Calmodulin binds to p21(Cip1) and is involved in the regulation of its nuclear localization

Thr55 phosphorylation of p21 by MPK38/MELK ameliorates defects in glucose, lipid, and energy metabolism in diet-induced obese mice

Murine protein serine-threonine kinase 38 (MPK38)/maternal embryonic leucine zipper kinase (MELK), an AMP-activated protein kinase (AMPK)-related kinase, has previously been shown to interact with p53 and to stimulate downstream signaling. p21, a downstream target of p53, is also known to be involved in adipocyte and obesity metabolism. However, little is known about the mechanism by which p21 mediates obesity-associated metabolic adaptation. Here, we identify MPK38 as an interacting partner of p21. p21 and MPK38 interacted through the cyclin-dependent kinase (CDK) binding region of p21 and the C-terminal domain of MPK38. MPK38 potentiated p21-mediated apoptosis and cell cycle arrest in a kinase-dependent manner by inhibiting assembly of CDK2-cyclin E and CDK4-cyclin D complexes via induction of CDK2-p21 and CDK4-p21 complex formation and reductions in complex formation between p21 and its negative regulator mouse double minute 2 (MDM2), leading to p21 stabilization. MPK38 phosphorylated p21 at Thr55, stimulating its nuclear translocation, which resulted in greater association of p21 with peroxisome proliferator-activated receptor γ (PPARγ), preventing the PPARγ transactivation required for adipogenesis. Furthermore, restoration of p21 expression by adenoviral delivery in diet-induced obese mice ameliorated obesity-induced metabolic abnormalities in a MPK38 phosphorylation-dependent manner. These results suggest that MPK38 functions as a positive regulator of p21, regulating apoptosis, cell cycle arrest, and metabolism during obesity.

The many faces of calmodulin in cell proliferation, programmed cell death, autophagy, and cancer

Calmodulin (CaM) is a ubiquitous Ca(2+) receptor protein mediating a large number of signaling processes in all eukaryotic cells. CaM plays a central role in regulating a myriad of cellular functions via interaction with multiple target proteins. This review focuses on the action of CaM and CaM-dependent signaling systems in the control of vertebrate cell proliferation, programmed cell death and autophagy. The significance of CaM and interconnected CaM-regulated systems for the physiology of cancer cells including tumor stem cells, and processes required for tumor progression such as growth, tumor-associated angiogenesis and metastasis are highlighted. Furthermore, the potential targeting of CaM-dependent signaling processes for therapeutic use is discussed.

Calmodulin activation of polo-like kinase 1 is required during mitotic entry

Polo-like kinase 1 (Plk1) is a conserved key regulator of the G2/M transition, but its upstream spatiotemporal regulators remain unknown. With the help of immunofluorescence, co-immunoprecipitation, and glutathione S-transferase (GST) pull-down assay, we found that calmodulin (CaM) is one such regulatory molecule that associates with Plk1 from G2 to metaphase. More importantly, this interaction results in considerable stimulation of Plk1 kinase activity leading to hyperphosphorylation of Cdc25C. Our results provide new insight into the role of CaM as an upstream regulator of Plk1 activation during mitotic entry.

Calmodulin regulates the translocation of Grb7 into the nucleus

We describe in this report the presence of a nuclear localization signal (NLS) overlapping the calmodulin-binding domain (CaM-BD) of the growth factor receptor bound protein 7 (Grb7). We show that deletion of the CaM-BD of Grb7 prevents its nuclear localization, and that its Src homology 2 (SH2) domain might participate as well in the translocation process. Also, treating cells with the CaM antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) enhances the presence of Grb7 in the nucleus. We propose that CaM inhibits the translocation of Grb7 to the nucleus after binding to its CaM-BD and therefore occluding its overlapping NLS.

Inhibition of SRY-calmodulin complex formation induces ectopic expression of ovarian cell markers in developing XY gonads

The transcription factor sex-determining region of the Y chromosome (SRY) plays a key role in human sex determination, because mutations in SRY cause disorders of sex development in XY individuals. During gonadal development, Sry in pre-Sertoli cells activates Sox9 gene transcription, committing the fate of the bipotential gonad to become a testis rather than an ovary. The high-mobility group domain of human SRY contains two independent nuclear localization signals, one bound by calmodulin (CaM) and the other by importin-β. Although XY females carry SRY mutations in these nuclear localization signals that affect SRY nuclear import in transfected cells, it is not known whether these transport mechanisms are essential for gonadal development and sex determination. Here, we show that mouse Sry protein binds CaM and that a CaM antagonist reduces CaM binding, nuclear accumulation, and transcriptional activity of Sry in transfected cells. CaM antagonist treatment of cultured, sexually indifferent XY mouse fetal gonads led to reduced expression of the Sry target gene Sox9, defects in testicular cord formation, and ectopic expression of the ovarian markers Rspondin1 and forkhead box L2. These results indicate the importance of CaM for SRY nuclear import, transcriptional activity, testis differentiation, and sex determination.

Calmodulin controls liver proliferation via interactions with C/EBPbeta-LAP and C/EBPbeta-LIP

A truncated isoform of C/EBPbeta, C/EBPbeta-LIP, is required for liver proliferation. This isoform is expressed at high levels in proliferating liver and in liver tumors. However, high levels of C/EBPbeta-LIP are also observed in non-proliferating livers during acute phase response (APR). In this paper we present mechanisms by which liver regulates activities of C/EBPbeta-LIP. We found that calmodulin (CaM) inhibits the ability of C/EBPbeta-LIP to promote liver proliferation during APR through direct interactions. This activity of CaM is under negative control of Ca(2+), which is reduced in nuclei of livers with APR, whereas it is increased in nuclei of proliferating livers. A mutant CaM, which does not interact with C/EBPbeta-LIP, also fails to inhibit the growth promotion activity of C/EBPbeta-LIP. Down-regulation of CaM in livers of LPS-treated mice causes liver proliferation via activation of C/EBPbeta-LIP. Overexpression of C/EBPbeta-LIP above levels of CaM also initiates liver proliferation in LPS-treated mice. In addition, CaM regulates transcriptional activity of another isoform of C/EBPbeta, C/EBPbeta-LAP, and might control liver biology through the regulation of both isoforms of C/EBPbeta. In searching for molecular mechanisms by which C/EBPbeta-LIP promotes cell proliferation, we found that C/EBPbeta-LIP releases E2F.Rb-dependent repression of cell cycle genes by a disruption of E2F1.Rb complexes and by a direct interaction with E2F-dependent promoters. CaM inhibits these growth promotion activities of C/EBPbeta-LIP and, therefore, supports liver quiescence. Thus, our findings discover a new pathway of the regulation of liver proliferation that involves calcium-CaM signaling.

