p27Kip1 represses the Pitx2-mediated expression of p21Cip1 and regulates DNA replication during cell cycle progression

The tumor suppressor p21 regulates cell cycle progression and peaks at mid/late G1. However, the mechanisms regulating its expression during cell cycle are poorly understood. We found that embryonic fibroblasts from p27 null mice at early passages progress slowly through the cell cycle. These cells present an elevated basal expression of p21 suggesting that p27 participates to its repression. Mechanistically, we found that p27 represses the expression of Pitx2 (an activator of p21 expression) by associating with the ASE-regulatory region of this gene together with an E2F4 repressive complex. Furthermore, we found that Pitx2 binds to the p21 promoter and induces its transcription. Finally, silencing Pitx2 or p21 in proliferating cells accelerates DNA replication and cell cycle progression. Collectively, these results demonstrate an unprecedented connection between p27, Pitx2 and p21 relevant for the regulation of cell cycle progression and cancer and for understanding human pathologies associated with p27 germline mutations.

Degradation of cyclin A is regulated by acetylation

Cyclin A accumulates at the onset of S phase, remains high during G(2) and early mitosis and is degraded at prometaphase. Here, we report that the acetyltransferase P/CAF directly interacts with cyclin A that as a consequence becomes acetylated at lysines 54, 68, 95 and 112. Maximal acetylation occurs simultaneously to ubiquitylation at mitosis, indicating importance of acetylation on cyclin A stability. This was further confirmed by the observation that the pseudoacetylated cyclin A mutant can be ubiquitylated whereas the nonacetylatable mutant cannot. The nonacetylatable mutant is more stable than cyclin A WT (cycA WT) and arrests cell cycle at mitosis. Moreover, in cells treated with histone deacetylase inhibitors cyclin A acetylation increases and its stability decreases, thus supporting the function of acetylation on cyclin A degradation. Although the nonacetylatable mutant cannot be ubiquitylated, it interacts with the proteins needed for its degradation (cdks, Cks, Cdc20, Cdh1 and APC/C). In fact, its association with cdks is increased and its complexes with these kinases display higher activity than control cycA WT-cdk complexes. All these results indicate that cyclin A acetylation at specific lysines is crucial for cyclin A stability and also has a function in the regulation of cycA-cdk activity.

Glyceraldehyde 3-phosphate dehydrogenase is a SET-binding protein and regulates cyclin B-cdk1 activity

We report here that glyceraldehyde-3-phosphate dehydrogenase (GAPDH) interacts in vitro and in vivo with the protein SET. This interaction is performed through the acidic domain of SET located at the carboxy terminal region. On analysing the functional relevance of SET-GAPDH interaction, we observed that GAPDH reverses in a dose-dependent manner, the inhibition of cyclin B-cdk1 activity produced by SET. Similarly to SET, GAPDH associates with cyclin B, suggesting that the regulation of cyclin B-cdk1 activity might be mediated not only by the interaction of GAPDH with SET but also with cyclin B. To analyse the putative role of GAPDH on cell cycle progression, HCT116 cells were transfected with a GAPDH expression vector. Results indicate that overexpression of GAPDH does not affect the timing of DNA replication but induces an increase in the number of mitosis, an advancement of the peak of cyclin B-cdk1 activity and an acceleration of cell cycle progression. All these results suggest that GAPDH might be involved in cell cycle regulation by modulating cyclin B-cdk1 activity.

Heterogeneous nuclear ribonucleoprotein A2 is a SET-binding protein and a PP2A inhibitor

The oncoprotein SET participates in a diversity of cellular functions including cell proliferation. Its role on cell cycle progression is likely mediated by inhibiting cyclin B-cdk1 and the protein phosphatase 2A (PP2A). On identifying new SET cellular partners, we found that SET interacts in vitro and in vivo with the heterogeneous nuclear ribonucleoprotein A2 (hnRNPA2); a protein involved in various aspects of mRNA biogenesis. The SET-binding region of hnRNPA2 is the RNP1 sequence that belongs to the RNA-binding domain (RBD) of this protein. We also found that hnRNPA2 has much higher affinity for single-standed DNA than for SET. On analysing the effect of hnRNPA2 on PP2A inhibition by SET, we observed that hnRNPA2 cooperates with SET on PP2A inhibition. This is because we found that hnRNPA2 is also a PP2A inhibitor. HnRNPA2 interacts with PP2A by the RNP1 sequence; however, to inhibit PP2A activity, the complete RBD is needed. We also observed that overexpression of hnRNPA2 inhibits PP2A activity and stimulates cell proliferation. Interestingly, the overexpression of the complete RBD is sufficient to stimulate proliferation. As hnRNPA2 is overexpressed in a variety of human tumors, our results suggest that hnRNPA2 might participate in oncogenesis by stimulating cell proliferation.

A new bis-indole, KARs, induces selective M arrest with specific spindle aberration in neuroblastoma cell line SH-SY5Y

KARs, new semisynthetic antitumor bis-indole derivatives, were found to be inhibitors of tubulin polymerization with lower toxicity than vinblastine or vincristine, used in chemotherapy. Here, we compare the effect of KARs with those of vinblastine and vincristine on cell viability, cell proliferation, and cell cycle in neuroblastoma cell line (SH-SY5Y). At concentrations of the different compounds equivalent in causing 50% of inhibition of cell growth, KARs induced a complete arrest in the G2/M phase, whereas vinblastine and vincristine induced a partial arrest in both G0/G1 and G2/M. Moreover, a combination of KAR-2 and W13 (an anticalmodulin drug) qualitatively caused a similar arrest in both G0/G1 and G2/M than vinblastine. Levels of cyclin A and B1 were higher in KARs-treated cells than in vinblastine- or vincristine-treated cells. Cdc2 activity was much higher in KAR-2 than in vinblastine-treated cells, indicating a stronger mitotic arrest. The effect of KAR2 and vinblastine on microtubules network was analyzed by immunostaining with anti-tubulin antibody. Results indicated that KAR-2-induces the formation of aberrant mitotic spindles, with not apparent effect on interphase microtubules, whereas vinblastine partially destroyed interphase microtubules coexisting with normal and aberrant mitotic spindles.

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.

Osmotic stress regulates the stability of cyclin D1 in a p38SAPK2-dependent manner

We report here that different cell stresses regulate the stability of cyclin D1 protein. Exposition of Granta 519 cells to osmotic shock, oxidative stress, and arsenite induced the post-transcriptional down-regulation of cyclin D1. In the case of osmotic shock, this effect was completely reversed by the addition of p38(SAPK2)-specific inhibitors (SB203580 or SB220025), indicating that this effect is dependent on p38(SAPK2) activity. Moreover, the use of proteasome inhibitors prevented this down-regulation. Thus, osmotic shock induces proteasomal degradation of cyclin D1 protein by a p38(SAPK2)-dependent pathway. The effect of p38(SAPK2) on cyclin D1 stability might be mediated by direct phosphorylation at specific sites. We found that p38(SAPK2) phosphorylates cyclin D1 in vitro at Thr(286) and that this phosphorylation triggers the ubiquitination of cyclin D1. These results link for the first time a stress-induced MAP kinase pathway to cyclin D1 protein stability, and they will help to understand the molecular mechanisms by which stress transduction pathways regulate the cell cycle machinery and take control over cell proliferation.

