:: Asian Journal of Andrology ::

Role of androgen receptor in prostate cancer

Hiroyoshi Suzuki, Haruo Ito

Department of Urology, Chiba University School of Medicine 1-8-1 Inohana, Chuo-ku, Chiba-shi, Chiba 260-8670, Japan

Asian J Androl 1999 Sep; 1: 81-85

Keywords: androgen receptors; prostatic neoplasms; genetic change; methylation

Abstract

The growth of prostate cancer is sensitive to androgen, and hormonal therapy has been used for treatment of advanced cancer. About 80% of prostate cancers initially respond to hormonal therapy, howerver, more than half of the responders gradually become resistant to this therapy. Changes in tumors from an androgen-responsive to an androgen-unresponsive state have been widely discussed. Since androgen action is mediated by androgen receptor (AR), abnormalities of AR is believed to play an important role of the loss of androgen responsiveness in prostate cancer. This article focused on the role of AR in the progression of prostate cancer.

1 Loss of hormone sensitivity of prostate cancer

During embryogenesis and puberty, androgens play an important role in the development of the male phenotype in genotypical 46, XY individuals^[1]. In adult males, they also control the function and structure of the male urogenital tissues, including the prostate. In the prostate, the enzyme 5-reductase type 2 is responsible for conversion of testosterone to the more potent 5-dihydrotestosterone. The action of both testosterone and 5-dihydrotestosterone is mediated by the intracellular androgen receptor (AR).

Prostate cancer is one of the most common cancers in Western countries and the number of patients is increasing in Asian countries. The growth of prostate is sensitive to androgen, and endocrine therapy has been used for metastatic prostate cancer since Huggins and Hodges' report in 1941^[2]. Androgen ablation can be achieved by orchiectomy, by the administration of antiandrogens, or, more recently, by luteinizing hormone-releasing hormone (LH-RH) agonist. Recent clinical trials have demonstrated the efficacy of combining antiandrogens with orchiectomy or an LH-RH agonist to block the remaining androgens produced by the adrenal glands (maximal androgen blockade: MAB)^[3]. About 80% of prostate cancer patients initially respond to endocrine therapy, but more than half of them gradually become resistant to the therapy. Changes in tumors from an androgen-responsive to an androgen-unresponsive state have been widely discussed. It is generally accepted that the mechanism of loss of androgen sensitivity can be explained by (1) selection of cancer clones, (2) adaptation of the cells to the environmental situation without androgen, (3) alternative pathway of signal transduction, and (4) involvement of AR. In this article, involvement of AR in the steps of loss of androgen sensitivity in prostate cancer is focused.

2 Structure and function of the AR

The AR is a member of the family of nuclear receptors including the steroid hormone receptors and other ligand-dependent transcription factors like thyroid hormone receptors and retinoic acid receptors. Steroid hormone receptors are characterized by a central DNA binding domain, that targets the receptor to specific DNA sequences (hormone responsive elements), a carboxy-terminal ligand binding domain, and an amino-terminal regulatory domain (Figure 1). The DNA binding domain and ligand binding domain are relatively conserved among the various steroid hormone receptors.

Figure 1.Structure and functional domain of androgen receptor.

The AR is encoded by an 11 kb-sized messenger RNA and located on the X chromosome[^4,5]. The open reading flame of AR is separated into 8 exons. Exon 1 encodes the large amino-terminal domain, exons 2 and 3 encode the DNA binding domain, and exons 4-8 encode the ligand binding domain, respectively.

3 Overexpression of AR in prostate cancer

Visakorpi et al^[6] found high-level AR amplification in 7 of 23 (30%) hormone-refractory prostate cancer cases and in none of the specimens taken from the same patients prior to therapy. However, almost all of the cases showing AR overexpression have received only surgical castration or the LH-RH agonist without the administration of antiandrogens. Thus, they have not received MAB therapy. From these results, the loss of androgen sensitivity in these cases were thought to be caused by the growth of cancer clones stimulated by the remaining androgen from the adrenal glands, suggesting the importance of MAB therapy.

4 AR gene mutations in prostate cancer tissue

The initial impulse to deal with AR gene mutations in prostate cancer came from the study of LNCaP cell line, derived from a metastatic lesion of the lymph node of a prostate cancer patient. AR of this cell line was reported to contain one mutation at codon 877 (Thr to Ala)^[7,8]. The growth of LNCaP cells is stimulated in vitro by the addition of androgens, estrogens, progestogens, and several antiandrogens, indicating a widely responsive property of LNCaP cells.

Mutations in the AR gene have been detected in prostate cancer specimens in about 10%-20% of cases with the general finding that the frequency of mutation appears to be higher in hormone-refractory, metastatic tumors compared with untreated lower grade primary tumors^[9,10]. Since the growth of early stage prostate cancer appears to be mediated by wild-type AR, it appears that receptor mutation may have a role in conferring growth advantage to cells during progression. Functional analysis of several AR mutations detected in hormone-refractory cancers showed a widely responsive property like LNCaP cells^[11] (Table 1). Therefore, AR gene mutation is thought to be one of the molecular mechanisms for loss of androgen dependency of prostate cancer.

