Novel extracellular role of REIC/Dkk-3 protein in PD-L1 regulation in cancer cells

The adenovirus-REIC/Dkk-3 expression vector (Ad-REIC) has been the focus of numerous clinical studies due to its potential for the quenching of cancers. The cancer-suppressing mechanisms of the REIC/DKK-3 gene depend on multiple pathways that exert both direct and indirect effects on cancers. The direct effect is triggered by REIC/Dkk-3-mediated ER stress that causes cancer-selective apoptosis, and the indirect effect can be classified in two ways: (i) induction, by Ad-REIC-mis-infected cancer-associated fibroblasts, of the production of IL-7, an important activator of T cells and NK cells, and (ii) promotion, by the secretory REIC/Dkk-3 protein, of dendritic cell polarization from monocytes. These unique features allow Ad-REIC to exert effective and selective cancer-preventative effects in the manner of an anticancer vaccine. However, the question of how the REIC/Dkk-3 protein leverages anticancer immunity has remained to be answered. We herein report a novel function of the extracellular REIC/Dkk-3-namely, regulation of an immune checkpoint via modulation of PD-L1 on the cancer-cell surface. First, we identified novel interactions of REIC/Dkk-3 with the membrane proteins C5aR, CXCR2, CXCR6, and CMTM6. These proteins all functioned to stabilize PD-L1 on the cell surface. Due to the dominant expression of CMTM6 among the proteins in cancer cells, we next focused on CMTM6 and observed that REIC/Dkk-3 competed with CMTM6 for PD-L1, thereby liberating PD-L1 from its complexation with CMTM6. The released PD-L1 immediately underwent endocytosis-mediated degradation. These results will enhance our understanding of not only the physiological nature of the extracellular REIC/Dkk-3 protein but also the Ad-REIC-mediated anticancer effects. KEY MESSAGES: • REIC/Dkk-3 protein effectively suppresses breast cancer progression through an acceleration of PD-L1 degradation. • PD-L1 stability on the cancer cell membrane is kept high by binding with mainly CMTM6. • Competitive binding of REIC/Dkk-3 protein with CMTM6 liberates PD-L1, leading to PD-L1 degradation.

Promising Gene Therapy Using an Adenovirus Vector Carrying REIC/Dkk-3 Gene for the Treatment of Biliary Cancer

BACKGROUND

We previously demonstrated that the reduced expression in immortalized cells (REIC)/dikkopf-3 (Dkk-3) gene was downregulated in various malignant tumors, and that an adenovirus vector carrying the REIC/Dkk-3 gene, termed Ad-REIC induced cancer-selective apoptosis in pancreatic cancer and hepatocellular carcinoma.

OBJECTIVE

In this study, we examined the therapeutic effects of Ad-REIC in biliary cancer using a second- generation Ad-REIC (Ad-SGE-REIC).

METHODS

Human biliary cancer cell lines (G-415, TFK-1) were used in this study. The cell viability and apoptotic effect of Ad-SGE-REIC were assessed in vitro using an MTT assay and Hoechst staining. The anti-tumor effect in vivo was assessed in a mouse xenograft model. We also assessed the therapeutic effects of Ad-SGE-REIC therapy with cisplatin. Cell signaling was assessed by Western blotting.

RESULTS

Ad-SGE-REIC reduced cell viability, and induced apoptosis in biliary cancer cell lines via the activation of the c-Jun N-terminal kinase pathway. Ad-SGE-REIC also inhibited tumor growth in a mouse xenograft model. This effect was further enhanced in combination with cisplatin.

CONCLUSION

Ad-SGE-REIC induced apoptosis and inhibited tumor growth in biliary cancer cells. REIC/Dkk-3 gene therapy using Ad-SGE-REIC is an attractive therapeutic tool for biliary cancer.