Effects of vitamin d and calcium on proliferation and differentiation in normal colon mucosa: a randomized clinical trial

To investigate the potential efficacy of calcium and vitamin D in reducing risk for colorectal neoplasms and to develop "treatable" phenotypic biomarkers of risk for colorectal neoplasms, we conducted a pilot, randomized, double-blind, placebo-controlled, 2 x 2 factorial clinical trial to test the effects of these agents on cell cycle markers in the normal colorectal mucosa. Ninety-two men and women with at least one pathology-confirmed colorectal adenoma were treated with 2 g/day calcium and/or 800 IU/day vitamin D(3) versus placebo over 6 months. Overall expression and distributions of p21(waf1/cip1) (marker of differentiation), MIB-1 (marker of short-term proliferation), and hTERT (marker of long-term proliferation) in colorectal crypts in the normal-appearing rectal mucosa were detected by automated immunohistochemistry and quantified by image analysis. In the calcium, vitamin D, and calcium plus vitamin D groups relative to the placebo, p21 expression increased by 201% (P = 0.03), 242% (P = 0.005), and 25% (P = 0.47), respectively, along the full lengths of colorectal crypts after 6 months of treatment. There were no statistically significant changes in the expression of either MIB-1 or hTERT in the crypts overall; however, the proportion of hTERT, but not MIB-1, expression that extended into the upper 40% of the crypts was reduced by 15% (P = 0.02) in the vitamin D plus calcium group relative to the placebo. These results indicate that calcium and vitamin D promote colorectal epithelial cell differentiation and may "normalize" the colorectal crypt proliferative zone in sporadic adenoma patients, and support further investigation of calcium and vitamin D as chemopreventive agents against colorectal neoplasms.

Residual structure within the disordered C-terminal segment of p21(Waf1/Cip1/Sdi1) and its implications for molecular recognition

Probably the most unusual class of proteins in nature is the intrinsically unstructured proteins (IUPs), because they are not structured yet play essential roles in protein-protein signaling. Many IUPs can bind different proteins, and in many cases, adopt different bound conformations. The p21 protein is a small IUP (164 residues) that is ubiquitous in cellular signaling, for example, cell cycle control, apoptosis, transcription, differentiation, and so forth; it binds to approximately 25 targets. How does this small, unstructured protein recognize each of these targets with high affinity? Here, we characterize residual structural elements of the C-terminal segment of p21 encompassing residues 145-164 using a combination of NMR measurements and molecular dynamics simulations. The N-terminal half of the peptide has a significant helical propensity which is recognized by calmodulin while the C-terminal half of the peptide prefers extended conformations that facilitate binding to the proliferating cell nuclear antigen (PCNA). Our results suggest that the final bound conformations of p21 (145-164) pre-exist in the free peptide even without its binding partners. While the conformational flexibility of the p21 peptide is essential for adapting to diverse binding environments, the intrinsic structural preferences of the free peptide enable promiscuous yet high affinity binding to a diverse array of molecular targets.

p21Waf1/Cip1REGULATES PROLIFERATION AND APOPTOSIS IN AIRWAY EPITHELIAL CELLS AND ALTERNATIVE FORMS HAVE ALTERED BINDING ACTIVITIES

p21(Waf1/Cip1) plays central roles in proliferation, differentiation, and apoptosis. Alterations in the expression and subcellular localisation of p21 occur during several lung diseases but the roles of p21 in the lung epithelium are unknown. The effects of p21 on proliferation and apoptosis in mouse airway epithelial cells (AECs) were examined using p21-null mice. AECs isolated from p21-null mice had increased proliferation and apoptotic rates compared to AECs from wild-type mice. Alterations in the subcellular localization of the cell cycle regulatory proteins p27, PCNA, and p53 were also evident in p21(-/-) cells. The nuclear and cytoplasmic forms of p21 present in AECs were also examined. Full-length p21 (20 kDa) was detected in nuclear fractions but a C-terminal truncated form (17 kDa) of p21 was present in cytoplasmic fractions. The binding activities of truncated p21 were altered compared to full-length p21. Although the latter was complexed with PCNA, Cdk2, Cdk4, Cdk6, cyclin D3, and cyclin E, truncated p21 was bound only to Cdk4 and cyclin D3. In conclusion, p21 regulates proliferation and protects against apoptosis in AECs. In addition, different forms of p21 are present in AECs and the subcellular localization of these forms reflects differences in p21 activity.

Investigation of phosphorylation site responsible for CaLP (P. fucata) nucleo-cytoplasmic shuttling triggered by overexpression of p21Cip1

Calmodulin (CaM) is a highly conserved and ubiquitous Ca(2+)-binding protein regulating intracellular Ca(2+) concentration by acting as a sensor of this divalent cation in eukaryotic cells. Being such a very important signal sensor, CaM is susceptible to undergo many posttranslational modifications. One of these important modifications is its phosphorylation. Our previous investigations showed that CaM and calmodulin-like protein (CaLP) cloned from Pinctada fucata have many different characteristics in spite of their high similarity to each other. We have narrowed down that the C-terminal domains of CaM and CaLP are responsible for their discrepant subcellular localizations and shuttling of CaLP when it is co-transfected with p21(Cip1), which is commonly considered as an important cell cycle regulating protein. In this study, we first predicted the potential phosphorylation site responsible for the shuttling and confirmed by fluorescence confocal microscopy. Together with fluorescence activated cell sorter analysis, we further investigated the releasing ability of wild type and point mutated CaLP from arrested cell cycle caused by p21(Cip1) overexpression. By performing pull-down analysis and phosphorylation status of CaLP in cytoplasm fraction of transfected COS-7 cells with CaLP alone and phosphorylation status of CaLP in nuclear fraction of co-transfected COS-7 cells with CaLP and p21(Cip), we propose that the CaLP staying in the cytoplasm is in the state of phosphorylation, but when p21(Cip1) is overexpressed in mammalian cells, some signal triggers CaLP dephosphorylation and translocation into the nucleus.

Negative regulation of ASK1 by p21Cip1 involves a small domain that includes Serine 98 that is phosphorylated by ASK1 in vivo

The cyclin-dependent kinase inhibitor p21(Cip1) regulates multiple cellular functions and protects cells from genotoxic and other cellular stresses. Activation of apoptosis signal-regulating kinase 1 (ASK1) induced by inhibition of mTOR signaling leads to sustained phospho-c-Jun that is suppressed in cells with functional p53 or by forced expression of p21(Cip1). Here we show that small deletions of p21(Cip1) around S98 abrogate its association with ASK1 but do not affect binding to Cdk1, hence distinguishing between the cell cycle-regulating functions of p21(Cip1) and its ability to suppress activation of the ASK1/Jun N-terminal protein kinase (JNK) pathway. p21(Cip1) is phosphorylated in vitro by both ASK1 and JNK1 at S98. In vivo phosphorylation of p21(Cip1), predominantly carried out by ASK1, is associated with binding to ASK1 and inactivation of ASK1 kinase function. Binding of p21(Cip1) to ASK1 requires ASK1 kinase function and may involve phosphorylation of S98.

