Integrins regulate cellular adhesion and transmit signals important for cell survival, proliferation and motility. They are expressed by
glioma cells and may contribute to their malignant phenotype.
Integrin inhibition may therefore represent a promising therapeutic strategy. GL-261 and SMA-560
glioma cells grown under standard conditions uniformly detached and formed large cell clusters after
integrin gene silencing or pharmacological inhibition using EMD-121974, a synthetic
Arg-Gly-Asp-motif
peptide, or
GLPG0187, a nonpeptidic
integrin inhibitor. After 120 h, the clusters induced by
integrin inhibition decayed and cells died. In contrast, when cells were cultured under stem cell (sphere) conditions, no disaggregation became apparent upon
integrin inhibition, and cell death was not observed. As
poly-HEMA-mediated detachment had similar effects on cell viability as
integrin inhibition, we postulated that cell death may result from detachment alone, which was confirmed using various permissive and nonpermissive substrates. No
surrogate markers of apoptosis were detected and electron microscopy confirmed that
necrosis represents the dominant morphology of detachment-induced cell death. In addition,
integrin inhibition resulted in the induction of autophagy that represents a survival signal. When
integrins were inhibited in nonsphere
glioma cells, the TGF-β pathway was strongly impaired, whereas no such effect was observed in
glioma cells cultured under sphere conditions. Cell death induced by
integrin inhibition was rescued by the addition of recombinant
transforming growth factor-β (TGF-β) and accelerated by exposure to the TGF-β receptor inhibitor,
SD-208. In summary, cell death following
integrin inhibition is detachment mediated, represents an atypical form of anoikis involving
necrosis as well as autophagy, and is modulated by TGF-β pathway activity.