The primary cilium (PC) is a microtubule-based antenna-like organelle projecting from the surface of the cell membrane. We previously reported that PC formation could be regulated by nephronophthisis 3 (NPHP3) expression followed by its interaction with thymosin β4. Here, we investigated whether cancer cell viability is regulated by NPHP3-mediated PC formation. The total and viable cell number were reduced by incubating cells under serum deprivation (SD) without fetal bovine serum (-FBS). PC frequency was increased by SD which enhanced NPHP3 expression and hypoxia inducible factor (HIF)-1α. The role of HIF-1α on NPHP3 expression and PC formation was confirmed by the binding of HIF-1α to the NPHP3 promoter and siRNA-based inhibition of HIF-1α (siHIF-1α), respectively. HIF-1α-stabilizing dimethyloxallyl glycine (DMOG) and hypoxic conditions increased NPHP3 expression and PC formation. In addition, as SD elevated the reactive oxygen species (ROS), PC frequency and NPHP3 expression were inhibited by a treatment with N-acetylcysteine (NAC), a ROS scavenger. PC formation was increased by H₂O₂ treatment, which was inhibited by siHIF-1α. The inhibition of ERK with P98059 decreased the frequency of PC formation and NPHP3 expression. Cell viability was reduced by a treatment with ciliobrevin A (CilioA) to inhibit PC formation, which was re-affirmed by using PC-deficient IFT88^−/− cells. Taken together, the results imply that PC formation in cancer cells could be controlled by NPHP3 expression through ROS-induced HIF-1α and ERK activation under SD conditions. It suggests that cancer cell viability under SD conditions could be maintained by NPHP3 expression to regulate PC formation.

Yun-Sook Lim, Men T.N. Nguyen, Thuy X. Pham, Trang T.X. Huynh, Eun-Mee Park, Dong Hwa Choi, Sang Min Kang, Dongseob Tark, and Soon B. Hwang. Mol. Cells 2022;45:148-57. https://doi.org/10.14348/molcells.2021.0167

RAD51 is an important factor involved in the homologous recombination and repair of DNA breaks, which has also been implicated in various virus replication processes. We have previously reported that hepatitis C virus (HCV) exploits cellular RAD51 to promote viral propagation. Since severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is also an RNA virus, we interrogated whether SARS-CoV-2 could coopt RAD51 for its propagation. Here, we showed that silencing of RAD51 impaired SARS-CoV-2 propagation. We further demonstrated that RAD51 colocalized with SARS-CoV-2 RNA in Vero E6 cells. Interestingly, RAD51 interacted with SARS-CoV-2 3CL protease. This suggests that RAD51 inhibitors may block SARS-CoV-2 propagation. Hence, we evaluated multiple RAD51 inhibitors as potential drug candidates for coronavirus disease 2019 (COVID-19). Among these, B02, 4 ' -diisothiocyanostilbene-2,2 ' -disulfonic acid (DIDS), IBR2, and RI(dl)-2 significantly decreased RNA, protein, and infectious virion levels of Wuhan and variants of SARS-CoV-2. Antiviral activity of DIDS was further confirmed in the Syrian hamster model. Molecular docking model showed that these chemicals interfered with RAD51 through dimerization interface. These data suggest that SARS-CoV-2 exploits host RAD51 to facilitate viral propagation, and hence, RAD51 inhibitor may serve as a putative novel therapeutic agent for the treatment of COVID-19. IMPORTANCE: Viruses are constantly evolving to promote propagation in the host. Here, we show that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) utilizes host RAD51 for replication. Silencing of RAD51 impaired SARS-CoV-2 propagation. Viral RNA colocalized with RAD51 in the cytoplasm of SARS-CoV-2-infected cells, suggesting that both viral RNA and RAD51 may form a replication complex. We, therefore, evaluated RAD51 inhibitors as possible therapeutic agents against SARS-CoV-2. Indeed, RAD51 inhibitors exerted antiviral activities against not only Wuhan but also variants of SARS-CoV-2. Molecular docking model shows that RAD51 inhibitors impede SARS-CoV-2 propagation by interfering with dimerization of RAD51. These data suggest that RAD51 may represent a novel host-based drug target for coronavirus disease 2019 treatment.

Primary cilia (PC) are microtubule-based organelles that function as cellular antennae to sense and transduce extracellular signals. Nephronophthisis 3 (NPHP3) is localized in the inversin compartment of PC. Mutations in NPHP3 are associated with renal-hepatic-pancreatic dysplasia. In this study, we investigated whether vinblastine (VBL), a microtubule destabilizer, induces anticancer drug resistance through NPHP3-associated PC formation in HeLa human cervical cancer cells. A considerable increase in PC frequency was observed in HeLa cells under serum-deprived (SD) conditions, which led to the inhibition of VBL-induced cell death. VBL-resistant cells were established by repetitive treatments with VBL and showed an increase in PC frequency. NPHP3 expression was also increased by VBL treatment under serum starvation as well as in VBL-resistant cells. NPHP3 expression and PC-associated resistance were positively correlated with apoptosis-antagonizing transcription factor (AATF) and negatively correlated with inhibition of NPHP3. In addition, AATF-mediated NPHP3 expression is associated with PC formation via the regulation of intraflagellar transport protein 88 (IFT88). VBL resistance ability was reduced by treating with ciliobrevin A, a well-known ciliogenesis inhibitor. Collectively, cancer cell survival following VBL treatment is regulated by PC formation via AATF-mediated expression of IFT88 and NPHP3. Our data suggest that the activation of AATF and IFT88 could be a novel regulator to induce anticancer drug resistance through NPHP3-associated PC formation.