Breast cancer is the most common cancer among women worldwide today. Nearly 9000 women are diagnosed with breast cancer in Sweden yearly and despite advantages in diagnostics and treatments approximately 1400 women still die from their disease every year. Breast cancer has a diverse etiology and hormonal factors and life-style factors contribute to an increased breast cancer risk. High mammographic density is also considered a risk factor but the underlying mechanisms are not fully understood. Inflammation is associated with poor survival in several malignancies and is considered a hallmark of cancer. There is evidence indicating that increased inflammation is associated with dense breast tissue and may contribute to an increased risk of breast cancer in these patients.

There is an urgent need to find risk reduction strategies in breast cancer prevention. Several studies have shown that antiestrogens significantly reduce breast cancer incidence in women with high risk of developing breast cancer and can be used for chemoprevention. These drugs may have potentially severe side effects and other strategies are needed. Dietary interventions may influence breast cancer risk without any major side effects. Studies indicate that dietary phytoestrogens may reduce breast cancer risk. The most common phytoestrogens in Western populations are lignans, mainly found in flaxseed, but results from several studies with lignans for breast cancer prevention have been inconsistent.

In this thesis we investigated the effects of tamoxifen and flaxseed on inflammatory mediators in normal breast tissue and in breast cancer. We used the microdialysis technique to sample proteins from the extracellular space in vivo. This technique gives us the opportunity to study proteins in their bioactive compartment in situ and to study changes in protein levels at different time points without affecting the tissue of interest. We also used experimental models and cell cultures to study tumor growth of human breast cancer xenografts, cancer cell proliferation and angiogenesis.

In paper I, we investigated whether tamoxifen, flaxseed, enterolactone or genestein reduced growth of human breast cancer xenografts and their association with pro-inflammatory cytokine interleukin 1β (IL-1β) and its antagonist interleukin 1 receptor antagonist (IL-1Ra). In paper II, we investigated whether tamoxifen and flaxseed exerted similar effects on inflammatory mediators in normal breast tissue in vivo. In paper III, we investigated whether osteopontin (OPN), a pro-inflammatory cytokine, was associated with dense breast tissue and breast cancer and if tamoxifen and flaxseed could alter OPN levels in normal breast tissue in vivo. We also investigated the correlation between OPN and inflammatory mediators in normal breast tissue and in breast cancer in vivo.

In conclusion, we showed that tamoxifen and flaxseed affected breast cancer growth in an experimental model and may exert an anti-inflammatory effect in breast cancer and normal breast tissue by increasing the IL-1Ra/IL-1β ratio in vivo. We showed that dense breast tissue and breast cancer were associated with increased levels of OPN. Circulating estrogen did not correlate to OPN and tamoxifen and flaxseed did not affect OPN levels suggesting an estrogen independent regulation of OPN in vivo. These finding contributes to our understanding of how tamoxifen and flaxseed affects inflammation and the role of inflammation in the pathogenesis of breast cancer.