Prostate cancer (PCa) is a common cancer in the male worldwide, and most of PCa can be cured by surgery or radiotherapy in the early stage [1]. However, up to 15% of initial patients are diagnosed to develop metastatic lesions, and recurrence rate of patients after conventional radical therapy is more than 40% [2]. Androgen deprivation therapy (ADT) has been developed for recurrent PCa, but some patients still relapse because of the progression of castration-resistant prostate cancer (CRPC) [3]. Since traditional therapies including chemotherapy and radiotherapy are not highly effective and have obvious side effects of cytotoxicity for CRPC, novel therapeutic strategies are urgently needed. Oncolytic adenovirus as a new viral therapy agent for CRPC has gained increasing attention due to several advantages such as high selectivity, low cytotixicity and oncolysis characteristics.

Adenovirus (Ad) has been the workhorse of virotherapy since 1950s. Unfortunately, due to the rapid development of chemotherapy and safety concern of virotherapy, the enthusiasm for virotherapy suddenly faded away later [4]. Only recently, virotherapy has become a hot topic with the development of Ad and retroviral vectors for the delivery of a variety of transgenes targeting different types of cancers [4]. Therefore, in this review we summarize current status on the design and application of oncolytic Ad vectors for PCa therapy.

To develop Ad vector as a novel approach of cancer therapy, it is important to understand the genome structure of Ad in order to design recombinant Ad vectors. Ad is a non-enveloped virus with double-stranded DNA genome. Total 103 Ads can be divided into seven ‘species’ named A to G based on their genotypes [5]. Among them, A and C species are the most prevalent, and Ad serotype 5 (Ad5) is mainly used in the studies on Ad [6].

There are five early transcription units (E1A, E1B, E2, E3 and E4), two delayed transcription units (IX and Iva2) and one late transcription unit (L1-L5) in Ad 5 coding region [7]. E1A conserved region 2 (E1A-CR2) interacts with retinoblastoma (Rb) in host cells to promote S-phase entry and viral DNA replication [7]. E1B contains E1B-19K and E1B-55K. E1B-19K is a functional Bcl-2 homologue and plays a dual role in apoptosis and autophagy [8]. E1B-55K could promote virus survival in tumor cells, but is not necessary for oncolytic effects of adenoviruses [9]. E2 region encodes polymerase, DNA binding protein (DBP) and preterminal protein (pTP), which are important for viral transformation and replication. E3 region encodes adenovirus death protein (ADP), which promotes the cytolysis of host cells and the spread of the virus to the surrounding cells [10]. E4 region plays an important role in transition and late viral gene expression, and is vital for viral replication and virion assembly (Fig. 1).

Fig. 1.

Illustration of the structure and function of Ad5 genome. LITR and RITR indicate left and right inverted terminal repeats, respectively. Abbreviation: E1A-CR2, E1A conserved region 2; Rb, retinoblastoma; ADP, adenovirus death protein; DBP, DNA-binding protein; pTP, precursor terminal protein; Pol, polymerase.

Ad5 is categorized into two vectors based on oncolytic character: conditionally replicating adenoviral (CRAd) vector which only propagates and lyse cancer cells, and could not replicate in normal cells; and replication-defective adenoviral (RDAd) vector with the deletion of E1, which could not replicate in cells but can carry therapeutic genes [11].

To achieve optimal anti-tumor efficacy of Ad vector, we need develop Ad vector with improved tropism and transduction. Therefore, we need identify cellular receptors that mediate the tropism and transduction of Ad. Cell attachment of oncolytic adenovirus is initiated by the attachment of the fiber protein to CAR receptor [12]. Next, the interaction between Arg-Gly-Asp (RGD) motifs in Ad penton-base protein and $\alpha$v$\beta$3 and $\alpha$v$\beta$5 integrin triggers virus internalization. Intravenous administration of Ad vectors is hindered by viral neutralizing antibodies (nAbs), other proteins in the circulating blood, inefficient transduction and hepatotoxicity [13]. It is reported that nAbs mainly hinder systemic administration of oncolytic Ads in preclinical and clinical studies [14]. In addition, Ad5 efficiently binds to lymphocytes and erythrocytes [15, 16]. Therefore, nonspecific tumor-selectivity and vector transduction are the main challenges, which may cause hepatotoxicity and low efficiency in specific tumor delivery.

