Interleukin-6 as a Potential Mediator of Breast Cancer Progression and Non-Melanoma Skin Carcinogenesis

Breast cancer is the second leading malignancy in women behind only non-melanoma skin cancer. Likewise, it is the second leading cause of cancer-specific female mortality behind only lung cancer.Breast tumor interleukin-6 (IL-6) expression increases with tumor grade, and elevated serum IL-6 levels are associated with poor breast cancer patient survival. However, the mechanisms driving these clinical anomalies are currently undefined. Whereas IL-6 is better known as an inflammatory cytokine which mediates hematopoiesis and lymphocyte activation, it is becoming increasingly appreciated as a mediator of cancer progression. Our previous studies demonstrated that IL-6 is necessary for breast, bone, and lung fibroblast-induced breast cancer cell growth enhancement and furthermore, showed that IL-6 alone is a potent growth factor for breast cancer cells. Thus, we set out to further characterize the molecular consequences of IL-6 signaling in breast cancer, focusing on those associated with poor clinical outcome.In Chapter 2, we highlight IL-6 as one of only few factors capable of inducing a carcinoma-associated epithelial-mesenchymal transition (EMT) phenotype. Such EMT phenotypes correlate with decreasedbreast cancer patient survival. In particular, we demonstrated E-cadherin repression among multiple estrogen receptor-alpha (ERα)-positive human breast cancer cell lines following IL-6 exposure. Furthermore, ectopic autocrine IL-6 signaling promoted breast cancer cell invasiveness. Intriguingly, Twist, a transcriptional repressor of E-cadherin, was shown to promote IL-6 production and constitutive STAT3 phophorylation. Ectopic IL-6 expression maintained an EMT phenotype in an orthotopic xenograft model, which exhibited increased tumor cell proliferative index, advanced histologic grade, and poor tumor cell differentiation.Chapter 3 exposes E-cadherin as a suppressor of breast cancer cell growth and IL-6 production. The objective of these studies was to characterize the mechanisms by which IL-6 promotes breast cancercell growth. As shown in Chapter 2, IL-6 induces repression of E-cadherin, a putative regulator of epithelial cell adhesion, phenotype, and consequent homeostasis. Hence, we hypothesized that IL-6-induced E-cadherin repression may promote breast cancer cell growth and moreover, that E-cadherin expression may inhibit breast cancer cell growth. IL-6 enhanced ERα-positive breast cancer cell growth in a 3-dimensional (3D) tumor growth assay (TGA) and promoted E-cadherin repression in a dose-dependent manner. Additionally, E-cadherin expression status was inversely associated withbreast cancer cell growth rates. Likewise, cells which lack E-cadherin due to ectopic Twist expression showed unaffected growth following IL-6 exposure. These findings are corroborated by our results in Chapter 2, which showed that Twist stimulated autocrine IL-6 production, which may negate the effects of additional IL-6 supplementation. To determine if E-cadherin inhibits breast cancer cell growth, we expressed either full-length E-cadherin or truncated E-cadherin lacking a β-catenin binding domain (mutant E-cadherin) in MDA-MB-231 cells, a commonly used aggressive E-cadherin-negative breast cancer cell line. Full-length E-cadherin completely abrogated IL-6 production, and mutant E-cadherin induced an intermediate impairment of IL-6 production, suggesting that β-catenin and to a lesser extent, E-cadherin adhesive activity, may mediate IL-6 expression. Full-length E-cadherin also inhibited MDA-MB-231 cell growth. Furthermore, mutant E-cadherin sustained slightly increased growth rates compared to full-length E-cadherin, suggesting that β-catenin may regulate MDA-MB-231 growth as well as IL-6 production.