Effects of Dickkopf-1 (DKK-1) on Prostate Cancer Growth and Bone Metastasis

Osteoblastic bone metastases are commonly detected in patients with advanced prostate cancer (PCa) and are associated with an increased mortality rate. Dickkopf-1 (DKK-1) antagonizes canonical WNT/β-catenin signaling and plays a complex role in bone metastases. We explored the function of cancer cell-specific DKK-1 in PCa growth, metastasis, and cancer-bone interactions using the osteoblastic canine PCa cell line, Probasco. Probasco or Probasco + DKK-1 (cells transduced with human DKK-1) were injected into the tibia or left cardiac ventricle of athymic nude mice. Bone metastases were detected by bioluminescent imaging in vivo and evaluated by micro-computed tomography and histopathology. Cancer cell proliferation, migration, gene/protein expression, and their impact on primary murine osteoblasts and osteoclasts, were evaluated in vitro. DKK-1 increased cancer growth and stimulated cell migration independent of canonical WNT signaling. Enhanced cancer progression by DKK-1 was associated with increased cell proliferation, up-regulation of NF-kB/p65 signaling, inhibition of caspase-dependent apoptosis by down-regulation of non-canonical WNT/JNK signaling, and increased expression of epithelial-to-mesenchymal transition genes. In addition, DKK-1 attenuated the osteoblastic activity of Probasco cells, and bone metastases had decreased cancer-induced intramedullary woven bone formation. Decreased bone formation might be due to the inhibition of osteoblast differentiation and stimulation of osteoclast activity through a decrease in the OPG/RANKL ratio in the bone microenvironment. The present study indicated that the cancer-promoting role of DKK-1 in PCa bone metastases was associated with increased growth of bone metastases, reduced bone induction, and altered signaling through the canonical WNT-independent pathway. DKK-1 could be a promising therapeutic target for PCa.

The Nuclear Localization Sequence and C-Terminus of Parathyroid Hormone-Related Protein Regulate Normal Pancreatic Islet Development and Function

Parathyroid hormone-related protein (PTHrP) is a pleiotropic hormone essential for morphogenesis, tissue differentiation, as well as cell regulation and function. PTHrP is expressed by pancreatic beta cells which are responsible for insulin secretion. Previous studies have reported that N-terminal PTHrP stimulated proliferation in beta cells in rodents. We have developed a knockin mouse model (PTHrP Δ/Δ) lacking the C-terminal and nuclear localization sequence (NLS) of PTHrP. These mice die at ∼day 5, are severely stunted in growth, weigh 54% less than control mice at day 1 -2 and eventually fail to grow. PTHrP Δ/Δ mice are also hypoinsulinemic and hypoglycemic yet have nutrient intake proportional to size. To characterize the pancreatic islets in these mice, islets (∼10-20) were isolated from 2-5 day-old-mice using collagenase digestion. Islets from PTHrP Δ/Δ mice were smaller in size but secreted more insulin than littermate controls. PTHrP Δ/Δ and control mice islets were exposed to various glucose concentrations and intracellular calcium, the trigger for insulin release, was elevated for glucose concentrations of 8-20 mM. Immunofluorescence staining showed less glucagon-stained area in islets from PTHrP Δ/Δ mice (∼250 µm²) compared to islets from control mice (∼900 µm²), and ELISA confirmed there was reduced glucagon content. These data collectively demonstrate increased insulin secretion and reduced glucagon at the islet level, which may contribute to the observed hypoglycemia and early death in PTHrP Δ/Δ mice. Thus, the C-terminus and NLS of PTHrP are crucial to life, including regulation of glucose homeostasis and islet function.

Abstract 1603: The effect of dickkopf-1 (DKK-1) on prostate cancer growth and bone metastasis using the canine osteoblastic Probasco cell line

