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J W Harper

Publications and source records attributed to J W Harper.

At least 37 records · Page 2Linked to original sources
PubMed1999

p21(CIP1) and p57(KIP2) control muscle differentiation at the myogenin step.

Cell-cycle arrest is thought to be required for differentiation of muscle cells. However, the molecules controlling cell-cycle exit and the differentiation step(s) dependent on cell-cycle arrest are poorly understood. Here we show that two Cdk inhibitors, p21(CIP1) and p57(KIP2), redundantly control differentiation of skeletal muscle and alveoli in the lungs. Mice lacking both p21 and p57 fail to form myotubes, display increased proliferation and apoptotic rates of myoblasts, and display endoreplication in residual myotubes. This point of arrest during muscle development is identical to that of mice lacking the myogenic transcription factor myogenin, indicating a role for cell-cycle exit in myogenin function. Expression of myogenin, p21, and p57 is parallel but independent, and in response to differentiation signals, these proteins are coordinately regulated to trigger both cell-cycle exit and a dependent muscle-specific program of gene expression to initiate myoblast terminal differentiation and muscle formation.

Animals↗
PubMed1999

SGT1 encodes an essential component of the yeast kinetochore assembly pathway and a novel subunit of the SCF ubiquitin ligase complex.

We have identified SGT1 as a dosage suppressor of skp1-4, a mutation causing defects in yeast kinetochore function. Sgt1p physically associates with Skp1p in vivo and in vitro. SGT1 is an essential gene, and different sgt1 conditional mutants arrest with either a G1 or G2 DNA content. Genetic and phenotypic analyses of sgt1-3 (G2 allele) mutants support an essential role in kinetochore function. Sgt1p is required for assembling the yeast kinetochore complex, CBF3, via activation of Ctf13p. Sgt1p also associates with SCF (Skp1p/Cdc53p/F box protein) ubiquitin ligase. sgt1-5 (G1 allele) mutants are defective in Sic1p turnover in vivo and Cln1p ubiquitination in vitro. Human SGT1 rescues an sgt1 null mutation, suggesting that the function of SGT1 is conserved in evolution.

Amino Acid Sequence↗
PubMed1999

Retinoblastoma protein contains a C-terminal motif that targets it for phosphorylation by cyclin-cdk complexes.

Stable association of certain proteins, such as E2F1 and p21, with cyclin-cdk2 complexes is dependent upon a conserved cyclin-cdk2 binding motif that contains the core sequence ZRXL, where Z and X are usually basic. In vitro phosphorylation of the retinoblastoma tumor suppressor protein, pRB, by cyclin A-cdk2 and cyclin E-cdk2 was inhibited by a short peptide spanning the cyclin-cdk2 binding motif present in E2F1. Examination of the pRB C terminus revealed that it contained sequence elements related to ZRXL. Site-directed mutagenesis of one of these sequences, beginning at residue 870, impaired the phosphorylation of pRB in vitro. A synthetic peptide spanning this sequence also inhibited the phosphorylation of pRB in vitro. pRB C-terminal truncation mutants lacking this sequence were hypophosphorylated in vitro and in vivo despite the presence of intact cyclin-cdk phosphoacceptor sites. Phosphorylation of such mutants was restored by fusion to the ZRXL-like motif derived from pRB or to the ZRXL motifs from E2F1 or p21. Phospho-site-specific antibodies revealed that certain phosphoacceptor sites strictly required a C-terminal ZRXL motif whereas at least one site did not. Furthermore, this residual phosphorylation was sufficient to inactivate pRB in vivo, implying that there are additional mechanisms for directing cyclin-cdk complexes to pRB. Thus, the C terminus of pRB contains a cyclin-cdk interaction motif of the type found in E2F1 and p21 that enables it to be recognized and phosphorylated by cyclin-cdk complexes.

Amino Acid Sequence↗
PubMed1999

Levels of expression of p27KIP1 protein in human prostate and prostate cancer: an immunohistochemical analysis.

