The tumor stroma is increasingly implicated in tumorigenesis, primarily through angiogenesis and inflammation. Most efforts to elucidate the mediators have focused on cytokines, giving little to no attention to non-proteinaceous autacoids in the tumor stroma. We recently discovered that the epoxyeicosatrienoic acids (EETs), which are cytochrome P450 (CYP) metabolites of arachidonic acid (AA), have potent tumor promoting activities. Overexpression of the CYP enzymes, or genetic abrogation of the enzyme soluble epoxide hydrolase (sEH) (which metabolizes EETs), resulted in increased EET levels in the endothelium of transgenic mice. In both cases, increased endothelial-derived EETs dramatically stimulated angiogenesis, primary tumor growth, and metastasis. Consistently, reducing endogenous EETs using specific EET-antagonists, or through endothelial-specific overexpression of sEH, inhibited tumor growth. Thus, we hypothesize that EETs in the endothelium are potent regulators of tumor growth and metastasis. Endothelial-derived EETs may be a key paracrine mediator of the tumor promoting role of the stroma. This is a novel concept for it attributes to endothelium a trophic and inflammation modulatory function in promoting tumors, in addition to its established role of providing blood supply. The overall goal of this project is to elucidate the mechanisms by which EETs stimulate tumor growth.
Aim 1 will determine whether endogenous tumor-promoting EETs are derived from the endothelium, tumor cells or macrophages in the stroma (which all express the EET producing CYPs and EET metabolizing sEH). This will be dissected primarily by differential overexpression of CYPs or sEH in the tumor vs transgenic host compartments, employing xenograft tumor models. The drastic increase of number, size and spread of distant metastases, triggered by high EET levels, that we observed is unprecedented. Therefore, Aim 2 will determine whether endothelial-derived EETs facilitate dissemination at the site of the primary tumor (invasion, migration), or at the metastatic site (homing, colonization, dormancy escape). This will be achieved using a parabiosis model (the surgical joining of two mice) in which the donor mouse carrying the primary tumor has high EETs in its endothelium, and the recipient mouse has normal EET levels. Comparing the rate of metastasis into organs of the tumor-carrying donor with that of the recipient (across the parabiosis junction) will reveal the site of action of EETs. Finally, Aim 3 will test if EETs can serve as a pharmacological target for cancer therapy by determining if small molecule antagonists of EETs inhibit tumor growth, and will begin to explore their mechanism of action. Understanding the role of EETs in tumorigenesis is of direct clinical relevance for two reasons. (1) Drugs which increase EETs are cardioprotective and in phase II clinical trials for hypertension;in view of our preliminary results, the potential of cancer risk must be carefully evaluated. (2) The pharmacologically accessible autacoid system, including EETs, may offer an entirely new target for anti-stromal and anti-angiogenesis strategies in cancer therapy.
Understanding the precise role in cancer of epoxyeicosatrienoic acids (EETs), which have become known as mediators of inflammation, is of immediate clinical importance because drugs which increase EETs are in phase II clinical trial for hypertension;but, our pilot studies show that elevated EETs promote growth of cancer and metastasis. Conversely, blocking the EET pathway may offer a new strategy to block tumor blood vessels and, hence, help combat cancer. Thus, novel drugs which block EETs will be tested in preclinical trials to confirm their activity.
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