Because the rodent prostate does not spontaneously develop prostate carcinoma and benign hypertrophy or hyperplasia, the usefulness of studying the mouse prostate as a model of human disease is frequently addressed.
However, the known heterogeneity of pathological prostate changes in the human prostate gland and the multifaceted nature of prostate disease have prompted the development of less complex, complementary model systems to study the etiology of prostate disease. Both prostate cancer and benign hypertrophy or hyperplasia can be induced in the rodent prostate through genetic modulation or chemical induction and several such models have been established. The advent of transgenic techniques in mice have put increasing
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focus on the mouse as a model organism for in vivo studies aiming at understanding gene function and by this gain insights into human pathophysiological conditions. Moreover, the mouse genome project will soon be completed which will enable a direct comparison between the mouse and human genes.
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RANSGENIC PROSTATE HYPERPLASIA MODELSMale mice overexpressing the rat PRL gene, Mt-PRL transgenic mice, develop a dramatic enlargement of the prostate gland, which shows similarities prostatic hyperplasia in humans. These animals were generated using a construct consisting of the rat PRL gene under the control of the ubiquitous metallothionein (Mt) promoter, which gives the transgene a general transcription in virtually all cell types. Expression of transgene was detected in all parts of the prostate (DP, VP, LP, AP). The prostate enlargement is mainly characterized by an expansion of the stromal compartment and areas of glandular hyperplasia with accumulation of secretory material [105]. Although dysplastic epithelial features were detected in individual prostates from older PRL-transgenic animals, no development of prostate carcinoma has been observed. The PRL-transgenic animals display, in addition to high serum levels of PRL, approximately a three-fold increase in serum androgen levels compared to wildtype littermates. The degree of prostate enlargement showed no correlation to circulating levels of PRL or testosterone.
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ODENT MODELS OF PROSTATE CANCERThere are several rodent models for human prostate cancer. One of the most well known is the Dunning-3327 rat prostatic adenocarcinoma model [145].
There are several recently established transgenic mouse models for use in prostate cancer studies [146]. The purpose of utilizing these animal models is to identify specific molecular changes in early malignant disease. As the mouse does not spontaneously develop prostate malignancy, different transgenic strategies for in vivo tumor induction have been developed including the use of the the SV40 early genes, such as the tumorigenic T antigen (Tag). Transgenes are usually under the control of a prostate-specific promoter region such as probasin or C3, directing expression to prostate epithelial cells.
The transgenic models of prostate cancer can be divided into two main types.
The first consists of models resulting from enforced expression of SV40
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early genes. Two frequently used models are the TRAMP (transgenic mouse model for prostate carcinoma) model and the C3(1)-Tag transgenic model, which utilizes the minimal rat probasin promoter to drive the expression of the Tag gene. In addition, a number of transgenic lines use the long probasin promoter to express large SV40 early genes. These models are well characterized and widely distributed, displaying progressive disease ranging from epithelial hyperplasia or PIN to adenocarcinoma and development of metastases [147].
The second type of transgenic mice utilizes the promoters mentioned above to express various “natural” molecules that have previously been suggested to play a role in development of prostate cancer. The list is extensive but includes c-myc, Bcl-2 and dominant negative transforming growth factor beta (TGFß). Interestingly, the majority of these models only display a relatively mild phenotype, primarily epithelial hyperplasia or PIN. Moreover, these phenotypes usually not arise until the mice are of advanced age.
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THER GENETICALLY ENGINEERED MOUSE MODELS WITH PROSTATE PHENOTYPEMouse models genetically engineered in the prolactin signaling pathway Null mutated mice have been generated both for the PRL ligand, PRL-/-[148], and the PRLR-/- [149]. PRL-/- males are reported fertile [148], whereas studies of male PRLR-/- mice have demonstrated both a subset of completely infertile males and a general latency to first successful mating [150].
Moreover, the studies of the prostate gland in PRLR-/- males did reveal only subtle histological alterations and the PRL-/- prostate has not been very well characterized. Taken together, the data from these two knockout mouse models indicate that PRL action is not of essential importance for male fertility and normal anatomical development of the prostate gland. However, studies of more functional aspects of the gland need to be carried out in these animals.
