Transgenic Expression of Cyclin-Dependent Kinase 4 Results in Epidermal Hyperplasia, Hypertrophy, and Severe Dermal Fibrosis

Abstract

In a previous report we have described the effects of expression of D-type cyclins in epithelial tissues of transgenic mice. To study the involvement of the Dtype cyclin partner cyclin-dependent kinase 4 (CDK4) in epithelial growth and differentiation, transgenic mice were generated carrying the CDK4 gene under the control of a keratin 5 promoter. As expected, transgenic mice showed expression of CDK4 in the epidermal basal-cell layer. Epidermal proliferation increased dramatically and basal cell hyperplasia and hypertrophy were observed. The hyperproliferative phenotype of these transgenic mice was independent of D-type cyclin expression because no overexpression of these proteins was detected. CDK4 and CDK2 kinase activities increased in transgenic animals and were associated with elevated binding of p27 Kip1 to CDK4. Expression of CDK4 in the epidermis results in an increased spinous layer compared with normal epidermis, and a mild hyperkeratosis in the cornified layer. In addition to epidermal changes, severe dermal fibrosis was observed and part of the subcutaneous adipose tissue was replaced by connective tissue. Normal cell growth and differentiation requires precise control of the mechanisms that govern the entry into, passage through, and exit from the cell cycle. Progress through the G 1 phase of the mammalian cell cycle is regulated by the ordered synthesis, assembly, and activation of distinct cyclin-dependent kinase (CDK)-cyclin holoenzymes.1,2 This process is mediated by the D-type cyclins (D1, D2, and D3), whose expression is modulated by growth-stimulatory signals. These cyclins associate with the closely related CDK4 and CDK6 kinases, resulting in their catalytic activation and substrate recognition. A key substrate for G 1 cyclin/CDK complexes is the retinoblastoma protein, pRb. Phosphorylation of pRb, a tumor suppressor gene product, has been attributed to cyclin/CDK complexes and implicated in the regulation of proliferation of keratinocytes and other cell types.

Expression of CDK4 or CDK2 in Mouse Oral Cavity Is Retained in Adult Pituitary with Distinct Effects on Tumorigenesis

“…We have previously described the phenotypical consequences of forced expression of hCDK4 under the K5 promoter (K5CDK4 mice; ref. 13). Similarly, overexpression of hCDK2 under the K5 promoter also drives the expression to stratified epithelium (our unpublished results).…”

“…6C, lane 6). We previously established that increased expression of CDK4 results in the activation of CDK2 through a mechanism involving sequestration of p21 Cip1 and p27 Kip1 in epithelial tissues (13,24), thus, we also assayed CDK2 kinase activity in pituitary lysates. Pituitary lysates from p27 À/À and K5CDK4/p27 À/À mice show strong CDK2 kinase activity compared with wild-type and K5CDK4 mice (Fig.…”

“…Previously, we reported the effects of transgenic expression of CDK4 and D-type cyclins in epithelial tissues driven by the keratin 5 (K5) promoter (13)(14)(15). Expression of reporter genes under the K5 promoter readily show expression in the basal cell layer of the epidermis, thymic reticulum, digestive tract, and the oral mucosa (16).…”

Enhanced malignant tumorigenesis in Cdk4 transgenic mice

“…Therefore, we have used the mouse skin two-stage carcinogenesis model as an experimental approach to determine how CDK4 overexpression influences tumor development. We used the previously described K5-CDK4 transgenic lines that showed 2.5-fold (K5-CDK4-2305) and 7.5-fold (K5-CDK4-2303) increased expression of CDK4 in the basal cell layer of epidermis (Miliani de Marval et al, 2001). The dorsal skin of K5-CDK4 transgenic mice and wildtype littermates were topically treated with a subcarcinogenic dose of the genotoxic carcinogen DMBA and later promoted with the phorbol ester TPA.…”

“…We have previously reported that forced expression of CDK4 in mouse epidermis led to phenotypic abnormalities, such as hyperplasia and hypertrophy and severe dermal fibrosis (Miliani de Marval et al, 2001). Here, we have investigated the susceptibility to chemically induced skin tumor formation.…”