Dysregulation of CDK8 and Cyclin C in tumorigenesis

Abstract

Appropriately controlled gene expression is fundamental for normal growth and survival of all living organisms. In eukaryotes, the transcription of protein-coding mRNAs is dependent on RNA polymerase II (Pol II). The multi-subunit transcription cofactor Mediator complex is proposed to regulate most, if not all, of the Pol II-dependent transcription. Here we focus our discussion on two subunits of the Mediator complex, cyclin-dependent kinase 8 (CDK8) and its regulatory partner Cyclin C (CycC), because they are either mutated or amplified in a variety of human cancers. CDK8 functions as an oncoprotein in melanoma and colorectal cancers, thus there are considerable interests in developing drugs specifically targeting the CDK8 kinase activity. However, to evaluate the feasibility of targeting CDK8 for cancer therapy and to understand how their dysregulation contributes to tumorigenesis, it is essential to elucidate the in vivo function and regulation of CDK8-CycC, which are still poorly understood in multi-cellular organisms. We summarize the evidence linking their dysregulation to various cancers and present our bioinformatics and computational analyses on the structure and evolution of CDK8. We also discuss the implications of these observations in tumorigenesis. Because most of the Mediator subunits, including CDK8 and CycC, are highly conserved during eukaryotic evolution, we expect that investigations using model organisms such as Drosophila will provide important insights into the function and regulation of CDK8 and CycC in different cellular and developmental contexts.

Fig. 1.

A model for Mediator complexes in regulating Pol II-dependent gene expression. A: Mediator complex serves as a molecular bridge between specific transcription factors (TFs) and the general transcription machinery, which is composed of the general transcription factors (GTFs) and RNA Polymerase II (RNA Pol II). B: CDK8 module represents a major difference between the small and the large Mediator complexes.

Fig. 2.

Context-specific functions of CDK8-CycC. Very little is known about the upstream regulators of CDK8-CycC. About 10 downstream effectors of CDK8 have been identified in multi-cellular organisms, and the transcriptional effect of their phosphorylation by CDK8 varies. It is likely that CDK8 has additional substrates that remain to be identified. Characterization of both upstream regulators and downstream effectors of CDK8-CycC is the key to understand how dysregulation of CDK8 contributes to tumorigenesis.

Fig. 3.

The sequence analyses of CDK8. A: the amino acid sequence of human CDK8. The amino acids of human CDK8 predicted to be involved in ATP binding are represented in red and highlighted in yellow; the amino acids of CDK8 predicted to have interactions with cyclin are in pink and highlighted in green; the predicted CDK8 activation loop is highlighted in blue. B: the sequence alignment of CDK8 in several representative species. The red rectangles highlight the SMSACRE motif and the activation loop.

Fig. 4.

The sequence alignment of the predicted activation loop of CDKs. A: the sequence alignment of the predicted CDK8 activation loops from the representative species. B: the sequence alignment of CDK8 and CDK7 activation loops from the representative species. C: the sequence alignment of human CDK activation loops.

Fig. 5.

The phylogenetic analyses of CDK8. A: the phylogenetic tree of CDK8 from yeast to humans. B: the phylogenetic tree of CDK8 and CDK19 in the representative species.

Fig. 6.

The hypothetical structure and the predicted activation loop of CDK8. A: the hypothetical structure of human CDK8 with the lowest energy from the molecular dynamics simulations. The molecular dynamics simulations revealed the heating phase to show a gradual increase in temperature with a few abrupt elevations; temperature at the equilibration phase was approximately 325 K, as expected. The energy profile (kinetic energy, potential energy, and total energy) was calculated. The kinetic energy was stable, as predicted by the stability of the temperature plot, which is directly proportional to the kinetic energy. The total and potential energy plots showed a gradual decrease, implying that the system had achieved an energy state more stable than the starting structure. The potential energy plot, which showed the structure with the lowest energy, was of greatest interest; its coordinates will be provided upon request. The backbone root-mean-square deviation (RMSD) plot, generated by using the lowest-energy structure as the reference, showed a decrease in RMSD during the simulation trajectory, indicating the equilibration steps are gradually approaching to the structure with the lowest energy. B: the models of human CDK8 activation loop. Cyan color represents the activation loop from the lowest potential energy structure (−7313 kcal/mol) during 50-ns simulation. Orange color represents the activation loop from the highest energy structure (−6677 kcal/mol) during 50-ns simulation. All the molecular dynamics simulation input, output and trajectory files are available upon request.