Regulation of CDK4

Cyclin-dependent kinase (CDK)4 is a master integrator that couples mitogenic and antimitogenic extracellular signals with the cell cycle. It is also crucial for many oncogenic transformation processes. In this overview, we address various molecular features of CDK4 activation that are critical but remain poorly known or debated, including the regulation of its association with D-type cyclins, its subcellular location, its activating Thr172-phosphorylation and the roles of Cip/Kip CDK "inhibitors" in these processes. We have recently identified the T-loop phosphorylation of CDK4, but not of CDK6, as a determining target for cell cycle control by extracellular factors, indicating that CDK4-activating kinase(s) might have to be reconsidered.

123I-labeled HIV-1 tat peptide radioimmunoconjugates are imported into the nucleus of human breast cancer cells and functionally interact in vitro and in vivo with the cyclin-dependent kinase inhibitor, p21WAF-1/Cip-1

PURPOSE

To evaluate the internalization and nuclear translocation of (123)I-tat-peptide radioimmunoconjugates in MDA-MB-468 breast cancer cells and their ability to interact with the cyclin-dependent kinase inhibitor, p21(WAF-1/Cip-1).

METHODS

Peptides [GRKKRRQRRRPPQGYGC] harboring the nuclear-localizing sequence from HIV tat domain were conjugated to anti-p21(WAF-1/Cip-1) antibodies. Immunoreactivity was assessed by Western blot using lysate from MDA-MB-468 cells exposed to EGF to induce p21(WAF-1/Cip-1). Internalization and nuclear translocation were measured. The ability of tat-anti-p21(WAF-1/Cip-1) to block G(1)-S phase arrest in MDA-MB-468 cells caused by EGF-induced p21(WAF-1/Cip-1) was evaluated. Tumor and normal tissue uptake were determined at 48 h p.i. in athymic mice implanted s.c. with MDA-MB-468 xenografts injected intratumorally with EGF.

RESULTS

There was 13.4+/-0.2% of radioactivity internalized by MDA-MB-468 cells incubated with (123)I-tat-anti-p21(WAF-1/Cip-1) and 34.6+/-3.1% imported into the nucleus. Tat-anti-p21(WAF-1/Cip-1)(8 muM) decreased the proportion of EGF-treated cells in G(1) phase from 81.9+/-0.7% to 46.1+/-0.7% (p<0.001), almost restoring the G(1) phase fraction to that of unexposed cells (25.8+/-0.2%). Non-specific tat-mouse IgG did not block EGF-induced G(1)-S phase arrest. Tumor uptake of radioactivity was higher in mice injected with EGF to induce p21(WAF-1/Cip-1) than in mice not receiving EGF (3.1+/-0.4% versus 1.8+/-0.2% ID/g; p=0.04). Western blot analysis of tumors revealed a threefold increase in the p21(WAF-1/Cip-1)/beta-actin ratio.

CONCLUSION

We conclude that intracellular and nuclear epitopes in cancer cells can be functionally targeted with tat-radioimmunoconjugates to exploit many more epitopes for imaging and radiotherapeutic applications than have previously been accessible.

A calmodulin-binding site on cyclin E mediates Ca2+-sensitive G1/s transitions in vascular smooth muscle cells

Calcium transients are known to control several transition points in the eukaryotic cell cycle. For example, we have previously shown that a coordinate elevation in the intracellular free calcium ion concentration is required for G1- to S-phase cell cycle progression in vascular smooth muscle cells (VSMC). However, the molecular basis for this Ca2+ sensitivity was not known. Using buffers with differing [Ca2+], we found that the kinase activity of mouse and human cyclin E/CDK2, but not other G1/S-associated cell cycle complexes, was responsive to physiological changes in [Ca2+]. We next determined that this Ca2+-responsive kinase activity was dependent on a direct interaction between calmodulin (CaM), one of the major Ca2+-signal transducers of eukaryotic cells, and cyclin E. Pharmacological inhibition of CaM abrogated the Ca2+ sensitivity of cyclin E/CDK2 and retarded mouse VSMC proliferation by causing G1 arrest. We next defined the presence of a highly conserved 22 amino acid N-terminal CaM-binding motif in mammalian cyclin E genes (dissociation constant, 1.5+/-0.1 micromol/L) and showed its essential role in mediating Ca2+-sensitive kinase activity of cyclin E/CDK2. Mutant human cyclin E protein, lacking this CaM-binding motif, was incapable of binding CaM or responding to [Ca2+]. Taken together, these findings reveal CaM-dependent cyclin E/CDK2 activity as a mediator of the known Ca2+ sensitivity of the G1/S transition of VSMC.

Alteration of gene expression in human hepatocellular carcinoma with integrated hepatitis B virus DNA

PURPOSE

Integration of hepatitis B virus (HBV) DNA into the human genome is one of the most important steps in HBV-related carcinogenesis. This study attempted to find the link between HBV DNA, the adjoining cellular sequence, and altered gene expression in hepatocellular carcinoma (HCC) with integrated HBV DNA.

EXPERIMENTAL DESIGN

We examined 15 cases of HCC infected with HBV by cassette ligation-mediated PCR. The human DNA adjacent to the integrated HBV DNA was sequenced. Protein coding sequences were searched for in the human sequence. In five cases with HBV DNA integration, from which good quality RNA was extracted, gene expression was examined by cDNA microarray analysis.

RESULTS

The human DNA sequence successive to integrated HBV DNA was determined in the 15 HCCs. Eight protein-coding regions were involved: ras-responsive element binding protein 1, calmodulin 1, mixed lineage leukemia 2 (MLL2), FLJ333655, LOC220272, LOC255345, LOC220220, and LOC168991. The MLL2 gene was expressed in three cases with HBV DNA integrated into exon 3 of MLL2 and in one case with HBV DNA integrated into intron 3 of MLL2. Gene expression analysis suggested that two HCCs with HBV integrated into MLL2 had similar patterns of gene expression compared with three HCCs with HBV integrated into other loci of human chromosomes.

CONCLUSIONS

HBV DNA was integrated at random sites of human DNA, and the MLL2 gene was one of the targets for integration. Our results suggest that HBV DNA might modulate human genes near integration sites, followed by integration site-specific expression of such genes during hepatocarcinogenesis.