p27KIP1 is a member of the CIP/KIP family of cyclin-dependent kinase inhibitory proteins that negatively regulate cell proliferation. Recent studies reported decreased p27 expression in breast and colon carcinomas and found that the loss of p27 is associated with a poor prognosis. We report here the results of our immunohistochemical analysis of p27 in human prostate cancer. Formalin-fixed, paraffin-embedded, whole-mount sections of prostate cancer from 73 selected patients treated by radical retropubic prostatectomy were obtained from the Department of Pathology, The Methodist Hospital, Houston, Texas. Ten histologically normal and nine high-grade prostatic intraepithelia neoplasia foci were selected from these whole-mount sections, and nine cases of transplant donor prostates were chosen as controls. Also, 10 prostate cancer metastatic lymph nodes were used to compare with the primary cancer group. Sections were immunostained with a monoclonal antibody against p27 protein using the avidin-biotin complex immunohistochemical method. Immunoactivity was evaluated without knowledge of follow-up and recorded as the p27 labeling index (LI) (defined as the percentage of p27-positive cells among epithelia of the same category). The p27 (LI) in normal prostatic epithelia was 86.4+/-3.5% (the mean +/- the standard error of the mean). In contrast, the p27 immunoreactivity was significantly lower in cancers (LI: 43.5 +/-3.7%, P < .001) and in the high-grade prostatic intraepithelial neoplasia group (LI: 59.3 +/- 3.2%, P < .05). Expression of p27 in the metastatic lymph node group was significantly lower than in the other groups, including the prostate cancer cases and the cases of high-grade intraepithelial neoplasia (LI, 7.0%; P = .05). There was no association of the mean p27 LI with progression after radical prostatectomy. Nonrecurrent cases, with a mean follow-up time of greater than 5 years (n = 45), equalled 41.9%; recurrent cases, with a mean follow-up time of 18.3 months (n = 28), equalled 40.0%. The mean p27 LI was not associated with pathologic stage. Organ-confined specimens (n = 21) equalled 34.2%; cases of extraprostatic extension (n = 24) equalled 46.5%; and samples showing seminal vesicle involvement (n = 14) equalled 47.6%. In 14 cases with lymph node metastases, the mean p27 LI was 48.1% in the primary cancer (P = .2322). There was no association of the mean p27 LI with the Gleason score (P = .4747) nor with the clinical stage (P = .9914).

Cell Cycle Proteins↗
PubMed1998

Cooperation between the Cdk inhibitors p27(KIP1) and p57(KIP2) in the control of tissue growth and development.

Cell cycle exit is required for terminal differentiation of many cell types. The retinoblastoma protein Rb has been implicated both in cell cycle exit and differentiation in several tissues. Rb is negatively regulated by cyclin-dependent kinases (Cdks). The main effectors that down-regulate Cdk activity to activate Rb are not known in the lens or other tissues. In this study, using multiple mutant mice, we show that the Cdk inhibitors p27(KIP1) and p57(KIP2) function redundantly to control cell cycle exit and differentiation of lens fiber cells and placental trophoblasts. These studies demonstrate that p27(KIP1) and p57(KIP2) are critical terminal effectors of signal transduction pathways that control cell differentiation.

Animals↗
PubMed1998

The role of protein stability in the cell cycle and cancer.

In addition to the examples mentioned above, other important regulators of cell proliferation such as cyclin D, cyclin E, p21, p27 are all potentially controlled by ubiquitin-mediated proteolysis. In several of these, phosphorylation has been shown to play a role in targeting the proteins for degradation. It remains to be seen how important the SCF pathway and ubiquitin-mediated proteolysis, in general, will become in cancer research. However, it appears that we have only just scratched the surface on the role of these pathways in the control of important regulatory genes. We suspect there will be much more to come from analysis of these fascinating pathways.

Animals↗
PubMed1998

G1 cyclin-dependent kinases are sufficient to initiate DNA synthesis in quiescent human fibroblasts.

Mammalian fibroblasts require mitogens in order to exit from G0 (quiescence) and progress through the G1 phase of the cell cycle, although once they pass the restriction point late in G1 they can enter S phase and complete the cell cycle without mitogens [1]. Mitogenic signals are integrated through the GTPase Ras, which regulates the levels of cyclin D1 [2-5], a component of the cell cycle machinery that operates during G1 phase by activating cyclin-dependent kinase 4 (Cdk4). The accumulation of active cyclin E-Cdk2 complexes also requires Ras [6]. These two G1 cyclin-Cdk complexes act on a family of E2F-associated transcriptional repressors typified by the retinoblastoma protein (Rb) to bring about a transcriptional program that promotes passage through S phase [7-9], but can also activate DNA replication independently of Rb-E2F [10-12]. Although G1 cyclin-Cdk complexes are required for S-phase entry and can shorten G1 phase when overexpressed [13-15], it is not known whether they are sufficient for this transition. Here, we report that serum-starved (G0) diploid human fibroblasts initiate DNA synthesis upon microinjection of active G1 cyclin-Cdk complexes, but not upon microinjection of an S-phase cyclin-Cdk complex. These data indicate that G1 Cdk activation is rate-limiting for S-phase entry, and that Cdk activation is likely to be the primary function of growth factor signalling pathways that lead to DNA synthesis.

Carrier Proteins↗
PubMed1998

Functional interaction of human immunodeficiency virus type 1 Vpu and Gag with a novel member of the tetratricopeptide repeat protein family.

Viral protein U (Vpu) is a protein encoded by human immunodeficiency virus type 1 (HIV-1) that promotes the degradation of the virus receptor, CD4, and enhances the release of virus particles from cells. We isolated a cDNA that encodes a novel cellular protein that interacts with Vpu in vitro, in vivo, and in yeast cells. This Vpu-binding protein (UBP) has a molecular mass of 41 kDa and is expressed ubiquitously in human tissues at the RNA level. UBP is a novel member of the tetratricopeptide repeat (TPR) protein family containing four copies of the 34-amino-acid TPR motif. Other proteins that contain TPR motifs include members of the immunophilin superfamily, organelle-targeting proteins, and a protein phosphatase. UBP also interacts directly with HIV-1 Gag protein, the principal structural component of the viral capsid. However, when Vpu and Gag are coexpressed, stable interaction between UBP and Gag is diminished. Furthermore, overexpression of UBP in virus-producing cells resulted in a significant reduction in HIV-1 virion release. Taken together, these data indicate that UBP plays a role in Vpu-mediated enhancement of particle release.

