Role of the ubiquitin-proteasome pathway in regulating abundance of the cyclin-dependent kinase inhibitor p27

Role of p27Kip1 and cyclin-dependent kinase 2 in the proliferation of non-small cell lung cancer

The cell cycle is governed by a family of cyclin-dependent kinases (Cdks). Cdk2 forms a functional complex with cyclin E and plays a pivotal role in the regulation of G1/S transition. Cdk2 activity is negatively regulated by interactions with inhibitors. p27Kip1, one of the most potent inhibitors of Cdk2, was recently identified as a powerful negative prognostic marker in non-small cell lung cancer as well as in colorectal and breast cancer. In the present study, the expression of p27 and Ki-67 antigen in nonneoplastic and cancerous lung tissues was determined by immunohistochemistry. After establishing that the antibody-measured p27 labeling index was a good reflection of the level of p27 expression measured by Western blotting, we show that p27 labeling index is decreased in cancerous lung tissues, compared with nonneoplastic lung tissues, and exhibits a significant inverse relation to the proliferation marker Ki-67 antigen, detected with monoclonal antibody MIB-1. Consistent with these data, all cancerous lung tissues showed enhanced degradation activity of p27 compared with nonneoplastic lung tissues and, in addition, increased levels of the phosphorylated form of Cdk2, as determined with Western blot analysis. The H1 histone kinase activity associated with Cdk2 was also increased in non-small cell lung cancers. Statistical analysis showed that proliferative activity as measured by MIB-1 labeling index was highly correlated with Cdk2 activity (r = 0.767, P < 0.0015). These results suggest that p27 and Cdk2 may play an important role in the proliferation of non-small cell cancer.

Peptidase activities of the multicatalytic protease in rat liver after voluntary and intragastric ethanol administration

Ethanol consumption slows down the rate of hepatic protein catabolism. The present study was conducted to determine whether ethanol consumption, given by voluntary (pair) feeding or by intragastric administration, affected the peptidase activities of the proteasome in rat liver. Rats were pair-fed liquid diets containing either ethanol or isocaloric maltose-dextrin. A separate group of animals was intragastrically infused continuously with similar liquid diets containing either ethanol or isocaloric dextrose. Crude liver homogenates and their cytosolic fractions were assayed for their chymotrypsin-like (Cht-L), trypsin-like (T-L), and peptidyl-glutamyl-peptide hydrolase (PGPH) activities, using specific fluorogenic peptides as substrates. Voluntary ethanol feeding did not affect the three peptidase activities of the proteasome. However, intragastric ethanol administration caused a 35% to 40% decline in the Cht-L and the T-L activities, but did not significantly change the PGPH activity. The lower peptidase activities in cytosol samples from intragastrically ethanol-fed rats were not restored to control levels by overnight dialysis, nor by the inclusion of low levels of sodium dodecyl sulfate (SDS) or of 0.5 mmol/L adenosine triphosphate (ATP) in the proteasome assay mixture. Immunoblot analyses using anti-rat liver proteaseome exhibited equal levels of immunoreactive proteasome subunits in livers of control and ethanol-fed rats. Similar results were obtained when blots were probed with antibody made specifically against the proteasome subunit, LMP-7. The results indicate that intragastric, but not voluntary, ethanol consumption differentially affects the separate catalytic activities of the proteasome without affecting its steady-state levels. Such changes may be related to the degree of ethanol-induced oxidative stress.

E-Cadherin-dependent growth suppression is mediated by the cyclin-dependent kinase inhibitor p27(KIP1)

Recent studies have demonstrated the importance of E-cadherin, a homophilic cell-cell adhesion molecule, in contact inhibition of growth of normal epithelial cells. Many tumor cells also maintain strong intercellular adhesion, and are growth-inhibited by cell- cell contact, especially when grown in three-dimensional culture. To determine if E-cadherin could mediate contact-dependent growth inhibition of nonadherent EMT/6 mouse mammary carcinoma cells that lack E-cadherin, we transfected these cells with an exogenous E-cadherin expression vector. E-cadherin expression in EMT/6 cells resulted in tighter adhesion of multicellular spheroids and a reduced proliferative fraction in three-dimensional culture. In addition to increased cell-cell adhesion, E-cadherin expression also resulted in dephosphorylation of the retinoblastoma protein, an increase in the level of the cyclin-dependent kinase inhibitor p27(kip1) and a late reduction in cyclin D1 protein. Tightly adherent spheroids also showed increased levels of p27 bound to the cyclin E-cdk2 complex, and a reduction in cyclin E-cdk2 activity. Exposure to E-cadherin-neutralizing antibodies in three-dimensional culture simultaneously prevented adhesion and stimulated proliferation of E-cadherin transfectants as well as a panel of human colon, breast, and lung carcinoma cell lines that express functional E-cadherin. To test the importance of p27 in E-cadherin-dependent growth inhibition, we engineered E-cadherin-positive cells to express inducible p27. By forcing expression of p27 levels similar to those observed in aggregated cells, the stimulatory effect of E-cadherin-neutralizing antibodies on proliferation could be inhibited. This study demonstrates that E-cadherin, classically described as an invasion suppressor, is also a major growth suppressor, and its ability to inhibit proliferation involves upregulation of the cyclin-dependent kinase inhibitor p27.

