UNCovering the molecular machinery of dependence receptor signaling

Polyomavirus Small T Antigen Induces Apoptosis in Mammalian Cells through the UNC5B Pathway in a PP2A-Dependent Manner

UNC5B is a dependence receptor that promotes survival in the presence of its ligand, netrin-1, while inducing cell death in its absence. The receptor has an important role in the development of the nervous and vascular systems. It is also involved in the normal turnover of intestinal epithelium. Netrin-1 and UNC5B are deregulated in multiple cancers, including colorectal, neuroblastoma, and breast tumors. However, the detailed mechanism of UNC5B function is not fully understood. We have utilized the murine polyomavirus small T antigen (PyST) as a tool to study UNC5B-mediated apoptosis. PyST is known to induce mitotic arrest followed by extensive cell death in mammalian cells. Our results show that the expression of PyST increases mRNA levels of UNC5B by approximately 3-fold in osteosarcoma cells (U2OS) and also stabilizes UNC5B at the posttranslational level. Furthermore, UNC5B is upregulated predominantly in those cells that undergo mitotic arrest upon PyST expression. Interestingly, although its expression was previously reported to be regulated by p53, our data show that the increase in UNC5B levels by PyST is p53 independent. The posttranslational stabilization of UNC5B by PyST is regulated by the interaction of PyST with PP2A. We also show that netrin-1 expression, which is known to inhibit UNC5B apoptotic activity, promotes survival of PyST-expressing cells. Our results thus suggest an important role of UNC5B in small-T antigen-induced mitotic catastrophe that also requires PP2A.IMPORTANCE UNC5B, PP2A, and netrin-1 are deregulated in a variety of cancers. UNC5B and PP2A are regarded as tumor suppressors, as they promote apoptosis and are deleted or mutated in many cancers. In contrast, netrin-1 promotes survival by inhibiting dependence receptors, including UNC5B, and is upregulated in many cancers. Here, we show that UNC5B-mediated apoptosis can occur independently of p53 but in a PP2A-dependent manner. A substantial percentage of cancers arise due to p53 mutations and are insensitive to chemotherapeutic treatments that activate p53. Unexpectedly, treatment of cancers having functional p53 with many conventional drugs leads to the upregulation of netrin-1 through activated p53, which is counterintuitive. Therefore, understanding the p53-independent mechanisms of the netrin-UNC5B axis, such as those involving PP2A, assumes greater clinical significance. Anticancer strategies utilizing anti-netrin-1 antibody treatment are already in clinical trials.

Intranasal administration of recombinant Netrin-1 attenuates neuronal apoptosis by activating DCC/APPL-1/AKT signaling pathway after subarachnoid hemorrhage in rats

Neuronal apoptosis is a crucial pathological process in early brain injury after subarachnoid hemorrhage (SAH). The effective therapeutic strategies to ameliorate neuronal apoptosis are still absent. We intended to determine whether intranasal administration of exogenous Netrin-1 (NTN-1) could attenuate neuronal apoptosis after experimental SAH, specifically via activating DCC-dependent APPL-1/AKT signaling cascade. Two hundred twenty-five male Sprague-Dawley rats were subjected to the endovascular perforation model of SAH. Recombinant human NTN-1 (rNTN-1) was administered intranasally. NTN-1 small interfering RNA (siRNA), APPL-1 siRNA, and AKT inhibitor MK2206 were administered through intracerebroventricular (i.c.v.) injection. SAH grade, neurological score, neuronal apoptosis assessed by cleaved caspase-3 (CC-3) expression and Fluoro-Jade C (FJC) staining, double immunofluorescence staining, and Western blot were examined. Our results revealed that endogenous NTN-1 level was increased after SAH. Administration of rNTN-1 improved neurological outcomes at 24 h and 72 h after SAH, while knockdown of endogenous NTN-1 worsened neurological impairments. Furthermore, exogenous rNTN-1 treatment promoted APPL-1 activation, increased phosphorylated-AKT and Bcl-2 expression, as well as decreased apoptotic marker CC-3 expression and the number of FJC-positive neurons, thereby alleviated neuronal apoptosis. Conversely, APPL-1 siRNA and MK2206 abolished the anti-apoptotic effect of exogenous rNTN-1 at 24 h after SAH. Collectively, intranasal administration of exogenous rNTN-1 attenuated neuronal apoptosis and improved neurological function in SAH rats, at least in apart via activating DCC/APPL-1/AKT signaling pathway.

Netrin-1 rescues neuron loss by attenuating secondary apoptosis in ipsilateral thalamic nucleus following focal cerebral infarction in hypertensive rats

Neurological deficit following cerebral infarction correlates with not only primary injury, but also secondary neuronal apoptosis in remote loci connected to the infarction. Netrin-1 is crucial for axonal guidance by interacting with its receptors, deleted in colorectal cancer (DCC) and uncoordinated gene 5H (UNC5H). DCC and UNC5H are also dependence receptors inducing cell apoptosis when unbound by netrin-1. The present study is to investigate the role of netrin-1 and its receptors in ipsilateral ventroposterior thalamic nucleus (VPN) injury secondary to stroke in hypertensive rats. Renovascular hypertensive Sprague-Dawley rats underwent middle cerebral artery occlusion (MCAO). Continuous intracerebroventricular infusion of netrin-1 (600 ng/d for 7 days) or vehicle (IgG/Fc) was given 24h after MCAO. Neurological function was evaluated by postural reflex 8 and 14 days after MCAO. Then, immunoreactivity was determined in the ipsilateral VPN for NeuN, glial fibrillary acidic protein, netrin-1 and its receptors (DCC and UNC5H2), apoptosis was detected with Terminal deoxynucleotidyl transferase-mediated digoxigenin-dUTP-biotin nick-end labeling (TUNEL) assay, and the expressions of caspase-3, netrin-1, DCC, and UNC5H2 were quantified by western blot analysis. MCAO resulted in the impaired postural reflex after 8 and 14 days, with decreased NeuN marked neurons and increased TUNEL-positive cells, as well as an up-regulation in the levels of cleaved caspase-3 and UNC5H2 protein in the ipsilateral VPN, without significant change in DCC or netrin-1 expression. By exogenous netrin-1 infusion, the number of neurons was increased in the ipsilateral VPN, and both TUNEL-positive cell number and caspase-3 protein level were reduced, while UNC5H2 expression remained unaffected, simultaneously, the impairment of postural reflex was improved. Taken together, the present study indicates that exogenous netrin-1 could rescue neuron loss by attenuating secondary apoptosis in the ipsilateral VPN after focal cerebral infarction, possibly via its receptor UNC5H2, suggesting that relative insufficiency of endogenous netrin-1 be an underlying mechanism of secondary injury in the VPN post stroke.