Binding of calmodulin to the carboxy-terminal region of p21 induces nuclear accumulation via inhibition of protein kinase C-mediated phosphorylation of Ser153

Intracellular localization plays an important role in the functional regulation of the cell cycle inhibitor p21. We have previously shown that calmodulin binds to p21 and that calmodulin is essential for the nuclear accumulation of p21. Here, we analyze the mechanism of this regulation. We show that calmodulin inhibits in vitro phosphorylation of p21 by protein kinase C (PKC) and that this inhibition is dependent upon calmodulin binding to p21. Two-dimensional electrophoresis analysis of cells expressing the p21 wild type or p21S153A, a nonphosphorylatable mutant of p21 at position 153, indicates that Ser153 of p21 is a phosphorylable residue in vivo. Furthermore, Western blot analysis using phospho-Ser153-specific antibodies indicates that Ser153 phosphorylation in vivo is induced when PKC is activated and calmodulin is inhibited. The mutation of Ser153 to aspartate, a pseudophosphorylated residue, inhibits the nuclear accumulation of p21. Finally, whereas wild-type p21 translocates to the cytoplasm after PKC activation in the presence of calmodulin inhibitors, p21 carrying a nonphosphorylatable residue at position 153 remains in the nucleus. We propose that calmodulin binding to p21 prevents its phosphorylation by PKC at Ser153 and consequently allows its nuclear localization. When phosphorylated at Ser153, p21 is located at the cytoplasm and disrupts stress fibers.

Recombinant p21 protein inhibits lymphocyte proliferation and transcription factors

Cellular proliferation determines the events leading to the initiation and development of inflammation, immune activation, cancer, atherogenesis, and other disorders associated with aberrant cell proliferation. Cyclin inhibitor p21 plays a unique role in limiting cell cycle progression. However, its effectiveness can only be demonstrated with direct in vitro and in vivo delivery to control aberrant proliferation. We demonstrate that using a protein-transducing domain p21 protein a) localizes within the nuclear compartments of cells, b) interacts with transcription factors, NF-kappaB, and NFATs (NFATc and NFATp), and c) inhibits lymphocyte proliferation and expression of proinflammatory cytokines. This study using lymphocyte proliferation as a model suggests that the recombinant p21 protein can directly be delivered as a therapeutic protein to provide a novel, viable, and powerful strategy to limit proliferation, inflammation, alloimmune activation, cancer, and vascular proliferative disorders such as atherosclerosis.

Defective Calmodulin-Mediated Nuclear Transport of the Sex-Determining Region of the Y Chromosome (SRY) in XY Sex Reversal

The sex-determining region of the Y chromosome (SRY) plays a key role in human sex determination, as mutations in SRY can cause XY sex reversal. Although some SRY missense mutations affect DNA binding and bending activities, it is unclear how others contribute to disease. The high mobility group domain of SRY has two nuclear localization signals (NLS). Sex-reversing mutations in the NLSs affect nuclear import in some patients, associated with defective importin-β binding to the C-terminal NLS (c-NLS), whereas in others, importin-β recognition is normal, suggesting the existence of an importin-β-independent nuclear import pathway. The SRY N-terminal NLS (n-NLS) binds calmodulin (CaM) in vitro, and here we show that this protein interaction is reduced in vivo by calmidazolium, a CaM antagonist. In calmidazolium-treated cells, the dramatic reduction in nuclear entry of SRY and an SRY-c-NLS mutant was not observed for two SRY-n-NLS mutants. Fluorescence spectroscopy studies reveal an unusual conformation of SRY.CaM complexes formed by the two n-NLS mutants. Thus, CaM may be involved directly in SRY nuclear import during gonadal development, and disruption of SRY.CaM recognition could underlie XY sex reversal. Given that the CaM-binding region of SRY is well-conserved among high mobility group box proteins, CaM-dependent nuclear import may underlie additional disease states.

Identification of the functional domain of p21(WAF1/CIP1) that protects cells from cisplatin cytotoxicity

The p21 cyclin-dependent kinase (cdk) inhibitor protects cells from cisplatin cytotoxicity in vivo and in vitro. However, the mechanism of protection is not known. Separate p21 domains are known to interact with several different proteins having proapoptotic functions. To investigate the mechanism of protection by p21, we have constructed adenoviruses encoding the different domains of p21. We were able to localize the protective activity to a region of 54 amino acids containing the cyclin-cdk interacting moiety. Other protein binding domains of p21, including the NH2-terminal procaspase-3 interactive region and the COOH-terminal region containing the proliferating cell nuclear antigen binding domain and the nuclear localization signal, had little protective effect on cisplatin cytotoxicity. The dependence of cisplatin cytotoxicity on cdk2 activity was also demonstrated because 1) cisplatin caused a marked increase in cdk2 activity, which was prevented by the p21 expression adenovirus, and 2) a cdk2 dominant-negative adenovirus also protected cells from cisplatin-induced apoptosis. Thus the data suggest that the mechanism of p21 protection is by direct inhibition of cdk2 activity and that cisplatin-induced apoptosis is caused by a cdk2-dependent pathway.

Physical and functional interaction of androgen receptor with calmodulin in prostate cancer cells

Ca(2+) and calmodulin (CaM) play a critical role in proliferation and viability of a wide variety of cells, including prostate cancer cells. We examined two prostate cancer cell lines, androgen-sensitive LNCaP and androgen-independent PC-3. Proliferation of LNCaP cells was six to eight times more sensitive to the inhibitory effect of the CaM antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) than were PC-3 cells. Because LNCaP cell proliferation is sensitive to stimulation by androgen, we assessed the physical and functional interaction between androgen receptor (AR) and CaM. We observed tight binding of AR to CaM when LNCaP cell extracts were subjected to CaM-affinity column chromatography. AR binding to CaM was Ca(2+)-dependent and was inhibited by pretreatment of the cell extracts with W-7. Using immunofluorescence staining and confocal microscopy, we demonstrated colocalization of AR and CaM in the nucleus of LNCaP cells. Furthermore, the functional relevance of AR-CaM interactions in intact cells was revealed by the observation that W-7 was as effective as Casodex, an antiandrogen, in blocking AR-regulated expression of prostate-specific antigen in LNCaP cells. AR seems to interact with CaM directly because purified human AR could bind to CaM-agarose, and CaM could be detected in AR-immunoprecipitate prepared from purified soluble proteins. These studies provide direct evidence for physical and functional interaction between AR and CaM and suggest the potential usefulness of CaM antagonists in blocking AR activity in prostate cancer.

The structural plasticity of the C terminus of p21Cip1 is a determinant for target protein recognition

The cyclin-dependent kinase inhibitory protein p21(Cip1) might play multiple roles in cell-cycle regulation through interaction of its C-terminal domain with a defined set of cellular proteins such as proliferating cell nuclear antigen (PCNA), calmodulin (CaM), and the oncoprotein SET. p21(Cip1) could be described as an intrinsically unstructured protein in solution although the C-terminal domain adopts a well-defined extended conformation when bound to PCNA. However, the molecular mechanism of the interaction with CaM and the oncoprotein SET is not well understood, partly because of the lack of structural information. In this work, a peptide derived from the C-terminal domain of p21(Cip1) that covers the binding domain of the three above-mentioned proteins was used to demonstrate that the C-terminal domain of p21 recognizes multiple ligands through its ability to adopt multiple conformations. The conformation is dictated by tertiary contacts rather than by the primary sequence of the protein. Our results suggest that the C-terminal domain of p21(Cip1) adopts an extended structure when bound to PCNA and probably when bound to the oncoprotein SET, but an alpha helix when bound to CaM.