Interaction of p27 with E1A and its effect on CDK kinase activity

The interaction of p27 with adenovirus (Ad) E1A was investigated to study its possible role in cell-cycle regulation and transformation by E1A. In in vitro binding assays, recombinant p27 proteins were shown to bind 12S and 13S E1A products of both Ad12 and Ad5. The amino-terminal region of p27, but not the carboxyl-terminal region, was responsible for the E1A binding. In the Ad12 E1A proteins, the C-terminal region showed relative importance in p27 binding. Phosphorylation of histone H1 or E1A proteins by CDK2 complex was inhibited by p27, but, in contrast, p27 stimulated the phosphorylation of E1A proteins by CDK4. Thus, the interaction of p27 and E1A proteins may modulate the function of E1A in cell-cycle control by regulating E1A phosphorylation.

Protection from oxidative inactivation of the 20S proteasome by heat-shock protein 90

Heat-shock protein 90 (Hsp 90) has been implicated in both protection against oxidative inactivation and inhibition of the multicatalytic proteinase (MCP, also known as 20 S proteasome). We report here that the protective and inhibitory effects of Hsp 90 depend on the activation state of the proteasome. Hsp 90 (and also alpha-crystallin) inhibits the N-Cbz-Leu-Leu-Leu-MCA-hydrolysing activity (Cbz=benzyloxycarbonyl; MCA=7-amido-4-methylcoumarin) when the rat liver MCP is in its latent form, but no inhibitory effects are observed when the MCP is in its active form. Metal-catalysed oxidation of the active MCP inactivates the Ala-Ala-Phe-MCA-hydrolysing (chymotrypsin-like), N-Boc-Leu-Ser-Thr-Arg-MCA-hydrolysing (trypsin-like; Boc=t-butyloxycarbonyl), N-Cbz-Leu-Leu-Glu-beta-naphthylamine-hydrolysing (peptidylglutamyl-peptide hydrolase) and N-Cbz-Leu-Leu-Leu-MCA-hydrolysing activities, whereas these activities are actually increased when the MCP is in its latent form. Hsp 90 protects against oxidative inactivation of the trypsin-like and N-Cbz-Leu-Leu-Leu-MCA-hydrolysing activities of the MCP active form, and alpha-crystallin protects the trypsin-like activity. The specificity of the Hsp 90-mediated protection was assessed by a quantitative analysis of the two-dimensional electrophoretic pattern of MCP subunits before and after oxidation of the MCP, in the presence or absence of Hsp 90. Treatment of the FAO hepatoma cell line with iron and ascorbate was found to inactivate the MCP. Hsp 90 overexpression obtained by challenging the cells with iron was associated with a decreased susceptibility to oxidative inactivation of the MCP trypsin-like activity. Depletion of Hsp 90 by using antisense oligonucleotides resulted in an increased susceptibility to oxidative inactivation of the MCP trypsin-like activity, providing evidence for the physiological relevance of Hsp 90-mediated protection of the MCP.

Mantle cell lymphomas lack expression of p27Kip1, a cyclin-dependent kinase inhibitor

p27Kip1 is a cyclin-dependent kinase inhibitor that regulates the decision to enter S phase or withdraw from the cell cycle. In resting cells, the level of p27Kip1 provides an inhibitory threshold above which G1 cyclin D/E/cyclin-dependent kinases accumulate before activation; however, in cycling cells, p27Kip1 protein is sequestered by high levels of active cyclin D/cyclin-dependent kinase 4 complexes. As a group, the cyclin-dependent kinase inhibitors have been proposed to act as tumor suppressor genes, and several members have been implicated in the pathogenesis of a variety of human cancers. We examined p27Kip1 expression in 116 non-Hodgkin's lymphomas including 50 cases of MCL (40 typical and 10 blastic variants), 21 follicular lymphomas, 20 diffuse large B-cell lymphomas, 16 chronic lymphocytic leukemias, 8 marginal zone B-cell lymphomas, and 1 splenic marginal zone lymphoma, and correlated its expression with that of the proliferation marker Ki67 (MiB1) and with p53. p27Kip1 gene structure was analyzed by Southern blot in the group of MCLs. In all cases of non-Hodgkin's lymphoma other than MCL, p27Kip1 expression was inversely related to the proliferation index as measured by Ki67. In contrast, in typical MCL, p27Kip1 expression was negative in 35 of 40 (88%) cases, irrespective of the proliferative rate (median 15%; range 2 to 90%). Paradoxically, in the blastic variant of MCL, 8 of 10 (80%) cases showed expression of p27Kip1, despite a high proliferation rate (median 60%; range 32 to 100%). However, the staining in most of the cases was less intense than in the reactive T lymphocytes. Deletions of p27Kip1 gene were not found in any of the 25 cases examined. p53 expression was found in 15 of 50 cases of MCL: 7 of 10 (70%) in the blastic variant and 8 of 40 (20%) in the typical MCL (70% vs. 20%, P < 0.0045). These results demonstrate that MCLs, in contrast to other non-Hodgkin's lymphomas and normal lymphoid tissue, fail to correlate p27Kip1 expression with the proliferation rate. This peculiar uncoupling of p27Kip1 protein expression from the proliferation rate may be related to the high levels of cyclin D1 expressed in MCL and is likely to have profound effects on cell cycle regulation and contribute to the pathogenesis of MCL.

