Disease relevance of CDK6

• Androgen-sensitive LNCaP prostate cancer cells engineered to stably overexpress CDK6 display increased expression of the prostate-specific antigen and enhanced growth in the presence of dihydrotestosterone [1].
• Cyclin from herpesvirus saimiri (Vcyclin) preferentially forms complexes with cyclin-dependent kinase 6 (CDK6) from primate host cells [2].
• The D-type cyclin-dependent kinases, CDK4 and CDK6, have been strongly implicated in the control of G1 progression and the phosphorylation of the retinoblastoma protein, pRb [3].
• These neuroblastoma cell lines express both CDK4 and CDK6 mRNA and protein, but only significant CDK6 protein kinase activity was detected in this study [4].
• Sixty-four percent of the seminomas and 23% of the non-seminomas had decreased expression of CDK6 by a mean factor of 5 (P = 0.009) [5].

High impact information on CDK6

• The point in G1 at which cells irrevocably commit to DNA synthesis is controlled by protein complexes consisting of cyclin-dependent kinases (CDK4 or CDK6) and cyclins (D1, D2 or D3) [6].
• The crystal structure of the cyclin D-dependent kinase Cdk6 bound to the p19 INK4d protein has been determined at 1.9 A resolution [7].
• Here we present the 2.9-A crystal structure of the inactive ternary complex between Cdk6, the INK4 inhibitor p18(INK4c), and a D-type viral cyclin [8].
• Recombinant p18 inhibits the kinase activity of cyclin D-CDK6 [9].
• Activation of p18 by IL-6 was rapid, concomitant with marked enhancement of its association with CDK6 and cell cycle arrest [10].

Chemical compound and disease context of CDK6

• Indole-3-carbinol (I3C), a compound naturally occurring in Brassica vegetables, can induce a G(1) cell cycle arrest of human MCF-7 breast cancer cells that is accompanied by the selective inhibition of cyclin-dependent kinase 6 (CDK6) expression [11].
• The I3C-mediated cell cycle arrest and repression of CDK6 production were also observed in estrogen receptor-deficient MDA-MB-231 human breast cancer cells, which demonstrates that this indole can suppress the growth of mammary tumor cells independent of estrogen receptor signaling [12].
• An activated complex of human CDK6 with a viral cyclin from herpesvirus saimiri was purified to homogeneity and crystallized using polyethylene glycol 3350 as precipitant [13].
• Here, we show that p21Cip1 is associated with K cyclin both in overexpression models and in primary effusion lymphoma cells and is a substrate of the K cyclin/cdk6 complex, resulting in phosphorylation of p21Cip1 on serine 130 [14].
• A new indole-3-carbinol tetrameric derivative inhibits cyclin-dependent kinase 6 expression, and induces G1 cell cycle arrest in both estrogen-dependent and estrogen-independent breast cancer cell lines [15].

Biological context of CDK6

• CDK6 is present in the chromatin structure of these cells in association with the AR and the promoter region of the prostate-specific antigen gene [1].
• The D-type cyclins and their major kinase partners CDK4 and CDK6 regulate G0-G1-S progression by contributing to the phosphorylation and inactivation of the retinoblastoma gene product, pRB [16].
• The regulation of the D-type cyclin-dependent kinase (CDK4 and CDK6) activity appears to be the key step in the progression of eukaryotic cells through the G1 cell cycle phase [17].
• Our findings suggest that during the cell division cycle the members of the p16(INK4) protein family and cyclin Ds compete for binding to CDK4/CDK6 and that their quantitative ratio is essential for G1 --> S transition [17].
• In cells, expression of CDK6 or to a lesser degree of a nonphosphorylatable CDK6(T177A) together with KSHV-cyclin induced apoptosis, indicating that CDK6 activation by KSHV-cyclin can proceed in the absence of phosphorylation by CAK in vivo [18].

Anatomical context of CDK6

• To determine whether mutation of the CDK6 gene, or some other mechanism, is responsible for the CDK6 kinase activity in these cell lines, several complementary analyses were performed [4].
• All tested cases of T-cell lymphoblastic lymphoma/leukemia (T-LBL/ALL) showed strong CDK6 expression in the majority (up to 100%) of neoplastic lymphoid cells [19].
• In this study we have investigated CDK6 expression in normal and neoplastic lymphoid tissues using immunohistochemistry and flow cytometry [19].
• Finally, kinase activity of CDK2 and CDK4 decreases as C2C12 cells differentiate, whereas the CDK6 kinase activity is low in both proliferating myoblasts and differentiated myotubes [20].
• Retinoblastoma protein-overexpressing HL60 cells resistant to 1,25-dihydroxyvitamin D3 display increased CDK2 and CDK6 activity and shortened G1 phase [21].