Osteoblastic bone metastasis is the predominant phenotype observed in advanced prostate cancer and is associated with high patient mortality and morbidity. WNT signaling is necessary for both bone formation and tumor progression. Dickkopf-1 (DKK-1), a secretory antagonist of the canonical WNT/β-catenin signaling pathway, plays a complex role in bone tumors since it can either promote or decrease tumor occurrence and metastasis. The present study explored the molecular role of DKK-1 in prostate tumor progression, metastasis, and tumor-bone interactions using the osteoblastic canine prostate cancer cell line, Probasco. Previously, we found that DKK-1 can inhibit bone mineralization of mouse calvaria by Probasco conditioned medium (CM). Now we stably expressed DKK-1 in Probasco cells (Probasco-DKK-1), which were injected into the tibias or left ventricle of athymic mice. Bone metastases were monitored by bioluminesent imaging, microCT, and histopathology. Our results showed that Probasco-DKK-1 bone metastases had increased tumor growth, increased osteoclastic bone resorption, and decreased intramedullary woven bone formation in vivo. In vitro, DKK-1 had an autocrine effect on Probasco cells, which altered the cell morphology, increased cell proliferation, and induced EMT. Mechanistically, DKK-1 had little effect on the canonical WNT/β-catenin signaling while dramatically down-regulated the non-canonical WNT/JNK signaling, which inhibited caspase-dependent apoptosis in Probasco-DKK-1 cells. To investigate the effect of Probasco-DKK-1 on bone cells, primary osteoblasts and osteoclasts were treated by CM collected from cancer cells. We found that DKK-1 did not inhibit the mineralization of osteoblasts nor stimulate the activity of osteoclasts directly, but the expression of RNAKL was significantly higher in Probasco-DKK-1 CM-treated osteoblasts. Therefore, the Probasco-DKK-1 cells enhanced the activity of osteoclasts indirectly via the regulation of osteoblasts, which induced bone resorption. In conclusion, we showed that DKK-1 promoted prostate tumor growth by stimulating cell proliferation and inhibiting apoptosis in a canonical WNT-independent manner and attenuated the osteoblastic activity of prostate cancer. These findings provide new insights into the molecular mechanisms of DKK-1 in tumor progression.

Citation Format: Shiyu Yuan, Nathan K. Hoggard, Noriko Kantake, Blake E. Hildreth, Thomas J. Rosol. The effect of dickkopf-1 (DKK-1) on prostate cancer growth and bone metastasis using the canine osteoblastic Probasco cell line [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1603.

Canine prostatic cancer cell line (LuMa) with osteoblastic bone metastasis

BACKGROUND

Osteoblastic bone metastasis represents the most common complication in men with prostate cancer (PCa). During progression and bone metastasis, PCa cells acquire properties similar to bone cells in a phenomenon called osteomimicry, which promotes their ability to metastasize, proliferate, and survive in the bone microenvironment. The mechanism of osteomimicry resulting in osteoblastic bone metastasis is unclear.

METHODS

We developed and characterized a novel canine prostatic cancer cell line (LuMa) that will be useful to investigate the relationship between osteoblastic bone metastasis and osteomimicry in PCa. The LuMa cell line was established from a primary prostate carcinoma of a 13-year old mixed breed castrated male dog. Cell proliferation and gene expression of LuMa were measured and compared to three other canine prostatic cancer cell lines (Probasco, Ace-1, and Leo) in vitro. The effect of LuMa cells on calvaria and murine preosteoblastic (MC3T3-E1) cells was measured by quantitative reverse-transcription polymerase chain reaction and alkaline phosphatase assay. LuMa cells were transduced with luciferase for monitoring in vivo tumor growth and metastasis using different inoculation routes (subcutaneous, intratibial [IT], and intracardiac [IC]). Xenograft tumors and metastases were evaluated using radiography and histopathology.

RESULTS

After left ventricular injection, LuMa cells metastasized to bone, brain, and adrenal glands. IT injections induced tumors with intramedullary new bone formation. LuMa cells had the highest messenger RNA levels of osteomimicry genes (RUNX2, RANKL, and Osteopontin [OPN]), CD44, E-cadherin, and MYOF compared to Ace-1, Probasco, and Leo cells. LuMa cells induced growth in calvaria defects and modulated gene expression in MC3T3-E1 cells.

CONCLUSIONS

LuMa is a novel canine PCa cell line with osteomimicry and stemness properties. LuMa cells induced osteoblastic bone formation in vitro and in vivo. LuMa PCa cells will serve as an excellent model for studying the mechanisms of osteomimicry and osteoblastic bone and brain metastasis in prostate cancer.