Progression of LNCaP prostate tumor cells during androgen deprivation: hormone-independent growth, repression of proliferation by androgen, and role for p27Kip1 in androgen-induced cell cycle arrest

The molecular mechanism of androgen-independent growth of prostate cancer after androgen ablation was explored in LNCaP cells. An androgen-dependent clonal subline of the LNCaP human prostate carcinoma cell line, LNCaP 104-S, progressed to a slow growing stage (104-R1) and then to a faster growing stage (104-R2) during more than 2 yr of continuous culture in the absence of androgen. Androgen-induced proliferation of 104-S cells is inhibited by the antiandrogen Casodex, while proliferation of 104-R1 and 104-R2 cells is unaffected by Casodex. This indicates that proliferation of 104-R1 and 104-R2 cells is not supported by low levels of androgen in the culture medium. Compared with LNCaP 104-S cells, both 104-R1 and 104-R2 cells express higher basal levels of androgen receptor (AR), and proliferation of these two cell lines is paradoxically repressed by androgen. After continuous passage in androgen-containing medium, 104-R1 cells reverted back to an androgen-dependent phenotype. The mechanism of androgenic repression of 104-R1 and 104-R2 sublines was further evaluated by examining the role of critical regulatory factors involved in the control of cell cycle progression. At concentrations that repressed growth, androgen transiently induced the expression of the cyclin-dependent kinase (cdk) inhibitor p21waf1/cip1 in 104-R1 cells, while expression of the cdk inhibitor p27Kip1 was persistently induced by androgen in both 104-R1 and 104-R2 cells. Induced expression of murine p27Kip1 in 104-R2 cells resulted in G1 arrest. Specific immunoprecipitates of Cdk2 but not Cdk4 from androgen-treated 104-R1 cells contained both p21waf1/cip1 and p27Kip1. This observation was confirmed by in vitro assay of histone H1 and Rb (retinoblastoma protein) phosphorylation by the proteins associated with the immune complex. Furthermore, inhibition of Cdk2 activity correlated with the accumulation of p27Kip1 and not p21waf1/cip1. From these results we conclude that androgenic repression of LNCaP 104-R1 and 104-R2 cell proliferation is due to the induction of p27Kip1, which in turn inhibits Cdk2, a factor critical for cell cycle progression and proliferation.

Protein expression of the cell-cycle inhibitor p27Kip1 in malignant melanoma: inverse correlation with disease-free survival

In the present study we analyzed, by immunohistochemistry, a panel of human melanomas for protein expression of the cyclin-dependent kinase (cdk) inhibitor p27Kip1 and evaluated whether deregulated expression correlates with clinical outcome for this type of cancer. We found that p27Kip1 was strongly expressed by normal melanocytes and benign nevi, whereas in malignant melanoma, a heterogeneous expression pattern was observed. In the case of nodular melanomas, the level of p27Kip1 was found to correlate significantly with the thickness of the tumor, with less protein expressed in thicker lesions. We also found that patients having tumors with fewer than 5% p27Kip1-staining cells had a significantly higher risk of early relapse of their disease compared with those expressing moderate or high levels. In contrast, the level of p27Kip1 did not correlate with tumor thickness or disease-free survival in patients with superficial spreading melanomas, suggesting that p27Kip1 may play different roles in these two major pathological subgroups of malignant melanoma. Furthermore, p27Kip1 did not appear to have an influence on overall survival for either subgroup. When we examined the combined effect of p21WAF1/CIP1 (another cdk inhibitor) and p27Kip1 on clinical outcome, we found that analysis of these two cdk inhibitors together may have greater prognostic potential than either alone. In conclusion, our results suggest that virtually complete loss of p27Kip1 protein expression has potential importance as a prognostic indicator of early relapse in patients with nodular melanoma The results, furthermore, underscore the value of analyzing multiple cell cycle regulatory proteins to obtain the most reliable indication of prognosis.