Differential association of p21Cip1 and p27Kip1 with cyclin E-CDK2 during rat liver regeneration

BACKGROUND/AIMS

The cell cycle inhibitors p21Cip1 and p27Kip1 regulate liver regeneration by modulating the activity of cyclin-dependent kinases (CDKs). However, the specific role of these inhibitors in the regulation of CDK2 activity during liver regeneration remains unknown. The aim of this study was to examine the association of p21Cip1 and p27Kip1 with cyclin E-CDK2 and cyclin A-CDK2 complexes during rat liver regeneration and to correlate the association of both inhibitors with CDK2 activity.

METHODS

The association of p21Cip1 or p27Kip1 with cyclin E-CDK2 or cyclin A-CDK2 and the activities of these complexes were analyzed by immunoprecipitation of rat liver homogenates obtained at different times after a partial hepatectomy (PH), followed by Western blotting or kinase assays.

RESULTS

High amounts of p27Kip1 bound to cyclin E-CDK2 were observed during the first 13 h after PH, when CDK2 activity was very low. At 24 h, when CDK2 activity was maximal, the amount of bound-p27Kip1 decreased strongly. The amount of p21Cip1 bound to these complexes was low during the first 13 h but subsequently increased. No cyclin A-CDK2 complexes were found during the first 13 h after PH. At 24 h, complexes containing low levels of both inhibitors were detected and at 28 h, a significant increase in p21Cip1 and p27Kip1 associated with cyclin A-CDK2 was observed.

CONCLUSIONS

p27Kip1 acts as a brake on cyclin E-CDK2 activity during the first 13 h after a PH. Both p21Cip1 and p27Kip1 down-regulate cyclin A-CDK2 activity at 28 h after PH, after its maximal activation.

[Lys61]N-Ras is able to induce full activation and nuclear accumulation of Cdk4 in NIH3T3 cells

The elements of the cell cycle regulatory machinery activated by the oncogenic form of Ras, [Lys61]N-Ras, have been analysed in NIH3T3 cells. We demonstrate that [Lys61]N-Ras expression is able to induce full cdk4 activation. As already reported, oncogenic Ras expression was sufficient to induce cyclin D1 and p21cip1 expression and their association with cdk4. Furthermore, serum-starved [Lys61]N-Ras NIH3T3 cells showed nuclear accumulation of cyclin D1 and cdk4 not observed in serum-starved NIH3T3 cells. This accumulation of cdk4 into the cell nucleus observed in serum-starved [Lys61]N-Ras NIH3T3 cells was inhibited by a microinjection of neutralizing anti-Ras antibodies. Thus, active [Lys61]N-Ras was a sufficient signal to induce nuclear accumulation of cyclin D1/cdk4, leading to its full activation. Transfection of [Lys61]N-Ras NIH3T3 cells with an inactive form of MEK or their treatment with PD 98059, showed that nuclear translocation of cdk4 was MEK dependent. Interestingly, cells constitutively expressing [Lys61]N-Ras did not inactivate pRb and did not proliferate in the absence of serum. This may be due to the fact that although association of cdk2 with cyclin E and the translocation of those complexes to the nucleus were achieved, [Lys61]N-Ras expression was not sufficient to induce cdk2 activation. The high levels of p27(kip1) that were found in cyclin E/cdk2 complexes may be responsible for the inability of oncogenic Ras to activate this kinase. In consequence, oncogenic alterations that lead to a decrease in p27kip1 bound to cyclin E may cooperate with Ras to induce full cdk2 activation, pRb inactivation and thus cell proliferation.

Novel, potent calmodulin antagonists derived from an all-D hexapeptide combinatorial library that inhibit in vivo cell proliferation: activity and structural characterization

Calmodulin is known to bind to various amphipathic helical peptide sequences, and the calmodulin-peptide binding surface has been shown to be remarkably tolerant sterically. D-Amino acid peptides, therefore, represent potential nonhydrolysable intracellular antagonists of calmodulin. In the present study, synthetic combinatorial libraries have been used to develop novel D-amino acid hexapeptide antagonists to calmodulin-regulated phosphodiesterase activity. Five hexapeptides were identified from a library containing over 52 million sequences. These peptides inhibited cell proliferation both in cell culture using normal rat kidney cells and by injection via the femoral vein following partial hepatectomy of rat liver cells. These hexapeptides showed no toxic effect on the cells. Despite their short length, the identified hexapeptides appear to adopt a partial helical conformation similar to other known calmodulin-binding peptides, as shown by CD spectroscopy in the presence of calmodulin and NMR spectroscopy in DMSO. The present peptides are the shortest peptide calmodulin antagonists reported to date showing potential in vivo activity.

DCC and SMAD4 alterations in human colorectal and pancreatic tumor dissemination

Chromosome 18q is lost a high proportion of colorectal and pancreatic cancers. Three candidate tumor suppressor genes, DCC, Smad4 and Smad2 have been identified in this chromosome region. DCC and Smad4 aberrations have been previously identified in pancreatic and colorectal tumors. The aim of this study was to compare the presence of concurrent genetic aberrations in DCC and neighboring Smad4 and Smad2 genes during colorectal and pancreatic distal dissemination. We have used a panel of orthotopically implanted colorectal and pancreatic xenografts and corresponding metastases. We have shown that while LOH at DCC locus occurred at a similar frequency in both tumors, diminished DCC protein expression was exclusively present in colorectal tumors harboring intragenic DCC LOH. In contrast, in pancreatic xenografts loss of DCC protein and mRNA expression was restricted to metastases. Smad4 gene aberrations were detected at a similar frequency in both tumors and were selected for during distal dissemination. Acquisition of alterations in both genes occurred independently. Our results suggest that both DCC and Smad4 contribute to pancreatic and colorectal distal dissemination. However, the role of DCC may differ between both tumor types.

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.

