Background/Objectives: Long non-coding RNAs (lncRNAs) are integral to the regulation of viral tumorigenesis. We have previously identified that the chicken lncRNA-803, which responds to Marek’s disease virus (MDV), inhibits apoptosis in the chicken embryonic fibroblast cell line DF-1, accompanied by changes in the expression of the p53 protein. Nonetheless, the molecular mechanism of lncRNA-803 in apoptosis has yet to be elucidated. Methods: In this study, through lentivirus-mediated overexpression and knockdown experiments, we determined that the overexpression of lncRNA-803 induces elevated expression levels of murine double minute 2 (MDM2), murine double minute 4 (MDM4), tumor protein p53 (p53), and tumor protein p53 binding protein 1 (TP53BP1) within the p53 signaling pathway. Results: This modulation subsequently leads to an upregulation of B-cell lymphoma-2 (Bcl-2) expression, while concurrently resulting in the downregulation of cysteinyl aspartate specific proteinase 8 (Caspase-8), cysteinyl aspartate specific proteinase 9 (Caspase-9), Bcl-2 associated protein X (Bax), and cysteinyl aspartate specific proteinase 9 (Caspase-3) in the apoptosis pathway. In terms of its mechanism, lncRNA-803 functions as a molecular sponge for miR-6555-3p. lncRNA-803 engages in competitive binding with miR-6555-3p, thereby diminishing its inhibitory effect on MDM4. Conclusions: These results elucidate that lncRNA-803 modulates apoptosis in DF-1 cells through a novel competing endogenous RNA mechanism involving the miR-6555-3p/MDM4/p53 axis. These findings provide new insights into the molecular pathogenesis of MDV.

The oncogenic Marek’s disease virus (MDV) triggers Marek’s disease (MD), which is a substantial threat to poultry as it transforms infected T cells into tumors. Our research identified that long non-coding RNA 9802 (lncRNA-9802) exhibits increased expression in the chicken spleen following MDV infection, with its expression being strongly associated with the expression of tumor p53-binding protein 1 (TP53BP1). The function of lncRNA-9802 in T cells transformed by MDV remains unclear. Consequently, the expression levels of lncRNA-9802 were either over-expressed or knocked down in MDV-transformed T cells, MDCC-MSB1, through lentivirus-mediated over-expression and knock down experiments. Our findings demonstrate that lncRNA-9802 induces proliferation disturbances in MDCC-MSB1 cells by causing arrest in the S phase, which is accompanied by increased expression levels of TP53BP1, p53, and p21. Activation of the p53 pathway results in elevated levels of Cyclin E and Cyclin-dependent kinase 2 (CDK2), thereby facilitating the entry of MDCC-MSB1 cells into the S phase. Concurrently, the reduced levels of Cyclin A inhibit the exit of MDCC-MSB1 cells from the S phase. By modulating the TP53BP1/p53/p21 pathway, lncRNA-9802 induces S phase arrest in MDCC-MSB1 cells, characterized by upregulation of Cyclin E and CDK2 and downregulation of Cyclin A. This research enhances the understanding of the pathogenic mechanisms of MDV and provides a foundation for identifying potential targets for antiviral drug development.