Antigen receptor signaling induces MAP kinase-mediated phosphorylation and degradation of the BCL-6 transcription factor

The bcl-6 proto-oncogene encodes a POZ/zinc finger transcriptional repressor expressed in germinal center (GC) B and T cells and required for GC formation and antibody affinity maturation. Deregulation of bcl-6 expression by chromosomal rearrangements and point mutations of the bcl-6 promoter region are implicated in the pathogenesis of B-cell lymphoma. The signals regulating bcl-6 expression are not known. Here we show that antigen receptor activation leads to BCL-6 phosphorylation by mitogen-activated protein kinase (MAPK). Phosphorylation, in turn, targets BCL-6 for rapid degradation by the ubiquitin/proteasome pathway. These findings indicate that BCL-6 expression is directly controlled by the antigen receptor via MAPK activation. This signaling pathway may be crucial for the control of B-cell differentiation and antibody response and has implications for the regulation of other POZ/zinc finger transcription factors in other tissues.

CD40-mediated induction of p21 accumulation in resting and cycling B cells

The accumulation of G1 cell cycle-related proteins by resting or cycling B cells stimulated with B cell antigen receptor (BCR)- and T helper (Th) cell-derived signals is documented. Resting B cells constitutively express cyclin dependent kinase (cdk)4, cdk2 and the cyclin dependent kinase inhibitor (CKI), p27. The initiation of optimal proliferation with F(ab')2 anti-mu plus paraformaldehyde-fixed CD40 ligand-baculovirus-infected Sf9 cells (CD40L/Sf9 cells) increases accumulation of both cdk4 and cdk2 while decreasing p27 levels. B cells express cyclin D2 early during cycle progression, while cyclin D3 and E are not expressed until 18 h poststimulation and cyclin A by 24 h poststimulation. Cycling B cells express heightened levels of all these cyclins and cdks. Although neither BCR- nor CD40-mediated signals appreciably alter cycling B cell accumulation of cyclins D2, cdk4 and cdk2, the absence of BCR-derived signals results in a decreased accumulation of cyclins D3 and E. Finally, CD40-mediated signals induce resting B cells to accumulate the CKI, p21, while cycling B cells require both BCR- and CD40-mediated signals to maintain increased expression of p21. Thus, a Th cell-derived signal may impact upon both resting and cycling B cell cycle progression, at least in part, by regulating the accumulation of p21. The functional consequences of p21 accumulation as cells enter and move through the cell cycle are discussed.

Human CUL1 forms an evolutionarily conserved ubiquitin ligase complex (SCF) with SKP1 and an F-box protein

The SCF ubiquitin ligase complex of budding yeast triggers DNA replication by catalyzing ubiquitination of the S phase cyclin-dependent kinase inhibitor SIC1. SCF is composed of three proteins-ySKP1, CDC53 (Cullin), and the F-box protein CDC4-that are conserved from yeast to humans. As part of an effort to identify components and substrates of a putative human SCF complex, we isolated hSKP1 in a two-hybrid screen with hCUL1, the closest human homologue of CDC53. Here, we show that hCUL1 associates with hSKP1 in vivo and directly interacts with both hSKP1 and the human F-box protein SKP2 in vitro, forming an SCF-like particle. Moreover, hCUL1 complements the growth defect of yeast cdc53(ts) mutants, associates with ubiquitination-promoting activity in human cell extracts, and can assemble into functional, chimeric ubiquitin ligase complexes with yeast SCF components. Taken together, these data suggest that hCUL1 functions as part of an SCF ubiquitin ligase complex in human cells. Further application of biochemical assays similar to those described here can now be used to identify regulators/components of hCUL1-based SCF complexes, to determine whether the hCUL2-hCUL5 proteins also are components of ubiquitin ligase complexes in human cells, and to screen for chemical compounds that modulate the activities of the hSKP1 and hCUL1 proteins.

Prognostic Significance of the Cell Cycle Inhibitor p27Kip1 in Chronic B-Cell Lymphocytic Leukemia

B-cell chronic lymphocytic leukemia (B-CLL) is characterized by the accumulation of resting lymphocytes. The identification of p27kip1, a cyclin-dependent kinase inhibitor that contributes to cell cycle arrest and represents a link between extracellular signals and cell cycle, prompted us to study p27 protein in the lymphocytes from 88 patients with B-CLL and 32 patients with other chronic B-lymphoproliferative disorders. The expression of p27 protein was higher in B-CLL samples with variations among them. In B-CLL, p27 levels were independent of absolute number of circulating lymphocytes, but strongly correlated with both lymphocyte and total tumor mass (TTM) doubling time. High p27 expression was associated with a poorer overall prognosis. In vitro, there was an increased spontaneous survival of B-CLL cells expressing high p27 levels. Interleukin-4 (IL-4) upregulated p27 levels in B-CLL cells, while fludarabine decreased p27 levels. Thus, our results indicate that p27 may be a valuable kinetic marker in B-CLL by providing instantaneous estimation of the disease doubling time. In addition, these results suggest that there is a link between p27 expression and the ability of CLL cells to undergo apoptosis.