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

p21(Cip1), first described as an inhibitor of cyclin-dependent kinases, has recently been shown to have a function in the formation of cyclin D-Cdk4 complexes and in their nuclear translocation. The dual behavior of p21(Cip1) may be due to its association with other proteins. Different evidence presented here indicate an in vitro and in vivo interaction of p21(Cip1) with calmodulin: 1) purified p21(Cip1) is able to bind to calmodulin-Sepharose in a Ca(2+)-dependent manner, and this binding is inhibited by the calmodulin-binding domain of calmodulin-dependent kinase II; 2) both molecules coimmunoprecipitate when extracted from cellular lysates; and 3) colocalization of calmodulin and p21(Cip1) can be detected in vivo by electron microscopy immunogold analysis. The carboxyl-terminal domain of p21(Cip1) is responsible for the calmodulin interaction, since p21(145-164) peptide is also able to bind calmodulin and to compete with full-length p21(Cip1) for the calmodulin binding. Because treatment of cells with anti-calmodulin drugs decreases the nuclear accumulation of p21(Cip1), we hypothesize that calmodulin interaction with p21(Cip1) is important for p21(Cip1), and in consequence for cyclin D-Cdk4, translocation into the cell nucleus.

Tumour suppressor protein p53 released by nuclease digestion increases at the onset of rat liver regeneration

BACKGROUND/AIMS

Protein kinase CK2 (CK2) increases when cells are committed to proliferate, as in liver regeneration. This enzyme phosphorylates the tumour suppressor protein p53, whose expression controls the levels of many other cell cycle proteins. The aim of this study was to determine if CK2 was affected by p53.

METHODS

Male Sprague-Dawley rats (200-250 g) were subjected to either partial hepatectomy or laparotomy and the levels and subcellular distribution of p53 were studied, following the approach used earlier for CK2. The levels of both proteins were also studied in the human cell lines HL-60 (devoid of p53) and HepG2 (with normal p53 levels) and in fibroblasts from transgenic p53-deficient mice (p53-/-) or homozygous for wild-type p53 (p53+/+). Computer-assisted search was used to detect p53 consensus sequences in genes for CK2 subunits Binding of p53 protein to some of these sequences was assayed by electrophoretic mobility shift assay.

RESULTS

Rat liver p53 protein was present mainly in the fraction extracted from intact nuclei by nucleases (S1) and showed a transient increase at 6 h post partial hepatectomy, as observed previously with nuclear CK2. The human CK2a gene presents the consensus sequence for trans-activation by p53 and specific binding of p53 protein to some of these sequences was detected in vitro. Total CK2a was higher in HepG2 than in HL-60 cells but total CK2 and its cytosolic/ nuclear distribution was similar in mice (p53+/+) fibroblasts and (p53-/-) fibroblasts.

CONCLUSIONS

p53 is present in the nuclease-extracted S1 fraction from liver cells, as described for CK2, and undergoes similar changes at the beginning of rat liver regeneration. However, the data on cultured cells suggest that the expression of CK2 and its subcellular localization are p53-independent events.

Heterogeneous nuclear ribonucleoprotein A2 interacts with protein kinase CK2

The catalytic subunit of protein kinase CK2 (CK2alpha) was found associated with heterogeneous nuclear ribonucleoprotein particles (hnRNPs) that contain the core proteins A2 and C1-C2. High levels of CK2 activity were also detected in these complexes. Phosphopeptide patterns of hnRNP A2 phosphorylated in vivo and in vitro by protein kinase CK2 were similar, suggesting that this kinase can phosphorylate hnRNPA2 in vivo. Binding experiments using human recombinant hnRNP A2, free human recombinant CK2alpha or CK2beta subunits, reconstituted CK2 holoenzyme and purified native rat liver CK2 indicated that hnRNP A2 associated with both catalytic and regulatory CK2 subunits, and that the interaction was independent of the presence of RNA. However, the capability of hnRNP A2 to bind to CK2 holoenzyme was lower than its binding to the isolated subunits. These data indicate that the association of CK2alpha with CK2beta interferes with the subsequent binding of hnRNP A2. HnRNP A2 inhibited the autophosphorylation of CK2beta. This effect was stronger with reconstituted human recombinant CK2 than with purified native rat liver CK2.

Activation of cdk4 and cdk2 during rat liver regeneration is associated with intranuclear rearrangements of cyclin-cdk complexes

Partial hepatectomy (PH) triggers the entry of rat liver cells into the cell cycle. The signals leading to cell-cycle activation converge into a family of kinases named cyclin-dependent kinases (cdks). Specific cyclin-cdk complexes are sequentially activated during the cell cycle. Cyclin D-cdk4 and cyclin E-cdk2 are activated during the G1 phase, cyclin A-cdk2 is activated during the S phase, and cyclin B-cdk1 during mitosis. In the present study, we have examined the timing of the activation of cdk4 and cdk2, the intracellular location of G1/S cyclins and cdks, and the relationship between location and cdk4 and cdk2 activities during rat liver regeneration after a PH. Results showed that the activity of both kinases started at 13 hours and showed maximal levels at 24 hours after hepatectomy. In quiescent cells, cyclin D3 and cdk4 were cytoplasmatic, whereas cyclin D1 was nuclear. At 5 hours after hepatectomy, cyclin D3 and cdk4 began to move into the nucleus, and at 13 hours, they were mostly nuclear. During the first 13 hours after hepatectomy, significant amounts of cyclin D1-cdk4 and cyclin D3-cdk4 complexes were formed, but they were mostly inactive. At 24 hours, these complexes were maximally activated. This activation was associated with the accumulation of cyclin D1, cyclin D3, and cdk4 in a nuclear subfraction extractable with nucleases. At 28 hours, the activity of cdk4 in this nuclear subfraction decreased when cyclin D1 moved from this fraction to the nuclear matrix (NM) and the levels of cyclin D3 diminished. The maximal activation of cdk2 at 24 hours was also associated with the accumulation of cyclin E, cyclin A, and cdk2 in this nuclease-sensitive fraction. The inactivation of cdk2 at 28 hours was associated with a strong decrease in cdk2 in this nuclear subfraction. Thus, results reported here indicate that the activation of cdk4 and cdk2 observed in rat liver cells after a PH is associated with a specific intranuclear location of these cdks and their associated cyclins.

Calmodulin is essential for cyclin-dependent kinase 4 (Cdk4) activity and nuclear accumulation of cyclin D1-Cdk4 during G1

Although it is known that calmodulin is involved in G1 progression, the calmodulin-dependent G1 events are not well understood. We have analyzed here the role of calmodulin in the activity, the expression, and the intracellular location of proteins involved in G1 progression. The addition of anti-calmodulin drugs to normal rat kidney cells in early G1 inhibited cyclin-dependent kinase 4 (Cdk4) and Cdk2 activities, as well as retinoblastoma protein phosphorylation. Protein levels of cdk4, cyclin D1, cyclin D2, cyclin E, p21, and p27 were not affected after CaM inhibition, whereas decreases in the amount of cyclin A and Cdc2 were observed. The decrease of Cdk4 activity was due neither to changes in its association to cyclin D1 nor to changes in the amount of p21 or p27 bound to cyclin D1-Cdk4 complexes. Calmodulin inhibition also produced a translocation of nuclear cyclin D1 and Cdk4 to the cytoplasm. This translocation could be responsible for the decreased Cdk4 activity upon calmodulin inhibition. Immunoprecipitation, calmodulin affinity chromatography, and direct binding experiments indicated that calmodulin associates with Cdk4 and cyclin D1 through a calmodulin-binding protein. The facts that Hsp90 interacts with Cdk4 and that its inhibition induced Cdk4 and cyclin D1 translocation to the cytoplasm point to Hsp90 as a good candidate for being the calmodulin-binding protein involved in the nuclear accumulation of Cdk4 and cyclin D1.

