The effects of Class-3 semaphorins on the development and progression of brain tumorsMalignant brain tumors, and in particular the various types of the gliomas which are the most common primary tumors arising in the central nervous system, are extremely difficult to treat successfully. Gliomas arise when various types of glial cells, a name encompassing several types of central nervous system cells, start to proliferate uncontrollably and turn malignant. Despite enormous efforts, the median survival time after initial diagnosis of the most aggressive form of glioma, glioblastoma multiforme, is still only 50 weeks. Glioblastoma multiforme tumors are highly invasive and form highly vascularized diffuse tumors in the brain.
When a tumor develops, the cells of the tumor compete with the healthy cells of the tissue in which it forms for nutrients and oxygen delivered by the blood vessels. This limits the expansion and growth of the tumor. However, over time, some tumor cells start to produce diffusible factors which induce the growth of new blood vessels into the growing mass of the tumor in a process known as tumor angiogenesis. This event, termed the angiogenic switch, enables the rapid proliferation of the tumor cells. Inhibition of tumor angiogenesis is therefore a promising strategy in the fight against cancer. Indeed, Avastin™, the first antiangiogenic drug approved by the FDA, is an antibody that inhibits the activity of the potent angiogenesis promoting factor vascular endothelial growth factor (VEGF). Avastin is currently used to treat several types of malignant diseases.
VEGF induces angiogenesis by binding to two cell surface receptors found on endothelial cells, the cell type that covers the inner side of all blood vessels. As a result of the binding, these two receptors, VEGFR- 1 and VEGFR-2, activate a signal transduction chain that results in the induction of endothelial cell proliferation and migration and growth of new blood vessels. Of these two receptors, VEGFR-2 is considered to be the receptor that is most important for the induction of VEGF induced angiogenesis. We have identified several years ago two additional novel types of VEGF receptors that do not transduce VEGF signals by themselves, but serve as accessory receptors that form complexes with VEGFR-1 and VEGFR-2. The complex they form with VEGFR-2 strongly enhances responses to VEGF. These accessory receptors are encoded by the neuropilin-1 and by the neuropilin-2 genes. We have recently found that even though the two neuropilins do not transduce VEGF signals independently, their presence is nevertheless crucial for VEGF induced angiogenesis since in their absence VEGF is unable to induce proliferation of endothelial cells.
Interestingly, in addition to their role as VEGF receptors, the neuropilins also function as receptors for six of the seven class-3 semaphorin family members (sema3A to sema3G). These semaphorins regulate the formation of correct contacts between nerve cells during the embryonic development of the central nervous system. Some of the genes encoding class-3 semaphorins, such as sema3F and sema3B, were originally identified as tumor suppressor genes which are lost during the development of certain types of malignant tumors. For example, the genes encoding sema3B and sema3F are lost or inactivated during the development of some types of lung cancer. The connection of these factors to cancer was strengthened when we found that sema3F inhibits the proliferation of endothelial cells and induce apoptosis (programmed cell death) of endothelial cells. In addition, it was observed that endothelial cells are repelled by semaphorins suggesting that semaphorins may repel blood vessels
in-vivo. These observations suggested that semaphorins may function as anti-angiogenic agents, and we have indeed found that sema3F inhibits angiogenesis
in-vivo and as a result can inhibit the development of tumors in mice. We have recently obtained additional evidence indicating that both sema3A and sema3F potently inhibit the development of tumors derived from malignant breast cancer cells and that these two semaphorins can function synergistically, suggesting that combinations of semaphorins may perhaps function as better inhibitors of tumor angiogenesis and tumor development than single semaphorins. Since many types of tumor cells also contain neuropilins on their cell surface, it is therefore not surprising that semaphorins such as sema3A and sema3F were also found to inhibit directly the migration and attachment of several types of neuropilin expressing tumor cells, indicating that semaphorins could also inhibit tumor development as a result of a direct effect on tumor cells as well as a result of their anti-angiogenic effects.
Based on our preliminary results, as well as results obtained by other research groups, we propose to find out if members of the class-3 semaphorin family have the potential to inhibit the development of brain tumors. We propose to find out if semaphorins can inhibit the development of tumors from glioblastoma multiforme derived cells implanted in mice, and whether semaphorins can cause shrinkage of pre-existing tumors that develop in mice from such cells.
Malignant brain tumors are more difficult to treat than other types of tumors. One of the reasons is because many types of drugs cannot be delivered to the brain through the vasculature because the blood vessels of the central nervous system are contain a blood brain barrier unique to vessels that feed the brain. The blood brain barrier limits the uptake of charged molecules and large molecules from blood vessels to the brain tissues. An interesting feature of the class-3 semaphorins is that they cause very rapid contraction of endothelial cells grown in cell culture and compromise their attachment to substrate. This observation raises the possibility that semaphorins may be able to increase the permeability of blood vessels since the transient contraction of endothelial cells of blood vessels feeding the brain may open gaps between the endothelial cells and open a breach through which drugs and large molecules such as antibodies may enter into the brain. We therefore propose in addition to determine if any of the semaphorins, alone or in conjunction with small molecules such as certain alkyl-glycerols that were previously found to partially break down the blood brain barrier, can increase the permeability of brain blood vessels to large proteins such as semaphorins and antibodies such as Avastin™.
To summarize, our preliminary studies suggest that class-3 semaphorins have the potential to function as potent inhibitors of brain tumor development and progression as a result of their anti-angiogenic properties. It is also possible that the proliferation and migration of glioblastoma multiforme cells themselves will be inhibited directly by semaphorins since we already know that at least some of these cells express semaphorin receptors. Finally, the effects that semaphorins have on endothelial cell morphology suggest that semaphorins may perhaps be able to break down the blood brain barrier, and may therefore find use as substances that enable the delivery of drugs that do not normally cross the blood brain barrier into the brain. We propose to put these questions and hypotheses to the test.
