Researchers studying the role of a cancer-associated protein called promyelocytic leukemia protein (PML) in glioblastoma have demonstrated that PML regulates growth and invasion through distinct cellular mechanisms—suppressing cell proliferation and maintaining invasive capacity when overexpressed and losing invasive capacity when suppressed.
The findings, which were published in the International Journal of Molecular Sciences, indicate that PML could serve as a potential molecular biomarker of glioblastoma progression, and the PML pathway may represent a therapeutic target in this aggressive and incurable brain cancer.
The investigators studied PML in glioblastoma cells using a combined in vitro-in silico approach wherein biological results could be differentiated and interpreted with cancer-predictive computational algorithms.
They genetically modified a human primary glioblastoma cell line (U87MG) to conditionally overexpress PML isoform IV, which is the most well-studied isoform and has been shown to act as a tumor suppressor, and targeted the histone methyltransferase Enhancer of Zeste Homolog 2 (EZH2), which is a mediator of PML activity, with an inhibitory drug, 3-Deazaneplanocin A (DZNeP), at various concentrations. The glioblastoma-derived cells can form multicellular spheroids; thus, spatiotemporal cell growth was monitored both in 2-dimensional space (D) and 3D. The invasive properties of the cells into an extracellular-like matrix were monitored in 3D.
The cells overexpressing PML exhibited reduced aggregation capacity, generating smaller (occasionally deformed) spheroids, and increased proliferation time compared with control cells, consistent with tumor suppressor function. The invasive capacity of the cells was similar between those overexpressing PML and the controls.
When treated with DZNeP, spheroid growth was similar between the cells overexpressing PML and the control cells. In contrast, the PML-overexpression cells completely lost their ability to migrate upon EZH2 inhibition, while control cells retained their invasive capacity. In silico simulations suggested that PML drives invasion of glioblastoma cells by differential modulation of cell-to-cell adhesion and migratory capacity of the cells.
“Under the light of precision medicine, variation in the expression levels of PML within a tumor and/or across different patients is expected, exploring the differential expression and effect of PML in patient-derived glioblastoma cell lines comprises an important next step,” the authors wrote. “In this line, computational models capable of predicting tumor evolution and treatment, while accounting for phenotypic heterogeneity based on data derived from patient-derived GB cell lines, are both highly in demand and valuable.”
Limitations of the study included differences between in vitro and in vivo conditions and those associated with mathematical models, such as restrictive assumptions and potential confounding effects of unmeasured variables.
Tampakaki M, Oraiopoulou ME, Tzamali E, et al. PML differentially regulates growth and invasion in brain cancer. Int J Mol Sci. 2021;22(12):6289. doi:10.3390/ijms22126289