Tumour-associated cell cycle defects are often mediated by alterations in cyclin-dependent kinase (CDK) activity. Misregulated CDKs induce unscheduled proliferation as well as genomic and chromosomal instability. According to current models, mammalian CDKs are essential for driving each cell cycle phase, so therapeutic strategies that block CDK activity are unlikely to selectively target tumour cells. However, recent genetic evidence has revealed that, whereas CDK1 is required for the cell cycle, interphase CDKs are only essential for proliferation of specialized cells. Emerging evidence suggests that tumour cells may also require specific interphase CDKs for proliferation. Thus, selective CDK inhibition may provide therapeutic benefit against certain human neoplasias.

Cell cycle, CDKs and cancer: a changing paradigm

The onset of M-phase is regulated by a mechanism common to all eukaryotic cells. Entry into M-phase is determined by activation of the p34cdc2 protein kinase which requires p34cdc2 dephosphorylation and association with cyclin.

Universal control mechanism regulating onset of M-phase

Human papillomavirus type 16 (HPV-16) is a DNA tumor virus that is associated with human anogenital cancers and encodes two transforming proteins, E6 and E7. The E7 protein has been shown to bind to the retinoblastoma tumor suppressor gene product, pRB. This study shows that the E6 protein of HPV-16 is capable of binding to the cellular p53 protein. The ability of the E6 proteins from different human papillomaviruses to form complexes with p53 was assayed and found to correlate with the in vivo clinical behavior and the in vitro transforming activity of these different papillomaviruses. The wild-type p53 protein has tumor suppressor properties and has also been found in association with large T antigen and the E1B 55-kilodalton protein in cells transformed by SV40 and by adenovirus type 5, respectively, providing further evidence that the human papillomaviruses, the adenoviruses, and SV40 may effect similar cellular pathways in transformation.

http://science.sciencemag.org/content/sci/248/4951/76.full.pdf

Association of human papillomavirus types 16 and 18 E6 proteins with p53.

Cells prepare for S phase during the G1 phase of the cell cycle. Cell biological methods have provided knowledge of cycle kinetics and of substages of G1 that are determined by extracellular signals. Through the use of biochemical and molecular biological techniques to study effects of growth factors, oncogenes, and inhibitors, intracellular events during G1 that lead to DNA synthesis are rapidly being discovered. Many cells in vivo are in a quiescent state (G0), with unduplicated DNA. Cells can be activated to reenter the cycle during G1. Similarly, cells in culture can be shifted between G0 and G1. These switches in and out of G1 are the main determinants of post-embryonic cell proliferation rate and are defectively controlled in cancer cells.

https://science.sciencemag.org/content/sci/246/4930/603.full.pdf

G1 events and regulation of cell proliferation.

The retinoblastoma gene is mutated in several types of human cancer and is the best characterized of the tumour-suppressor genes. A mouse strain has been constructed in which one allele of Rb is disrupted. These heterozygous animals are not predisposed to retinoblastoma, but some display pituitary tumours arising from cells in which the wild-type Rb allele is absent. Embryos homozygous for the mutation die between days 14 and 15 of gestation, exhibiting neuronal cell death and defective erythropoiesis.

Effects of an Rb mutation in the mouse

https://www.cell.com/cell/pdf/0092-8674(89)90517-5.pdf

Phosphorylation of the retinoblastoma gene product is modulated during the cell cycle and cellular differentiation.

https://www.cell.com/cell/pdf/0092-8674(89)90415-7.pdf

Reversible tyrosine phosphorylation of cdc2: Dephosphorylation accompanies activation during entry into mitosis

In Philadelphia chromosome-positive human leukemias, which include chronic myelogenous leukemia and some acute lymphocytic leukemias, the c-abl proto-oncogene on chromosome 9 becomes fused to the bcr gene on chromosome 22, and Bcr-Abl fusion proteins are produced The Bcr sequences activate the Abl tyrosine kinase which is required for the transforming function of Bcr-Abl The Bcr sequences also enhance an F-actin-binding activity associated with c-Abl Here, we show that binding of c-Abl and Bcr-Abl proteins to actin filaments in vivo and in vitro is mediated by an evolutionarily conserved domain at the C-terminal end of c-Abl The c-Abl F-actin-binding domain contains a consensus motif found in several other actin-crosslinking proteins Mutations in the consensus motif are shown to abolish binding to F-actin Bcr-Abl proteins unable to associate with F-actin have a reduced ability to transform Rat-1 fibroblasts and to abrogate the requirement for interleukin-3 in the lymphoblastoid cell line Ba/F3 In transformed cells, Bcr-Abl induces a redistribution of F-actin into punctate, juxtanuclear aggregates The binding to actin filaments has important implications for the pathogenic and physiological functions of the Bcr-Abl and c-Abl proteins

An actin-binding function contributes to transformation by the Bcr-Abl oncoprotein of Philadelphia chromosome-positive human leukemias

The retinoblastoma gene product (RB) is a nuclear protein which has been shown to function as a tumor suppressor. It is phosphorylated from S to M phase of the cell cycle and dephosphorylated in G1. This suggests that the function of RB is regulated by its phosphorylation in the cell cycle. Ten phosphotryptic peptides are found in human RB proteins. The pattern of RB phosphorylation does not change from S to M phases of the cell cycle. Hypophosphorylated RB prepared from insect cells infected with an RB-recombinant baculovirus is used as a substrate for in vitro phosphorylation reactions. Of several protein kinases tested, only cdc2 kinase phosphorylates RB efficiently and all 10 peptides can be phosphorylated by cdc2 in vitro. Removal of cdc2 from mitotic cell extracts by immunoprecipitation causes a concomitant depletion of RB kinase activity. These results indicate that cdc2 or a kinase with similar substrate specificity is involved in the cell cycle-dependent phosphorylation of the RB protein.

Retinoblastoma cancer suppressor gene product is a substrate of the cell cycle regulator cdc2 kinase

The requirements for the oncogenic conversion of the c-abl proto-oncogene have been determined by the expression of N-terminal deleted forms and viral gag-fused forms of the c-abl proteins from a selectable retroviral vector. To activate the transforming potential of c-abl, it is necessary that (i) specific N-terminal amino acids are deleted to release the kinase from negative regulation in vivo; (ii) an N-terminal myristylation site is part of the activated kinase; (iii) the fatty-acylated, activated kinase is overproduced. The N-terminal amino acids found to be necessary for the cellular inhibition of c-abl tyrosine phosphorylation are part of a homologous region present in many non-receptor tyrosine kinases, the v-crk oncogene and phospholipase C-II. Overproduction of a deregulated and myristylated c-abl tyrosine kinase induces the transformation of NIH 3T3 cells.

/pdf/deletion-of-an-n-terminal-regulatory-domain-of-the-c-abl-45g7pmtebw.pdf

J. Y. J. Wang

Papers

Phosphorylation of the retinoblastoma gene product is modulated during the cell cycle and cellular differentiation.

Reversible tyrosine phosphorylation of cdc2: Dephosphorylation accompanies activation during entry into mitosis

An actin-binding function contributes to transformation by the Bcr-Abl oncoprotein of Philadelphia chromosome-positive human leukemias

Retinoblastoma cancer suppressor gene product is a substrate of the cell cycle regulator cdc2 kinase

Deletion of an N-terminal regulatory domain of the c-abl tyrosine kinase activates its oncogenic potential.