The PI3K/Akt/mTOR pathway has been well known to interact with the estrogen receptor (ER)-pathway and to be also frequently upregulated in aromatase inhibitor (AI)-resistant breast cancer patients. Intracellular levels of free amino acids, especially leucine, regulate the mammalian target of rapamycin complex 1 (mTORC1) activation. L-type amino acid transporters such as LAT1 and LAT3 are associated with the uptake of essential amino acids. LAT1 expression could mediate leucine uptake, mTORC1 signaling, and cell proliferation. Therefore, in this study, we explored amino acid metabolism, including LAT1, in breast cancer and clarified the potential roles of LAT1 in the development of therapeutic resistance and the eventual clinical outcome of the patients. We evaluated LAT1 and LAT3 expression before and after neoadjuvant hormone therapy (NAH) and examined LAT1 function and expression in estrogen deprivation-resistant (EDR) breast carcinoma cell lines. Tumors tended to be in advanced stages in the cases whose LAT1 expression was high. LAT1 expression in the EDR cell lines was upregulated. JPH203, a selective LAT1 inhibitor, demonstrated inhibitory effects on cell proliferation in EDR cells. Hormone therapy changed the tumor microenvironment and resulted in metabolic reprogramming through inducing LAT1 expression. LAT1 expression then mediated leucine uptake, enhanced mTORC1 signaling, and eventually resulted in AI resistance. Therefore, LAT1 could be the potential therapeutic target in AI-resistant breast cancer patients.

Kallikrein-related peptides (KLKs) form an evolutionally conserved subgroup of secreted serine proteases that consists of 15 members (KLK1-15). Previous studies have shown that KLKs regulate diverse biological processes, but the clinical significance of KLKs remains largely unclear in human breast cancers. We examined the expression profile of 15 KLK genes in breast carcinomas using microarray data. Next, we immunolocalized KLK12 in 140 breast carcinomas and evaluated its clinical significance. Subsequently, we examined the effects of KLK12 on proliferation and migration in breast cancer cell lines. From microarray analyses, it turned out that KLK12 was the most strongly associated with low-grade malignancy in breast carcinomas among the 15 KLK members. Immunohistochemical KLK12 status was positively associated with ER and PR status, while it was inversely associated with stage, pathological T factor, lymph node metastasis, and distant metastasis. Prognostic analyses demonstrated that KLK12 was a favorable prognostic factor for both disease-free and breast cancer-specific survival of the patients. Furthermore, the knockdown of KLK12 significantly increased cell proliferation activity and cell migration of breast cancer cells. These results suggest that KLK12 has antitumorigenic effects associated with proliferation and migration and immunohistochemical KLK12 status as a potent favorable prognostic factor in breast carcinoma patients.

It is known that estrogen receptor (ER) has extranuclear signaling functions in addition to classical genomic pathway, and estrogenic actions have been reported in ER-negative breast carcinoma cells. However, significance of cytoplasmic-ER immunoreactivity has not been reported in ER-negative breast carcinoma tissues. We immunolocalized cytoplasmic ER in 155 ER-negative breast carcinoma tissues and evaluated its clinicopathological significance including the prognosis. As a comparative cohort set, we also used 142 ER-positive breast carcinomas. Cytoplasmic-ER immunoreactivity was detected in the carcinoma cells, but not in the non-neoplastic mammary epithelium. Cytoplasmic-ER immunoreactivity was positive in the 35 out of 155 (23%) ER-negative breast carcinoma cases, whereas it was detected only in 2 out of 142 (1.4%) ER-positive cases. Cytoplasmic ER status was positively associated with cytoplasmic-PR status, but inversely associated with Ki67 labeling index or distant free-relapse survival rate. Moreover, cytoplasmic-ER status turned out to be an independent good prognostic factor for both distant relapse-free survival and breast cancer specific survival. These findings suggested that cytoplasmic ER plays important roles in the ER-negative breast carcinoma, and cytoplasmic ER is a potent good prognostic factor. Among the ER-negative breast cancer patients, clinical benefit of chemotherapy may be limited in the cytoplasmic-ER positive cases.

Background/Objectives: HER2, a critical diagnostic marker and therapeutic target in breast cancer, is a membrane receptor that forms diverse signaling complexes, the constituents of which have not been fully characterized in actual breast cancer tissues. Methods: In this study, we applied the Rapid Immunoprecipitation Mass Spectrometry of Endogenous Proteins (RIME) method, originally developed to explore transcription factor complexes, to identify the complexes formed by HER2 in HER2-positive breast cancer specimens. Results: Through our approach, we successfully identified multiple complex components, including MARCKS, a novel HER2-interacting partner, which we verified using both proximal ligation assay in cultured cells and immunohistochemistry in tissue sections. TCGA analysis further revealed that high MARCKS expression significantly correlates with ER negativity, as confirmed by multivariate analysis, suggesting its potential role as a prognostic indicator in aggressive breast cancer subtypes. Conclusions: These results demonstrate the capability of RIME to elucidate interactomes of membrane proteins such as HER2 in clinical tissue specimens. Furthermore, this study highlights its broader applicability beyond nuclear proteins, underscoring its potential for discovering novel prognostic and diagnostic clinical markers in diverse cancer types.

Breast cancer is one of the most representative sex-steroid-dependent cancers and both estrogens and androgens are locally synthesized in breast cancer tissues by sex-steroid producing enzymes. 5α-reductase (5α-Red) is an enzyme which reduces testosterone to biologically active dihydrotestosterone (DHT), serving as a regulator of intratumoral DHT levels. To date, three 5α-Red isozymes have been identified: 5α-Red types 1-3. However, 5α-Red type 3 is not fully examined in breast cancer, and its contribution to DHT synthesis is yet to be elucidated. We therefore immunolocalized 5α-Red type 3 in breast cancer tissues and correlated its immunoreactivity with intratumoral DHT levels as well as clinicopathological parameters. In the present study, 5α-Red type 3 immunoreactivity was not correlated with intratumoral DHT level. Additionally, the immunoreactivity of 5α-Red type 3 was negatively correlated with that of 17β-hydroxysteroid dehydrogenase type 5, which converts androstenedione to testosterone. In the prognostic analysis, although 5α-Red type 3 immunoreactivity was not correlated with patients' clinical outcomes in the entire cohort of 172 breast cancer cases, it was significantly correlated with better clinical outcomes in the patients with non-luminal A type breast cancer or in those who received chemotherapy. These findings suggest that 5α-Red type 3 does not contribute to intratumoral DHT synthesis, while served as a potent predictive marker for efficacy of chemotherapy in breast cancer independent of androgen action.