Double jeopardy: Ras and CDK4 co-expression in skin cancer

Abstract

The deregulation of pathways that control cellular growth and death can lead to tumourigenesis. Ras is one of the key oncogenes in these cell growth pathways, and is mutated in approximately 15% of human cancers. Activation of this gene by mutation or transcriptional upregulation can lead to uncontrolled Ras signalling and, hence, increased proliferation. However, Ras activation does not always lead to cancer, suggesting that there are cellular safety mechanisms in place that protect against deregulation; other genetic alterations are required, in addition to Ras activation, for neoplastic transformation of these cells. The bypassing of inhibition by the Rb (retinoblastoma) protein is another element of Ras tumourigenesis. Rb functions in the G1 phase of the cell cycle to prevent cell division and, hence, progression of the cell cycle requires removal of the Rb block, a process facilitated by cyclins and cyclin-dependent kinases (CDKs), such as CDK4. In a recent paper, Lazarov et al. [1xCDK4 coexpression with Ras generates malignant human epidermal tumorigenesis. Lazarov, M. et al. Nat. Med. 2002; 8: 1105–1114Crossref | PubMed | Scopus (138)See all References[1] describe the effect of Ras expression on regulators of the G1–S transition in human keratinocytes. They also discuss the effect of the combined expression of Ras and CDK4 in skin tumourigenesis.In this study, human keratinocytes were transfected with an inducible Ras vector and, after induction of Ras expression, the expression profiles of numerous G1 regulators were determined by immunoblotting. It was found that the expression of CDK4 was suppressed post-transcriptionally, whereas levels of CDK inhibitors, cyclins, and other CDKs and G1 regulators were unaffected. Furthermore, CDK4 was redistributed from the nucleus to the cytoplasm, away from its main substrates.Transduction of cells with both Ras and CDK4 overcame G1 growth arrest. Lazarov et al. then tested whether these cells could induce neoplasia similar to human epidermal tumours. CDK4–Ras-transduced cells were seeded on human dermis, and then used to regenerate human skin tissue on severe-combined-immunodeficient scid/scid mice. Tumours developed rapidly and demonstrated many features similar to human cutaneous squamous-cell carcinomas (SCCs), including hyperplasia, failure of terminal differentiation, angiogenesis and epithelial islands invading through subcutaneous fat.In the Ras–CDK4 co-expressing cells, proliferation was 20-fold greater than in normal epidermal cells, and there was increased expression of vascular endothelial growth factor and matrix-metalloproteinase-2, and decreased expression of E-Cadherin. Furthermore, telomere length was preserved for over eight weeks, and there was increased expression of the telomerase catalytic subunit, which is important for sustained cell division.Ras and CDK4 were also shown to be over-expressed in a small subset of human tumours. Further studies are required to ascertain what proportion of SCCs and other tumours have similar levels of over-expression of these two proteins, and whether other oncogenes and tumour-suppressor genes in these pathways, such as Raf and p16ink4a, can be substituted to give the same effect.This model for the development of skin cancer is not perfect because, for example, the effects were only seen in immunocompromized mice, and the tumour cells failed to display complete immortalization in culture. Despite this, cells co-expressing CDK4–Ras do give significant clues to the key mechanistic steps involved in the development of skin cancer.