Amino Acid Sequence↗
PubMed1997

F-box proteins are receptors that recruit phosphorylated substrates to the SCF ubiquitin-ligase complex.

We have reconstituted the ubiquitination pathway for the Cdk inhibitor Sic1 using recombinant proteins. Skp1, Cdc53, and the F-box protein Cdc4 form a complex, SCFCdc4, which functions as a Sic1 ubiquitin-ligase (E3) in combination with the ubiquitin conjugating enzyme (E2) Cdc34 and E1. Cdc4 assembled with Skp1 functions as the receptor that selectively binds phosphorylated Sic1. Grr1, an F-box protein involved in Cln destruction, forms complexes with Skp1 and Cdc53 and binds phosphorylated Cln1 and Cln2, but not Sic1. Because the constituents of the SCF complex are members of protein families, SCFCdc4 is likely to serve as the prototype for a large class of E3s formed by combinatorial interactions of related family members. SCF complexes couple protein kinase signaling pathways to the control of protein abundance.

Anaphase-Promoting Complex-Cyclosome↗
PubMed1997

S-Phase entry upon ectopic expression of G1 cyclin-dependent kinases in the absence of retinoblastoma protein phosphorylation.

In mammalian cells, the retinoblastoma protein (Rb) is thought to negatively regulate progression through the G1 phase of the cell cycle by its association with the transcription factor E2F [1-3]. Rb-E2F complexes suppress transcription of genes required for DNA synthesis ([4], reviewed in [3,5]), and the prevailing view is that phosphorylation of Rb by complexes of cyclin-dependent kinases (Cdks) and their regulatory cyclin subunits, and the subsequent release of active E2F, is required for S-phase entry [1-3]. This view is based, in part, on the fact that ectopic expression of cyclin-Cdks leads to Rb phosphorylation and that this modification correlates with S-phase entry [6-8]. In Drosophila, however, cyclin E expression can bypass a requirement for E2F, suggesting that cyclins may activate replication independently of the Rb/E2F pathway [9]. We sought to examine whether Rb phosphorylation is a prerequisite for S-phase entry in Rb-deficient SAOS-2 osteosarcoma cells, using a commonly used cotransfection assay [6-8,10]. We find that a G1 arrest in SAOS-2 cells mediated by an Rb mutant lacking all 14 consensus Cdk phosphorylation sites is bypassed by coexpressing G1-specific E-type or D-type cyclin-Cdk complexes, and that injection of purified cyclin-Cdks during G1 accelerates S-phase entry. Our results indicate that Rb phosphorylation is not essential for S-phase entry when G1 cyclin-Cdks are overexpressed, and that other substrates of these kinases can be rate-limiting for the G1 to S-phase transition. These data also reveal that the SAOS-2 cotransfection assay is complicated by Rb-independent effects of the coexpressed Cdks.

Animals↗
PubMed1997

Altered cell differentiation and proliferation in mice lacking p57KIP2 indicates a role in Beckwith-Wiedemann syndrome.

Mice lacking the imprinted Cdk inhibitor p57(KIP2) have altered cell proliferation and differentiation, leading to abdominal muscle defects; cleft palate; endochondral bone ossification defects with incomplete differentiation of hypertrophic chondrocytes; renal medullary dysplasia; adrenal cortical hyperplasia and cytomegaly; and lens cell hyperproliferation and apoptosis. Many of these phenotypes are also seen in patients with Beckwith-Wiedemann syndrome, a pleiotropic hereditary disorder characterized by overgrowth and predisposition to cancer, suggesting that loss of p57(KIP2) expression may play a role in the condition.

Abdomen↗
PubMed1997

Cyclin D1/Cdk4 regulates retinoblastoma protein-mediated cell cycle arrest by site-specific phosphorylation.

The retinoblastoma protein (pRb) inhibits progression through the cell cycle. Although pRb is phosphorylated when G1 cyclin-dependent kinases (Cdks) are active, the mechanisms underlying pRb regulation are unknown. In vitro phosphorylation by cyclin D1/Cdk4 leads to inactivation of pRb in a microinjection-based in vivo cell cycle assay. In contrast, phosphorylation of pRb by Cdk2 or Cdk3 in complexes with A- or E-type cyclins is not sufficient to inactivate pRb function in this assay, despite extensive phosphorylation and conversion to a slowly migrating "hyperphosphorylated form." The differential effects of phosphorylation on pRb function coincide with modification of distinct sets of sites. Serine 795 is phosphorylated efficiently by Cdk4, even in the absence of an intact LXCXE motif in cyclin D, but not by Cdk2 or Cdk3. Mutation of serine 795 to alanine prevents pRb inactivation by Cdk4 phosphorylation in the microinjection assay. This study identifies a residue whose phosphorylation is critical for inactivation of pRb-mediated growth suppression, and it indicates that hyperphosphorylation and inactivation of pRb are not necessarily synonymous.

Amino Acid Sequence↗