Phenotypic and mechanistic elucidation for the role of adipocyte in breast cancer development

Abstract

The unique microenvironment of breast cancer consists of various types of adipocytes and plays a critical role in tumorigenesis and metastasis. But the effects of different types of adipocytes on breast cancer behavior are undetermined due to their different features, rendering the addition of adipocytes during post-operation lipofilling controversial. Additionally, the underlying mechanisms are not fully elucidated. This study aims to understand the role of adipocytes in breast cancer invasiveness at cellular and molecular levels.

In this study, various types of adipocytes including preadipocytes, adipocyte-like adipocytes (AL), brown adipocytes (BAT), white adipocytes (WAT), and adipose-derived stem cell (ASC) were co-cultured with breast cancer cells in direct and indirect manners. The invasiveness of breast cancer cells was assessed regarding their proliferation, migration, and invasion. The alteration of co-cultured cells as well as the underlying mechanisms at morphological, organellar and molecular levels at serial time points were examined. Furthermore, the functional role of adipocytes on breast cancer behavior was explored in vitro and in a xenograft breast tumor mouse model. Tumor growth, proliferation rate and overall survival of mice were compared among groups.

The in vitro study suggested that luminal A breast cancer cell line (MCF7) co-cultured with all types of adipocytes showed a higher cancer invasiveness than MCF7 alone, among which MCF7-BAT co-cultivation was the highest, especially in direct co-cultivation. Furthermore, the tumorigenic potential of the co-cultivated cells was observed in the xenograft tumor mouse model, where the mixture of BAT and MCF7 showed increased tumor growth and decreased animal survival, but adipose-derive stem cell (ASC) showed no significant hazardous effects on both MCF7 and triple-negative breast cancer (MDA-MB-231) growth. Morphologically, MCF7-adipocyte cell-cell fusion was observed in all types of co-cultivation, and the fused cell proportion was significantly higher in MCF7-BAT than in other co-cultures. Mechanistically, the transcriptional levels of empirical cell fusion, adipogenesis, mitochondrial dynamics related molecules were substantially higher in MCF7-BAT fusion hybrid cells, particularly IRX3. In addition, IRX3 knock down significantly ameliorated the cancer invasiveness via modulated mitochondrial functions.

In conclusion, breast cancer microenvironment was significantly educated by various types of adipocytes, which exerted heterogeneous effects on breast cancer behavior. Among all types of adipocytes, brown adipocytes could facilitate breast cancer invasiveness by spontaneously forming fusion hybrid cells with breast cancer cells. Upregulated adipogenesis was the critical molecular mechanism that mediated cancer behavior of fusion hybrid cells through mitochondrial functional regulation. The altered molecular, organellar, and cellular mechanisms potentialized the development of new targeted therapy and cell therapy for breast cancer adjuvant therapy, as well as new surrogate biomarkers for precisely predicting long-term outcomes.