Preface Cancer is characterized by uncontrolled proliferation resulting from aberrant activity of various cell cycle proteins; therefore, cell cycle regulators are considered attractive targets in cancer therapy. Intriguingly, animal models demonstrated that some of these proteins are not essential for proliferation of non-transformed cells and development of most tissues. In contrast, many cancers are uniquely dependent on these proteins and are hence selectively sensitive to their inhibition. After decades of research on the physiological functions of cell cycle proteins and their relevance for cancer, this knowledge recently translated into the first approved cancer therapeutic targeting of a direct regulator of the cell cycle. Here, we review the role of cell cycle proteins in cancer, the rationale for targeting them in cancer treatment and results of clinical trials, as well as future therapeutic potential of various inhibitors. We focus only on proteins that directly regulate cell cycle progression. Cyclin-dependent kinases with transcriptional functions, as well as PARP inhibitors, which are highly successful in targeting BRCA1/BRCA2-mutant tumours, are not covered by this review.

Mice lacking cyclin D1 have been generated by gene targeting in embryonic stem cells. Cyclin D1-deficient animals develop to term but show reduced body size, reduced viability, and symptoms of neurological impairment. Their retinas display a striking reduction in cell number due to proliferative failure during embryonic development. In situ hybridization studies of normal mouse embryos revealed an extremely high level of cyclin D1 in the retina, suggesting a special dependence of this tissue on cyclin D1. In adult mutant females, the breast epithelial compartment fails to undergo the massive proliferative changes associated with pregnancy despite normal levels of ovarian steroid hormones. Thus, steroid-induced proliferation of mammary epithelium during pregnancy may be driven through cyclin D1.

Breast cancer is the most common malignancy among women. Most of these cancers overexpress cyclin D1, a component of the core cell-cycle machinery. We previously generated mice lacking cyclin D1 using gene targeting. Here we report that these cyclin D1-deficient mice are resistant to breast cancers induced by the neu and ras oncogenes. However, animals lacking cyclin D1 remain fully sensitive to other oncogenic pathways of the mammary epithelium, such as those driven by c-myc or Wnt-1. Our analyses revealed that, in mammary epithelial cells, the Neu-Ras pathway is connected to the cell-cycle machinery by cyclin D1, explaining the absolute dependency on cyclin D1 for malignant transformation in this tissue. Our results suggest that an anti-cyclin D1 therapy might be highly specific in treating human breast cancers with activated Neu-Ras pathways.