Antiangiogenic activities of 2,5-dimethyl-celecoxib on the tumor vasculature.

Our laboratory has previously shown that a novel compound, 2,5-dimethyl-celecoxib (DMC), which is structurally similar to the cyclooxygenase-2 (COX-2) inhibitor celecoxib but lacks the COX-2-inhibitory function, mimics the antitumor effects of celecoxib. Most studies on DMC, however, focused on its effects on tumor cells. Here, we investigated the activities of DMC as an antiangiogenic agent in both in vitro and in vivo systems. Using primary cultures of human glioma specimens, we found that DMC treatment was cytotoxic to tumor-associated brain endothelial cells (TuBEC), which was mediated through the endoplasmic reticulum stress pathway. In contrast, confluent cultures of quiescent human BEC did not undergo cell death. DMC potently suppressed the proliferation and migration of the TuBEC. DMC caused no apparent effects on the secretion of vascular endothelial growth factor and interleukin-8 but inhibited the secretion of endothelin-1 in tumor-associated EC. DMC treatment of glioma xenografts in mice resulted in smaller tumors with a pronounced reduction in microvessel density compared with untreated mice. In vitro and in vivo analyses confirmed that DMC has antivascular activity. Considering that DMC targets both tumor cells and tumor-associated ECs, this agent is a promising anticancer drug.

Glioma-associated endothelial cells are chemoresistant to temozolomide.

Temozolomide is considered the standard of care and drug of choice for the treatment of initially diagnosed malignant gliomas. Although well tolerated, temozolomide still has limited clinical efficacy. Following drug treatment, patient prognosis still remains poor; tumor recurrence is almost universal. We hypothesized that this lack of effectiveness with temozolomide is because this drug does not target the glioma microenvironment, which is highly vascular in malignant gliomas. To test this hypothesis we analyzed the effects of temozolomide on the tumor vasculature in vitro and in vivo. We found that this drug did not affect the viability or proliferation rate of endothelial cells isolated from human glioma specimens, although temozolomide was highly cytotoxic to the glioma cell lines U87MG and U251. Furthermore, temozolomide did not inhibit the migration of these glioma-associated endothelial cells, a key mechanism responsible for tumor angiogenesis. In in vivo studies, using the intracranial glioma mouse model, temozolomide did not cause a pronounced effect on microvessel density. Our findings show that temozolomide has no apparent effect on the glioma vascular microenvironment. Thus combination therapy with anti-vascular agents may enhance temozolomide effectiveness as glioma therapeutic protocol.

Stress chaperone GRP78/BiP confers chemoresistance to tumor-associated endothelial cells.

The tumor vasculature is essential for tumor growth and survival and is a key target for anticancer therapy. Glioblastoma multiforme, the most malignant form of brain tumor, is highly vascular and contains abnormal vessels, unlike blood vessels in normal brain. Previously, we showed that primary cultures of human brain endothelial cells, derived from blood vessels of malignant glioma tissues (TuBEC), are physiologically and functionally different from endothelial cells derived from nonmalignant brain tissues (BEC) and are substantially more resistant to apoptosis. Resistance of TuBEC to a wide range of current anticancer drugs has significant clinical consequences as it represents a major obstacle toward eradication of residual brain tumor. We report here that the endoplasmic reticulum chaperone GRP78/BiP is generally highly elevated in the vasculature derived from human glioma specimens, both in situ in tissue and in vitro in primary cell cultures, compared with minimal GRP78 expression in normal brain tissues and blood vessels. Interestingly, TuBEC constitutively overexpress GRP78 without concomitant induction of other major unfolded protein response targets. Resistance of TuBEC to chemotherapeutic agents such as CPT-11, etoposide, and temozolomide can be overcome by knockdown of GRP78 using small interfering RNA or chemical inhibition of its catalytic site. Conversely, overexpression of GRP78 in BEC rendered these cells resistant to drug treatments. Our findings provide the proof of principle that targeting GRP78 will sensitize the tumor vasculature to chemotherapeutic drugs, thus enhancing the efficacy of these drugs in combination therapy for glioma treatment.

