Vitamin D Signaling Pathway and Breast Cancer

Abstract

Although the mortality rate of breast cancer has continued to decrease over the past several decades, it remains the most frequently diagnosed cancer and the leading cause of cancer death in women worldwide. Accumulating epidemiological and clinical evidence suggest that vitamin D insufficiency is associated with increased breast cancer incidence and poor clinical outcomes in patients with breast cancer, which makes vitamin D supplementation a potential preventive or therapeutic option for the management of breast cancer. However, the detailed mechanisms on how vitamin D protects against breast cancer remains largely unknown. This thesis mainly investigates the role of vitamin D signaling pathway in the prevention of breast cancer. There are two major themes. The first theme focuses on identifying target genes and molecular pathways of vitamin D in human breast cancer, while the second theme aims to characterize the role of the vitamin D inactivating enzyme Cyp24a1 in the development of mouse mammary gland morphogenesis. This thesis is in the publication format with Chapter III as a published article, Chapter IV, V, and VI as manuscripts in preparation, while the remaining parts of this thesis (Chapter I and II) are regarded as an introduction. Theme 1: Identification of vitamin D3 target genes in human breast cancer tissue: Chapter I summarizes the basic knowledge of vitamin D metabolism in breast tissue, our current understandings of the anti-tumor effects of vitamin D, and the role of vitamin D in breast cancer prevention based on epidemiological and clinical studies. Target genes of the vitamin D signaling pathway mainly generated from investigation of breast cancer cell lines are also reviewed. Chapter II presents the evidence from randomized clinical trials that vitamin D, plus or minus calcium supplementation, has the potential to prevent breast cancer. Our analyses indicate that the available data are insufficient to confirm any protective effect of vitamin D supplementation, with or without calcium, on the risk of breast cancer. More participants in future trials are required to make a reliable and conclusive assessment. Chapter III, published as an original research article in the Journal of Steroid Biochemistry and Molecular Biology, determines the target genes and molecular pathways influenced by 1,25(OH)2D in normal human breast and cancer tissues in an ex vivo explant system. RNA-Seq revealed 523 genes that were differentially expressed in breast cancer tissues in response to 1,25(OH)2D treatment, and 127 genes with altered expression in normal breast tissues. The major finding of the present study is that exposure of both normal and malignant breast tissue to 1,25(OH)2D results in changes in cellular adhesion, metabolic pathways and tumor suppressor-like Chapter II presents the evidence from randomized clinical trials that vitamin D, plus or minus calcium supplementation, has the potential to prevent breast cancer. Our analyses indicate that the available data are insufficient to confirm any protective effect of vitamin D supplementation, with or without calcium, on the risk of breast cancer. More participants in future trials are required to make a reliable and conclusive assessment. Chapter III, published as an original research article in the Journal of Steroid Biochemistry and Molecular Biology, determines the target genes and molecular pathways influenced by 1,25(OH)2D in normal human breast and cancer tissues in an ex vivo explant system. RNA-Seq revealed 523 genes that were differentially expressed in breast cancer tissues in response to 1,25(OH)2D treatment, and 127 genes with altered expression in normal breast tissues. The major finding of the present study is that exposure of both normal and malignant breast tissue to 1,25(OH)2D results in changes in cellular adhesion, metabolic pathways and tumor suppressor-like pathways, which support epidemiological data suggesting that adequate vitamin D3 levels may improve breast cancer outcome. Theme 2: Characterization of the vitamin D-inactivating enzyme Cyp24a1 in pubertal mouse mammary gland morphogenesis. Chapter IV, presented as a manuscript in preparation, summarizes our current understanding from transgenic mouse models of the vitamin D receptor (Vdr) and vitamin D associated metabolic enzymes in mammary gland development, cancer initiation, and progression and discusses the implications of these findings for human breast cancer. An improved understanding of the mechanisms of action of vitamin D signaling pathway derived from these mouse models provides support that activation of vitamin D signaling pathway is a potential approach for human breast cancer prevention. Chapter V, also presented as a manuscript in preparation, characterizes the role of vitamin D-inactivating enzyme Cyp24a1 in mammary gland development. A novel mouse model was available with conditional knockout of the Cyp24a1 gene specifically in the mammary epithelium (MMTV-Cre x Cyp24a1lox/lox). Our major finding is that mammary glands from virgin Cyp24a1 knockout females display impaired ductal morphogenesis compared with age- and weight-matched wild-type mice, which is due to reduced proliferation of mammary epithelial cells with the ablation of Cyp24a1 activity. The final chapter (VI), presented as a manuscript in preparation, explores the regulatory network of RANKL expression in human breast cells. Although RANKL is well characterized as a target gene of vitamin D in bone tissue, it becomes a different story when it comes to breast tissue. Taken together, although the detailed mechanisms of how women with adequate levels of vitamin D have a lower incidence of breast cancer are yet to be defined, our findings definitely contribute to a better understanding of the role of vitamin D signaling pathway in the prevention of breast cancer. Our work focuses on analysing data from human breast cancer tissue and transgenic mouse models, which could facilitate translation from the bench to the bedside and ultimately benefit the welfare of people in general.