A SOX9 defect of calmodulin-dependent nuclear import in campomelic dysplasia/autosomal sex reversal

During mammalian sex determination, SOX9 is translocated into the nuclei of Sertoli cells within the developing XY gonad. The N-terminal nuclear localization signal (NLS) is contained within a SOX consensus calmodulin (CaM) binding region, thereby implicating CaM in nuclear import of SOX9. By fluorescence spectroscopy and glutaraldehyde cross-linking, we show that the SOX9 HMG domain and CaM interact in vitro. The formation of a SOX9.CaM binary complex is calcium-dependent and is accompanied by a conformational change in SOX9. A CaM antagonist, calmidazolium chloride (CDZ), was observed to block CaM recognition of SOX9 in vitro and inhibit both nuclear import and consequent transcriptional activity of SOX9 in treated cells. The significance of the SOX9-CaM interaction was highlighted by analysis of a missense SOX9 mutation, A158T, identified from a XY female with campomelic dysplasia/autosomal sex reversal (CD/SRA). This mutant binds importin beta normally despite defective nuclear import. Fluorescence and quenching studies indicate that in the unbound state, the A158T mutant shows a similar conformation to that of the WT SOX9, but in the presence of CaM, the mutant undergoes unusual conformational changes. Furthermore, SOX9-mediated transcriptional activation by cells expressing the A158T mutant is more sensitive to CDZ than cells expressing WT SOX9. These results suggest first that CaM is involved in the nuclear transport of SOX9 in a process likely to involve direct interaction and second, that CD/SRA can arise, at least in part, from a defect in CaM recognition, ultimately leading to reduced ability of SOX9 to activate transcription of cartilage and testes-forming genes.

Defective importin beta recognition and nuclear import of the sex-determining factor SRY are associated with XY sex-reversing mutations

The architectural transcription factor SRY (sex-determining region of the Y chromosome) plays a key role in sex determination as indicated by the fact that mutations in SRY are responsible for XY gonadal dysgenesis in humans. Although many SRY mutations reduce DNA-binding/bending activity, it is not clear how SRY mutations that do not affect interaction with DNA contribute to disease. The SRY high-mobility group domain harbors two nuclear localization signals (NLSs), and here we examine SRY from four XY females with missense mutations in these signals. In all cases, mutant SRY protein is partly localized to the cytoplasm, whereas wild-type SRY is strictly nuclear. Each NLS can independently direct nuclear transport of a carrier protein in vitro and in vivo, with mutations in either affecting the rate and extent of nuclear accumulation. The N-terminal NLS function is independent of the conventional NLS-binding importins (IMPs) and requires unidentified cytoplasmic transport factors, whereas the C-terminal NLS is recognized by IMPbeta. The SRY-R133W mutant shows reduced IMPbeta binding as a direct consequence of the sex-reversing C-terminal NLS mutation. Of the N-terminal NLS mutants examined, SRY-R62G unexpectedly shows a marked reduction in IMPbeta binding, whereas SRY-R75N and SRY-R76P show normal IMPbeta binding, suggesting defects in the IMP-independent pathway. We conclude that SRY normally requires the two distinct NLS-dependent nuclear import pathways to reach sufficient levels in the nucleus for sex determination. This study documents cases of human disease being explained, at a molecular level, by the impaired ability of a protein to accumulate in the nucleus.

IQGAP1 as signal integrator: Ca2+, calmodulin, Cdc42 and the cytoskeleton

A family of proteins known as IQGAPs have been identified in yeast, amebas and mammals. IQGAPs are multidomain molecules that contain several protein-interacting motifs which mediate binding to target proteins. Mammalian IQGAP1 is a component of signaling networks that are integral to maintaining cytoskeletal architecture and cell-cell adhesion. Published data suggest that IQGAP1 is a scaffolding protein that modulates cross-talk among diverse pathways in complex regulatory circuits. These pathways include modulating the actin cytoskeleton, mediating signaling by Rho family GTPases and calmodulin, regulating E-cadherin and beta-catenin function and organizing microtubules.

Utility of peptide-protein affinity complexes in proteomics: identification of interaction partners of a tumor suppressor peptide

We used a N-biotinylated peptide analog of the C-terminal domain of the tumor suppressor protein, p21cip1/waf1 to elucidate peptide/protein interacting partners. The C-terminal domain of p21cip1/waf1 protein spanning 141-160 amino acid residues is known to bind PCNA and this interaction is important in many biological processes including cell-cycle control. This C-terminal 20-mer efficiently extracts PCNA in the presence of a variety of N- or C-terminally attached affinity tags. Using difference silver stained 2D gels combined with in-gel tryptic digests, we identified the difference spots using MALDI-TOF mass spectrometry-based peptide mass fingerprinting followed by a database search using PROFOUND against NCBIs human nonredundant protein sequence data bank. Identified spots include the p48 subunit of chromatin assembly factor-1, the heat shock 70 protein analog BiP, calmodulin, nucleolin and a spot similar in size to dimeric PCNA. In contrast, microcapillary ion-trap LC-MS/MS analysis of a tryptic digest of entire affinity extracts derived from both control and experimental runs followed by database searches using SEQUEST confirmed the presence of most of the above proteins. This strategy also identified hnRNPA1, HPSP90alpha, HSP40 and T-complex protein 1, a protein similar to prothymosin, and a possible allelic variant of the p21cip1/waf1 protein. The use of N-biotinylated peptide derived from the C-terminal domain of p21cip1/waf1 protein in proteomic analysis exemplified here suggests that peptides obtained from intracellular functional screens could also potentially serve as efficient baits to discover new drug targets.

Cholesteatoma epithelium is characterized by increased expression of Ki-67, p53 and p21, with minimal apoptosis

OBJECTIVE

To investigate differences in cell proliferation, cell cycle arrest and apoptosis between cholesteatoma and control skin.

MATERIAL AND METHODS

Immunohistochemical sections of 15 cholesteatoma and 15 paired control retro-auricular skin samples were examined for Ki-67, p53, p21 and active caspase 3, using image analysis, as well as for DNA fragmentation.

RESULTS

The retro-auricular skin samples contained 5.7% +/- 3.6%, Ki-67-positive cells and showed a normal expression pattern. In the cholesteatoma epithelium 11.7% +/- 9.5% of the cells were Ki-67-positive and these cells were dominantly expressed in the basal and parabasal cell layers. Retro-auricular skin contained 5.8% +/- 5.4% p53-positive cells and 1.0% +/- 0.9%, p21-positive cells. In the cholesteatoma epithelium 17.8% +/- 12.3% of the cells were p53-positive and 14.3% +/- 11.6% were p21-positive The expression of Ki-67, p53 and p21 differed significantly between the two groups (p < 0.05). In the cholesteatoma epithelium a positive correlation was found between p53 and p21 expression (p = 0.016). Active caspase 3 positivity and DNA fragmentation were rarely seen in the cholesteatoma epithelium.

