Blood-brain barrier permeability increases with the differentiation of glioblastoma cells in vitro

Background: Glioblastoma multiforme (GBM) is an aggressive tumor, difficult to treat pharmacologically because of the blood-brain barrier (BBB), which is rich in ATP-binding cassette (ABC) transporters and tight junction (TJ) proteins. The BBB is disrupted within GBM bulk, but it is competent in brain-adjacent-to-tumor areas, where eventual GBM foci can trigger tumor relapse. How GBM cells influence the permeability of BBB is poorly investigated. Methods: To clarify this point, we co-cultured human BBB models with 3 patient-derived GBM cells, after separating from each tumor the stem cell/neurosphere (SC/NS) and the differentiated/adherent cell (AC) components. Also, we set up cultures of BBB cells with the conditioned medium of NS or AC, enriched or depleted of IL-6. Extracellular cytokines were measured by protein arrays and ELISA. The intracellular signaling in BBB cells was measured by immunoblotting, in the presence of STAT3 pharmacological inhibitor or specific PROTAC. The competence of BBB was evaluated by permeability assays and TEER measurement. Results: The presence of GBM cells or their conditioned medium increased the permeability to doxorubicin, mitoxantrone and dextran-70, decreased TEER, down-regulated ABC transporters and TJ proteins at the transcriptional level. These effects were higher with AC or their medium than with NS. The secretome analysis identified IL-6 as significantly more produced by AC than by NS. Notably, AC-conditioned medium treated with an IL-6 neutralizing antibody reduced the BBB permeability to NS levels, while NS-conditioned medium enriched with IL-6 increased BBB permeability to AC levels. Mechanistically, IL-6 released by AC GBM cells activated STAT3 in BBB cells. In turn, STAT3 down-regulated ABC transporter and TJ expression, increased permeability and decreased TEER. The same effects were obtained in BBB cells treated with STA-21, a pharmacological inhibitor of STAT3, or with a PROTAC targeting STAT3. Conclusions: Our work demonstrates for the first time that the degree of GBM differentiation influences BBB permeability. The crosstalk between GBM cells that release IL-6 and BBB cells that respond by activating STAT3, controls the expression of ABC transporters and TJ proteins on BBB. These results may pave the way for novel therapeutic tools to tune BBB permeability and improve drug delivery to GBM.