Downregulated in renal cell carcinoma 1 (DRR1) has important roles in tumor cell growth, neuron survival and spine formation, and was recently shown to bind actin. However, the roles of nuclear DRR1 remain largely unexplored. Here, we identified an interaction between filamentous actin (F-actin) and DRR1 in the nucleus, and demonstrated that copper metabolism MURR1 domain-containing 1 (COMMD1) is another binding partner of DRR1. Accordingly, DRR1, F-actin and COMMD1 were shown to form a complex in the nucleus, and the stability of COMMD1 was enhanced in this complex. Increased nuclear COMMD1 in turn promoted the degradation of NF-κB. In addition, DRR1 and COMMD1 suppressed the cyclin D1 expression, G1/S transition and cell proliferation of neuroblastoma cells. The binding between DRR1 and F-actin in the nucleus was required for these events. Consistent with these facts, low expressions of DRR1 were associated with tumorigenesis of human neuroblastoma and its mouse model. This study has thus revealed a novel nuclear complex of F-actin, DRR1 and COMMD1 that is involved in NF-κB degradation and cell cycle suppression in neuroblastoma cells.

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Neuroblastoma is derived from the sympathetic neuronal lineage of neural crest cells, and is the most frequently observed of the extracranial pediatric solid tumors. The neuronal differentiation factor, NeuroD1, has previously been shown to promote cell motility in neuroblastoma by suppressing the expression of Slit2. Here we report that NeuroD1 is also involved in the proliferation of neuroblastoma cells, including human cell lines and primary tumorspheres cultured from the tumor tissues of model mice. Interestingly, the growth inhibition of neuroblastoma cells induced by knockdown of NeuroD1 was accompanied by a reduction of ALK expression. ALK is known to be one of the important predisposition genes for neuroblastoma. The phenotype resulting from knockdown of NeuroD1 was suppressed by forced expression of ALK and, therefore, NeuroD1 appears to act mainly through ALK to promote the proliferation of neuroblastoma cells. Furthermore, we showed that NeuroD1 directly bound to the promoter region of ALK gene. In addition, the particular E-box in the promoter was responsible for NeuroD1-mediated ALK expression. These results indicate that ALK should be a direct target gene of NeuroD1. Finally, the expressions of NeuroD1 and ALK in the early tumor lesions of neuroblastoma model mice coincided in vivo. We conclude that the novel mechanism would regulate the expression of ALK in neuroblastoma and that NeuroD1 should be significantly involved in neuroblastoma tumorigenesis.

The cyclic-AMP response element-binding protein (CREB), a well-known nuclear transcription factor, has been shown to play an essential role in many cellular processes, including differentiation, cell survival, and cell proliferation, by regulating the expression of downstream genes. Recently, increased expression of CREB was frequently found in various tumors, indicating that CREB is implicated in the process of tumorigenesis. However, the effects of CREB on Hodgkin lymphoma (HL) remain unknown. To clarify the role of CREB in HL, we performed knockdown experiments in HL. We found that downregulation of CREB by short hairpin RNA (shRNA) resulted in enhancement of cell proliferation and promotion of G1/S phase transition, and these effects can be rescued by expression of shRNA-resistant CREB. Meanwhile, the expression level of cell cycle-related proteins, such as cyclin D1, cyclin E1, cyclin-dependent kinase 2 (CDK2), and CDK4, was elevated in response to depletion of CREB. Furthermore, we performed chromatin immunoprecipitation (ChIP) assay and confirmed that CREB directly bound to the promoter regions of these genes, which consequently contributed to the regulation of cell cycle. Consistent with our results, a clinical database showed that high expression of CREB correlates with favorable prognosis in B-cell lymphoma patients, which is totally different from the function of CREB in other cancers such as colorectal cancer, acute myeloid leukemia, and some endocrine cancers. Taken together, all of these features of CREB in HL strongly support its role as a tumor suppressor gene that can decelerate cell proliferation by inhibiting the expression of several cell cycle-related genes. Our results provide new evidence for prognosis prediction of HL and a promising therapeutic strategy for HL patients.