Investigating a novel CTRP8-DLK1 interaction and its impact on glioma stemness
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Background: The C1Q-tumor necrosis factor related protein (CTRP) family has functions in metabolism, immunity, and cancer. Among the CTRP family, CTRP8 remains the least well characterized in terms of function and receptors. In the brain tumor glioblastoma (GBM), CTRP8 impacts migration and treatment resistance through its receptor RXFP1. We investigate a new interaction between CTRP8 and the EGF-like protein Delta-like homolog 1 (DLK1). DLK1 is a well-known regulator of differentiation and is considered a marker of stemness in several cancers. In GBM, elevated DLK1 expression promotes migration, stemness and has been correlated with poor prognosis. There is interest in further understanding complex mechanisms of DLK1 in a cancer context. Methodology: A yeast-two hybrid (Y2H) screen and co-immunoprecipitation (co-IP) were used to characterize CTRP8-DLK1 interaction domains. The U251 cell line was used to assess stemness using tumor sphere formation and glioma stem cell (GSC) marker expression. In the same U251 model we investigate mechanisms responsive to CTRP8-DLK1 that regulate GSCs. Results: The Y2H identified DLK1 as a CTRP8 interaction partner. In co-IP CTRP8 binds full-size DLK1 and a truncated, soluble form of DLK1. DLK1 enhanced sphere formation and GSC marker expression in U251. The pro-stemness effect of DLK1 was counteracted by CTRP8, resulting in an overall reduction in stemness. Reduced stemness in cells expressing a cleavage-resistant DLK1 form revealed potential functions of DLK1 cleavage products as factors regulating the GSC population. In U251 cells expressing DLK1, CTRP8 treatment increased DLK1 intracellular domain (ICD) release and nuclear translocation. DLK1 ICD may act as a transcriptional co-factor revealing a new function for DLK1 ICD. Conclusions: This is the first study describing CTRP8 as a DLK1 ligand. Our findings reveal DLK1 as a promoter of glioma stemness, and CTRP8 as an antagonist. The CTRP8-DLK1 interaction may be part of a novel regulatory system impacting the glioma stem cell niche that can be of clinical relevance.