UBPY: a growth-regulated human ubiquitin isopeptidase

The ubiquitin pathway has been implicated in the regulation of the abundance of proteins that control cell growth and proliferation. We have identified and characterized a novel human ubiquitin isopeptidase, UBPY, which both as a recombinant protein and upon immunoprecipitation from cell extracts is able to cleave linear or isopeptide-linked ubiquitin chains. UBPY accumulates upon growth stimulation of starved human fibroblasts, and its levels decrease in response to growth arrest induced by cell-cell contact. Inhibition of UBPY accumulation by antisense plasmid microinjection prevents fibroblasts from entering S-phase in response to serum stimulation. By increasing or decreasing the cellular abundance of UBPY or by overexpressing a catalytic site mutant, we detect substantial changes in the total pattern of protein ubiquitination, which correlate stringently with cell proliferation. Our results suggest that UBPY plays a role in regulating the overall function of the ubiquitin-proteasome pathway. Affecting the function of a specific UBP in vivo could provide novel tools for controlling mammalian cell proliferation.

Prognostic Significance of the Cell Cycle Inhibitor p27Kip1 in Chronic B-Cell Lymphocytic Leukemia

B-cell chronic lymphocytic leukemia (B-CLL) is characterized by the accumulation of resting lymphocytes. The identification of p27kip1, a cyclin-dependent kinase inhibitor that contributes to cell cycle arrest and represents a link between extracellular signals and cell cycle, prompted us to study p27 protein in the lymphocytes from 88 patients with B-CLL and 32 patients with other chronic B-lymphoproliferative disorders. The expression of p27 protein was higher in B-CLL samples with variations among them. In B-CLL, p27 levels were independent of absolute number of circulating lymphocytes, but strongly correlated with both lymphocyte and total tumor mass (TTM) doubling time. High p27 expression was associated with a poorer overall prognosis. In vitro, there was an increased spontaneous survival of B-CLL cells expressing high p27 levels. Interleukin-4 (IL-4) upregulated p27 levels in B-CLL cells, while fludarabine decreased p27 levels. Thus, our results indicate that p27 may be a valuable kinetic marker in B-CLL by providing instantaneous estimation of the disease doubling time. In addition, these results suggest that there is a link between p27 expression and the ability of CLL cells to undergo apoptosis.

Rapamycin inhibition of the G1 to S transition is mediated by effects on cyclin D1 mRNA and protein stability

The immunosuppressant rapamycin has been shown previously to inhibit the G1/S transition in several cell types by prolonging the G1 phase of the cell cycle. This process appears to be controlled, in part, by the rapamycin-sensitive FK506-binding protein-rapamycin-associated protein-p70 S6 kinase (p70(S6k)) pathway and the cyclin-dependent kinases (Cdk). We now show that in serum-stimulated NIH 3T3 cells, rapamycin treatment delays the accumulation of cyclin D1 mRNA during progression through G1. Rapamycin also appears to affect stability of the transcript. The combined transcriptional and post-transcriptional effects of the drug ultimately result in decreased levels of cyclin D1 protein. Moreover, degradation of newly synthesized cyclin D1 protein is accelerated by rapamycin, a process prevented by inclusion of the proteasome inhibitor, N-acetyl-Leu-Leu-norleucinal. The overall effect of rapamycin on cyclin D1 leads, in turn, to impaired formation of active complexes with Cdk4, a process which triggers retargeting of the p27(Kip1) inhibitor to cyclin E/Cdk2. In view of this novel experimental evidence, we discuss a possible mechanism for the rapamycin-induced cell cycle arrest at the G1/S transition.

Oncogenic Abl and Src tyrosine kinases elicit the ubiquitin-dependent degradation of target proteins through a Ras-independent pathway

Oncogenic forms of the Abl and Src tyrosine kinases trigger the destruction of the Abi proteins, a family of Abl-interacting proteins that antagonize the oncogenic potential of Abl after overexpression in fibroblasts. The destruction of the Abi proteins requires tyrosine kinase activity and is dependent on the ubiquitin-proteasome pathway. We show that degradation of the Abi proteins occurs through a Ras-independent pathway. Significantly, expression of the Abi proteins is lost in cell lines and bone marrow cells isolated from patients with aggressive Bcr-Abl-positive leukemias. These findings suggest that loss of Abi proteins may be a component in the progression of Bcr-Abl-positive leukemias and identify a novel pathway linking activated nonreceptor protein tyrosine kinases to the destruction of specific target proteins through the ubiquitin-proteasome pathway.

