Elucidating roles for caveolin-1 in transformation, tumorigenesis, and metastasis: Molecular, cellular and animal model approaches

Abstract

Caveolin-1 (Cav-1) is the principal marker protein for caveolae, 50--100 nm flask-shaped membrane invaginations, and was first identified as a v-Src substrate in transformed fibroblasts. Subsequent research yielded additional evidence that caveolin-1 has roles in proliferation, survival, and transformation. Here, I explore roles for caveolin-1 with regard to tumorigenesis and metastasis in vivo using both cellular and animal model approaches. First, I interbred INK4a (-/-) mice with Cav-1 (-/-) mice to establish immortalized mouse embryonic fibroblasts (MEFs) that either express or lack caveolin-1. Combined loss of INK4a and Cav-1 imparts a significant proliferative advantage, as demonstrated by increased growth rates, altered cell cycle fractions, and altered molecular expression of cell cycle-related proteins (cyclin D1, PCNA, p21Cap1, and phospho-ERK-1/2). Additionally, I evaluated the effects of caveolin-1 on tumorigenesis by transforming INK4a (-/-) and INK4a (-/-)/caveolin-1 (-/-) MEF cell lines with activated oncogenes (H-RasG12V and v-Src). Injections of these cells into the flanks of nude mice revealed that transformed INK4a (-/-)/caveolin-1 (-/-) MEFs form dramatically larger tumors. Analysis of INK4a (-/-)/caveolin-1 (-/-) mice further revealed an aberrant morphological phenotype in the mammary gland (increased side-branching, hyperplasia, and fibrosis). Next, I examined roles for caveolin-1 in mammary tumorigenesis by interbreeding caveolin-1 (-/-) mice to MMTV-Polyomavirus middle T antigen (PyMT) transgenic mice to promote mammary adenocarcinoma development. At 3 and 4 weeks of age, complete loss of caveolin-1 in PyMT mice resulted in 3- to 5-fold increases in both the frequency and total area occupied by dysplastic foci (areas of transformed epithelium). The acceleration of dysplastic foci formation in PyMT/caveolin-1 (-/-) mice was substantiated by histo-pathological findings (increased histological grade) and an altered expression profile of the mammary proto-oncogene cyclin D1. In terms of tumor and metastasis development, loss of caveolin-1 decreases tumor latency, increases tumor multiplicity and tumor burden in female and male PyMT transgenic mice. Genetic ablation of caveolin-1 is also associated with cyclin D1 overexpression, ERK-1/2 hyperactivation, and increased phospho-Rb levels in PyMT/Cav-1 (-/-) tumors. Finally, PyMT/Cav-1 (-/-) mice demonstrate increased pulmonary metastatic disease, and caveolin-1 overexpression in a mammary tumor-derived cell line inhibits invasion, matrix metalloproteinase activity, and experimental metastasis.