Cyclin D3/CDK11p58 complex is involved in the repression of androgen receptor

Mislocalization of CDK11/PITSLRE, a regulator of the G2/M phase of the cell cycle, in Alzheimer disease

Post-mitotic neurons are typically terminally differentiated and in a quiescent status. However, in Alzheimer disease (AD), many neurons display ectopic re-expression of cell cycle-related proteins. Cyclin-dependent kinase 11 (CDK11) mRNA produces a 110-kDa protein (CDK11^p110) throughout the cell cycle, a 58-kDa protein (CDK11^p58) that is specifically translated from an internal ribosome entry site and expressed only in the G₂/M phase of the cell cycle, and a 46-kDa protein (CDK11^p46) that is considered to be apoptosis specific. CDK11 is required for sister chromatid cohesion and the completion of mitosis. In this study, we found that the expression patterns of CDK11 vary such that cytoplasmic CDK11 is increased in AD cellular processes, compared to a pronounced nuclear expression pattern in most controls. We also investigated the effect of amyloid precursor protein (APP) on CDK11 expression in vitro by using M17 cells overexpressing wild-type APP and APP Swedish mutant phenotype and found increased CDK11 expression compared to empty vector. In addition, amyloid-β_25–35 resulted in increased CDK11 in M17 cells. These data suggest that CDK11 may play a vital role in cell cycle re-entry in AD neurons in an APP-dependent manner, thus presenting an intriguing novel function of the APP signaling pathway in AD.

CDK11p46 and RPS8 associate with each other and suppress translation in a synergistic manner

CDK11p46, a 46kDa isoform of the PITSLRE kinase family, is a key mediator of cell apoptosis, while the precise mechanism remains to be elucidated. By using His pull-down and mass spectrometry analysis, we identified the ribosomal protein S8 (RPS8), a member of the small subunit ribosome, as an interacting partner of CDK11p46. Further analysis confirmed the association of CDK11p46 and RPS8 in vitro and in vivo, and revealed that RPS8 was not a substrate of CDK11p46. Moreover, RPS8 and CDK11p46 synergize to inhibit the translation process both in cap- and internal ribosomal entry site (IRES)-dependent way, and sensitize cells to Fas ligand-induced apoptosis. Taken together, our results provide evidence for the novel role of CDK11p46 in the regulation of translation and cell apoptosis.

Thr-370 is responsible for CDK11(p58) autophosphorylation, dimerization, and kinase activity

CDK11(p58), a member of the p34(cdc2)-related kinase family, is associated with cell cycle progression, tumorigenesis, and proapoptotic signaling. It is also required for the maintenance of chromosome cohesion, the maturation of centrosome, the formation of bipolar spindle, and the completion of mitosis. Here we identified that CDK11(p58) interacted with itself to form homodimers in cells, whereas D224N, the kinase-dead mutant, failed to form homodimers. CDK11(p58) was autophosphorylated, and the main functions of CDK11(p58), such as kinase activity, transactivation of nuclear receptors, and proapoptotic signal transduction, were dependent on its autophosphorylation. Furthermore, the in vitro kinase assay indicated that CDK11(p58) was autophosphorylated at Thr-370. By mutagenesis, we created CDK11(p58) T370A and CDK11(p58) T370D, which mimic the dephosphorylated and phosphorylated forms of CDK11(p58), respectively. The T370A mutant could not form dimers and be phosphorylated by the wild type CDK11(p58) and finally lost the kinase activity. Further functional research revealed that T370A failed to repress the transactivation of androgen receptor and enhance the cell apoptosis. Overall, our data indicated that Thr-370 is responsible for the autophosphorylation, dimerization, and kinase activity of CDK11(p58). Moreover, Thr-370 mutants might affect CDK11(p58)-mediated signaling pathways.