The regulation of cyclin-dependent kinase inhibitors (CKIs)

Inhibitors of cyclin-dependent kinases (CKIs) play key roles in coordinating cell proliferation and development. They also function to control critical cell cycle transitions and as effectors of checkpoint pathways. The activity of CKIs is tightly controlled through the cell cycle and in response to various signals. Regulation generally affects the levels or availability of the CKIs rather than changing their intrinsic activities. Mechanisms controlling CKI function include the regulation of transcription, translation and proteolysis. In addition some signals appear to induce sequestration of CKIs within the cells, thereby changing their ability to interact with specific targets.

Aberrations of the G1- and G1/S-regulating genes in human cancer

Deregulated cell proliferation is the hallmark of cancer, and convergent data from the fields of cell-cycle research and molecular oncology have revealed the key role played by abnormalities of the cell-cycle control genes in multistep tumorigenesis. Along with the p53-mediated DNA damage checkpoint, the G1-governing pathway of D-type cyclins, their partner cyclin-dependent kinases (Cdk), Cdk inhibitors, and the retinoblastoma protein constitute a functional unit and prominent oncogenic target. We have learned a great deal about the molecular basis of G1 phase progression and G1/S transition, their proto-oncogenic defects, and potential clinical significance including diagnostic and prognostic applications and new approaches to gene therapy of cancer.

G1/S regulatory mechanisms from yeast to man

Cyclin-dependent kinases play a key role in promoting and regulating the transition from G1 to S phase in all eukaryotic organisms. The kinase activities involved are distinguished from those participating in other cell cycle phase transitions in that they are driven by a class of specialised G1-specific cyclins. Although the G1 regulatory components have diverged structurally in the course of evolution, the regulatory mechanisms and principles remain highly conserved from yeast to vertebrates. An important issue that remains is that of identifying the principal targets phosphorylated by G1 cyclin-dependent kinases.

A temporal and biochemical link between growth factor-activated MAP kinases, cyclin D1 induction and cell cycle entry

Cell cycle re-entry requires the growth factor-stimulation of at least two distinct classes of protein kinases: (i) the p42/p44 MAP kinases activated by the Ras > Raf > MKK cascade and (ii) the G1 cyclin-dependent protein kinases (CDKs). Specific inactivation of either class of kinase arrests fibroblasts in G1. Growth factors promote nuclear translocation and persistent activation of p42/p44 MAP kinases during the entire G0/G1 period. Here, we demonstrate that induction of cyclin D1, and therefore cdk4/6 activity associated with, is positively controlled by the p42/p44 MAP kinase cascade whereas the parallel cytokines/stress-activated p38MAP kinase cascade is antagonistic. Finally, using an antisense approach we demonstrate that p27Kip1 plays a key role in setting the growth factor-dependency of the G0 state.

Regulation of G1 progression in fission yeast by the rum1+ gene product

Recently it has been found that B-type cyclins in fission yeast regulate the activation of the cdc2 kinase to promote the onset of both DNA replication and mitosis. cig2 is the major G1 cyclin while cdc13 is the principal mitotic cyclin. cdc13 also has an additional function in G2 phase, preventing more than one round of DNA replication per cell cycle. In opposition to these cyclins the rum1 inhibitor, a protein present exclusively in G1, prevents premature activation of the cdc2/cig2 and the cdc2/cdc13 complexes until cells have reached the critical cell size required to pass Start and initiate a new cell cycle.

Differential expression of the p27Kip1 mRNA in IFN-sensitive and resistant cell lines

IFNs arrest the growth of a small cell lung cancer (SCLC) cell line NCI-H82 in the G1 phase but not the growth of the derived cell line NCI-H82R. Progression through the G1 phase is controlled by positive and negative regulatory genes. Oncoviral genes can override this control. In this study, we compared the effects of human IFN alpha 2b on the mRNA levels of the Cdk inhibitor p27Kip1 in NCI-H82, NCI-H82R and HPV 16 E7-transfected NCI-H82 cell lines. Induction of the 2-5 Oligoadenylate synthetase (2-5 OAS) gene was used as a marker of IFN-dependent signal transduction The expression of p27Kip1 mRNA increased at 48 and 72 hr after IFN alpha 2b addition in sensitive cells. In contrast, p27Kip1 mRNA had only slight variations in both the resistant and E7-transfected cells. Interestingly, the E7-transfected NCI-H82 cells became resistant to the IFN alpha 2b anti-proliferative effect. Our results suggest that p27Kip1 could be a key mediator of the IFN alpha 2b-induced growth arrest and that HPV 16 E7 might affect p27Kip1 inducibility, originating IFN alpha 2b-resistant cells.