Disruption of the antiproliferative TGF-beta signaling pathways in human pancreatic cancer cells

Resistance to TGF-beta1 occurred in pancreatic cancer cells suggesting that inactivation of TGF-beta inhibitory signaling pathways may play an important role in human pancreatic cancer. The aim of our study was to determine the presence of alterations in the main putative components of the TGF-beta inhibitory signaling pathways (p15, Smad4, Smad2, TGFbeta-RII, CDC25A). A panel of human carcinomas of the exocrine pancreas orthotopically implanted and perpetuated in nude mice and pancreatic cancer cell lines were studied. p15 gene alterations, mainly homozygous deletions that involved exons 1 and/or 2, were found in the 62.5% (5 of 8) of pancreatic xenografts whereas Smad4 gene aberrations were found in one of eight xenografts and in two of seven cell lines. Additional aberrations in these genes were acquired during in vivo perpetuation and distal dissemination. Paradoxically, TGFbeta-RII overexpression and a decrease in CDC25A protein levels were found in all tumors and cell lines. In one cell line, resistance to TGF-beta1 occurred in the absence of alterations in the genes analysed so far. We conclude that all human pancreatic tumor cells analysed herein have non-functional TGF-beta pathways. The majority of cells harbor alterations in at least one of the putative components of TGF-beta pathways, mainly in p15 and Smad4 genes. These results suggest that inactivation of TGF-beta signaling pathways plays an important role in human pancreatic tumorigenesis.

Calmodulin inhibitor W13 induces sustained activation of ERK2 and expression of p21(cip1)

One of the major signaling pathways by which extracellular signals induce cell proliferation and differentiation involves the activation of extracellular signal-regulated kinases (ERKs). Because calmodulin is essential for quiescent cells to enter cell cycle, the role of calmodulin on ERK2 activation was studied in cultured fibroblasts. Serum, phorbol esters, or active Ras induced ERK2 activation in NIH 3T3 fibroblasts. This activation was not inhibited by calmodulin blockade. Surprisingly, inhibition of calmodulin prior to fetal bovine serum addition prolonged activation of ERK2. Furthermore, inactivation of calmodulin in serum-starved cells induced ERK2 phosphorylation that was dependent on MAP kinase kinase (MEK). Inactivation of calmodulin in serum-starved cells also induced activation of Ras, Raf, and MEK. On the contrary, tyrosine phosphorylation of tyrosine kinase receptors was not observed. These results indicate that calmodulin inhibits ERK2 activation pathway at the level of Ras. Calmodulin inhibition induced overexpression of p21(cip1) which was dependent on MEK activity. We propose that inhibition of Ras by calmodulin prevents the activation of ERK2 at low serum concentration. Thus, entering into the cell cycle after serum addition would imply the overcoming of the inhibitory effect of calmodulin and consequently ERK2 activation. Furthermore, down-regulation of Ras by calmodulin may be also important to determine the duration of ERK2 activation and to prevent a high p21(cip1) expression that would lead to an inhibition of cell proliferation.

New nuclear functions for calmodulin

The data reported here summarize a series of results which reveal new functions for nuclear calmodulin (CaM). The addition of CaM inhibitors to cultures of proliferating NRK cells blocked the activity of the cyclin-dependent protein kinases 4 (cdk4) and 2 (cdk2), which are enzymes implicated in the progression of G1 and in the onset of DNA replication, respectively. CaM modulates the activity of cdk4 by regulating the nuclear location of both cdk4 and cyclin D, its associated regulatory subunit. By using CaM-affinity chromatography, we have recently identified two new nuclear CaM-binding proteins: (i) the protein La/SSB, which is an autoantigen implicated in several autoimmune diseases such as lupus erythematosus and Sjögren's syndrome (since La/SSB participates in the process of transcription mediated by RNA polymerase III, CaM could be involved in the regulation of this process); and (ii) the protein SAP145, a member of the spliceosome-associated proteins (SAPs) which is a subunit of the splicing factor SF3(b). This finding suggests the involvement of CaM in pre-mRNA splicing. Finally, a screening for new CaM-binding proteins in the fission yeast performed by using the phage display analysis, revealed that several nucleolar-ribosomal proteins associate to CaM, suggesting that CaM modulates ribosomal assembly and/or function.

The cell cycle inhibitor p21CIP is phosphorylated by cyclin A-CDK2 complexes

We report here experimental evidence indicating that the p21CIP, the universal inhibitor of cyclin-dependent protein kinases, general inhibitor CDKs is a substrate oof cyclin A-cdk2. The evidence comes from phosphorylation experiments in which the endogenous p21CIP present in using the original cyclin A-cdk2 complexes immunoprecipitated from HeLa cells extracts can be phosphorylated by the cdk2 of the same complexes. In vitro experiments showing that reconstituted GSTcyclin A-GSTcdk2 complexes from phosphorylate recombinant GSTp21CIP confirms that p21CIP is a cyclin A-cdk2 substrate.

Cyclin A is present in the endocytic compartment of rat liver cells and increases during liver regeneration

Recent studies have implicated the cell cycle kinase cdc2 and cyclin A in the inhibition of the fusion of endocytic vesicles in vitro during mitosis. However, the presence of cyclins or their associated cyclin dependent kinases (cdks) in the endocytic fractions have not been reported. Using Western-blotting and immunocytochemistry approaches with different anticyclin A antibodies we have detected cyclin A in the endocytic compartment of the rat liver. During the pre-replicative phase of liver regeneration the amount of cyclin A in endosomes increases significantly with a peak around 12 hours after partial hepatectomy. Cyclin A-dependent kinases, cdc2 and cdk2, were also found in isolated endosome fractions, showing a distinct kinetics of accumulation during the regenerative period. Finally, histone H1 kinase activity was detected associated with cyclin A in endocytic vesicles and increased in regenerating liver. These results suggest that changes in the organization and in the function of the endocytic compartment during the hepatocellular proliferation may be modulated by proteins involved in the regulation of the cell cycle.

