell type have been compared with those of normal prostatic epithelium.

Growth factor receptors and oncogene expression in prostate cells

Specimens of benign prostatic hypertrophy (BPH) and prostate carcinoma and prostate cells in culture were assessed for their capacity to bind androgens, radioiodinated EGF, and IGF-I, and to express certain cellular protooncogenes. Prostate cell lines contained receptors for both EGF and IGF-I. Similarly, clinical samples of human diseased prostate contained receptors for both of these factors. Prostate carcinoma contained higher concentrations of EGF receptors based on DNA than did BPH, although it is accepted that BPH may not be the appropriate comparison for carcinoma. Increased EGF receptors were associated circumstantially with a decline in androgen receptors with deteriorating differentiation status and with an increase in expression of c-myc. Androgen receptor concentration correlated with increased expression of c-fos. Deteriorating differentiation status was associated with the appearance or increase in secondary sites with lower affinity for IGF-I. Whereas c-myc expression was increased in all grades of carcinoma compared to BPH, expression of c-H-ras accompanied loss of differentiation. Although those alterations are hindered by tissue heterogeneity and correlations are essentially circumstantial, they may provide clues to the progression of prostate cancer that can be validated in prostate cell lines with similar growth response capabilities.

Epithelial and fibroblastoid cell lines derived from the normal canine prostate. I. Separation and characterization of epithelial and stromal components

Cell cultures displaying exclusively epithelial or fibroblastoid morphology have been isolated by spillage and collagenase digest techniques, respectively. Primary cultures of both cell types have been readily subcultured. The use of a type I collagen substrate has been shown to be essential to the growth of normal prostatic epithelium in monolayer cultures. The ability to generate replicate subcultures of both cell types has allowed the quantitative characterization of the mitogenicity of fetal bovine serum and insulin in early subcultures. The control of culture conditions has permitted uniform cell population growth in early subculture with regular population doubling times in log phase of growth. Epithelial cultures have been shown to display many ultrastructural characteristics common to the normal epithelium of the canine prostate.