Inactivation of the tumor suppressor PTEN/MMAC1 in advanced human prostate cancer through loss of expression

The recently identified PTEN/MMAC1 gene is a candidate tumor suppressor implicated in multiple tumor types based on mutations or homozygous deletions of the gene in certain human cancers. No studies of PTEN/MMAC1 mRNA or protein expression in cancer cells have been reported, primarily because of significant numbers of normal cells contaminating most tumor samples and because of the lack of antibody reagents. We examined PTEN/MMAC1 in advanced prostate cancer for gene mutations or abnormalities in expression by using a series of recently derived xenografts free of normal human cells and a PTEN/MMAC1-specific antibody. Only 1 of 10 tumors contained a homozygous deletion of PTEN/MMAC1, and no mutations were detected in the entire coding region of the remaining nine xenografts. However, five of these showed reduced or absent PTEN/MMAC1 expression by Northern analysis and reverse transcription-PCR of mRNA. PTEN/MMAC1 mRNA expression was restored in nonexpressing prostate cancer cells by in vitro treatment with the demethylating agent 5-azadeoxycytidine. Alterations in PTEN/MMAC1 expression were confirmed at the protein level by immunoblot analysis, and immunohistochemical studies show that the endogenous wild-type PTEN/MMAC1 protein is localized exclusively in the cytoplasm. These results demonstrate that loss of PTEN/MMAC1 expression occurs frequently in advanced prostate cancer.

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.

Effects of Guanine Nucleotide Depletion on Cell Cycle Progression in Human T Lymphocytes

Depletion of guanine nucleotide pools after inhibition of inosine monophosphate dehydrogenase (IMPDH) potently inhibits DNA synthesis by arresting cells in G1 and has been shown to induce the differentiation of cultured myeloid and erythroid cell lines, as well as chronic granulocytic leukemic cells after blast transformation. Inhibitors of IMPDH are also highly effective as immunosuppressive agents. The mechanism underlying these pleiotropic effects of depletion of guanine nucleotides is unknown. We have examined the effects of mycophenolic acid (MPA), a potent IMPDH inhibitor, on the cell cycle progression of activated normal human T lymphocytes. MPA treatment resulted in the inhibition of pRb phosphorylation and cell entry into S phase. The expression of cyclin D3, a major component of the cyclin-dependent kinase (CDK) activity required for pRb phosphorylation, was completely abrogated by MPA treatment of T cells activated by interleukin-2 (IL-2) and leucoagglutinin (PHA-L), whereas the expression of cyclin D2, CDK6, and CDK4 was more mildly attenuated. The direct kinase activity of a complex immunoprecipitated with anti-CDK6 antibody was also inhibited. In addition, MPA prevented the IL-2–induced elimination of p27Kip1, a CDK inhibitor, and resulted in the retention of high levels of p27Kip1 in IL-2/PHA-L–treated T cells bound to CDK2. These results indicate that inhibition of the de novo synthesis of guanine nucleotides blocks the transition of normal peripheral blood T lymphocytes from G0 to S phase in early- to mid-G1 and that this cell cycle arrest results from inhibition of the induction of cyclin D/CDK6 kinase and the elimination of p27Kip1 inhibitory activity.

p27Kip1 alters the response of cells to mitogen and is part of a cell-intrinsic timer that arrests the cell cycle and initiates differentiation

BACKGROUND

In many vertebrate cell lineages, precursor cells divide a limited number of times before they arrest and terminally differentiate into postmitotic cells. It is not known what causes them to stop dividing. We have been studying the 'stopping' mechanism in the proliferating precursor cells that give rise to oligodendrocytes, the cells that make myelin in the central nervous system. We showed previously that the cyclin-dependent kinase inhibitor p27Kip1 (p27) progressively accumulates in cultured precursor cells as they proliferate and that the time course of the increase is consistent with the possibility that p27 accumulation is part of a cell-intrinsic timer that arrests the cell cycle and initiates differentiation at the appropriate time.

RESULTS

We now provide direct evidence that p27 is part of the intrinsic timer. We show that although p27-/- precursor cells stop dividing and differentiate almost as fast as wild-type cells when deprived of mitogen, when stimulated by saturating amounts of mitogen they have a normal cell-cycle time but tend to go through one or two more divisions than wild-type cells before they stop and differentiate. Cells that are p27+/- behave in an intermediate way, going through at most one extra division, indicating that the levels of p27 matter in the way the timer works. We also show that p27-/- precursor cells are more sensitive than wild-type cells to the mitogenic effect of platelet-derived growth factor.

CONCLUSIONS

These findings demonstrate that p27 is part of the normal timer that determines when oligodendrocyte precursor cells stop dividing and differentiate, at least in vitro. It seems likely that p27 plays a similar role in many other cell lineages, which could explain the phenotypes of the p27-/- and p27+/- mice.