Increased survivin expression confers chemoresistance to tumor-associated endothelial cells.

Growing evidence suggests that survivin, a member of the inhibitor of apoptosis gene family, is responsible for drug resistance in cancer cells, yet little is known about its role in the endothelial cells of the tumor vasculature. We have previously reported that tumor-associated endothelial cells derived from gliomas (TuBECs) are resistant to anticancer chemotherapy whereas normal brain endothelial cells (BECs) are sensitive. The focus of this study is to investigate the mechanism behind this chemoresistance. Here we show that survivin is constitutively overexpressed in the glioma vasculature but not in the blood vessels of normal brain. To determine whether survivin contributes to TuBEC chemoresistance, we used a lentiviral siRNA system or the drug roscovitine to down-regulate survivin expression. Reduced levels of survivin sensitized TuBECs to the chemotherapeutic agents VP-16, paclitaxel, thapsigargin, and temozolomide. This cell death was mediated through caspases 7 and 4. Conversely, forced expression of survivin in BECs was protective against drug cytotoxicity. These data suggest that overexpression of survivin in endothelial cells serves as a protective mechanism that defends the vasculature from drug cytotoxicity. Our studies demonstrate that targeting survivin may be an effective approach to chemosensitization and anti-vascular therapy for brain tumors.

Critical role of the stress chaperone GRP78/BiP in tumor proliferation, survival, and tumor angiogenesis in transgene-induced mammary tumor development.

The unfolded protein response (UPR) is an evolutionarily conserved mechanism that activates both proapoptotic and survival pathways to allow eukaryotic cells to adapt to endoplasmic reticulum (ER) stress. Although the UPR has been implicated in tumorigenesis, its precise role in endogenous cancer remains unclear. A major UPR protective response is the induction of the ER chaperone GRP78/BiP, which is expressed at high levels in a variety of tumors and confers drug resistance in both proliferating and dormant cancer cells. To determine the physiologic role of GRP78 in in situ-generated tumor and the consequence of its suppression on normal organs, we used a genetic model of breast cancer in the Grp78 heterozygous mice where GRP78 expression level was reduced by about half, mimicking anti-GRP78 agents that achieve partial suppression of GRP78 expression. Here, we report that Grp78 heterozygosity has no effect on organ development or antibody production but prolongs the latency period and significantly impedes tumor growth. Our results reveal three major mechanisms mediated by GRP78 for cancer progression: enhancement of tumor cell proliferation, protection against apoptosis, and promotion of tumor angiogenesis. Importantly, although partial reduction of GRP78 in the Grp78 heterozygous mice substantially reduces the tumor microvessel density, it has no effect on vasculature of normal organs. Our findings establish that a key UPR target GRP78 is preferably required for pathophysiologic conditions, such as tumor proliferation, survival, and angiogenesis, underscoring its potential value as a novel therapeutic target for dual antitumor and antiangiogenesis activity.

Glioma-associated endothelial cells show evidence of replicative senescence.

The innately programmed process of replicative senescence has been studied extensively with respect to cancer, but primarily from the perspective of tumor cells overcoming this stringent innate barrier and acquiring the capacity for unlimited proliferation. In this study, we focus on the potential role of replicative senescence affecting the non-transformed endothelial cells of the blood vessels within the tumor microenvironment. Based on the well-documented aberrant structural and functional features of blood vessels within solid tumors, we hypothesized that tumor-derived factors may lead to premature replicative senescence in tumor-associated brain endothelial cells (TuBEC). We show here that glioma tissue, but not normal brain tissue, contains cells that express the signature of replicative senescence, senescence-associated beta-galactosidase (SA-beta-gal), on CD31-positive endothelial cells. Primary cultures of human TuBEC stain for SA-beta-gal and exhibit characteristics of replicative senescence, including increased levels of the cell cycle inhibitors p21 and p27, increased resistance to cytotoxic drugs, increased growth factor production, and inability to proliferate. These data provide the first demonstration that tumor-derived brain endothelial cells may have reached an end-stage of differentiation known as replicative senescence and underscore the need for anti-angiogenic therapies to target this unique tumor-associated endothelial cell population.