Tumor suppressor REIC/Dkk-3 and its interacting protein SGTA inhibit glucocorticoid receptor to nuclear transport

REIC/Dkk-3 is a tumor suppressor, and its expression is significantly downregulated in a variety of human cancer types. A previous study performed yeast two-hybrid screening and identified the small glutamine-rich tetratricopeptide repeat-containing protein α (SGTA), known as a negative modulator of cytoplasmic androgen receptor (AR) signaling, which is a novel interacting partner of REIC/Dkk-3. The previous study also indicated that the REIC/Dkk-3 protein interferes with the dimerization of SGTA and then upregulates the AR transport and signaling in human prostate cancer PC3 cells. Since the transport of some steroid receptors to nucleus is conducted similarly by dynein motor-dependent way, the current study aimed to investigate the role of SGTA and REIC/Dkk-3 in the transport of other glucocorticoid receptors (GR). In vitro reporter assays for the cytoplasmic GR transport were performed in human prostate cancer PC3 cells and 293T cells. As for the SGTA protein, a suppressive effect on the GR transport to the nucleus was observed in the cells. As for the REIC/Dkk-3 protein, an inhibitory effect was observed for the GR transport in PC3 cells. Under the depleted condition of SGTA by short-hairpin (sh)RNA, the downregulation of GR transport by REIC/Dkk-3 was significantly enhanced compared with the non-depleted condition in PC3 cells, suggesting a compensatory role of REIC/Dkk-3 in the SGTA mediated inhibition of GR transport. The current study therefore demonstrated that SGTA inhibited the cytoplasmic transport of GR in 293T and PC3 cells, and REIC/Dkk-3 also inhibited the cytoplasmic transport of GR in PC3 cells. These results may be used to gain novel insight into the GR transport and signaling in normal and cancer cells.

The Downregulation of the Expression of CD147 by Tumor Suppressor REIC/Dkk-3, and Its Implication in Human Prostate Cancer Cell Growth Inhibition

The cluster of differentiation 147 (CD147), also known as EMMPRIN, is a key molecule that promotes cancer progression. We previously developed an adenoviral vector encoding a tumor suppressor REIC/Dkk-3 gene (Ad-REIC) for cancer gene therapy. The therapeutic effects are based on suppressing the growth of cancer cells, but, the underlying molecular mechanism has not been fully clarified. To elucidate this mechanism, we investigated the effects of Ad-REIC on the expression of CD147 in LNCaP prostate cancer cells. Western blotting revealed that the expression of CD147 was significantly suppressed by Ad-REIC. Ad-REIC also suppressed the cell growth of LNCaP cells. Since other researchers have demonstrated that phosphorylated mitogen-activated protein kinases (MAPKs) and c-Myc protein positively regulate the expression of CD147, we investigated the correlation between the CD147 level and the activation of MAPK and c-Myc expression. Unexpectedly, no positive correlation was observed between CD147 and its possible regulators, suggesting that another signaling pathway was involved in the downregulation of CD147. This is the first study to show the downregulation of CD147 by Ad-REIC in prostate cancer cells. At least some of the therapeutic effects of Ad-REIC may be due to the downregulation of the cancer-progression factor, CD147.

Overexpression of REIC/Dkk-3 suppresses the expression of CD147 and inhibits the proliferation of human bladder cancer cells

Our group previously developed an adenoviral vector encoding the REIC/Dkk-3 gene (Ad-REIC), a tumor suppressor, for cancer gene therapy. The Ad-REIC agent induces apoptosis and inhibits invasion in a number of cancer cell lines; however, the molecular mechanisms underlying its effects remain unclear. Cluster of differentiation 147 (CD147), also known as extracellular matrix metalloproteinase inducer (EMMPRIN), is a key molecule that promotes cancer proliferation and invasion. In order to elucidate the therapeutic mechanism of Ad-REIC, its effect on the expression of CD147 in human bladder cancer KK47 cells was investigated. Treatment with Ad-REIC markedly downregulated the expression of CD147 and significantly inhibited cellular proliferation. Since the expression of CD147 is reported to be under the positive control of mitogen-activated protein kinase (MAPK) signaling and the c-Myc protein, the correlations between the expression of CD147 and the activation of MAPKs or the expression of c-Myc were examined. Unexpectedly, no positive correlation was observed between the level of CD147 and the potential regulators that were assessed, indicating that another signaling pathway is responsible for the downregulation of CD147. The results from the present study demonstrate that Ad-REIC treatment can significantly downregulate the expression of CD147 in bladder cancer cells. Downregulation of the cancer-progression factor CD147 may be a novel mechanism that underlies the therapeutic effects of Ad-REIC treatment.