Associations of CDK6 with chemical compounds

• Fisetin binds to the active form of CDK6, forming hydrogen bonds with the side chains of residues in the binding pocket that undergo large conformational changes during CDK activation by cyclin binding [22].
• We describe here the crystal structure of human CDK6 in complex with a viral cyclin and a flavonol inhibitor, fisetin [22].
• Expression of CDK6 and beta-actin was not affected by LY-294002 [23].
• Substitution of aspartic acid 92 by alanine instead increases the binding of the peptide to cdk4 and cdk6 and the kinase inhibitory activity [24].
• Substitution of two valine residues corresponding to amino acids 95 and 96 (V95A and V96A) of the p16 peptide reduces the binding to cdk4 and cdk6 and increases its IC0.5 for kinase inhibition approximately threefold when linked to the Antennapedia homeodomain carrier sequence [24].

Physical interactions of CDK6

• A bona fide CDK6 mutation that disrupts p16INK4a binding and prevents inhibition of CDK6 protein kinase activity was identified in 1 of 17 neuroblastoma cell lines [4].
• The present study indicates that, in human prostate cancer cells, CDK6 can also bind to the androgen receptor (AR) and stimulate its transcriptional activity in the presence of dihydrotestosterone [1].
• This mutation abrogates the ability of p18 to interact with CDK6 and renders p18 deficient in suppressing cell growth in a colony formation assay [25].
• Moreover, only cyclin D3/cdk6 complexes were functional as observed in both naive and GC/M B cells stimulated in the presence of IL-4 [26].
• These data suggest that IL-4 controls B cell proliferation by action during at least two steps of the regulation of the cell cycle, cyclin D3/cdk6 complex regulation and p27 inhibitor expression [27].

Enzymatic interactions of CDK6

• The cyclin encoded by Kaposi's sarcoma-associated herpesvirus stimulates cdk6 to phosphorylate the retinoblastoma protein and histone H1 [28].
• Viral cyclin/cdk6 complexes interact with and phosphorylate human Orc1, a component of the origin recognition complex (ORC) that functions in DNA replication [29].

Regulatory relationships of CDK6

• This effect of CDK6 does not require its kinase activity and is inhibited by cyclin D1 and p16INK4a [1].
• These findings suggest that CDK6 may play an important role in the development and/or progression of a subset of human prostate cancers by stimulating the activity of the AR [1].
• I3C selectively inhibited the expression of CDK6 protein and transcripts and strongly stimulated the production of the p16 CDK inhibitor [30].
• In contrast, Cdk4 was expressed at high levels in several tumor cell lines and Cdk6 was ectopically expressed in two sarcoma lines, suggesting a possible involvement of these two Cdks in oncogenesis [31].
• Thus, expression of the viral cyclin enables cells to subvert the cell cycle inhibitory function of p21Cip1 by promoting cdk6-dependent phosphorylation of this antiproliferative protein [14].

Other interactions of CDK6

• Both in vivo and in vitro, p18 interacts strongly with CDK6, weakly with CDK4, and exhibits no detectable interaction with the other known CDKs [9].
• Immunoprecipitation of CDK4 and CDK6 and their associated proteins from radiolabeled extracts from senescent HDFs showed no other CDK inhibitors [32].
• Therefore, testicular GCTs were analyzed as to the expression of CDK2, CDK4, CDK6, and the expression of their catalytic partners cyclins D1, D2 and E by semiquantitative reverse transcription-PCR [5].
• In addition, CDK6 was not present in p16INK4a immune complexes in cells with significant kinase activity, although p16INK4a levels were high [4].
• In addition, cyclin D1- or cyclin D3-expressing multiple myeloma cells are uniformly distributed in the bone marrow, whereas cdk6-specific phosphorylation of Rb occurs in discrete foci of bone marrow multiple myeloma cells before proliferation early in the clinical course and is then heightened with proliferation and disease progression [33].

Analytical, diagnostic and therapeutic context of CDK6

• Western blot analysis confirmed the expected CDK6 protein size (40 kd) [19].
• According to Southern blot analysis, CDK6 overexpression in neoplastic T lymphoblasts was not due to gene amplification [19].
• In addition, the quantitative difference of CDK6 expression between neoplastic and non-neoplastic cortical thymocytes can be potentially useful in the differential diagnosis of thymic neoplasms on histological and cytological specimens [19].
• Costimulation resulted in a rapid translocation of cdk6 to the nucleus, as demonstrated by both confocal immunofluorescence microscopy and biochemical fractionation techniques [34].
• CONCLUSION: We identified CDK6 as a novel molecular marker that can be determined by immunohistochemistry on routinely processed tissue specimens and may facilitate the prognostic assessment of medulloblastoma patients [35].

References