Increased expression of cyclooxygenase-2 in malignant pheochromocytomas

Pheochromocytomas are rare tumors of the adrenal medulla or the paraganglion system. There are no histological or chemical markers available that define the malignant behavior of these tumors; so far only the discovery of metastases reveals malignancy. Cyclooxygenase (Cox) is the key enzyme in conversion of arachidonic acid to PGs, and two isoforms, Cox-1 and Cox-2, have been identified. Cox-2 has been associated with carcinogenesis, and it is overexpressed in many human malignancies. We have now investigated the expression of Cox-2 in normal adrenal gland, in 92 primary pheochromocytomas and in six metastases using immunohistochemistry and Northern blot and Western blot analyses. Cox-2 protein was expressed in the adrenal cortex, whereas the medulla was negative as detected by immunohistochemistry. Interestingly, all malignant pheochromocytomas (n = 8), regardless of the primary location of the tumor, showed moderate or strong Cox-2 immunoreactivity, whereas 75% of the benign adrenal tumors (n = 36) showed no or only weak immunopositivity. The staining was negative or weak in 79% of the adrenal tumors that showed histologically suspicious features (n = 24), but had not metastasized. Most of the pheochromocytoma samples studied also expressed low levels of Cox-2 mRNA. Our data show that normal adrenal medulla does not express Cox-2 immunohistochemically. However, strong Cox-2 protein expression was found in malignant pheochromocytomas, whereas most benign tumors expressed Cox-2 only weakly. To our knowledge, this is the first report on Cox-2 expression in pheochromocytomas and enhanced expression in malignant pheochromocytomas. These findings suggest that negative or weak Cox-2 expression in pheochromocytomas favors benign diagnosis.