CONCLUSION

Our results indicate that increased cell proliferation in cholesteatoma epithelium is accompanied by an increase in p53 and p21 protein levels, whilst apoptosis is minimal.

The SET protein regulates G2/M transition by modulating cyclin B-cyclin-dependent kinase 1 activity

The SET protein and the cell cycle inhibitor p21(Cip1) interact in vivo and in vitro. We identified here the domain (157)LIF(159) of p21(Cip1) as essential for the binding of SET. We also found that SET contains at least two domains of interaction with p21(Cip1), one located in the fragment amino acids 81-180 and the other one in the fragment including amino acids 181-277. SET and p21(Cip1) co-localize in the cell nucleus in a temporal manner. Overexpression of SET blocks the cell cycle at the G(2)/M transition in COS and HCT116 cells. Moreover, SET inhibits cyclin B-CDK1 activity both in vivo and in vitro in both cell types. This effect is specific for these complexes since SET did not inhibit either cyclin A-CDK2 or cyclin E-CDK2 complexes. SET and p21(Cip1) cooperate in the inhibition of cyclin B-CDK1 activity. The inhibitory effect of SET resides in its acidic C terminus, as demonstrated by the ability of this domain to inhibit cyclin B-CDK1 activity and by the lack of blocking G(2)/M transition when a mutated form of SET lacking this C terminus domain was overexpressed in COS cells. These results indicate that SET might regulate G(2)/M transition by modulating cyclin B-CDK1 activity.

Phosphorylation of the cell cycle inhibitor p21Cip1/WAF1 by Pim-1 kinase

The serine/threonine kinase, Pim-1, appears to be involved in regulating proliferation, differentiation and cell survival of lymphoid and myeloid cells. In this study, we have found that amino acid residues 140-147 (RKRRQTSM) at the C-terminal end of p21(Cip1/WAF1), a cyclin-dependent kinase (CDK) inhibitor, constitute an ideal phosphorylation consensus sequence for Pim-1. We demonstrate that Pim-1 efficiently phosphorylates this peptide sequence as well as the p21 protein in vitro. We also demonstrate by pull-down assay and by immunoprecipitation that Pim-1 associates with p21. During phorbol ester-induced differentiation of U937 cells, both Pim-1 and p21 expression levels increase with Pim-1 levels increasing in both the nucleus and cytoplasm while p21 remains primarily cytoplasmic. Co-transfection of wild type p21 with wild type Pim-1 results in cytoplasmic localization of p21 while co-transfection of wild type p21 with kinase dead Pim-1 results in nuclear localization of p21. Consistent with the results from the phosphoamino acid assay, Pim-1 phosphorylates transfected p21 only on Thr(145) in p21-deficient human fibroblasts and this phosphorylation event results in the cytoplasmic localization of p21. These findings demonstrate that Pim-1 associates with and phosphorylates p21 in vivo, which influences the subcellular localization of p21.

Identification of the nuclear localization signal of p21(cip1) and consequences of its mutation on cell proliferation

Overexpression of p21(cip1) induces cell cycle arrest. Although this ability has been correlated with its nuclear localization, the evidence is not conclusive. The mutants that were used to inhibit its nuclear translocation could no longer bind to several proteins known to interact with the last 25 amino acids of p21(cip1). Here we used point mutation analysis and fusion of the proteins to DsRed to identify which amino acids are essential for the nuclear localization of p21(cip1). We conclude that amino acids RKR(140-142) are essential for nuclear translocation of p21(cip1). While wild-type DsRed-p21 induces cell cycle arrest in 95% of transfected cells, overexpression of cytoplasmatic p21AAA(140-142) arrested only 20% of transfected cells. We conclude that cytoplasmatic p21, with no deletion in the C-terminal region, had a much lower capacity to arrest the cell cycle.

Modulation of the Ras/Raf/MEK/ERK pathway by Ca(2+), and calmodulin

Ras activation induces a variety of cellular responses that depend on the specific activated effector, the intensity and amplitude of its activation, and the cellular type. Transient activation followed by a sustained but low signal of the Ras/Raf/MEK/ERK pathway is a common feature of cell proliferation in many systems. On the contrary, sustained, high activation is linked with either senescence or apoptosis in fibroblasts and to differentiation in neurones and PC12 cells. The temporal regulation of the pathway is relevant and not only depends on the specific receptor activated but also on the presence of diverse modulators of the pathway. We review here evidence showing that calcium (Ca(2+)) and calmodulin (CaM) are able to regulate the Ras/Raf/MEK/ERK pathway. CaM-binding proteins (CaMBPs) as Ras-GRF and CaM-dependent protein kinase IV (CaMKIV) positively modulate ERK1/2 activation induced by either NGF or membrane depolarisation in neurones. In fibroblasts, CaM binding to EGF receptor and K-Ras(B) may be involved in the downregulation of the pathway after its activation, allowing a proliferative signalling.

The p21(Cip1) protein, a cyclin inhibitor, regulates the levels and the intracellular localization of CDC25A in mice regenerating livers

Liver cells from p21(Cip1-/-) mice subjected to partial hepatectomy (PH) progress into DNA synthesis faster than those from wild-type mice. These cells also show a premature induction of cyclin E/cyclin-dependent kinase (CDK) 2 activity. We studied the mechanisms whereby cells lacking p21(Cip1) showed a premature induction of this activity. Whereas the levels of CDK2, cyclin E, and p27(Kip1) were similar in both wild-type and p21(Cip1-/-) mice, those of the activator CDC25A were much higher in p21(Cip1-/-) quiescent and regenerating livers than in wild-type animals. Moreover, p21(Cip1-/-) cells also showed a premature translocation of CDC25A from cytoplasm into the nucleus. The ectopic expression of p21(Cip1) into mice embryo fibroblasts from p21(Cip1-/-) mice decreased the levels of CDC25A and delayed its nuclear translocation. The levels of CDC25A messenger RNA in p21(Cip1-/-) cells were higher than in wild-type cells, suggesting that this increase might be responsible, at least in part, for the high levels of CDC25A protein in these cells. Thus, the results reported here indicate that p21(Cip1) regulates the levels and the intracellular localization of CDC25A. We also found a good correlation between CDC25A nuclear translocation and cyclin E/CDK2 activation. In conclusion, premature translocation of CDC25A to the nucleus might be involved in the advanced induction of cyclin E/CDK2 activity and DNA replication in cells from animals lacking p21(Cip1).