Nuclear protein patterns in normal T-lymphocytes and lymphoblastoid cells

We compared the two-dimensional patterns of nuclear proteins obtained from normal quiescent T lymphocytes with those from normal proliferating T lymphocytes and three lymphoblastoid cell lines (CEM, Namalwa, and Molt-4). We identified sets of nuclear proteins which are specific for normal quiescent or normal proliferating T lymphocytes, or shared by the three lymphoblastoid cell lines and absent from the normal T cells. The protein patterns from two nuclear subfractions, i.e., S1 fraction, obtained after nuclease extraction, and the nuclear matrix, were also analyzed. In S1 nuclear fraction, 6 proteins of 75 kDa [isoelectric point (pI) 4.4], 55 kDa (pI 6.7), 41 kDa (pI 4.1), 39 kDa (pI 5.0), 32 kDa (pI 5.5), and 29 kDa (pI 6.6) were found to be specifically present in normal quiescent cells but not in normal proliferating or lymphoblastoid cell lines. Five proteins of 23 kDa (pI 4.2), 23 kDa (pI 4.3), 22 kDa (pI 4.4), 21 kDa (pI 4.5), and 21 kDa (pI 4.6) were observed only in the S1 fraction of normal proliferating lymphocytes, whereas they were absent in normal quiescent cells and in the transformed cell lines. Eight proteins of 56 kDa (pI 4.7), 50 kDa (pI 4.6), 45 kDa (pI 4.4), 43 kDa (pI 4.3), 42 kDa (pI 4.3), 41 kDa (pI 4.3), 43 kDa (pI 4.2), and 42 kDa (pI 4.1) were found only in the nuclear matrix of normal quiescent cells. Moreover, two doublets of proteins of 31-33 kDa (pI 4.3) and 31-33 kDa (pI 4.2) were found only in the nuclear matrix of the normal proliferating cells and three proteins of 37 kDa (pI 3.8), 37 kDa (pI 3.7), and 35 kDa (pI 4.5) were specifically present in the nuclear matrix of the lymphoblastoid cells lines, but not in normal quiescent or activated lymphocytes.

The autoantigen La/SSB is a calmodulin-binding protein

The work reported here has been directed to the identification of new nuclear calmodulin-binding proteins. To achieve this goal, nuclei from rat hepatocytes were purified and a fraction enriched in DNA- and RNA-binding proteins was extracted using DNase I and RNase A. Calmodulin-binding proteins present in this nuclear subfraction were purified by chromatography using first a DEAE-Sephacel column and subsequently a calmodulin-Sepharose column. Four major polypeptides of 118, 107, 48 and 45 kDa were found to bind to the calmodulin column in a Ca(2+)-dependent way. [125I]-calmodulin overlay analysis confirmed that the proteins of 118, 48 and 45 kDa are calmodulin-binding proteins. These proteins bind single-stranded and also double-stranded DNA. A partial amino acid sequence obtained from the 48 kDa protein revealed a 100% identity with the La/SSB protein, an autoantigen implicated in several autoimmune diseases, such as lupus erythematosus and Sjögren's syndrome. Two-dimensional gel electrophoresis, Western blot analysis and experiments of binding to poly(U), also supports the identity of p48 as La/SSB. CaM and La/SSB protein colocalize in the heterochromatinic regions within the nucleus of rat hepatocytes. Preincubation of La/SSB with calmodulin in the presence of Ca2+ resulted in an increase in the binding of ssDNA to La/SSB, suggesting that calmodulin can play a role in the regulation of the association of La/SSB with DNA.

Putative nuclear cdk2 substrates in normal and transformed cells

The presence of putative substrates of cdk2 in a nuclear fraction obtained by DNase plus RNase extraction (S1 fraction) has been analyzed by immunoprecipitation using specific anti-cdk2 antibodies, followed by phosphorylation assays. S1 nuclear fractions from four different cellular types, two normal (rat hepatocytes and human T lymphocytes) and two transformed (HeLa and Namalwa cells), have been studied. Results indicate that the normal cells share three putative nuclear cdk2 substrates of 21, 37 and 57 kDa. On the other hand, only a substrate of 20 kDa is shared by the two transformed cell lines. On comparing the proliferating normal lymphocytes with the lymphoblastoid cell line Namalwa, it can be observed that they share two proteins of 40 and 70 kDa.

Changes in the activity of nuclear protein kinase CK2 during rat liver regeneration

Protein kinase CK2 has been found in two nuclear fractions obtained after treatment of purified rat liver nuclei with nucleases (S1 fraction) and subsequently with 1.6 M NaCl (S2 fraction). In both fractions three isoforms of the alpha subunit were identified. Two of them corresponded to the classical alpha and alpha' subunits, whereas the identity of the third one (alpha 3) remains unknown. In the S1 fraction two peaks of CK2 activity were detected at 6 h (5.5 fold) and 24 h (1.9 fold) after partial hepatectomy, whereas no significant changes were found in the S2 fraction. At 6 h after laparatomy a much lower increase of CK2 in S1 fraction was also detected (2.5 fold). The increases in CK2 activity found at 6 h after hepatectomy or laparatomy were accompanied with rises in the amount of the alpha subunit.

Calmodulin is involved in the induction of DNA polymerases alpha and delta activities in normal rat kidney cells activated to proliferate

Normal rat kidney cells that reenter the cell cycle from quiescence start DNA synthesis at 12 h following serum addition and reach a maximum after 20 h. We have previously shown that the activation of DNA polymerase alpha, and the expression of the proliferating cell nuclear antigen were inhibited when the anti-calmodulin drug W13 is added to the cell cultures. Here we have analyzed the effect of W13 on the activity of DNA polymerase delta and on the expression of replication protein A. The results showed that the blockade of calmodulin by W13 produced an almost complete inhibition of DNA polymerase delta activity whereas the activity of DNA polymerase alpha was only partially inhibited. Finally, the expression of replication protein A was not affected after W13 treatment. Our data suggest that calmodulin might regulate DNA replication through the control of the activities of DNA polymerases alpha and delta and the expression of proliferating cell nuclear antigen.

Differential distribution of heterogeneous nuclear ribonucleoproteins in rat tissues

Heterogeneous nuclear ribonucleoproteins bind to RNA as long as it is transcribed. Since their binding can be sequence-specific, it has been suggested that their expression in different tissues could vary depending on the specific mRNA processing requirements. In order to better establish this possibility we studied the presence of the heterogeneous nuclear ribonucleoproteins A1, A2/B1, C and D in the cell nuclei of different rat tissues by one- and two-dimensional immunoblotting. We found that these proteins were heterogeneously distributed among tissues and that they were found in different proportions.