Toll-like receptor 3 is critical for coxsackievirus B4-induced type 1 diabetes in female NOD mice

Group B coxsackieviruses (CVBs) are involved in triggering some cases of type 1 diabetes mellitus (T1DM). However, the molecular mechanism(s) responsible for this remain elusive. Toll-like receptor 3 (TLR3), a receptor that recognizes viral double-stranded RNA, is hypothesized to play a role in virus-induced T1DM, although this hypothesis is yet to be substantiated. The objective of this study was to directly investigate the role of TLR3 in CVB-triggered T1DM in nonobese diabetic (NOD) mice, a mouse model of human T1DM that is widely used to study both spontaneous autoimmune and viral-induced T1DM. As such, we infected female wild-type (TLR3(+/+)) and TLR3 knockout (TLR3(-/-)) NOD mice with CVB4 and compared the incidence of diabetes in CVB4-infected mice with that of uninfected counterparts. We also evaluated the islets of uninfected and CVB4-infected wild-type and TLR3 knockout NOD mice by immunohistochemistry and insulitis scoring. TLR3 knockout mice were markedly protected from CVB4-induced diabetes compared with CVB4-infected wild-type mice. CVB4-induced T-lymphocyte-mediated insulitis was also significantly less severe in TLR3 knockout mice compared with wild-type mice. No differences in insulitis were observed between uninfected animals, either wild-type or TLR3 knockout mice. These data demonstrate for the first time that TLR3 is 1) critical for CVB4-induced T1DM, and 2) modulates CVB4-induced insulitis in genetically prone NOD mice.

Dynamic regulatory interactions of rad51, rad52, and replication protein-a in recombination intermediates

Rad51, Rad52, and replication protein-A (RPA) play crucial roles in the repair of DNA double-strand breaks in Saccharomyces cerevisiae. Rad51 mediates DNA strand exchange, a key reaction in DNA recombination. Rad52 recruits Rad51 into single-stranded DNAs (ssDNAs) that are saturated with RPA. Rad52 also promotes annealing of ssDNA strands that are complexed with RPA. Specific protein-protein interactions are involved in these reactions. Here we report new biochemical characteristics of these protein interactions. First, Rad52-RPA interaction requires multiple molecules of RPA to be associated with ssDNA, suggesting that multiple contacts between the Rad52 ring and RPA-ssDNA filament are needed for stable binding. Second, RPA-t11, which is a recombination-deficient mutant of RPA, displays a defect in interacting with Rad52 in the presence of salt above 50 mM, explaining the defect in Rad52-mediated ssDNA annealing in the presence of this mutation. Third, ssDNA annealing promoted by Rad52 is preceded by aggregation of multiple RPA-ssDNA complexes with Rad52, and Rad51 inhibits this aggregation. These results suggest a regulatory role for Rad51 that suppresses ssDNA annealing and facilitates DNA strand invasion. Finally, the Rad51-double-stranded DNA complex disrupts Rad52-RPA interaction in ssDNA and titrates Rad52 from RPA. This suggests an additional regulatory role for Rad51 following DNA strand invasion, where Rad51-double-stranded DNA may inhibit illegitimate second-end capture to ensure the error-free repair of a DNA double-strand break.

Rad51 protein controls Rad52-mediated DNA annealing

In Saccharomyces cerevisiae, Rad52 protein plays an essential role in the repair of DNA double-stranded breaks (DSBs). Rad52 and its orthologs possess the unique capacity to anneal single-stranded DNA (ssDNA) complexed with its cognate ssDNA-binding protein, RPA. This annealing activity is used in multiple mechanisms of DSB repair: single-stranded annealing, synthesis-dependent strand annealing, and cross-over formation. Here we report that the S. cerevisiae DNA strand exchange protein, Rad51, prevents Rad52-mediated annealing of complementary ssDNA. Efficient inhibition is ATP-dependent and involves a specific interaction between Rad51 and Rad52. Free Rad51 can limit DNA annealing by Rad52, but the Rad51 nucleoprotein filament is even more effective. We also discovered that the budding yeast Rad52 paralog, Rad59 protein, partially restores Rad52-dependent DNA annealing in the presence of Rad51, suggesting that Rad52 and Rad59 function coordinately to enhance recombinational DNA repair either by directing the processed DSBs to repair by DNA strand annealing or by promoting second end capture to form a double Holliday junction. This regulation of Rad52-mediated annealing suggests a control function for Rad51 in deciding the recombination path taken for a processed DNA break; the ssDNA can be directed to either Rad51-mediated DNA strand invasion or to Rad52-mediated DNA annealing. This channeling determines the nature of the subsequent repair process and is consistent with the observed competition between these pathways in vivo.