Sex with two SOX on: SRY and SOX9 in testis development

Although gonads are not required for development or survival, defects in gonadal development undoubtedly have a profound influence on affected individuals. Recent complementary studies in the fields of cytology, biochemistry and molecular genetics have revealed that normal gonad development involves an exquisitely regulated network of gene expression and protein-protein interactions. The initial event of gonadogenesis, in both males and females, involves the formation of a bipotential primordium. A Y chromosome then activates the male-specific pathway. The demonstration that mutations in the SOX proteins, SRY and SOX9, are responsible for disorders associated with male-to-female sex reversal showed dramatically that SRY and SOX9 have an essential role in male sex differentiation. This was emphasized when it was shown that female mice carrying transgenes that encode these proteins developed as males. SRY and SOX9 proteins have been characterized extensively and aspects of their function and regulation are now known.

Characterization of fortilin, a novel antiapoptotic protein

Apoptosis is meticulously controlled in living organisms. Its dysregulation has been shown to play a key role in a number of human diseases, including neoplastic, cardiovascular, and degenerative disorders. Bcl-2 family member proteins and inhibitors of apoptosis proteins are two major negative regulators of apoptosis. We report here the characterization of novel antiapoptotic protein, fortilin, which we identified through yeast two-hybrid library screening. Sequence analysis of fortilin revealed it to be a 172-amino acid polypeptide highly conserved from mammals to plants. Fortilin is structurally unrelated to either Bcl-2 family member proteins or inhibitors of apoptosis proteins. Northern blot analysis showed the fortilin message to be ubiquitous in normal tissue but especially abundant in the liver, kidney, and small intestine. Western blot analysis using anti-fortilin antibody showed more extensive expression in cancerous cell lines (H1299, MCF-7, and A549) than in cell lines derived from normal tissue (HEK293). Immunocytochemistry using HeLa cells transiently expressing FLAG-tagged fortilin and immunohistochemistry using human breast ductal carcinoma tissue and anti-fortilin antibody both showed that fortilin is predominantly localized in the nucleus. Functionally, the transient overexpression of fortilin in HeLa cells prevented them, in a dose-dependent fashion, from undergoing etoposide-induced apoptosis. Consistently, U2OS cells stably expressing fortilin protected the cells from cell death induced by etoposide over various concentrations and durations of exposure. In addition, fortilin overexpression inhibited caspase-3-like activity as assessed by the cleavage of fluorogenic substrate benzyloxycarbonyl-DEVD-7-amido-4-(trifluoromethyl)coumarin. Furthermore, the antisense depletion of fortilin from breast cancer cell line MCF-7 was associated with massive cell death. These data suggest that fortilin represents a novel antiapoptotic protein involved in cell survival and apoptosis regulation.

Solution structure of p21(Waf1/Cip1/Sdi1) C-terminal domain bound to Cdk4

Cyclin-dependent kinase (Cdk) inhibitor p21(Waf1/Cip1/Sdi1), a multifunctional protein, has a major role as tumor suppressor, mediating G1/S arrest through inhibition of Cdks. Recent biological studies of Cyclin D1/Cdk4 have proposed that p21 C-terminal domain (p21(CT)) plays a key role as a potent Cdk4 inhibitor. We report here solution structures of p21(CT) for both the free and Cdk4-bound forms using 2D transferred NOE spectroscopy and dynamical simulated annealing calculations. Even though p21(CT) peptide is very flexible in the free state, when it bound to Cdk4, the structure becomes well structured in the binding domain. Therefore we propose that p21(CT) experiences an extensive conformational change upon Cdk4 binding. This structural change of p21(CT) may suggest the molecular mechanism of p21 for specificity and inhibition mode to assemble different cyclin-Cdk complexes. Especially, our data suggests that the D(149)FYHSKRR(156) region of p21 is critical for Cdk4 binding, indicating that the major driving force for complex originates from hydrophobic interaction between p21 and Cdk4.

Calmodulin binds to K-Ras, but not to H- or N-Ras, and modulates its downstream signaling

Activation of Ras induces a variety of cellular responses depending on the specific effector activated and the intensity and amplitude of this activation. We have previously shown that calmodulin is an essential molecule in the down-regulation of the Ras/Raf/MEK/extracellularly regulated kinase (ERK) pathway in cultured fibroblasts and that this is due at least in part to an inhibitory effect of calmodulin on Ras activation. Here we show that inhibition of calmodulin synergizes with diverse stimuli (epidermal growth factor, platelet-derived growth factor, bombesin, or fetal bovine serum) to induce ERK activation. Moreover, even in the absence of any added stimuli, activation of Ras by calmodulin inhibition was observed. To identify the calmodulin-binding protein involved in this process, calmodulin affinity chromatography was performed. We show that Ras and Raf from cellular lysates were able to bind to calmodulin. Furthermore, Ras binding to calmodulin was favored in lysates with large amounts of GTP-bound Ras, and it was Raf independent. Interestingly, only one of the Ras isoforms, K-RasB, was able to bind to calmodulin. Furthermore, calmodulin inhibition preferentially activated K-Ras. Interaction between calmodulin and K-RasB is direct and is inhibited by the calmodulin kinase II calmodulin-binding domain. Thus, GTP-bound K-RasB is a calmodulin-binding protein, and we suggest that this binding may be a key element in the modulation of Ras signaling.

Neuronal survival induced by neurotrophins requires calmodulin

It has been reported that phosphoinositide 3-kinase (PI 3-kinase) and its downstream target, protein kinase B (PKB), play a central role in the signaling of cell survival triggered by neurotrophins (NTs). In this report, we have analyzed the involvement of Ca2+ and calmodulin (CaM) in the activation of the PKB induced by NTs. We have found that reduction of intracellular Ca2+ concentration or functional blockade of CaM abolished NGF-induced activation of PKB in PC12 cells. Similar results were obtained in cultures of chicken spinal cord motoneurons treated with brain-derived neurotrophic factor (BDNF). Moreover, CaM inhibition prevented the cell survival triggered by NGF or BDNF. This effect was counteracted by the transient expression of constitutive active forms of the PKB, indicating that CaM regulates NT-induced cell survival through the activation of the PKB. We have investigated the mechanisms whereby CaM regulates the activation of the PKB, and we have found that CaM was necessary for the proper generation and/or accumulation of the products of the PI 3-kinase in intact cells.

CCAAT/enhancer-binding protein-alpha cooperates with p21 to inhibit cyclin-dependent kinase-2 activity and induces growth arrest independent of DNA binding

CCAAT/enhancer-binding protein-alpha (C/EBP alpha) is a basic leucine zipper protein that controls transcription of genes important for liver function, white adipose tissue development, and granulocyte differentiation. In addition to its function in controlling gene expression in differentiated tissues, C/EBP alpha is also associated with an antimitotic activity. We have previously demonstrated that C/EBP alpha interacts with p21, a cyclin-dependent kinase (CDK) inhibitor, and that C/EBP alpha inhibits proliferation when expressed in several different cell types (Timchenko, N. A., Harris, T. E., Wilde, M., Bilyeu, T. A., Burgess-Beusse, B. L., Finegold, M. J., and Darlington, G. J. (1997) Mol. Cell. Biol. 17, 7353--7361). Here we define the regions of C/EBP alpha required for interaction with p21 and demonstrate that CDK2 also interacts with C/EBP alpha. We show that C/EBP alpha can cooperate with p21 to inhibit CDK2 activity in vitro. The effect of C/EBP alpha on CDK2 activity requires the p21 and CDK2 interaction sites within C/EBP alpha. C/EBP alpha mutants incapable of inhibiting CDK2 activity in vitro do not inhibit proliferation in cultured cells. However, C/EBP alpha mutants defective in DNA binding inhibit proliferation as effectively as the wild-type protein. These findings show that C/EBP alpha-mediated growth arrest occurs through protein interactions and is independent of its transcriptional activity.