Addition of calmodulin antagonists to NRK cells during G1 inhibits proliferating cell nuclear antigen expression

The mRNAs of most proteins involved in DNA synthesis show an S phase correlated expression when mammalian cells are stimulated to proliferate from G0. This is the case for proliferating cell nuclear antigen (PCNA), a cofactor of DNA polymerase delta that is essential for the synthesis of the leading and lagging strands of DNA. Normal rat kidney cells re-entering the cell cycle from quiescence start DNA synthesis at 12 h and reach a maximum at 20 h. The expression of PCNA parallels the synthesis of DNA. Progression through the S phase was inhibited by addition of the anticalmodulin drug W13 to the cells during G1, 5 h after activation. W13 also inhibited the increase in both PCNA protein and mRNA indicating that calmodulin regulates its expression. Using TK-ts13 cells transfected with a plasmid containing the thymidine kinase gene under the control of the human 2.8 kb PCNA promoter, we demonstrated that this promoter is not regulated by calmodulin. The half-life of PCNA mRNA during G1/S transition was not modified by the treatment with W13, indicating that the decrease in the mRNA found when calmodulin was inhibited is not due to changes in its stability. Run-on assays revealed that control cells produced predominantly complete PCNA transcripts during S phase, while short incomplete transcripts were generated in W13-treated cells at the same time. These results indicate that calmodulin participates in a more direct or indirect way during G1 in the activation of PCNA expression. From data presented here it can be suggested that calmodulin activates the release of a transcriptional block leading to an increase in the amount of PCNA during S phase.

Phosphorylation of rat liver heterogeneous nuclear ribonucleoproteins A2 and C can be modulated by calmodulin

It was previously reported that the phosphorylation of three proteins of 36, 40 to 42, and 50 kDa by casein kinase 2 is inhibited by calmodulin in nuclear extracts from rat liver cells (R. Bosser, R. Aligué, D. Guerini, N. Agell, E. Carafoli, and O. Bachs, J. Biol. Chem. 268:15477-15483, 1993). By immunoblotting, peptide mapping, and endogenous phosphorylation experiments, the 36- and 40- to 42-kDa proteins have been identified as the A2 and C proteins, respectively, of the heterogeneous nuclear ribonucleoprotein particles. To better understand the mechanism by which calmodulin inhibits the phosphorylation of these proteins, they were purified by using single-stranded DNA chromatography, and the effect of calmodulin on their phosphorylation by casein kinase 2 was analyzed. Results revealed that whereas calmodulin inhibited the phosphorylation of purified A2 and C proteins in a Ca(2+)-dependent manner, it did not affect the casein kinase 2 phosphorylation of a different protein substrate, i.e., beta-casein. These results indicate that the effect of calmodulin was not on casein kinase 2 activity but on specific protein substrates. The finding that the A2 and C proteins can bind to a calmodulin-Sepharose column in a Ca(2+)-dependent manner suggests that this association could prevent the phosphorylation of the proteins by casein kinase 2. Immunoelectron microscopy studies have revealed that such interactions could also occur in vivo, since calmodulin and A2 and C proteins colocalize on the ribonucleoprotein particles in rat liver cell nuclei.

Protein kinase C regulates calmodulin expression in NRK cells activated to proliferate from quiescence

We have investigated the levels of calmodulin protein and calmodulin mRNA species during proliferative activation of NRK cells. Cells activated to proliferate from quiescence started to replicate DNA at 15 h, reaching a maximum at 20 h after serum addition. The maximum of mitosis was observed at 24 h. Quiescent cells showed a calmodulin concentration of 1.5 ng/micrograms of protein. At 10 h after serum addition the amount of calmodulin started to increase, reaching values of 3.0 ng/micrograms of protein at 24 h. NRK cells expressed predominantly 3 species of calmodulin transcripts: the 1.7 kb from CaM I, the 1.4 kb from CaM II and the 2.3 kb from CaM III. The amount of all the 3 transcripts was low in quiescent cells and 10 h after activation the levels were already high, reaching a maximum around 20 h. At the latter time the amount of the 3 calmodulin mRNAs was 5-10-fold higher than in serum starved cells. Run-on experiments showed that at 20 h after activation the transcription rates of the 3 calmodulin genes were higher than in quiescent cells. The addition of protein kinase C inhibitors to the cultures blocked the increase of the calmodulin transcripts while inhibitors of protein kinase A did not have any effect. Moreover, the addition of submitogenic doses of phorbol 12-tetradecanoate induced the increase of all 3 calmodulin transcripts. These results indicate that protein kinase C regulates calmodulin expression when NRK cells are activated to proliferate.

Nuclear calmodulin/62 kDa calmodulin-binding protein complexes in interphasic and mitotic cells

We report here that a 62 kDa calmodulin-binding protein (p62), recently identified in the nucleus of rat hepatocytes, neurons and glial cells, consists of four polypeptides showing pI values between 5.9 and 6.1. By using a DNA-binding overlay assay we found that the two most basic of the p62 polypeptides bind both single- and double-stranded DNA. The intranuclear distribution of calmodulin and p62 was analysed in hepatocytes and astrocyte precursor cells, and in proliferating and differentiated astrocytes in primary cultures by immunogold-labeling methods. In non-dividing cells nuclear calmodulin was mostly localized in heterochromatin although it was also present in euchromatin and nucleoli. A similar pattern was observed for p62, with the difference that it was not located in nucleoli. p62/calmodulin complexes, mainly located over heterochromatin domains were also observed in interphasic cells. These complexes remained associated with the nuclear matrix after in situ sequential extraction with nucleases and high-salt containing buffers. In dividing cells, both calmodulin and p62 were found distributed over all the mitotic chromosomes but the p62/calmodulin aggregates were disrupted. These results suggest a role for calmodulin and p62 in the condensation of the chromatin.

Calmodulin expression during rat liver regeneration

We have investigated the messenger RNAs expressed from the three calmodulin genes during rat liver regeneration. The results revealed that all the calmodulin transcripts increased from 8 hr after a partial hepatectomy, although differences in the timing and the level of expression from the three genes were observed. Calmodulin I transcripts peaked at 16 hr, whereas calmodulin II and calmodulin III progressively increased from 8 to 24 hr. At 24 hr after surgery, calmodulin I, calmodulin II and the 2.3 kb calmodulin III transcripts reached values of a 6-fold increase, whereas the 0.8 kb product of calmodulin III increased 25-fold. At 30 hr the levels of all the calmodulin transcripts were similar to those observed at 24 hr. The transcription rates of the three calmodulin genes augmented after hepatectomy (calmodulin I and calmodulin II twofold and calmodulin III fourfold), indicating that the elevation of the calmodulin transcripts could be, at least partially, the result of this increase in the transcription rates. The total calmodulin concentration also increased twofold at 24 hr after hepatectomy. We also report that the administration of the beta-adrenergic blocker, D,L-propranolol inhibited the accumulation of calmodulin protein without significantly affecting the increase of the messenger RNAs. These results indicate that the expression of calmodulin observed during liver regeneration could be regulated by cyclic AMP at the translational or posttranslational level.

