Obesity, Adipocytes and Breast Cancer – Insights from Translational Studies

Background: Being overweight is becoming the new normal, and more than half of the adult Swedish population is overweight which poses a risk to public health. Overweight and obese women have both an increased risk and a worse prognosis for breast cancer, compared with women of normal weight. The breast cancer incidence in Sweden is increasing, and about one in nine women will be diagnosed with breast cancer during her lifetime. Adipose tissue is in close proximity to tumor cells in the breast microenvironment, and excess body fat and an altered metabolic state may lead to local and systemic molecular changes favoring tumor progression. The underlying biological mechanisms, however, are still not fully understood. The overall objective of this thesis was to bring new insights into the biological processes linking obesity to breast cancer, both through preclinical experimental studies of the interactions of adipocytes and breast cancer cells, as well as through epidemiological studies of women and their body constitution in relation to breast cancer risk and subsequent clinical outcome.
Methods: In Paper I-II, an in vitro model mimicking the microenvironment in normal- and obese-like breast cancer patients was established. Effects of the adipocytes secretome on breast cancer cell morphology, proliferation, and motility were investigated. The adipokine secretome was analyzed by proteome profiler array for putative biological mediators. The phosphorylation patterns of protein kinases in breast cancer cells in response to normal- and obese-like adipocyte secretome were analyzed and potential signaling pathways highlighted. The adipokine receptor CAP1 was silenced using small interfering RNA knockdown. In Paper I, the association between CAP1 mRNA expression, in a set of 1,881 breast cancer patients and prognosis were investigated. In Paper III, tumor-specific CAP1 protein expression in 718 primary breast cancers from the Malmö Diet and Cancer Study (MDCS) were analyzed with immunohistochemistry in relation to body constitution and breast cancer outcome. In Paper IV, prediagnostic NLR levels, body constitution and risk of breast cancer was explored among the 16,459 women in MDCS.
Results: In Paper I-II: Adipokines induced a more aggressive phenotype, higher proliferation, increased motility, and induced phosphorylation of proteins within key cellular processes in the breast cancer cells, effects that were more pronounced in obese-like conditions compared with normal-like. In a panel of adipokines, resistin was found upregulated in the adipocyte secretome during obese-like conditions. The receptor for resistin, CAP1 had a higher mRNA expression in estrogen receptor-negative breast cancer cells and was associated with shorter overall and relapse-free survival among breast cancer patients. Knockdown of CAP1 decreased the breast cancer cell proliferation and reduced the expression of the majority of phosphokinases. In Paper III, low tumor-specific CAP1 protein expression in patients was associated with older age at diagnosis, higher adiposity, unfavorable tumor characteristics, and poor breast cancer-specific and overall survival compared to women with tumors of high expression. In Paper IV, high prediagnostic NLR was associated with established breast cancer risk factors at study inclusion, but not with breast cancer risk overall, nor by specific tumor characteristics or by body constitution.
Conclusion/Implications: Adipocyte secretome stimulates molecular and cellular features in breast cancer cell associated with tumor progression. The adipokine receptor CAP1 displayed a divergent role for breast cancer prognosis where high CAP1 gene expression and low tumor-specific CAP1 protein level were associated with poor breast cancer prognosis. Prediagnostic NLR was not associated with overall breast cancer risk. Further studies regarding adipokines’ roles in obesity-related breast cancer, the posttranslational regulation of CAP1, and studies of NLR in terms of potential short-term effects on breast cancer risk are needed.