Calmodulin enhances the stability of the estrogen receptor

The estrogen receptor mediates breast cell proliferation and is the principal target for chemotherapy of breast carcinoma. Previous studies have demonstrated that the estrogen receptor binds to calmodulin-Sepharose in vitro. However, the association of endogenous calmodulin with endogenous estrogen receptors in intact cells has not been reported, and the function of the interaction is obscure. Here we demonstrate by co-immunoprecipitation from MCF-7 human breast epithelial cells that endogenous estrogen receptors bind to endogenous calmodulin. Estradiol treatment of the cells had no significant effect on the interaction. However, incubation of the cells with tamoxifen enhanced by 5-10-fold the association of calmodulin with the estrogen receptor and increased the total cellular content of estrogen receptors by 1.5-2-fold. In contrast, the structurally distinct calmodulin antagonists trifluoperazine and CGS9343B attenuated the interaction between calmodulin and the estrogen receptor and dramatically reduced the number of estrogen receptors in the cell. Neither of these agents altered the amount of estrogen receptor mRNA, suggesting that calmodulin stabilizes the protein. This hypothesis is supported by the observation that, in the presence of Ca2+, calmodulin protected estrogen receptors from in vitro proteolysis by trypsin. Furthermore, overexpression of wild type calmodulin, but not a mutant calmodulin incapable of binding Ca2+, increased the concentration of estrogen receptors in MCF-7 cells, whereas transient expression of a calmodulin inhibitor peptide reduced the estrogen receptor concentration. These data demonstrate that calmodulin binds to the estrogen receptor in intact cells in a Ca2+-dependent, but estradiol-independent, manner, thereby modulating the stability and the steady state level of estrogen receptors.

TOK-1, a novel p21Cip1-binding protein that cooperatively enhances p21-dependent inhibitory activity toward CDK2 kinase

A p21(Cip1/Waf1/Sdi1) is known to act as a negative cell-cycle regulator by inhibiting kinase activity of a variety of cyclin-dependent kinases. In addition to binding of the cyclin-dependent kinase to the N-terminal region of p21, p21 is also bound at its C-terminal region by proliferating cell nuclear antigen (PCNA), SET/TAF1, and calmodulin, indicating the versatile function of p21. In this study, we cloned cDNA encoding a novel protein named TOK-1 as a p21 C-terminal-binding protein by a two-hybrid system. Two splicing isoforms of TOK-1, TOK-1alpha and TOK-1beta, comprising 322 and 314 amino acids, respectively, were co-localized with p21 in nuclei and showed a similar expression profile to that of p21 in human tissues. TOK-1alpha, but not TOK-1beta, directly bound to the C-terminal proximal region of p21, and both were expressed at the G(1)/S boundary of the cell cycle. TOK-1alpha also preferentially bound to an active form of cyclin-dependent kinase 2 (CDK2) via p21, and these made a ternary complex in human cells. Furthermore, the results of three different types of experiments showed that TOK-1alpha enhanced the inhibitory activity of p21 toward histone H1 kinase activity of CDK2. TOK-1alpha is thus thought to be a new type of CDK2 modulator.

Requirement of cyclin D1 in mesangial cell mitogenesis

Abstract. Hyperplasia of mesangial cells (MC) is a frequent finding in glomerulonephritis. The control and function of cyclin D1, a regulator of cell cycle progression, in MC proliferation in vivo and in vitro were investigated. In a rat model of mesangioproliferative glomerulonephritis, increases in the number of cyclin D1-positive MC nuclei were prominent on day 5 of the disease, preceding the peak of MC hyperplasia. In growth-arrested rat MC in culture, mitogenic stimulation with serum or platelet-derived growth factor (PDGF) led to rapid increases in cyclin D1 protein expression. Transforming growth factor-beta1 inhibited PDGF induction of cyclin D1 protein at 12 h. In an examination of the subcellular distribution of cyclin D1, it was observed that stimulation of MC with PDGF for 6 h caused translocation of cyclin D1 from the cytoplasm into the nucleus. Coincubation with PDGF and transforming growth factor-beta1 completely inhibited this effect, without altering the cellular cyclin D1 protein abundance at that time point. To test whether reduction of cyclin D1 protein levels was sufficient to inhibit mitogenesis, MC were transfected with antisense oligonucleotides (ODN) complementary to rat cyclin D1 mRNA. Antisense ODN against cyclin D1 reduced the serum- or PDGF-induced protein expression of cyclin D1 to 27 or 10% of control levels, respectively. These inhibitory effects were correlated with diminished cyclin-dependent kinase 4 activity. Antisense ODN against cyclin D1 also decreased the PDGF-induced increase in p21(Waf-1) protein levels. The MC proliferation caused by serum or PDGF was markedly inhibited by antisense ODN against cyclin D1, as measured by [(3)H]thymidine uptake and cell counts. It is concluded that increased cyclin D1 protein expression of MC is required for MC proliferation. Targeting cyclin D1 expression may represent an effective means to inhibit MC proliferation in vitro and in vivo.

The protein SET regulates the inhibitory effect of p21(Cip1) on cyclin E-cyclin-dependent kinase 2 activity

The cyclin-dependent kinase (CDK) inhibitor p21(Cip1) has a dual role in the regulation of the cell cycle; it is an activator of cyclin D1-CDK4 complexes and an inhibitor of cyclins E/A-CDK2 activity. By affinity chromatography with p21(Cip1)-Sepharose 4B columns, we purified a 39-kDa protein, which was identified by microsequence analysis as the oncoprotein SET. Complexes containing SET and p21(Cip1) were detected in vivo by immunoprecipitation of Namalwa cell extracts using specific anti-p21(Cip1) antibodies. We found that SET bound directly to p21(Cip1) in vitro by the carboxyl-terminal region of p21(Cip1). SET had no direct effect on cyclin E/A-CDK2 activity, although it reversed the inhibition of cyclin E-CDK2, but not of cyclin A-CDK2, induced by p21(Cip1). This result is specific for p21(Cip1), since SET neither bound to p27(Kip1) nor reversed its inhibitory effect on cyclin E-CDK2 or cyclin A-CDK2. Thus, SET appears to be a modulator of p21(Cip1) inhibitory function. These results suggest that SET can regulate G(1)/S transition by modulating the activity of cyclin E-CDK2.