Given the observation that certain mutations can clearly alter AR ligand specificity, AR mutation has been postulated as playing a key role in the antiandrogen withdrawal syndrome^[12]. This phenomenon occurs in a subset of patients experiencing relapse of tumor growth, measured by increasing serum PSA concentration, after a long-term antiandrogen treatment. After the cessation of antiandrogen treatment, symptoms improve, and serum PSA level drops, suggesting that the antiandrogen is acting agonistically in the tumor cells to promote growth. Our previous study^[13] reported that in 2 of 4 patients who experienced an improvement after antiandrogen withdrawal (out of total 22 prostatic cancer patients), AR mutations occurred during antiandrogen treatment. These mutations, which were identical to the LNCaP cell AR mutation (T877A), were not observed in untreated tumors. From these results, the antiandrogen withdrawal syndrome seems to be caused by AR mutation.

Table 1. Responsive properties of mutated AR detected in prostate cancer patients.

	DHT	Estradiol	Progesterone	Flutamide/Niltamide
Wild-type	+	-	-	-
V715M^*	+	-	-	+
A721T	+	++	++	++
H874Y^*	+	++	++	++
T877A^#	+	++	++	++
T877S^*		-	-	-
Q902R	+++	-	-	++

^*These mutated ARs were found in the patients treated with flutamide.
This mutation is identical to that found in LNCaP cell line.

5 Down-regulation of AR in endocrine therapy-resistant prostatic cancers

Immunohistochemical studies of AR for prostate cancer showed a heterogeneous expression of AR in cancer tissues. AR was noted in not only normal prostate, benign prostatic hypertrophy but also prostate cancers including endocrine therapy-resistant cases. In prostate cancer tissues, the ratio of AR-positive cells was related with histological grade (i.e., Gleason score). Also, comparison of AR status before and after endocrine therapy within the same patient showed the down-regulation of AR during the loss of androgen responsiveness. Although the exact mechanism for this down-regulation of AR is still unclear, two possible pathways are described here.

5.1Hypermethylation of AR promotor region

In many types of malignancies, DNA hypermethylation of some tumor suppressor genes (ie, VHL, RB, p16/MTS1/CDK4I, etc.) has been found^{[14, 15]}. A main target of the regional hypermethylation is normally unmethylated CpG islands located in gene promotor regions. This hypermethylation correlates with transcriptional repression that can serve as an alternative to coding region mutations for inactivations of the genes. Also, hypermethylation of CpG island located at estrogen receptor promotor was found in breast cancer tissues. A recent study by Jarrard et al^[16] showed that in vitro DNA methylation of AR promotor CpG island was associated with loss of AR expression in human prostate cancer cells. Further study should be necessary for confirmation of this hypothesis.

5.2 Involvement of co-regulators (co-factors)

Several co-regulators between AR and transcriptional complex have been cloned. Yeh et al^[17]have cloned ARA70 as a specific co-regulator for AR and demonstrated that ARA70 functioned as 10 times transcriptional activator in DU145, which is an AR-negative prostate cancer cell line, in the presence of androgen. Figure 2 and Table 2 showed co-regulators related to AR. Although the main cause of androgen insensitivity syndrome (AIS) is AR gene mutation^[18], small number of AIS patients revealed no AR mutations. It is suggested that abnormality of co-regulator may contribute to the down-regulation of AR expression in AIS patients. Similar mechanism is suggested to occur in prostate cancer as well. Further studies should be performed to clarify the functional role of co-regulators in prostate cancer.

Table 2. Co-regulators (co-factors) of AR

Name	Size	Type	Region of interaction	Function
ARA55	444 aa	Co-activator	?	Ligand-dependent binding, TGF- inducible
ARA70	70 kDa	Co-activator	LBD or DBD-LBD	AR mutation can increase or decrease ARA70 interaction
CBP		Co-activator	LBD	Histone acetyl transferase
F-SRC-1		Co-activator	LBD	Enhances rAR transact
GRIP-1		Co-activator	LBD	?
RAF	110 kDa	Co-activator	N-terminal-DBD	Enhances AR-DNA binding
RIP140	194 aa	Co-activator	DBD	Overexpression increases transactivation
TIF2		Co-activator	LBD	?
ARIP3	64 kDa	Co-repressor	Zinc finger region	Expressed in testis
c-jun		Co-repressor	DBD-hinge and LBD	Inhibits formation of AR-ARE
Calreticulin		Co-repressor	DBD	Inhibits DNA binding and transactivationby AR by binding to DBD
Ets		Co-repressor	Aa536-918	A transcription factor
MCM-7		Co-repressor	N-terminal	Cell cycle control
SRC-1		Co-repressor	LBD	Inhibits AR function

Figure 2. Co-regulators (co-factors) of AR.

6 Shorter CAG repeats in the N-terminal domain of AR

The incidence of prostate cancer is highly variable among races. The highest incidence is observed in black peoples and the lowest incidence is observed in Asian peoples. Several reports have shown that the shorter poly-glutamine and polyglycine repeat length has been correlated with the higher transactivational function or expression level of AR, which has been associated with the higher risk of prostate cancer. Hardy et al^[19] found a significant correlation between the reduced CAG repeat length and the age at prostate cancer onset, suggesting that CAG repeat length may impinge on mechanisms involved in tumor initiation but not in progression of the localized to advanced stage.

7 Concluding remarks

Androgen ablation therapy has been an important modality for the treatment of disseminating prostate cancer for nearly 60 years. However, still loss of androgen dependency in prostate cancer has been one of the big problems for the treatment of this malignancy. Since prostate cancer is very heterogenous^[20], several mechanisms such as mentioned above may contribute to the loss of androgen dependency simultaneously within the same patient.

References

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Correspondence to Prof. Haruo Ito, The President of Japanese Society of Andrology.
Tel: +81-43-226 2134 Fax: +81-43-226 2136
E-mail: itoh@med.m.chiba-u.ac.jp
Received 1999-05-19 Accepted 1999-08-19