Cyclin/cdk2 complexes in the nucleus of HeLa cells

Two different fractions of cdk2 and cdc2 have been found in the nucleus of HeLa cells. One, which can be extracted by nuclease treatment, possibly associated with DNA- or RNA-containing structures and another one, which is bound to the nuclear matrix. Nuclear cdk2 forms high molecular weight complexes which migrate at the same position as DNA polymerase alpha and proliferating cell nuclear antigen in sucrose gradient centrifugation experiments. These results suggest that nuclear cdk2 complexes could be associated with the replication factories. Immunoprecipitation experiments reveal that nuclear cdk2 complexes display histone H1-kinase activity and phosphorylate a protein of 18 kDa which is present in these complexes.

Casein kinase 2 and protein substrates are released from rat liver cells nuclei by DNase or RNase digestion

Casein kinase 2 was released from rat liver cells nuclei by digestion with DNase I plus RNase A. This treatment also released three major substrates of 50, 40-42, and 37 kDa. Casein kinase 2 and substrates were also extracted by DNase or RNase separately. However, in DNase extracts only the 37 kDa protein was phosphorylated by casein kinase 2, whereas in RNase extracts all three substrates were phosphorylated. When the DNase extracts were subsequently treated with RNase the 40-42 substrates were then phosphorylated, indicating that their interaction with RNA prevents their phosphorylation by casein kinase 2. The ratio of B: alpha subunits of casein kinase 2 present in the nuclease extracts was higher than that of the purified enzyme, which is assumed to be 1:1. A further analysis by sucrose gradient centrifugation revealed that under physiological salt conditions casein kinase 2 from nuclease extracts formed large aggregates (higher than 300 kDa) which were disrupted at 400 mM KCl. At the latter KCl concentration CK-2 activity was localized at a position corresponding to a M(r) of 230-250 kDa, which is still higher than the typical tetrameric form of the enzyme.

Microsomal localization of cyclin A and cdk2 in proliferating rat liver cells

The expression and intracellular localization of cyclin A and cdk2 have been analyzed in rat liver cells proliferatively activated in vivo by a partial hepatectomy. Western blot analysis revealed that cyclin A started to increase during G1 (at 6 h after hepatectomy) reaching maximal levels during S phase (at 18 h). Cdk2 began to increase during late G1 (at 12 h) peaking also at 18 h. At the latter time cyclin A was mainly localized in the microsomal fraction, although it was also present in cytosol, plasma membrane and nucleus. Active cyclin/cdks complexes containing cyclin A and cdk2 were obtained by precipitation with p13-Sepharose after solubilization of microsomes with triton X-100. The presence of active cyclin A/cdk2 complexes in microsomes was confirmed by immunoprecipitation experiments with anti-cdk2 antibodies. These results suggest a putative role of cyclin A/cdk2 during S phase which would be related with microsomal function.

Expression of calmodulin and calmodulin binding proteins in lymphoblastoid cells

Calmodulin is encoded in vertebrates by three different genes: CALM1, CALM2, and CALM3. We have examined the mRNAs expressed from these three genes in eight lines of human lymphoblastoid cells (Namalwa, Raji, Ramos, JY, Molt-4, Jurkat, CEM, and HPB-ALL). We found that all these cell lines (except Ramos) overexpressed CALM3 transcripts, which led to an increase of total CaM protein with respect to quiescent normal T lymphocytes. The nuclear concentration of calmodulin was measured in two of these lymphoblastoid cell lines (JY and HPB-ALL) and compared to quiescent and phytohemagglutinin-activated T lymphocytes. Activated lymphocytes showed a 2-fold increase of nuclear calmodulin with respect to quiescent cells, whereas in the two lymphoblastoid cell lines, nuclear calmodulin remained similar to that of quiescent cells. The levels of a calmodulin-binding protein of 150 kDa in the homogenates of the eight lymphoblastoid lines was found to be higher than those of quiescent and activated lymphocytes. Likewise, the amount of three calmodulin-binding proteins of 240, 200, and 170 kDa was also increased in several of the cell lines, but not in all of them. The 170-kDa protein was only expressed by activated lymphocytes and lymphoblastoid cells, suggesting that it could be specific for proliferating cells. In the nuclei of activated lymphocytes and lymphoblastoid cells, a decrease of a calmodulin-binding protein of 110 kDa and increases of three other of 240, 180 and 170 kDa were also detected.

Calmodulin regulates the expression of cdks, cyclins and replicative enzymes during proliferative activation of human T lymphocytes

Cell cycle is regulated by the activation of complexes of cyclins and cyclin-dependent protein kinases at specific points. Quiescent cells lack both cyclins and cyclin-dependent kinases but their expression is induced after proliferative activation. Cyclin A/cdk2 complexes are involved in the onset of DNA replication whereas cyclin B/cdc2 trigger mitosis. We report here that Ca2+ and calmodulin regulate the expression of cdk2, cdc2, cyclin B and the proliferating cell nuclear antigen (a co-factor of DNA polymerase-delta) in human T lymphocytes. Likewise, the expression of cdk4, cyclin A and DNA polymerase-alpha is dependent of the synergistic effect of both the Ca2+/calmodulin and the protein kinase C pathways. Thus, calmodulin controls DNA synthesis by regulating the levels of cdk2 and proliferating cell nuclear antigen and mitosis entry by modulating the expression of cyclin B and cdc2.

Calmodulin expression during proliferative activation of human T lymphocytes

We have investigated the levels of calmodulin mRNA species and calmodulin protein during proliferation of human T lymphocytes. Quiescent lymphocytes expressed the 1.7 kb transcript of CaM I, the 1.4 kb of CaM II and the 2.3 kb of CaM III. Phytohaemagglutinin added to peripheral blood lymphocytes induced DNA replication which started at 48 h and reached a maximum at 72 h after activation. All the species of calmodulin mRNAs, including the 4.0 kb transcript of CaM I and the 0.8 kb of CaM III which were not detected in quiescent cells, increased during lymphocyte proliferation. At 72 h after activation, the increase of CaM I and CaM II transcripts were found to be 2-fold whereas CaM III mRNAs increased 9-fold. The cellular content of calmodulin protein was also found to increase during proliferation and calmodulin accumulations in cytosol and nuclei of activated cells were observed. Two calmodulin binding proteins of 180 and 170 kD were found to increase in the nuclei of proliferating lymphocytes, whereas on the contrary 3 other calmodulin binding proteins of 110, 62 and 60 kD decreased during proliferation.

