Osteosarcoma (OS) is the most common primary malignant bone tumor in children and adolescents, which is characterized by dysfunctional autophagy and poor differentiation. Our recent studies have suggested that the tripartite motif containing-21 (TRIM21) plays a crucial role in regulating OS cell senescence and proliferation via interactions with several proteins. Yet, its implication in autophagy and differentiation in OS is largely unknown. In the present study, we first showed that TRIM21 could promote OS cell autophagy, as determined by the accumulation of LC3-II, and the degradation of cargo receptor p62. Further, we were able to identify that Annexin A2 (ANXA2), as a novel interacting partner of TRIM21, was critical for TIRM21-induced OS cell autophagy. Although TRIM21 had a negligible effect on the mRNA and protein expressions of ANXA2, we did find that TRIM21 facilitated the translocation of ANXA2 toward plasma membrane (PM) in OS cells through a manner relying on TRIM21-mediated cell autophagy. This functional link has been confirmed by observing a nice co-expression of TRIM21 and ANXA2 (at the PM) in the OS tissues. Mechanistically, we demonstrated that TRIM21, via facilitating the ANXA2 trafficking at the PM, enabled to release the transcription factor EB (TFEB, a master regulator of autophagy) from the ANXA2-TFEB complex, which in turn entered into the nucleus for the regulation of OS cell autophagy. In accord with previous findings that autophagy plays a critical role in the control of differentiation, we also demonstrated that autophagy inhibited OS cell differentiation, and that the TRIM21/ANXA2/TFEB axis is implicated in OS cell differentiation through the coordination with autophagy. Taken together, our results suggest that the TRIM21/ANXA2/TFEB axis is involved in OS cell autophagy and subsequent differentiation, indicating that targeting this signaling axis might lead to a new clue for OS treatment.

骨肉瘤（OS）是儿童和青少年最常见的原发性恶性骨肿瘤，其特点是自噬功能障碍和分化差。我们最近的研究表明，含有 21 的三方基序（TRIM21）通过与几种蛋白质的相互作用在调节 OS 细胞衰老和增殖中起着至关重要的作用。然而，它在自噬和 OS 分化中的意义在很大程度上是未知的。在本研究中，我们首先表明 TRIM21 可以促进 OS 细胞自噬，这取决于 LC3-II 的积累和货物受体 p62 的降解。此外，我们能够确定膜联蛋白 A2（ANXA2）作为 TRIM21 的新型相互作用伙伴，对于 TIRM21 诱导的 OS 细胞自噬至关重要。尽管 TRIM21 对 ANXA2 的 mRNA 和蛋白表达的影响可以忽略不计，我们确实发现TRIM21通过依赖TRIM21介导的细胞自噬的方式促进了ANXA2向OS细胞中的质膜（PM）易位。通过观察 OS 组织中 TRIM21 和 ANXA2（在 PM 处）的良好共表达，已经证实了这种功能联系。从机制上讲，我们证明了 TRIM21 通过促进 PM 的 ANXA2 运输，能够从 ANXA2-TFEB 复合物中释放转录因子 EB（TFEB，自噬的主要调节因子），后者又进入细胞核调节OS细胞自噬。与先前关于自噬在控制分化中起关键作用的发现一致，我们还证明自噬抑制了 OS 细胞分化，并且 TRIM21/ANXA2/TFEB 轴通过与自噬的协调参与 OS 细胞分化。总之，我们的结果表明 TRIM21/ANXA2/TFEB 轴参与 OS 细胞自噬和随后的分化，表明靶向该信号轴可能会导致 OS 治疗的新线索。