Rad52-mediated DNA annealing after Rad51-mediated DNA strand exchange promotes second ssDNA capture

Rad51, Rad52, and RPA play central roles in homologous DNA recombination. Rad51 mediates DNA strand exchange, a key reaction in DNA recombination. Rad52 has two distinct activities: to recruit Rad51 onto single-strand (ss)DNA that is complexed with the ssDNA-binding protein, RPA, and to anneal complementary ssDNA complexed with RPA. Here, we report that Rad52 promotes annealing of the ssDNA strand that is displaced by DNA strand exchange by Rad51 and RPA, to a second ssDNA strand. An RPA that is recombination-deficient (RPA(rfa1-t11)) failed to support annealing, explaining its in vivo phenotype. Escherichia coli RecO and SSB proteins, which are functional homologues of Rad52 and RPA, also facilitated the same reaction, demonstrating its conserved nature. We also demonstrate that the two activities of Rad52, recruiting Rad51 and annealing DNA, are coordinated in DNA strand exchange and second ssDNA capture.

The recombination-deficient mutant RPA (rfa1-t11) is displaced slowly from single-stranded DNA by Rad51 protein

Replication protein-A (RPA) is involved in many processes of DNA metabolism, including DNA replication, repair, and recombination. Cells carrying a mutation in the largest subunit of RPA (rfa1-t11: K45E) have defects in meiotic recombination, mating-type switching, and survival after DNA damage caused by UV and methyl methanesulfonate, as well as increased genome instability; however, this mutant has no significant defect in DNA replication. We purified the RPA heterotrimer containing the rfa1-t11 substitution (RPA(rfa1-t11)). This mutant RPA binds single-stranded DNA (ssDNA) with the same site size, and the RPA(rfa1-t11).ssDNA complex shows a similar sensitivity to disruption by salt as the wild-type RPA.ssDNA complex. RPA(rfa1-t11) stimulates DNA strand exchange, provided that the Rad51 protein.ssDNA nucleoprotein complex is assembled prior to introduction of the mutant RPA. However, RPA(rfa1-t11) is displaced from ssDNA by Rad51 protein more slowly than wild-type RPA and, as a consequence, Rad51 protein-mediated DNA strand exchange is inhibited when the ssDNA is in a complex with RPA(rfa1-t11). Rad52 protein can stimulate displacement of RPA(rfa1-t11) from ssDNA by Rad51 protein, but the rate of displacement remains slow compared with wild-type RPA. These in vitro results suggest that, in vivo, RPA is bound to ssDNA prior to Rad51 protein and that RPA displacement by Rad51 protein is a critical step in homologous recombination, which is impaired in the rfa1-t11 mutation.

Escherichia coli RecO protein anneals ssDNA complexed with its cognate ssDNA-binding protein: A common step in genetic recombination

We present biochemical evidence for the functional similarity of Escherichia coli RecO protein and bacteriophage T4 UvsY protein to eukaryotic Rad52 protein. Although Rad52 protein is conserved in eukaryotes, no sequence homologue has been found in prokaryotes or archeabacteria. Rad52 protein has two unique activities: facilitation of replication protein-A (RPA) displacement by Rad51 protein and annealing of RPA-single-stranded DNA (ssDNA) complexes. Both activities require species-specific interaction between Rad52 protein and RPA. Both RecO and UvsY proteins also possess the former property with regard to their cognate ssDNA-binding protein. Here, we report that RecO protein anneals ssDNA that is complexed with only its cognate ssDNA-binding protein, suggesting the involvement of species-specific interactions. Optimal activity for RecO protein occurs after formation of a 1:1 complex with SSB protein. RecR protein, which is known to stimulate RecO protein to facilitate SSB protein displacement by RecA protein, inhibits annealing by RecO protein, suggesting that RecR protein may regulate the choice between the DNA strand invasion versus annealing pathways. In addition, we show that UvsY protein anneals ssDNA; furthermore, ssDNA, which is complexed only with its cognate ssDNA-binding protein, is annealed in the presence of UvsY protein. These results indicate that RecO and possibly UvsY proteins are functional counterparts of Rad52 protein. Based on the conservation of these functions, we propose a modified double-strand break repair model that includes DNA annealing as an important intermediate step.