Tamoxifen has been the most important therapeutic agent for the treatment of estrogen receptor (ER)-positive breast cancer for the past three decades. Tamoxifen is extensively metabolized by cytochrome P450 enzymes, and recent in vivo studies have shown that women with genetically impaired cytochrome P450 2D6 have reduced production of endoxifen and a higher risk of breast cancer recurrence. Despite these observations, the contribution of endoxifen to the overall drug effectiveness of tamoxifen remains uncertain. Here, we provide novel evidence that endoxifen is a potent antiestrogen that functions in part by targeting ERA for degradation by the proteasome in breast cancer cells. Additionally, we show that endoxifen blocks ERA transcriptional activity and inhibits estrogen-induced breast cancer cell proliferation even in the presence of tamoxifen, N-desmethyl-tamoxifen, and 4-hydroxytamoxifen. All of the effects of endoxifen are concentration dependent and do not occur at concentrations observed in human CYP2D6 poor metabolizers. These results support the theory that endoxifen is the primary metabolite responsible for the overall effectiveness of tamoxifen in the treatment of ER-positive breast cancer.

Age-related cataract, an opacity of the eye lens, is the leading cause of visual impairment in the elderly, the etiology of which is related to oxidative stress damage. Oxidation of methionine to methionine sulfoxide is a major oxidative stress product that reaches levels as high as 60% in cataract while being essentially absent from clear lenses. Methionine oxidation results in loss of protein function that can be reversed through the action of methionine sulfoxide reductase A (MsrA), which is implicated in oxidative stress protection and is an essential regulator of longevity in species ranging from Escherichia coli to mice. To establish a role for MsrA in lens protection against oxidative stress, we have examined the levels and spatial expression patterns of MsrA in the human lens and have tested the ability of MsrA to protect lens cells directly against oxidative stress. In the present report, we establish that MsrA is present throughout the human lens, where it is likely to defend lens cells and their components against methionine oxidation. We demonstrate that overexpression of MsrA protects lens cells against oxidative stress damage, whereas silencing of the MsrA gene renders lens cells more sensitive to oxidative stress damage. We also provide evidence that MsrA is important for lens cell function in the absence of exogenous stress. Collectively, these data implicate MsrA as a key player in lens cell viability and resistance to oxidative stress, a major factor in the etiology of age-related cataract.