Caveolin-1 expression is elevated in claudin-low mammary tumor cells

Abstract

Background: Caveolin-1 is a scaffolding protein found in plasma membrane invaginations known as caveolae. Caveolin-1 can regulate a number of intracellular processes such as signal transduction, cholesterol metabolism and vesicular transport. With respect to breast cancer caveolin-1 has been observed in both tumor cells and stromal cells surrounding tumors however most of the recent research has focused on how the loss of caveolin-1 in the stromal cells surrounding the tumor alters the tumor microenvironment.

Methods: Caveolin-1 expression was evaluated in (1) mammary tumors induced by the transgenic overexpression of the type I insulin-like growth factor receptor (IGF-IR), (2) mammary tumors that became independent of IGF-IR signalling and acquired a claudin-low genotype, (3) two murine mammary epithelial tumor cell lines and (4) two murine mammary claudin-low tumor cell lines.

Results: We found that mammary tumors induced by IGF-IR overexpression expressed low levels of caveolin-1 while mammary tumors that became independent of IGF-IR signalling expressed considerably higher levels of caveolin-1. Interestingly, pockets of caveolin-1 positive cells could be observed in some of the IGF-IR-induced mammary tumors and these caveolin-1 positive cells were associated with tumor cells that expressed basal cytokeratins (cytokeratins 5 and 14). This caveolin-1 expression pattern was maintained in the murine mammary tumor cell lines in that the epithelial mammary tumor cell lines expressed little or no caveolin-1 while the claudin-low cell lines expressed caveolin-1.

Conclusions: Our model indicates that mammary tumor cells with epithelial characteristics lack caveolin-1 while mesenchymal tumor cells express caveolin-1 suggesting that caveolin-1 may serve as a marker of mammary tumor cells with mesenchymal characteristics such as claudin-low breast tumors.

Figure 1

Western blot of Cav-1 protein in 4 independent wild type (WT) mammary tissue sample, 4 independent primary mammary tumor (PMT) tissue samples induced by transgenic overexpression of IGF-IR, and 4 independent recurrent spindle tumor (RST) tissue samples which developed after the downregulation of the IGF-IR transgene. β-actin served as a loading control

Figure 2

Cav-1 immunohistochemistry in PMT tissue (A, B), RST tissue (C, D) and two different lung metastases (E, F). The arrows in (A) indicate Cav-1 positive cells lining blood vessels in normal mammary tissue while T marks the mammary tumor. The arrowhead in (B) highlights Cav-1 positive tumor cells within a PMT sample. In panels (E, F) LM identifies the lung metastasis while NL identifies normal lung tissue. Scale bars, 100 μm

Figure 3

Immunohistochemistry for Cav-1 (A, D), cytokeratin 5 (B, E), and cytokeratin 14 (C, F) on serial sections from two different PMT samples. Scale bars, 100 μm

Figure 4

(A) Western blot of Cav-1 protein in two epithelial murine mammary tumor cell lines (4T1 and RJ345) and two murine mammary claudin-low cell lines (RJ348 and RM11A). β-actin served as a loading control. Immunofluorescence of Cav-1 (green) in RJ345 (B), 4T1 (C), RJ348 (D) and RM11A (E). Blue fluorescence indicates cell nuclei staining positive for DAPI; scale bars, 50 μm