The interaction between carcinoembryonic antigen-related cell adhesion molecule 6 and human epidermal growth factor receptor 2 is associated with therapeutic efficacy of trastuzumab in breast cancer

In Situ Proximity Ligation Assay to Visualize Protein-Protein Interactions in Tumor Specimens

Protein-protein interactions (PPI) are the basis of various biological phenomena, such as intracellular signal transduction, gene transcription, and metabolism. PPI are also considered to be involved in the pathogenesis and development of various diseases, including cancer. PPI phenomenon and their functions have been elucidated by gene transfection and molecular detection technologies. On the other hand, in histopathological analysis, although immunohistochemical analyses provide information pertaining to protein expression and their localization in pathophysiological tissues, it has been difficult to visualize the PPI of these proteins. An in situ proximity ligation assay (PLA) was developed as a microscopic visualization technique for PPI in formalin-fixed, paraffin-embedded (FFPE) tissues as well as in cultured cells and frozen tissues. PLA using histopathological specimens enables cohort studies of PPI, which can clarify the significance of PPI in pathology. We have previously shown the dimerization pattern of estrogen receptors and significance of HER2-binding proteins using breast cancer FFPE tissues. In this chapter, we describe a methodology for the visualization of PPI using PLA in pathological specimens.

The Visualization of Protein-Protein Interactions in Breast Cancer: Deployment Study in Pathological Examination

The therapeutic strategy is determined by protein expression using immunohistochemistry of estrogen receptor (ER), progesterone receptor, and human epidermal growth factor receptor 2 (HER2) in formalin-fixed paraffin-embedded (FFPE) breast cancer tissues. However, few proteins function independently, and many of them functions due to protein-protein interactions (PPIs) with other proteins. Therefore, it is important to focus on PPIs. This review summarizes the PPIs of ER and HER2 in breast cancer, especially those using a proximity ligation assay that can visualize PPIs in FFPE tissues. In particular, assessing the interaction of CEACAM6 with HER2 may serve as a surrogate marker for the efficacy of trastuzumab in patients with breast cancer. Therefore, in this review, the technique used to detect the interaction of CEACAM6 and HER2 in routinely processed pathological specimens will be applied to the clinical practice of drug selection. We showed the possibility as a novel pathological examination method using PPIs.

Heterogeneous Nuclear Ribonucleoprotein K Is Involved in the Estrogen-Signaling Pathway in Breast Cancer

Heterogeneous nuclear ribonucleoprotein K (hnRNPK) transcripts are abundant in estrogen receptor (ER)- or progesterone receptor (PR)-positive breast cancer. However, the biological functions of hnRNPK in the ER-mediated signaling pathway have remained largely unknown. Therefore, this study analyzes the functions of hnRNPK expression in the ER-mediated signaling pathway in breast cancer. We initially evaluated hnRNPK expression upon treatment with estradiol (E2) and ICI 182,780 in the ERα-positive breast carcinoma cell line MCF-7. The results revealed that E2 increased hnRNPK; however, hnRNPK expression was decreased with ICI 182,780 treatment, indicating estrogen dependency. We further evaluated the effects of hnRNPK knockdown in the ER-mediated signaling pathway in MCF-7 cells using small interfering RNAs. The results revealed that hnRNPK knockdown decreased ERα expression and ERα target gene pS2 by E2 treatment. As hnRNPK interacts with several other proteins, we explored the interaction between hnRNPK and ERα, which was demonstrated using immunoprecipitation and proximity ligation assay. Subsequently, we immunolocalized hnRNPK in patients with breast cancer, which revealed that hnRNPK immunoreactivity was significantly higher in ERα-positive carcinoma cells and significantly lower in Ki67-positive or proliferative carcinoma cells. These results indicated that hnRNPK directly interacted with ERα and was involved in the ER-mediated signaling pathway in breast carcinoma. Furthermore, hnRNPK expression could be an additional target of endocrine therapy in patients with ERα-positive breast cancer.

Co-expression of carcinoembryonic antigen-related cell adhesion molecule 6 and 8 inhibits proliferation and invasiveness of breast carcinoma cells

The carcinoembryonic antigen-related cell adhesion molecule (CEACAM) 6 and CEACAM8 form heterodimers and exert their effects. Therefore, we examined the effects of CEACAM6 and CEACAM8 co-expression in breast cancer. We first studied CEACAM6/8 expression using immunohistochemistry in 109 patients with breast cancer. We then established MCF-7 cells that were stably transfected with CEACAM8 and studied cell proliferation, invasion and adhesion. The number of CEACAM6 and CEACAM8 double-positive breast carcinoma cells significantly increased in patients with low histopathological grade and stage. Proximity ligation assay (PLA) confirmed high CEACAM6/8 expression in MCF-7 cells. CEACAM6/8 expression promoted the adhesion of MCF-7 cells to endothelial cell monolayers but inhibited their invasion and proliferation. Furthermore, CEACAM6 status in carcinoma cells was significantly higher in bone than in lung metastases. CEACAM6/8 expression is associated with the inhibition of vascular invasion and cell proliferation. CEACAM6 expression was also considered to be involved in bone metastases of breast cancer. This is the first study to demonstrate the possible role of CEACAM6/8 heterodimer and CEACAM6 expression in breast cancer patients.

Exploring Protein⁻Protein Interaction in the Study of Hormone-Dependent Cancers

Estrogen receptors promote target gene transcription when they form a dimer, in which two identical (homodimer) or different (heterodimer) proteins are bound to each other. In hormone-dependent cancers, hormone receptor dimerization plays pivotal roles, not only in the pathogenesis or development of the tumors, but also in the development of therapeutic resistance. Protein–protein interactions (PPIs), including dimerization and complex formation, have been also well-known to be required for proteins to exert their functions. The methods which could detect PPIs are genetic engineering (i.e., resonance energy transfer) and/or antibody technology (i.e., co-immunoprecipitation) using cultured cells. In addition, visualization of the target proteins in tissues can be performed using antigen–antibody reactions, as in immunohistochemistry. Furthermore, development of microscopic techniques (i.e., electron microscopy and confocal laser microscopy) has made it possible to visualize intracellular and/or intranuclear organelles. We have recently reported the visualization of estrogen receptor dimers in breast cancer tissues by using the in situ proximity ligation assay (PLA). PLA was developed along the lines of antibody technology development, and this assay has made it possible to visualize PPIs in archival tissue specimens. Localization of PPI in organelles has also become possible using super-resolution microscopes exceeding the resolution limit of conventional microscopes. Therefore, in this review, we summarize the methodologies used for studying PPIs in both cells and tissues, and review the recently reported studies on PPIs of hormones.