Identification of androgen receptor phosphorylation in the primate ovary in vivo

The androgen receptor (AR) is a member of the nuclear receptor superfamily, and is important for both male and female reproductive health. The receptor is a target for a number of post-translational modifications including phosphorylation, which has been intensively studied in vitro. However, little is known about the phosphorylation status of the receptor in target tissues in vivo. The common marmoset is a useful model for studying human reproductive functions, and comparison of the AR primary sequence from this primate shows high conservation of serines known to be phosphorylated in the human receptor and corresponding flanking amino acids. We have used a panel of phosphospecific antibodies to study AR phosphorylation in the marmoset ovary throughout the follicular phase and after treatment with GNRH antagonist or testosterone propionate. In normal follicular phase ovaries, total AR (both phosphorylated and non-phosphorylated forms) immunopositive staining was observed in several cell types including granulosa cells of developing follicles, theca cells and endothelial cells lining blood vessels. Receptor phosphorylation at serines 81, 308, and 650 was detected primarily in the granulosa cells of developing follicles, surface epithelium, and vessel endothelial cells. Testosterone treatment lead to a modest increase in AR staining in all stages of follicle studied, while GNRH antagonist had no effect. Neither treatment significantly altered the pattern of phosphorylation compared to the control group. These results demonstrate that phosphorylation of the AR occurs, at a subset of serine residues, in a reproductive target tissue in vivo, which appears refractory to hormonal manipulations.

Steroid receptor phosphorylation: Assigning function to site-specific phosphorylation

Steroid receptors (SRs) are hormone-activated transcription factors important for a wide variety of cellular functions. Post-translational modifications of SRs, including phosphorylation, ubiquitination, acetylation, and sumoylation regulate their expression and function. The remarkable number of phosphorylation sites in these receptors and the wide variety of kinases shown to modulate phosphorylation influence the integration between cell-signaling pathways and SR action. These phosphorylation sites have been identified in all of the functional domains with the majority being located within the amino-terminal portions of the receptors. The regulation of function is receptor specific, site specific, and often dependent on the cellular context. Numerous roles for site-specific phosphorylation have been elucidated including sensitivity of hormone response, DNA binding, expression, stability, subcellular localization, dimerization, and protein-protein interactions that can determine the regulation of specific target genes. This review summarizes the current knowledge regarding receptor site-specific phosphorylation and regulation of function. As functional assays become more sophisticated, it is likely that additional roles for phosphorylation in receptor function will be identified.

Coordinating cellular events during spermatogenesis: a biochemical model

Throughout spermatogenesis, a select pool of germ cells, the leptotene spermatocytes, must traverse the blood-testis barrier (BTB) to enter the adluminal compartment of the seminiferous epithelium. This event requires extensive restructuring of cell junctions, and it must also coincide with germ cell cycle progression in preparation for primary spermatocyte meiosis. Recent findings show that cell-cycle-associated kinases and phosphatases, including mitogen-activated protein kinases (MAPKs), participate in the pathways that also direct germ cell adhesion and movement. Our new biochemical model explains, in part, how two distinct cellular events, BTB restructuring and spermiation, are coordinated to maintain spermatogenesis and fertility. In this way, MAPKs would synchronize cell cycle progression in primary spermatocytes with junction remodeling and cell migration across the BTB.

CDK11p58 represses vitamin D receptor-mediated transcriptional activation through promoting its ubiquitin-proteasome degradation

Vitamin D receptor (VDR) is a member of the nuclear receptor superfamily and regulates transcription of target genes. In this study, we identified CDK11(p58) as a novel protein involved in the regulation of VDR. CDK11(p58), a member of the large family of p34cdc2-related kinases, is associated with cell cycle progression, tumorigenesis, and apoptotic signaling. Our study demonstrated that CDK11(p58) interacted with VDR and repressed VDR-dependent transcriptional activation. Furthermore, overexpression of CDK11(p58) decreased the stability of VDR through promoting its ubiquitin-proteasome-mediated degradation. Taken together, these results suggest that CDK11(p58) is involved in the negative regulation of VDR.

CDK11p58 phosphorylation of PAK1 Ser174 promotes DLC2 binding and roles on cell cycle progression

CDK11(p58), a CDK11 family Ser/Thr kinase, is a G2/M specific protein and contributed to regulation of cell cycle, transcription and apoptotic signal transduction. Recently, CDK11(p58) has been reported to exert important functions in mitotic process, such as the regulation of bipolar spindle formation and sister chromatid cohesion. Here, we identified p21 activated kinase 1 (PAK1) as a new CDK11(p58) substrate and we mapped a new phosphorylation site of Ser174 on PAK1. By mutagenesis, we created PAK1(174A) and PAK1(174E), which mimic the dephosphorylated and phosphorylated form of PAK1; further analysis showed PAK1(174E) could be recruited to myosin V motor complex through binding to dynein light chain 2 (DLC2). PAK1(174E) could accelerate the mitosis progression in a nocodazole blocked cell model, while PAK1(174A) exhibited an opposite role. Our results indicated PAK1 may serve as a downstream effector of CDK11(p58) during mitosis progression.

