Stromal cell expression of caveolin-1 predicts outcome in breast cancer

Abstract

Caveolin-1 has been linked to tumor progression and clinical outcome in breast cancer, but a clear resolution of its role as a prognostic marker is lacking. We assessed caveolin-1 levels in normal breast tissue and two breast cancer cohorts for which outcome data were available. We found that caveolin-1 was not expressed in normal breast luminal epithelium but was present in the epithelial compartment of some tumors. We found no association between caveolin-1 expression in the epithelial compartment and clinical outcome. However, high levels of caveolin-1 in the stromal tissue surrounding the tumor, rather than within tumor cells, associated strongly with reduced metastasis and improved survival (P < 0.0001). The onset of mammary tumors driven by Her2/neu overexpression was accelerated in mice lacking caveolin-1, thereby supporting the observation that the presence of caveolin-1 in the tumor microenvironment modulates tumor development. These studies suggest that stromal caveolin-1 expression may be a potential therapeutic target and a valuable prognostic indicator of breast cancer progression.

Figure 1

Caveolin-1 expression in normal human breast tissue. Normal breast tissue samples were immunostained with rabbit polyclonal (A–C) or mouse monoclonal (D–F) antibodies directed against caveolin-1 (A, C, D, F) or a non-specific protein (isotype) (B and E). Caveolin-1 expression in the stromal tissue surrounding the ductal epithelium and especially in the myoepithelial layer underlining the ducts is clearly visible in panels A and D. Specific reactivity against caveolin-1 in adipose tissue (arrowheads) and vascular endothelial cells (arrows) is shown in panels C and F. Scale bar = 50 μm.

Figure 2

Caveolin-1 expression in human breast cancer. A: Representative images of primary human breast tumors with tumor cells showing negative, weak or strong expression of caveolin-1 in tumor epithelium. Scale bar = 10 μm. B: Metastasis-free survival by caveolin-1 status (positive or negative) in tumor epithelium after adjusting for other prognostic factors. The samples scoring 1 or 2 for expression of caveolin-1 are pooled and shown as positive samples. C: Representative tumors with stroma staining positive or negative for caveolin-1. Stroma indicated by arrows. Scale bar = 40 μm.

Figure 3

Stromal expression of caveolin-1 correlates with improved survival. Kaplan-Meier plots for caveolin-1 expression in the tumor epithelium (A) or stroma (B) in a consecutive series of breast cancer patients. The relationship between overall survival and caveolin-1 expression is shown over time (years following diagnosis).

Figure 4

Loss of stromal expression accelerates tumor onset in the MMTV-neu mouse. Kaplan-Meier plots for tumor onset by caveolin-1 status in mice transgenic for the MMTV-neu gene. Mice were monitored for tumor onset (age at which tumor was first palpable) and tumor growth, and culled when the primary tumor reached 1500 mm³. Analysis of the trend using the log-rank test gave a P value of 0.005.

Figure 5

Caveolin-1 expression in the normal mammary gland of MMTV-neu transgenic mice and in MMTV-neu tumors. A: Caveolin-1 expression in the mammary gland of the MMTV-neu mouse before tumor onset. B: isotype matched antibody staining of the mammary gland. C, D: H&E staining to show the morphology of the tumors in caveolin-1 wild-type (C) and null (D) mice. E, G: caveolin-1 expression in tumors from caveolin-1 wild-type mouse (E) and caveolin-1 null mouse (G). The respective isotype control staining is shown in panels (F) and (H). Scale bar = 100 μm.