PRL can activate several of the Stat proteins, including Stat 1, 3, 5a, and 5b, but the two latter acts as the major mediator [55]. Stat5a-/- and Stat5b-/- knockout mice have confirmed these molecules as the major transducers of PRL signaling in both prostate and mammary gland [151], and also shown similar phenotype to those of the PRL-/- and PRLR-/- knockout mouse models, mainly emphasizing the irreplaceable role of PRL in reproduction and mammary gland development. PRL signaling in rat prostate tissue is primarily transduced via Stat5a and Stat5b, likely supporting the viability of
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prostate epithelial cells during long-term androgen deprivation [152]. In the prostate, studies in Stat5a-/- knockout mice have provided evidence for a direct role of Stat5a in the maintenance of normal tissue architecture and function of the mouse prostate [153]. Lack of Stat5a function results in a distinct prostatic phenotype characterized by an increased occurrence of cyst formation with disorganization and detachment of prostate epithelial cells. In addition to PRL, other polypeptide factors, such as GH, insulin-like growth factor I (IGF-I), epidermal growth factor (EGF) and interleukin-6 (IL-6) are known to activate Stat5.
Mouse models genetically engineered in other hormones
The AR transgenic mice overexpress the AR specifically in prostate secretory epithelium [154]. The earliest alteration observed in the AR transgenic mouse prostates was an extensive 5-fold increase in the proliferation of secretory epithelial cells, as evidenced by immunostaining of the proliferating marker Ki-67, in the absence of histological abnormalities.
Proliferation in these glands was associated with increased apoptosis, possibly accounting for the absence of hyperplasia. Older AR transgenic mice developed focal areas of intraepithelial neoplasia, resembling human high-grade PIN, but no further malignancy has been observed. A certain resistance to malignant transformation in the mouse prostate compared to humans has been suggested. No reports of any tumorigenic effects of exogenously added androgens in these models are available.
The recent generation and characterization of the various estrogen modulated mouse models (αERKO, βERKO, αβERKO and ArKO) have provided new insights regarding the role of estrogens in prostate growth and development [155]. A specific direct response to estrogens is the induction of changes in the prostatic epithelium, termed squamous metaplasia [156-159]. Tissue recombinant studies using epithelium and stroma from wildtype and transgenic mice lacking a functional ERα (αERKO) or ERβ (βERKO) have demonstrated that the development of squamous metaplasia is mediated through stromal ERα [160, 161]. Furthermore, a distinct phenotype of focal epithelial hyperplasia in the VP has been reported in aging mice lacking functional ERβ (βERKO) [162, 163], while no apparent prostate pathology or enlargement has yet been reported in αERKO or the double knockout αβERKO [155]. Altogether, these findings indicate an anti-proliferative role for epithelial ERβ and also suggest that an unbalanced stromal ERα in action could contribute to the phenotype observed.
The ArKO (aromatase knockout) mouse lacks endogenous estrogen
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production due to a non-functional aromatase enzyme. In the ArKO mouse, the combined effects of estrogen absence and elevated androgen and PRL levels result in a moderate prostate enlargement with hyperplasia evident in all lobes and tissue compartments [161]. Moreover, an associated up-regulation of epithelial AR was demonstrated in the ArKO mouse and has been suggested to contribute to the observed phenotype. In the absence of endogenous estrogen (ArKO) or ERs (αERKO and βERKO), prostate development occurs normally, suggesting that intact estrogen signaling is not essential for the initiation of neonatal prostate growth. The histological appearance of the prostate hyperplasia in ArKO male mice is strikingly similar to that of the Mt-PRL-transgenic mice.
In contrast, the AROM+ mice, which overexpress the aromatase gene, resulting in elevated estrogens levels, combined with significantly reduced testosterone and FSH levels, and elevated levels of PRL and corticosterone [164]. AROM+ males present a multitude of severe structural and functional alterations in the reproductive organs. Furthermore, squamous metaplasia has been seen in the prostatic collecting ducts, consistent with high levels of endogenous estrogens. Some of the abnormalities, such as non-descended testes and undeveloped prostate, resemble those observed in animals exposed perinatally to high levels of exogenous estrogen, indicating that the elevated aromatase activity results in excessive estrogen exposure during early phases of development.