To determine whether antioxidants influence lung cancer metastasis, we administered NAC and vitE to Kras2LSL/+ mice that had previously received a low dose of Cre-adenovirus in their lungs (to induce only a few tumors) and followed them for one year. Tumors in Kras2LSL/+ mice rarely metastasize to lymph nodes, and never to distant sites unless Trp53 or another tumor suppressor gene is inactivated. Under these experimental conditions, NAC and vitE markedly increased lymph node metastasis and even triggered distant metastasis.
In a second metastasis-prone Kras2LSL/+Trp53fl/fl model, the antioxidants further increased lymph node metastasis and also induced metastasis to the inner wall of the rib cage—a site where we rarely observe metastasis. Thus, antioxidants increased lung cancer metastasis to both common and unique sites and of both Trp53-proficient and -deficient starting conditions. Although the antioxidants clearly reduce ROS levels in the tumor cells, we don’t yet know which type of ROS they reduce and where in the cell these ROS operate to inhibit tumor metastasis. Understanding cell intrinsic and dietary factors that regulate metastasis is important because it is metastases, rarely the primary tumor, that kill cancer patients.
Few scientific fields are fraught with more controversy than that of the role of reactive oxygen species (ROS) in cancer. This field has spawned a wide-spread dogma: Because ROS induces DNA damage and pro-tumorigenic signaling that can contribute to cancer, antioxidants, which neutralize ROS, must protect against cancer. But clinical trials with antioxidants do not support this dogma, and some even showed that antioxidants increase cancer risks. However, those trials have done little to change the belief that antioxidant supplements protect against cancer—a belief that fuels a billion-dollar industry that markets its products to healthy people and cancer patients alike.
Results from our group shed light on this issue. We discovered that supplementing the diet with pharmacological and dietary antioxidants such as N-acetylcysteine (NAC), vitamin E (vitE), and beta-carotene accelerates lung cancer progression in mice. We also found that antioxidants markedly accelerate metastasis in mouse models of malignant melanoma and lung cancer (this project). These results, which gain support from numerous subsequent studies, sparked a world-wide discussion about the potential dangers of antioxidant supplementation for cancer patients, and provide a plausible explanation for the failed clinical trials with these compounds.
The key to these results is that we focused exclusively on antioxidant effects on tumor progression, not initiation. The results suggest that oxidative stress potently suppresses tumor progression and metastasis; which explains why antioxidants—by eliminating oxidative stress—accelerate those processes. But interestingly, the underlying mechanisms seem to be different in melanoma and lung cancer cells: Antioxidants acutely increased invasion of mouse and human melanoma cells indicating that posttranslational mechanisms are involved. In contrast, we recently found that antioxidants in vivo cause long-term programming of lung cancer cells into a metastatic phenotype that is preserved for up to 15 passages when the tumor cells are introduced into cell culture. Also, preincubating human lung cancer cells with antioxidants for >7 days, but not acute administration, induces a long-lasting invasive phenotype, suggesting that epigenetic mechanisms are involved.
This project, along with previous results we obtained, markedly increased our understanding of how oxidative stress affects tumor progression and metastasis and will allow us to identify tumor-specific vulnerabilities that can be exploited for new therapies.