Repression of estrogen receptor alpha by CDK11p58 through promoting its ubiquitin-proteasome degradation

Estrogen receptor alpha (ERalpha) is a ligand-dependent transcription factor that mediates physiological responses to 17beta-estradiol (E(2)). These responses of cells to estrogen are regulated in part by degradation of ERalpha. In this report, we found that CDK11(p58) repressed ERalpha transcriptional activity. And we further demonstrated that ERalpha protein level was down-regulated by CDK11(p58) in mammalian cells in a ligand independent manner. This effect could be abrogated by treatment with proteasome inhibitor MG132. Our results indicated that the ubiquitin/proteasome-mediated degradation of ERalpha was promoted by CDK11(p58). Furthermore, the interaction between ERalpha and CDK11(p58) was detected. This interaction was necessary for the polyubiquitination and degradation of ERalpha. On the contrary, the other isoform of CDK11, CDK11(p110) and the kinase dead mutant of CDK11(p58), D224N, did not associate with ERalpha and failed to reduce the ERalpha protein level. These data identified a new negative regulatory protein of ERalpha and provided a new pathway by which CDK11(p58) negatively regulated cells.

Characterization of cyclin L1 and L2 interactions with CDK11 and splicing factors: influence of cyclin L isoforms on splice site selection

Although it has been reported that cyclin L1alpha and L2alpha proteins interact with CDK11(p110), the nature of the cyclin L transcripts, the formation of complexes between the five cyclin L and the three CDK11 protein isoforms, and the influence of these complexes on splicing have not been thoroughly investigated. Here we report that cyclin L1 and L2 genes generate 14 mRNA variants encoding six cyclin L proteins, one of which has not been described previously. Using cyclin L gene-specific antibodies, we demonstrate expression of multiple endogenous cyclin L proteins in human cell lines and mouse tissues. Moreover, we characterize interactions between CDK11(p110), mitosis-specific CDK11(p58), and apoptosis-specific CDK11(p46) with both cyclin Lalpha and -beta proteins and the co-elution of these proteins following size exclusion chromatography. We further establish that CDK11(p110) and associated cyclin Lalpha/beta proteins localize to splicing factor compartments and nucleoplasm and interact with serine/arginine-rich proteins. Importantly, we also determine the effect of CDK11-cyclin L complexes on pre-mRNA splicing. Preincubation of nuclear extracts with purified cyclin Lalpha and -beta isoforms depletes the extract of in vitro splicing activity. Ectopic expression of cyclin L1alpha, L1beta, L2alpha, or L2beta or active CDK11(p110) individually enhances intracellular intron splicing activity, whereas expression of CDK11(p58/p46) or kinase-dead CDK11(p110)represses splicing activity. Finally, we demonstrate that expression of cyclins Lalpha and -beta and CDK11(p110) strongly and differentially affects alternative splicing in vivo. Together, these data establish that CDK11(p110) interacts physically and functionally with cyclin Lalpha and -beta isoforms and SR proteins to regulate splicing.

Cyclin D3 action in androgen receptor regulation and prostate cancer

Prostate cancer (PCa) cell proliferation is dependent on activation of the androgen receptor (AR), a ligand-dependent transcription factor. AR activation controls G1-S phase progression through fostering enhanced translation of the D-type cyclins, which promote cell cycle progression through activation of CDK4/6. However, the D-type cyclins harbor additional, CDK4/6 kinase-independent, functions through manipulation of transcription factors, including AR. It was previously established that cyclins D1 and D3 have the potential to modulate AR, and with regard to cyclin D1, disruption of this function occurs in human tumors. Therefore, it was essential to interrogate cyclin D3 function in this tumor type. Here, we show that cyclin D3 is found in association with AR in PCa cells, as mediated through a conserved motif. Cyclin D3 functions to attenuate AR activity through defined mechanisms that include modulation of ligand-dependent conformational changes and modulation of chromatin binding activity. Accumulated cyclin D3 slows cell proliferation in AR-dependent cells, thus suggesting that androgen-induced D-type cyclin production serves to temper the mitogenic response to androgen. Supporting this hypothesis, it is shown that cyclin D3 expression is reduced in primary PCas as a function of tumor grade, and inversely correlates with the proliferative index. In total, these data identify cyclin D3 as a critical modulator of the androgen response, whose deregulation may foster unchecked AR activity in PCa.