Flavopiridol downregulates hypoxia-mediated hypoxia-inducible factor-1α expression in human glioma cells by a proteasome-independent pathway: Implications for in vivo therapy

Abstract

Angiogenesis is a critical step required for sustained tumor growth and tumor progression. The stimulation of endothelial cells by cytokines secreted by tumor cells such as vascular endothelial growth factor (VEGF) induces their proliferation and migration. This is a prominent feature of high-grade gliomas. The secretion of VEGF is greatly upregulated under conditions of hypoxia because of the transcription factor hypoxiainducible factor (HIF)-1alpha, which controls the expression of many genes, allowing rapid adaptation of cells to their hypoxic microenvironment. Flavopiridol, a novel cyclin-dependent kinase inhibitor, has been attributed with antiangiogenic properties in some cancer cell lines by its ability to inhibit VEGF production. Here, we show that flavopiridol treatment of human U87MG and T98G glioma cell lines decreases hypoxia-mediated HIF-1alpha expression, VEGF secretion, and tumor cell migration. These in vitro results correlate with reduced vascularity of intracranial syngeneic GL261 gliomas from animals treated with flavopiridol. In addition, we show that flavopiridol downregulates HIF-1alpha expression in the presence of a proteasome inhibitor, an agent that normally results in the accumulation and overexpression of HIF-1alpha. The potential to downregulate HIF-1alpha expression with flavopiridol treatment in combination with a proteasome inhibitor makes this an extremely attractive anticancer treatment strategy for tumors with high angiogenic activity, such as gliomas.

Cyclin-dependent kinases regulate lysosomal degradation of hypoxia-inducible factor 1α to promote cell-cycle progression

“…Previous studies revealed that the Cdk inhibitor flavopiridol decreased HIF-1α levels and HIF target gene expression (55,56). This effect was noted to be proteasome-independent, although the molecular mechanism was not delineated (55).…”

Radiation Sensitivity of GL261 Murine Glioma Model and Enhanced Radiation Response by Flavopiridol

“…[43][44][45] Thus, agents that can block radiation-induced VEGF secretion or its binding to its receptor, VEGFR, are two strategies that have been used to increase anti-tumor effects of radiation. [21][22][23][44][45][46][47][48] Although the mechanisms by which flavopiridol enhanced anti-tumor effects when combined with radiation were not evaluated in this study, based on its known anti-angiogenic activity in a variety of in vivo models, 14,16,49,50 it is likely that one mechanism involved is due to its ability to inhibit angiogenesis. Recently, we demonstrated that flavopiridol inhibited the HIF-1 pathway by downregulation of HIF-1α expression, concomitant with decreased VEGF secretion by glioma cells.…”

“…Recently, we demonstrated that flavopiridol inhibited the HIF-1 pathway by downregulation of HIF-1α expression, concomitant with decreased VEGF secretion by glioma cells. 16 When we analyzed vascular density in intracranial GL261 tumors from control and flavopiridol treated animals, the tumors from drug-treated animals had significantly fewer blood vessels compared with tumors from control animals, suggesting that flavopiridol inhibited angiogenic activity. 16 This conclusion is further supported by two other recent in vivo studies.…”

“…16 When we analyzed vascular density in intracranial GL261 tumors from control and flavopiridol treated animals, the tumors from drug-treated animals had significantly fewer blood vessels compared with tumors from control animals, suggesting that flavopiridol inhibited angiogenic activity. 16 This conclusion is further supported by two other recent in vivo studies. Using zebrafish as an in vivo model to screen anti-angiogenic compounds for their effects on activated vascular endothelial cells, several known small molecule inhibitors of angiogenesis, were used to validate the potential of the animal model.…”

“…[12][13][14][15] Our work demonstrated that, as a monotherapy, flavopiridol was effective in delaying GL261 tumors growing in the flank as well as in the brain 11 and this was correlated with an anti-angiogenic effect on the tumor vasculature. 16 Flavopiridol has been investigated for its potential to enhance the effect of radiotherapy in several animal models including human colon, gastric, and lung xenografts and murine ovarian, breast and lymphoma syngeneic tumors. [17][18][19] In addition, the GL261 glioma tumor model has been used in several recent studies to screen potential radiosensitizers such as the mTOR inhibitors rapamycin and RAD001, the broad spectrum receptor tyrosine www.landesbioscience.com…”