Studies in mice by researchers in Finland, Sweden, and Switzerland have shown how the most aggressive form of brain cancer, glioblastoma, can be stopped in its tracks by an antihistamine drug that triggers a form of cell death caused by leaky lysosomes. Headed by Pirjo Laakkonen, PhD, at the University of Helsinki, the studies demonstrated an association between the fatty acid binding protein mammary-derived growth inhibitor (MDGI) and poorer prognosis in patients.
MDGI shuttles fatty acids into cells, and the team’s studies found that blocking the MDGI gene in glioblastoma cell lines disrupted fatty acid transport into cells and incorporation into lysosomal membranes, which compromised lysosomal membrane composition and integrity, resulting in lysosomal membrane permeabilization (LMP). LMP is an intracellular cell death pathway triggered when the lysosome contents leak into the cell. The team’s subsequent studies in cell lines and in live mice found that treatment with clemastine, an older type of antihistamine that can cross the blood-brain barrier, effectively mirrored the effects of MDGI, triggering LMP and causing glioblastoma cell death, without harming healthy cells.
The investigators say their findings could help scientists develop new treatments for glioblastoma. “Our research demonstrates that MDGI is a key factor regulating and maintaining the structure of the lysosomal membrane,” Laakkonen commented. “This is the first gene found to regulate the stability of the membrane … Our findings demonstrate that antihistamines and other drugs that increase the permeability of the lysosomal membrane can be considered as an enhancing therapy for patients with glioblastoma alongside established treatments.”
The findings are reported in EMBO Molecular Medicine, in a paper titled, “Vulnerability of invasive glioblastoma cells to lysosomal membrane destabilization.”
Glioblastoma is the most common, aggressive, and lethal form of glioma, which has a “dismal” prognosis, the authors noted. Current surgical approaches can’t remove all of the tumor, and remaining multidrug resistant cells and cancer stem-like cells propagate the tumor and cause relapse, even after radio- and chemotherapy. “Most probably, different approaches are required to eradicate the invasive cells and the cells that reside within the tumor bulk,” the team stated.
Their work had previously identified MDGI as a glioma biomarker of invasive gliomas, and linked increased expression of the protein with more severe glioma grade. Also known as heart-type fatty acid binding protein (FABP3), MDGI is a fatty acid binding protein (FABP) that is involved in the uptake of fatty acids, and particularly polyunsaturated fatty acids (PUFAs) uptake into glioma cells.
The team’s newly reported analyses of lower grades of gliomas confirmed a link between increased expression of MDGI and poorer patient survival. “… both MDGI expression and high tumor grade independently associated with unfavorable overall survival, increasing the risk of death by the factor of two,” the authors commented. They also found that significantly more MDGI was expressed in glioblastomas than in lower grade gliomas.
Through further experiments the investigators showed that MDGI was essential to glioma cell survival. Glioma cells engineered to overexpress MDGI also grew more aggressively and invasively than non-engineered tumor cells following implantation into experimental mice. Conversely, silencing the MDGI gene dramatically reduced the viability of patient-derived glioma cells and blocked cell proliferation. Human glioblastoma cells lacking MDGI were also unable to form tumors when transplanted into mice. “These results demonstrate a dose-dependent effect of MDGI silencing on glioblastoma cell growth and viability.”
The team’s studies in cell lines indicated that blocking MDGI prevented the transport of fatty acids—and particularly linoleic acid—into the cells for incorporation into lysosomal membranes. Linoleic acid is an essential PUFA, which cells can’t be made from scratch, and so it has to be obtained from food and imported into cells. Without MDGI the glioma cells couldn’t access enough linoleic acid, which was then lacking from the lysosomal membranes. “Our lipid analyses show that MDGI silencing impaired trafficking of polyunsaturated fatty acids (FA) into cells, resulting in significant alterations in the lipid composition of lysosomal membranes.” Without the correct fatty acid composition the lysosomal membranes become more permeable, and the enzymes and other components they leaked out into the cell cytoplasm, triggering cell death.
Previous work had suggested that some classes of antihistamines can trigger LMP, so the team carried out a series of tests to evaluate clemastine, a first generation cationic amphiphilic (CAD) antihistamine, on both cultured human glioblastoma cells and on the growth of human glioblastoma xenografts in experimental mice. Encouragingly, the drug effectively killed the lab-grown tumor cells at doses that weren’t harmful to any other cell type tested. “We observed a dramatic loss of the glioblastoma cell viability that was associated with the loss of lysosomal membrane integrity at doses that did not affect the proliferation or viability of several normal cells in vitro,” the investigators wrote.
Clemastine treatment also blocked tumor growth and increased survival in the tumor-bearing mice. “When we evaluated the preclinical efficacy of clemastine, the survival of animals bearing intracranial glioblastoma xenografts was significantly prolonged compared to controls due to the eradication of invasive glioma cells.” Interestingly, the authors pointed out, a prior study had shown that the use of CAD antihistamines was linked with significantly reduced all-cause mortality among cancer patients when compared with non-CAD antihistamines.
“Our study demonstrates the crucial role of MDGI in glioma cell survival, linking this fatty acid binding protein to the maintenance of lysosomal membrane integrity,” they concluded. “This unexpected fragility of the aggressive infiltrating cells to LMP provides new opportunities for clinical interventions, such as repositioning of an established antihistamine drug, to eradicate the inoperable, invasive, and chemo-resistant glioma cells from sustaining disease progression and recurrence … More widely, LMP-inducing agents should be considered as a possible novel treatment option for gliomas.”
