Long-Term Risks of Shutting Down VEGF from Inside the Cell
This article was originally published in Start Up
Executive Summary
New research shows that production of VEGF -- a master regulator of blood vessel growth -- from within a cell is an important survival mechanism used by that cell, and that shutting it down could pose long-term risks. The work raises a red flag about long-term use of VEGF-inhibiting small-molecule cancer drugs that travel inside a cell to prevent VEGF expression. It also reinforces the importance of advancing methods for direct delivery of these drugs to tumors.
Mark L. Ratner
It’s long been held that vascular endothelial growth factor (VEGF)--a master regulator of blood vessel growth--transmits its effects through interactions between adjacent cells. Now, a research team has shown that intracellular VEGF production in endothelial cells is an important survival mechanism, and that shutting it down could pose long-term risks. The work raises a flag about long-term use of small-molecule tyrosine kinase receptor (TKR) inhibitors that target VEGF (in many cases along with other TKRs) by penetrating cells to prevent VEGF activation. These include Bayer HealthCare Pharmaceuticals’ Nexavar (sorafenib, approved for treating advanced kidney cancer), Pfizer Inc.’s developmental axitinib, and a slew of other cancer drugs in the pipelines of both large pharmas and biotechs. It also reinforces the importance of advancing methods for direct delivery of these drugs--as well as their large-molecule counterparts such as Genentech Inc.’s antibody Avastin (bevacizumab)--to tumors instead of relying on systemic administration.
The anti-angiogenesis field has moved very quickly from the discovery of the importance of the VEGF pathway to actually generating drugs that interfere with it. As a result of this remarkable flurry of drug development activity, there’s been little opportunity to gather long-term data on the side effects of these novel compounds. That dearth of information sparked Luisa Iruela-Arispe, PhD, of the Jonsson Comprehensive Cancer Center and the Molecular Biology Institute at the University of California, Los Angeles (UCLA) to explore whether long-term exposure to angiogenesis inhibitors could be dangerous to other organs. The results of her work are reported in the August 24, 2007, issue of Cell.
As a first step, Iruela-Arispe’s group used a mouse model to track VEGF production under normal cellular conditions, when stress signals that trigger the growth factor’s production are not in play. Contrary to the belief that VEGF is not made by endothelial cells, the group saw plenty of expression in the endothelium. The researchers then created a mouse with a VEGF deficiency in the endothelium and discovered, very surprisingly, that the animals did not survive exposure to stress. "I thought perhaps we’d see some effect," says Iruela-Arispe, a past president of the North American Vascular Biology Association and an expert on angiogenesis. "But when we started to evaluate the large cohort of animals, my post-doc, Sunyoung Lee, came to me and said ‘I get drop-dead mice.’"
To determine whether the effect was paracrine--namely, from VEGF secreted and acting in adjacent cells--or from a deficiency of endogenous VEGF, the researchers labeled cells isolated from the knockout and wild-type mice, grew them together in culture, and looked for differences in survival after exposure to stress. They saw significant differences in survivability even after they added VEGF to the culture medium. "That experiment gave us a hint that it could not be paracrine, from endothelial cell to endothelial cell, because the addition of VEGF into the culture was incapable of rescuing the knockout cells," Iruela-Arispe explains. "The knockout cells have an intrinsic defect that is cell-autonomous."
The researchers then compared the effects of a small-molecule TKR inhibitor, which works from inside the cell, with that of Avastin, which binds VEGF at the cell surface. Only the TKR inhibitor substantially neutralized the activity of endogenous VEGF, by blocking phosphorylation of the receptor.
"Using Avastin to block the VEGF receptor from the outside doesn’t really lead to lethality," Iruela-Arispe concludes. "But if you block receptor activation, you see a significant degree of lethality when the mice are exposed to stresses," such as tumor starvation or hypoxia. "The stress stimuli show you the difference between the wild-type and the knockout cells," she adds. "We’re all exposed to stresses, and VEGF is one of the survival mechanisms in response to them." Indeed, promoting the growth of new blood vessels in response to stress is a primary role VEGF plays in the normal adult.
Although review papers all state that VEGF is a paracrine growth factor, this work makes it clear that intercellular VEGF signaling is an important mechanism that protects cells from the effects of stress. "This study puts a flag up that long-term exposure to a [VEGF tyrosine kinase] receptor 2 inhibitor will raise some caution," says Iruela-Arispe, adding that even with Avastin there’s a 5% chance of an arterio-thrombic event, as Genentech has disclosed, for reasons that are not yet clear.
"I don’t want to come across as saying that angiogenesis inhibitors are bad," she goes on. "But I have heard at meetings where the premise of anti-angiogenesis therapy was put forward that, to block metastasis, you have to ‘keep up the pressure’ of the anti-angiogenesis blockade. That may be true, but if you use a drug that also has an effect on normal tissues, you may be causing more harm than good." Improved drug targeting--for example, by using peptides to carry compounds to distinct "zip codes" on the surface of tumor cells--could go a long way toward minimizing these issues.