Angiogenesis inhibitors are increasingly being tested in combination with radiotherapy. Empirical evidence from animal tumor models and human clinical trials indicates that this combination is beneficial126
, but the mechanism remains controversial. On the one hand, tumors have hypoxic regions, and based on their ability to “normalize” the tumor vasculature, angiogenesis inhibitors may improve the overall oxygenation of tumors in an NO-dependent manner127
. Increased tissue oxygenation can increase radiosensitivity109, 128
. This scenario predicts that prior treatment with angiogenesis inhibitors would produce the best enhancement. However, some studies have shown enhanced radiosensitivity when angiogenesis inhibitors are administered either immediately before or after irradiation129
. In this case, mechanisms other than vascular normalization must account for their efficacy. Based on the radioresistance of tumors grown in mice lacking Bax or acid sphingomyelinase, which protect the tumor vasculature from radiation-induced apoptosis, tumor endothelial cells were proposed to be the critical radiosensitive component of tumors130
. Thus, combining angiogenesis inhibitors with radiation could enhance apoptosis within the tumor vasculature and thereby starve the tumor of nutrients. This could account for some acute radiosensitization by angiogenesis inhibitors. However, other studies have questioned the limiting role of tumor vasculature in tumor radioresistance131, 132
Given the many variables that can influence tumor tissue responses to radiation, it is important to first understand how angiogenesis inhibitors influence radiosensitivity in normal tissue. In addition to shedding light on cancer radiosensitivity, any manipulation that improves radioresistance of normal tissue could be exploited to locally protect surrounding healthy tissue and thereby maximize the radiation dose that can be delivered to an underlying tumor. If tumor radiosensitivity indeed proves to be more important than that of the tumor vasculature, systemic administration of a vascular radioprotectant could prevent radiation damage to healthy tissue without compromising tumor ablation.
Assessing the radiosensitivity of healthy soft tissue in mice, we recently found that Thbs1
-null mice are remarkably resistant to high dose (25 Gy) hindlimb irradiation133
. In the skin, alopecia[G] and wet desquamation[G] were decreased compared to that observed in wild type mice, and histological examination after 2 months confirmed preservation of skin architecture and function. Remarkably, underlying skeletal muscle in Thbs1
-null mice showed essentially no signs of necrosis or fibrosis following irradiation. Not only was muscle mitochondrial function preserved, but the null mice tended to have greater muscle mass in the irradiated hindlimb compared to nonirradiated control limbs. Responses to irradiation in CD47
-null mice paralleled those of the Thbs1
-null, indicating that this TSP1 receptor is critical to the radiosensitizing activity of endogenous TSP1. Thbs2
-null mice showed no protection from radiation injury, however, indicating that this activity is specific for TSP1.
Shortly following irradiation, apoptosis was also decreased in muscle and bone marrow of Thbs1
- and CD47
-null mice. Because CD47 and TSP1 are also implicated in recruitment of inflammatory cells at sites of infection or injury97, 134
, we considered whether the apparent radioprotection could be secondary to a decreased inflammatory response in the irradiated null limbs. To eliminate this variable, radiosensitivity was assessed in vascular cells isolated from the respective mice. Remarkably, Thbs1
- and CD47
-null cells in culture replicated the radioresistance of their hosts133
. The null cells exhibited less death following irradiation, and over time in culture, maintained proliferative capacity that was progressively lost by irradiated wild type cells. Thus, eliminating the signal from secreted TSP1 engaging its receptor CD47 dramatically protects the viability and proliferative capacity of cells following high dose irradiation. Cell-autonomous radioprotection in the null cells indicates that blocking this pathway in vivo could be protective independent of any effect of these proteins on the immune system. Although elevating NO is known to be radioprotective for whole body irradiation135
, ongoing experiments indicate that regulation of NO/cGMP signaling only partially explains this radioprotection. Additional molecular mechanisms are under investigation.
Returning to the controversy concerning the relative contribution of tumor cells versus host vasculature to radiosensitivity, we compared radiation growth delays for syngeneic B16 melanomas implanted in wild type versus Thbs1
. No decrease in irradiation-induced regrowth delay was observed in the Thbs1
-null mice. Therefore, antagonists of TSP1/CD47 interaction merit testing as selective radioprotectants and could permit higher radiation doses to be delivered to tumors while preserving critical adjacent tissue and organ function.