Properties of the feline tumour suppressor reduced expression in immortalized cells (REIC/Dkk-3)

Reduced expression in immortalized cells (REIC/Dkk-3), a member of the human Dickkopf (Dkk) family, is a growth suppressor in human and canine mammary tumours. Mammary gland tumours are common neoplasms with high malignancy in female cats. The purpose of this study was to clone the feline REIC/Dkk-3 homolog, investigate its expression in cell lines established from feline mammary gland tumours, and test its tumour suppressor function. Western blot analysis revealed that expression of the REIC/Dkk-3 protein was reduced in feline mammary carcinoma cell lines. Forced expression of REIC/Dkk-3 induced apoptosis in feline mammary tumour cell lines. These results demonstrate that REIC/Dkk-3 expression, which is downregulated in feline mammary tumour cell lines, results in the induction of apoptosis in these cells. Our findings suggest that feline REIC/Dkk-3 represents a potential molecular target for the development of therapies against feline mammary cancers.

Feasibility of Neoadjuvant Ad-REIC Gene Therapy in Patients with High-Risk Localized Prostate Cancer Undergoing Radical Prostatectomy

In a phase I/IIa study of in situ gene therapy using an adenovirus vector carrying the human REIC/Dkk‐3 gene (Ad‐REIC), we assessed the inhibitory effects of cancer recurrence after radical prostatectomy (RP), in patients with high risk localized prostate cancer (PCa). After completing the therapeutic interventions with initially planned three escalating doses of 1.0 × 10¹⁰, 1.0 × 10¹¹, and 1.0 × 10¹² viral particles (VP) in 1.0–1.2 mL (n = 3, 3, and 6), an additional higher dose of 3.0 × 10¹² VP in 3.6 mL (n = 6) was further studied. Patients with recurrence probability of 35% or more within 5 years after RP as calculated by Kattan's nomogram, were enrolled. They received two ultrasound‐guided intratumoral injections at 2‐week intervals, followed by RP 6 weeks after the second injection. Based on the findings of MRI and biopsy mapping, as a rule, one track injection to the most prominent cancer area was given to initial 12 patients and 3 track injections to multiple cancer areas in additional 6 patients. As compared to the former group, biochemical recurrence‐free survival of the latter showed a significantly favorable outcome. Neoadjuvant Ad‐REIC, mediating simultaneous induction of cancer selective apoptosis and augmentation of antitumor immunity, is a feasible approach in preventing cancer recurrence after RP. (199)

Adenovirus-mediated REIC/Dkk-3 gene therapy: Development of an autologous cancer vaccination therapy (Review)

Reduced expression in immortalized cells (REIC)/Dickkopf (Dkk)-3 is a tumor suppressor and therapeutic gene and has been studied with respect to the application of cancer gene therapy. Our previous studies demonstrated that the intratumoral injection of an adenovirus vector carrying the human REIC/Dkk-3 gene (Ad-REIC) suppresses tumor growth in mouse models of prostate, breast and testicular cancer and malignant mesothelioma. The mechanisms underlying these antitumor therapeutic effects have only been clarified recently. It has been demonstrated that Ad-REIC treatment inhibits cancer progression via the upregulation of systemic anticancer immunity. Under experimental conditions, autologous cancer vaccination via cancer-specific apoptosis and anticancer immune activation is a possible therapeutic mechanism. The robust anticancer effects observed in previous preclinical studies support the clinical utility of Ad-REIC. At present, a phase I–IIa study of Ad-REIC gene therapy in prostate cancer patients is ongoing. The current study reviews the observations of previous fundamental studies and summarizes the anticancer mechanisms of intratumoral Ad-REIC treatment in terms of cancer vaccination.

Molecular cloning and tumour suppressor function analysis of canine REIC/Dkk-3 in mammary gland tumours

REIC/Dkk-3, a member of the human Dickkopf (Dkk) family, plays a role as a suppressor of growth in several human cancers. In this study, the tumour suppression function of canine REIC/Dkk-3 was investigated. The full-length open reading frame of the canine REIC/Dkk-3 homologue was cloned and the tissue distribution of REIC/Dkk-3 mRNA was determined, along with the subcellular localisation of the REIC/Dkk-3 protein in canine cancer cell lines. Expression of REIC/Dkk-3 was lower in mammary gland tumours and in canine mammary carcinoma cell lines than in normal mammary gland tissue. Overexpression of REIC/Dkk-3 induced apoptosis in canine mammary carcinoma cell lines. These results show that expression of REIC/Dkk-3 is downregulated in canine mammary tumours and that one of the functions of this gene is induction of apoptosis.