Osteosarcoma, the most common primary malignant bone tumor, primarily affects children and adolescents, constituting approximately two-thirds of primary malignant bone tumors worldwide [1, 2]. Osteosarcoma is a high malignancy and early tendency for lung metastasis [3, 4]. Significant advancements in neoadjuvant chemotherapy and surgical techniques are effective in approximately 70% of patients with localized osteosarcoma. However, the overall 5-year survival rate remains only 20% for patients with metastasis at diagnosis or recurrence at presentation [5]. Notably, the key pathophysiological mechanism of osteosarcoma remains unclear. Therefore, it is urgent to explore the potential mechanism underlying osteosarcoma progression to find effective therapeutic targets.
RNA posttranscriptional modifications, including N6-methyladenosine (m6A), m5C, and pseudouridine (Ψ), have gained considerable attention for their role in regulating gene expression and cellular activities during physiological and pathological process [6–9]. m6A is the most abundant epitranscriptomic modification in mRNA, which mainly regulates mRNA translation and degradation [10, 11]. We have verified altered m6A regulators in various tumors, including breast cancer [12], hepatoblastoma [13], and osteosarcoma [14]. Recently, it has been discovered that m5C plays a crucial regulatory role in mRNA metabolism, affecting its target transcript stability, export, and translation [15]. m5C modification is dynamically regulated by m5C writers (NSUN, DNMT, and TRDMT family members) and erasers (TET families and ALKBH1), which induce m5C methylation and demethylation, respectively. Notably, m5C readers, such as ALYREF and YBX1, can recognize and bind to the m5C-modified mRNA, leading to a change in mRNA stability and export [16–18].
So far, emerging evidence suggests that m5C modification is involved in multiple types of cancer, such as those in the esophagus [19] liver [20], pancreas [21], stomach [22], colorecta [23] and prostate [24]. As the pivotal m5C methyltransferase, NOP2/Sun RNA methyltransferase family member 2 (NSUN2) plays a key role in tumorigenesis and cancer metastasis. For instance, Sun et al. demonstrated that NSUN2 promoted HCC development by stabilizing the m5C-modified FZR1 mRNA [25]. Furthermore, NSUN2-mediated m5C modification promoted ESCC progression by enhancing GRB2 mRNA stabilization in a LIN28B-dependent manner [19]. However, less is known about the role of NSUN2 in the progression of osteosarcoma.
In this study, we confirmed dramatically upregulated NSUN2 expression in osteosarcoma, and high NSUN2 expression predicted poor prognosis. Then the functional importance of NSUN2 in osteosarcoma was elucidated both in vitro and in vivo, affirming the essential pro-metastatic role of NSUN2. Mechanistically, UBE2S was identified as the potential target gene of NSUN2, while NSUN2 promoted osteosarcoma malignancies through enhancing UBE2S mRNA stabilization. In particular, UBE2S stabilized the β-catenin protein through ubiquitination and consequently induced EMT. Clinically, activated NSUN2/UBE2S axis predicted the poorer survival of osteosarcoma patients. Taken together, our findings provide a potential therapeutic strategy for targeting NSUN2 in the treatment of osteosarcoma.