Critical role of oxidative stress in estrogen-induced carcinogenesis

Mechanisms of estrogen-induced tumorigenesis in the target organ are not well understood. It has been suggested that oxidative stress resulting from metabolic activation of carcinogenic estrogens plays a critical role in estrogen-induced carcinogenesis. We tested this hypothesis by using an estrogen-induced hamster renal tumor model, a well established animal model of hormonal carcinogenesis. Hamsters were implanted with 17β-estradiol (βE2), 17α-estradiol (αE2), 17α-ethinylestradiol (αEE), menadione, a combination of αE2 and αEE, or a combination of αEE and menadione for 7 months. The group treated with βE2 developed target organ specific kidney tumors. The kidneys of hamsters treated with αE2, αEE, or menadione alone did not show any gross evidence of tumor. Kidneys of hamsters treated with a combination of αE2 and αEE showed early signs of proliferation in the interstitial cells. Kidneys of hamsters treated with a combination of menadione and αEE showed foci of tumor with congested tubules and atrophic glomeruli. βE2-treated tumor-bearing kidneys showed >2-fold increase in 8-iso-prostaglandin F_2α (8-iso-PGF_2α) levels compared with untreated controls. Kidneys of hamsters treated with a combination of menadione and αEE showed increased 8-iso-PGF_2α levels compared with untreated controls, whereas no increase in 8-iso-PGF_2α was detected in kidneys of αEE-treated group. A chemical known to produce oxidative stress or a potent estrogen with poor ability to produce oxidative stress, were nontumorigenic in hamsters, when given as single agents, but induced renal tumors, when given together. Thus, these data provide evidence that oxidant stress plays a crucial role in estrogen-induced carcinogenesis.