**Cyclin-Dependent Kinases (Cdk) as Targets for Cancer Therapy and Imaging**

Franziska Graf1, Frank Wuest2 and Jens Pietzsch*<sup>1</sup> 1Institute of Radiopharmacy, Helmholtz-Zentrum Dresden-Rossendorf 2Department of Oncology, University of Alberta 1Germany 2Canada* 

### **1. Introduction**

264 Advances in Cancer Therapy

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Aberration in proliferation and consequently in cell cycle control is a common aspect in carcinogenesis. As master cell cycle regulating proteins in all eukaryotic cells the Cyclindependent kinases (Cdk) were identified by Leland Hartwell, Paul Nurse, and Timothy Hunt in the 1970s and 1980s. Chronological activation of respective Cdk according to respective cell cycle phase G1, S, G2 or M is mediated through association with a regulatory Cyclin subunit, phosphorylation of Cdk and binding of endogenous activators and inhibitors, as well as subcellular localization (Shapiro, 2006).

In human cells four Cdk are essential components of the cell cycle machinery with key functions also in human cancer cells: Cdk1, Cdk2, Cdk4, and Cdk6 (Fig. 1) (Malumbres & Barbacid, 2009). First, Cyclin D-dependent kinases Cdk4 and Cdk6 are activated in human cell cycle in response to mitogenic signals to initiate G1 phase progression and prepare DNA duplication in S phase (Malumbres & Barbacid, 2005). Cdk4-Cyclin D or Cdk6-Cyclin D and later also Cdk2-Cyclin E complexes sequentially phosphorylate retinoblastoma proteins (Rb) on different serine and threonine residues. Resulting Rb protein inactivation is required for the transcriptional activation of genes in G1/S phase (Harbour & Dean, 2000). In G1 phase endogenous inhibitors of monomeric Cdk4 and Cdk6 like INK4 and inhibitors of Cdk2/Cdk4/Cdk6-Cyclin complexes like Cip and Kip proteins exert important influence on Cdk catalytic activity (Blain, 2008; Sherr & Roberts, 1999). Once the cell irreversibly passed restriction point R at the end of G1 phase, Cdk2-Cyclin A complex is formed, facilitating orderly execution of S phase events like DNA replication and centrosome cycle through phosphorylation of various proteins (Malumbres & Barbacid, 2005). Activation of Cdk1 by Cyclin A is required for DNA damage checkpoint control, later Cdk1-Cyclin B for G2/ M phase transition and initiation of mitosis, especially chromosome condensation and microtubule dynamics (Malumbres & Barbacid, 2009). Therefore, active Cdk1-Cyclin complexes mediate phosphorylation of about 70 substrates, e. g., minichromosome maintenance (MCM), p53, lamins, and dyneins.

Initiation of cell re-entrance from G0 to G1 phase and early inactivation of Rb is assigned to Cdk3-Cyclin C (Ren & Rollins, 2004). Another Cyclin-dependent kinase, Cdk5, is involved in the regulation of neuronal function (Cruz & Tsai, 2004).

Cyclin-Dependent Kinases (Cdk) as Targets for Cancer Therapy and Imaging 267

human cancer including mutations and amplification of Cdk and positive regulatory Cyclin subunits, mutations or silencing of substrates (Rb) and endogenous Cdk inhibitors (INK4, Cip/ Kip proteins) lead to a hyperactivation of Cdk regulatory pathways (Table 1) (Deshpande et al., 2005; Malumbres & Barbacid, 2005). In consequence, critical cell cycle checkpoints are ignored resulting in abnormal cell proliferation and tumor progression. Although tumor cells exhibit rather infrequent mutations of *cdk* genes with the exception of G1 kinases Cdk4 and Cdk6 amplification, overexpression or hyperactivation of basic cell cycle regulators is a general feature of human tumors (Easton et al., 1998; Kim et al., 1999; Sotillo et al., 2001; Wolfel et al., 1995). Cdk hyperactivation is often affected by mutations of Cdk regulatory subunits. In consequence, overexpression of Cyclin A, Cyclin B, Cyclin E, and Cyclin D were reported in a wide spectrum of tumors, like leukemia or carcinomas and were associated with poor prognosis (Johansson & Persson, 2008; Ko et al., 2009). A common alteration in human tumors was demonstrated for tumor suppressor gene *rb*. Altered Rb proteins, momentous for transcriptional control, are insensitive to Cdk regulation and accelerate cell cycle progression (Nevins, 2001). Finally, abnormal regulation or inactivation of Cdk endogenous inhibitors p15*INK4B*, p16*INK4A* and p27*Kip1* was described in numerous human tumors leading to enhanced Cdk activity (Ruas & Peters, 1998; Tsihlias et al., 1999).

alteration occurrence in cancer type

hepatoma, carcinoma, leukemia

hepatoma, carcinoma, leukemia

melanoma, insulinoma, sarcoma carcinoma, glioma, sarcoma, …

carcinoma, glioma, sarcoma, …

retinoblastoma, osteosarcoma, carcinoma

carcinoma, melanoma, neuroblastoma

carcinoma, lymphoma, melanoma, …

glioblastoma, carcinoma, melanoma, …

carcinoma, leukemia, … lymphoma, adenoma

carcinoma, lymphoma

neuroblastoma lymphoma, leukemia

brain tumors

melanoma

**Cdk1** upregulation/

**Cdk2** upregulation/

**Cyclins (A, B, E, D)** amplification/

**Rb** mutation

**p15INK4B, p16INK4A** 

**Cdk4** point mutation R24C

**Cdk6** analogous R24C mutation

overexpression

overexpression

amplification/ overexpression

amplification/ overexpression

overexpression

sequestration

deletion

(Graf et al., 2010; Ortega et al., 2002; Weinberg, 2007)

**p27Kip1** decreased transcription

chromosomal translocations

chromosomal translocations

promoter methylation

promoter methylation

increased degradation

Table 1. Genetic and epigenetic alterations of Cdk pathway components in human cancer

The second group of proteins belonging to Cdk family Cdk7 to Cdk13 are involved in the activation of cell cycle kinases and transcriptional regulation (Akoulitchev et al., 2000; Chen et al., 2006; Chen et al., 2007; Garriga & Grana, 2004; Hu et al., 2007; Kasten & Giordano, 2001). Cdk7 in complex with Cyclin H is given a special importance since it is the only Cdk activating kinase (CAK) in mammalian cells phosphorylating a threonine residue in the conserved T-loop of Cdk (Lolli & Johnson, 2005).

Fig. 1. Overview of human cell cycle activation and transcriptional regulation through Cdk-Cyclin complexes