Calmodulin can modulate protein phosphorylation in rat liver cells nuclei

This report describes the immunological identification of a 60-kDa calmodulin-binding protein, previously detected in the nuclei of rat liver cells (Bachs, O., Lanini, L., Serratosa, J., Coll, M.J., Bastos, R., Aligué, R., Rius, E., and Carafoli, E. (1990) J. Biol. Chem. 265, 18595-18600), as the calmodulin-dependent protein phosphatase calcineurin. Calcineurin could be extracted from the nuclei by incubation with DNase and RNase, indicating that it is associated with nuclear structures sensitive to the action of nucleases (chromatin or/and ribonucleoproteins). The presence of calcineurin in the nuclei of rat liver cells indicates that calmodulin may modulate the phosphorylation level of nuclear proteins by promoting their dephosphorylation. This report also shows that calmodulin inhibits the activity of casein kinase-2 in the nuclear fractions obtained by nuclease extraction. Phosphorylation experiments indicate that casein kinase-2 phosphorylates three major substrates of 100, 42-44, and 37 kDa as well as other minor proteins in the nuclease extracts. Calmodulin reduces the phosphorylation level of the two latter major proteins and of a minor band of 50 kDa. Thus, nuclear calmodulin in rat liver cells could regulate phosphorylation of nuclear proteins by at least two mechanisms: 1) activation of calcineurin and 2) inhibition of casein kinase-2.

Nuclear calmodulin-binding proteins in rat neurons

By using a 125I-calmodulin overlay assay, three major high-affinity calmodulin-binding proteins, showing apparent molecular masses of 135, 60, and 50 kDa, have been detected in purified nuclear fractions isolated from rat neurons. It has been shown that after extraction of the nuclei with nucleases and high salt, all these proteins remain strongly associated with the nuclear matrix. The 60- and 50-kDa proteins have been previously identified as subunits of the calmodulin-dependent protein kinase II. We report here the immunoblot identification of the 135-kDa calmodulin-binding protein as myosin light chain kinase. We also show that the calmodulin-dependent protein phosphatase calcineurin is present in the neuronal nuclei and associated with the nuclear matrix. The nuclear localization of both calcineurin and myosin light chain kinase has been confirmed by immunocytochemical studies.

Effect of alpha 1-adrenergic blockade on nucleolar growth, chromatin relaxation, and histone H1(0) content in regenerating liver

alpha 1-Adrenergic agonists are known to be involved in the regulation of hepatocyte proliferation after a partial hepatectomy. The blockade of alpha 1-adrenergic receptors with the specific antagonist prazosin inhibits DNA synthesis which peaks at 24 h after surgery. In this report we have studied the effects of prazosin administration on several events occurring during liver regeneration. The results show that the nuclear volume and nucleolar volume density of hepatocytes were increased and that the relative amount of heterochromatin decreased at 24 h. The increase in hepatocyte nucleolar volume density and the decrease in the relative amount of heterochromatin were partially abolished by prazosin administration while the increase in the nuclear volume was not affected. The relative amount of the histone H1 variant H1(0) was reduced in 24-h regenerating liver and prazosin treatment prevented this reduction.

Increase of cytokeratin D during liver regeneration: association with the nuclear matrix

An increase of a 45 kD protein (p45) in the nuclear matrix has been observed when rat liver cells were proliferatively activated in vivo by a partial hepatectomy. The maximal levels of the association of p45 with the nuclear matrix have been detected 24 hr after hepatectomy just at the time when DNA replication is also maximal. By amino acid sequence analysis, immunoblotting and immunocytochemical methods, it has been demonstrated that p45 is identical to rat cytokeratin D. Immunogold staining of nuclear matrix-intermediate filament preparations from cultured hepatocytes indicated that p45 is associated with cytoskeletal filaments that are strongly interconnected to the lamina, whereas no intranuclear localization of the protein has been detected. With an overlay assay a specific binding of labeled p45 to two nonidentified high-molecular weight proteins and also to lamin B has been observed. Northern blot analysis revealed a biphasic pattern of expression of the messenger RNA for cytokeratin D during liver regeneration. A sharp increase in the messenger RNA levels occurred in the prereplicative phase of liver regeneration a few hours before the accumulation of the protein in the nuclear matrix fraction, and a second peak occurred 48 hr after partial hepatectomy.

The calcium pump of the liver nuclear membrane is identical to that of endoplasmic reticulum

The envelope membrane of rat liver nuclei contains a P-type Ca(2+)-transporting pump, revealed by the presence of a Ca(2+)-stimulated phosphoenzyme. The level of the nuclear phosphoenzyme in autoradiographed polyacrylamide gels was decreased by lanthanum, as typically observed in the endoplasmic reticulum Ca2+ pump. It was also decreased by thapsigargin and 2,5-di-(tert-butyl)-1,4-benzohydroquinone, two accepted inhibitors of the endoplasmic reticulum Ca(2+)-ATPase. Comparative proteolysis of the phosphorylated enzyme of liver microsomes (endoplasmic reticulum) and nuclear membranes revealed an identical cleavage pattern. In addition, antibodies raised against the endoplasmic reticulum Ca2+ pump cross-reacted with the pump in the nuclear membranes. The findings show that nuclear membranes contain a Ca(2+)-transporting pump closely related to that of the endoplasmic reticulum, if not identical to it. The pump is likely to be involved in the control of nuclear free calcium.

Calmodulin regulates DNA polymerase alpha activity during proliferative activation of NRK cells

When Normal Rat Kidney cells are allowed to reenter the cell cycle after quiescence they start to replicate DNA around 12 h, reaching a maximum at 20 h. Activation of DNA polymerase alpha parallels the increase in DNA synthesis. The addition of two different anti-calmodulin drugs, trifluoroperazine (7.5 microM) or W13 (10 micrograms/ml), to the media at 4 h after proliferative activation, inhibits DNA synthesis by 55% and 80%, respectively. The blockade of calmodulin produced by trifluoroperazine allows the cells to progress through G1 phase but stops progression through S phase as determined by 5-Bromo deoxyuridine labeling. Both anti-calmodulin drugs also inhibit by more than 50% the increase in DNA polymerase alpha activity observed at 20 h. These results indicate that a calmodulin-dependent event, essential for the activation of DNA polymerase alpha and subsequently for DNA replication, is produced during G1. Therefore, the control of DNA polymerase alpha activation is one of the ways by which calmodulin is regulating the progression of NRK cells through S phase.

Presence of calmodulin and calmodulin-binding proteins in the nuclei of brain cells

The nuclear calmodulin levels have been measured in rat neurons and glial cells. The values are 1.0 and 1.1 micrograms/mg of protein, respectively. These levels are about threefold higher than those in the nuclei of rat liver cells. We have also investigated the presence of several calmodulin-binding proteins in the nuclei of both brain cellular types. As similarly observed in the nuclei of liver cells, we detected the presence of alpha-spectrin and a 62-kDa calmodulin-binding protein (p62) in the nuclei of neurons and glial cells by immunoblotting and immunocytochemical methods. Both proteins are enriched in the purified nuclear matrix samples from both cellular types. In contrast to that occurring in rat hepatocytes, we have not been able to detect, by immunoblotting methods, caldesmon in the nuclear matrices of neurons and glial cells. The immunocytochemical studies suggest, however, that caldesmon can be present in the nuclei but in a fraction distinct from the nuclear matrices.