Absence of Caveolin-1 Sensitizes Mouse Skin to Carcinogen-Induced Epidermal Hyperplasia and Tumor Formation

Abstract

Caveolin-1 is the principal protein component of caveolae membrane domains, which are located at the cell surface in most cell types. Evidence has accumulated suggesting that caveolin-1 may function as a suppressor of cell transformation in cultured cells. The human CAV-1 gene is located at a putative tumor suppressor locus (7q31.1/D7S522) and a known fragile site (FRA7G) that is deleted in a variety of epithelial-derived tumors. Mechanistically, caveolin-1 is known to function as a negative regulator of the Ras-p42/44 MAP kinase cascade and as a transcriptional repressor of cyclin D1, possibly explaining its transformation suppressor activity in cultured cells. However, it remains unknown whether caveolin-1 functions as a tumor suppressor gene in vivo. Here, we examine the tumor suppressor function of caveolin-1 using Cav-1 (-/-) null mice as a model system. Cav-1 null mice and their wild-type counterparts were subjected to carcinogen-induced skin tumorigenesis, using 7,12-dimethylbenzanthracene (DMBA). Mice were monitored weekly for the development of tumors. We demonstrate that Cav-1 null mice are dramatically more susceptible to carcinogen-induced tumorigenesis, as they develop skin tumors at an increased rate. After 16 weeks of DMBA-treatment, Cav-1 null mice showed a 10-fold increase in tumor incidence, a 15-fold increase in tumor number per mouse (multiplicity), and a 35-fold increase in tumor area per mouse, as compared with wild-type littermate mice. Moreover, before the development of tumors, DMBA-treatment induced severe epidermal hyperplasia in Cav-1 null mice. Both the basal cell layer and the suprabasal cell layers were expanded in treated Cav-1 null mice, as evidenced by immunostaining with cell-type specific differentiation markers (keratin-10 and keratin-14). In addition, cyclin D1 and phospho-ERK1/2 levels were up-regulated during epidermal hyperplasia, suggesting a possible mechanism for the increased susceptibility of Cav-1 null mice to tumorigenesis. However, the skin of untreated Cav-1 null mice appeared normal, without any evidence of epidermal hyperplasia, despite the fact that Cav-1 null keratinocytes failed to express caveolin-1 and showed a complete ablation of caveolae formation. Thus, Cav-1 null mice require an appropriate oncogenic stimulus, such as DMBA treatment, to reveal their increased susceptibility toward epidermal hyperplasia and skin tumor formation. Our results provide the first genetic evidence that caveolin-1 indeed functions as a tumor suppressor gene in vivo.

The Caveolin-1 Connection to Cell Death and Survival

“…Also, Caveolin-1 expression is reduced in several human tumors, including lung [29], mammary [30], colon [15,31] and ovarian carcinomas [32], as well as sarcomas [32] and re-expression of Caveolin-1 often (but not always) results in reversal of characteristics associated with the transformed phenotype. Consistent with these results, lung hyperplasia and predisposition to mammary, as well as carcinogen-induced skin hyperplasia and tumor formation are observed in Caveolin-1 knock-out mice [4][5][33][34][35]. These results indicate that Caveolin-1 displays properties and characteristics of a tumor suppressor molecule in a variety of cellular settings.…”

“…In NIH3T3 fibroblasts, down-regulation of Caveolin-1 using specific siRNA is sufficient to hyperactivate the MAPK/ERK pathway and induce cell transformation [78]. Also, in Caveolin-1 knock-out mice, signaling via the MAPK/ERK pathway is increased and associated with increased sensitivity to topically applied carcinogens, cardiac hypertrophy and neointimal hyperplasia [33,[87][88]. Moreover, in human head and neck squamous cell cancer, Caveolin-1 inhibits the ERK pathway and cell growth [89], and in human laryngeal carcinoma cell lines, Caveolin-1 interaction with the EGFR is associated with reduced MAPK/ERK phosphorylation and increased apoptotic cell death [90].…”

Lipid Rafts and Caveolae in Signaling by Growth Factor Receptors

“…However, caveolin-1 has also been found to be downregulated in tumours themselves, and several independent studies have shown that the re-expression of caveolin-1 in transformed cells inhibits features of the transformed phenotype, such as anchorage-independent growth. Caveolin-1 knockout mice also support the hypothesis, as they display hyperproliferative disorders and hyperactivation of the MAPK cascade, and are at greater risk from carcinogeninduced tumorigenesis [188]. Although these mice do not spontaneously form tumours, knockouts of important cell cycle control agents, such as the cyclin-dependent kinase inhibitor p21, also do not form tumours [206].…”

“…In addition, Capozza et al [188] found that caveolin-1 knockout mice were more susceptible to skin carcinogenesis induced by a known carcinogen, 7,12-dimethylbenzanthracene (DMBA).…”

Ethanol exposure induces the cancer-associated fibroblast phenotype and lethal tumor metabolism

“…54 In addition, Cav-1-deficient mice show the same phenotype, with increased ketone body production systemically 48 and increased numbers of stem cells in the skin, gut, mammary gland and brain. [55][56][57][58][59][60] In the brain, astrocytes produce mitochondrial fuels (L-lactate, ketone bodies and glutamine), which are then consumed by oxidative mitochondrial metabolism in neurons. 61 In addition, Figure 13.…”