Tubercidin Enhances Apoptosis in Serum-Starved and Hypoxic Mouse Cardiomyocytes by Inducing Nuclear Speckle Condensation

Tubercidin, known for its antimicrobial, antiparasitic, and anticancer effects, faces clinical limitations due to adverse effects, especially cardiotoxicity risks for those with ischemic cardiomyopathy. This study aims to clarify the molecular pathways of Tubercidin-induced cardiotoxicity, focusing on nuclear speckles (NSs) disruption in cardiomyocytes under serum deprivation and/or hypoxia. To simulate ischemic cardiomyopathy in vitro, we utilized FMC84 and HL-1 murine cardiomyocyte cell lines, exposing them to conditions of serum limitation and/or hypoxia to evaluate the cardiotoxic impact of Tubercidin and the contributing mechanisms. Apoptosis was quantified using flow cytometry, NSs condensation was visualized via immunofluorescence with an anti-SC35 antibody, and the expression levels of key apoptotic transcripts (RFFL, RIF1, and RNF144B) were analyzed by RT-PCR. Our findings revealed that Tubercidin significantly increased apoptosis in both HL-1 and FMC84 cell lines under conditions mimicking serum deprivation (21% O2 with 1% FBS), hypoxia (1% O2 with 10% FBS), or a combination of both. Furthermore, Tubercidin treatment led to a pronounced enlargement of NSs, as detected by immunofluorescence. Concurrently, we documented significant alterations in the expression of critical apoptotic regulatory genes, implying that Tubercidin may modulate the apoptotic pathway in stressed cardiomyocytes. It is hypothesized that Tubercidin induces NSs condensation, affecting alternative splicing of cell death genes, potentially worsening ischemic cardiomyocytes' damage. Therefore, a cautious clinical use of Tubercidin for ischemic cardiomyopathy patients is advised to reduce cardiotoxicity risks.