Both CAR and $\alpha$v$\beta$ integrin are the tropism determinants of Ad5 [17]. The liver is susceptible to the hepatotoxicity of Ad5, which may be due to high expression of CAR in the liver although CAR expression is low in PCa [18]. A novel Ad5 vector containing two amino acid mutations in the AB loop of the fiber-modified Ad5 fiber-knob reduced liver tropism and increased the anti-tumor efficacy of the vector in low CAR expression or CAR deficient cancer cells following intravascular delivery [19]. Therefore, CAR-independent targeting strategy has promise for the treatment of CAR deficient PCa.

The upregulation of $\alpha$v$\beta$6 integrin has been suggested to correlate with tumor progression [20, 21]. A trial targeting $\alpha$v$\beta$6 integrin was conducted. Ad5.HI.A20 and Ad5/kn48.DG.A20 were generated by inserting A20 (a 20-amino acid peptide) into penton base protein loop targeting $\alpha$v$\beta$6 integrin, leading to 160 and 180 fold increase in the transduction of BT-20 breast carcinoma cells with high expression of $\alpha$v$\beta$6 [22].

Coagulation factor X (FX) is an adapter for coagulation factors involved in liver tropism following systemic delivery [23]. Ad5 recognizes FX via hexon hypervariable region, and FX then interacts with heparan sulfate proteoglycans (HSPGs) on the surface of liver cells [24]. Ablating the binding of FX to Ad5 can diminish virus localization to the liver. Warfarin pretreatment significantly reduced liver sequestration and hepatic toxicity of Ad vectors [25].

On the other hand, Ad shows high affinity to scavenger receptor-A (SR-A) and scavenger receptor expressed on endothelial cell-I (SREC-I). Kupffer cells (KCs) and liver sinusoidal endothelial cells (LSECs) recognize Ad and remove them from the circulation to inhibit efficient hepatocyte transduction (Fig. 2). The efficiency of hepatocyte transduction by Ad could be increased by blocking SR-A and SREC-I [26]. Two peptides PP1 and PP2 have been designed to block SR-A and SREC-I, respectively, and they significantly improved Ad-mediated hepatocyte transduction efficiency [27].

Fig. 2.

The ligands and receptors involved in Ad tropism and transduction. (1) Ad is picked by SR-A and SREC-I on KC and LSEC, and then is internalized through pinocytosis and then passed to lysosome. Most Ads are released from hepatocyte by first-pass effect which reduces the tropism and transduction and protects the hepatocytes from lysis. (2) Ad binds to FX and then FX binds to HSPGs on liver cells. Warfarin significantly reduces liver sequestration and hepatic toxicity. (3) Ad binds to CAR on PCa cells and then is internalized via $\alpha$v$\beta$6. Ad replicates in PCa cells and induces cells lysis. Abbreviation: Ad, adenovirus; SR-A, scavenger receptor-A; SREC-I, scavenger receptor expressed on endothelial cell-I; KCs, Kupffer cells; LSECs, liver sinusoidal endothelial cells; FX, Coagulation factor X; HSPGs,heparan sulfate proteoglycans; CAR, coxsackievirus and adenovirus receptor; PCa, prostate cancer.

Elucidating the molecular mechanism of virus-host interactions of oncolytic adenovirus is essential to the development of better therapy for PCa. The identification of FX, $\alpha$v$\beta$6, SR-A and SREC-I receptors increases our understanding of tropism and transduction of Ads. A loop in the penton base protein incorporated by a 20-amino acid peptide (A20) reveals a possibility for PCa virotherapy following systemic delivery [60]. Further optimization of Ads with enhanced tropism and transduction is important to achieve high efficacy and specificity to PCa with low toxicity. It is also important to develop synergistic therapy strategies based on oncolytic adenovirus and other treatments such as chemotherapy and radiotherapy.

On the other hand, we should pay attention to the variety in the phenotypes and surface receptors in PCa cells. Although primary cancer cell culture is a golden standard of in vitro model, it could not mimic in vivo situation of PCa perfectly. A patient-derived xenograft (PDX) model was developed based on direct engraftment of patient tumor into immunocompromised mice, and it retained most characteristics of primary tumor [61]. The design of novel PCa in vivo model will hope realize personalized therapy for PCa patients in near future.

LM and JW designed the study. CW, FW, ZX collected and analyzed the literatures. RW and JC analyzed the literatures and made the figures. All authors participated in writing the draft and approved the final manuscript.

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This study was supported by grants from the Project of Invigorating Health Care Through Science, Technology and Education (No. CXTDA2017034), and Jiangsu Provincial Medical Youth Talent (No. QNRC2016794).

The authors declare no conflict of interest.