We have previously reported that thyroid capsular
inflammation induced by
sulfadimethoxine (
SDM), a
goitrogen, might play a role in development of invasive follicular cell
adenocarcinomas in rats initiated with
N-bis(2-hydroxypropyl)nitrosamine (
DHPN). The present study was designed to examine the role of
cyclooxygenase (COX)-2, widely known to be up-regulated in inflammatory states, during chemically induced rat thyroid
carcinogenesis. Male F344 rats received a subcutaneous
DHPN (2800 mg/kg) injection, and 1 week later were allowed free access to
drinking water containing antithyroidal
propylthiouracil (PTU, 0.003%) or
SDM (0.1%) for 4 or 10 weeks. Control groups receiving
goitrogen alone and no treatment were also included. At week 4, diffuse follicular cell
hyperplasia was induced in all PTU- and
SDM-treated groups, along with fibrous capsular thickening and capsular thickening with
inflammation, respectively. Additionally, multiple focal follicular cell
hyperplasias and
adenomas were observed in the
DHPN + PTU and
DHPN +
SDM cases. At week 10,
adenocarcinomas invasive to the
capsule and restricted to the capsular adjacent region, were frequent in the
DHPN +
SDM group, but not observed in the animals given
DHPN + PTU. Western blots and immunohistochemistry revealed constitutive COX-2 expression in non-neoplastic follicular cells of the control and all of the PTU- and
SDM-treated rats. However, COX-2 reactivity was significantly reduced or negative in the preneoplastic/neoplastic lesions in the
DHPN-treated groups. In fibrous or inflamed thickened capsules, only a few component cells with inflammatory elements were positive for COX-2, and there was no significant difference in this regard between the PTU and
SDM treatments. The present results suggest that capsular
inflammation could play a role in development of invasive
carcinomas, but COX-2 expression does not make a major contribution.