Coupled transcriptional and translational control of cyclin-dependent kinase inhibitor p18INK4c expression during myogenesis

Terminal differentiation of many cell types involves permanent withdrawal from the cell division cycle. The p18INK4c protein, a member of the p16/INK4 cyclin-dependent kinase (CDK) inhibitor family, is induced more than 50-fold during myogenic differentiation of mouse C2C12 myoblasts to become the predominant CDK inhibitor complexed with CDK4 and CDK6 in terminally differentiated myotubes. We have found that the p18INK4c gene expresses two mRNA transcripts--a 2.4-kb transcript, p18(L), and a 1.2-kb transcript, p18(S). In proliferating C2C12 myoblasts, only the larger p18(L) transcript is expressed from an upstream promoter. As C2C12 cells are induced to differentiate into permanently arrested myotubes, the abundance of the p18(L) transcript decreases. The smaller p18(S) transcript expressed from a downstream promoter becomes detectable by 12 h postinduction and is the predominant transcript expressed in terminally differentiated myotubes. Both transcripts contain coding exons 2 and 3, but p18(L) uniquely contains an additional noncoding 1.2-kb exon, exon 1, corresponding exclusively to the 5' untranslated region (5' UTR). The expression pattern of the shorter p18(S) transcript, but not that of the longer p18(L) transcript, correlates with terminal differentiation of muscle, lung, liver, thymus, and eye lens cells during mouse embryo development. The presence of the long 5' UTR in exon 1 attenuated the translation of p18(L) transcript, while its absence from the shorter p18(S) transcript resulted in significantly more efficient translation of the p18 protein. Our results demonstrate that during terminal muscle cell differentiation, induction of the p18 protein is regulated by promoter switching coupled with translational control.

Stat proteins control lymphocyte proliferation by regulating p27Kip1 expression

The proliferation of lymphocytes in response to cytokine stimulation is essential for a variety of immune responses. Recent studies with signal transducer and activator of transcription 6 (Stat6)-deficient mice have demonstrated that this protein is required for the normal proliferation of lymphocytes in response to interleukin-4 (IL-4). In this report, we show that the impaired IL-4-induced proliferative response of Stat6-deficient lymphocytes is not due to an inability to activate alternate signaling pathways, such as those involving insulin receptor substrates, or to a failure to upregulate IL-4 receptor levels. Cell cycle analysis showed that the percentage of Stat6-deficient lymphocytes that transit from the G1 to the S phase of the cell cycle following IL-4 stimulation is lower than that of control lymphocytes. Although the regulation of many genes involved in the control of cytokine-induced proliferation is normal in Stat6-deficient lymphocytes, protein levels of the cdk inhibitor p27Kip1 were found to be markedly dysregulated. p27Kip1 is expressed at significantly higher levels in Stat6-deficient lymphocytes than in control cells following IL-4 stimulation. The higher level of p27Kip1 expression seen in IL-4-stimulated Stat6-deficient lymphocytes correlates with decreased cdk2-associated kinase activity and is the result of the increased accumulation of protein rather than altered mRNA expression. Similarly, higher levels of p27Kip1 protein expression are also seen following IL-12 stimulation of Stat4-deficient lymphocytes than are seen following stimulation of control cells. These data suggest that Stat proteins may control the cytokine-induced proliferative response of activated T cells by regulating the expression of cell cycle inhibitors so that cyclin-cdk complexes may function to promote transition from the G1 to the S phase of the cell cycle.

Expression of the cyclin kinase inhibitor, p27kip1, in developing and mature human kidney

It has been shown that glomerular visceral epithelial cells (VEC) proliferate during glomerulogenesis, but differentiated VEC of the fetal kidney do not. It is also recognized that the proliferative capacity of the VEC in mature kidneys is very limited, and according to some investigators, may be completely absent. The basis for this remains unknown. Cell proliferation is controlled by cell cycle-related proteins, of which one class, the cyclin kinase inhibitors (CKI), cause cell cycle arrest and inhibit proliferation. A role for CKI in kidney development is not known. Accordingly, we examined the expression of the CKI p27kip1 (p27) in developing and mature human kidney tissue. Concomitant expression of markers of cell proliferation, Ki-67-related antigen (Ki-67) and proliferating cell nuclear antigen (PCNA), also were examined in fetal and mature human kidney tissue by immunocytochemical techniques. In developing kidney, Ki-67 and PCNA expression are most pronounced in the nephrogenic zone where expression correlates inversely with increasing glomerular maturation. In well-differentiated glomeruli, Ki-67 and PCNA expression is present in some parietal epithelial cells but is absent in the VEC. In contrast, p27 staining exhibits a reverse gradient of expression. p27 is absent in the proliferating tissue exhibiting the earliest stages of differentiation, whereas expression is widespread in the differentiated epithelial cells of more mature glomeruli, in which detectable cell proliferation has ceased. Expression of p27 was not identified in fetal mesangial or glomerular endothelial cells. In the mature human kidney, the pattern of p27 expression identified in differentiated fetal glomeruli persists and appears to be constitutive and specific for glomerular VEC. This pattern of p27 expression in terminally differentiated VEC may explain their limited proliferative capacity in response to injury. This is the first demonstration of a potential role for p27 in human renal development.

Synergistic effect of cyclic AMP and insulin on the expression of cyclin A gene in Swiss 3T3 cells

Cyclic AMP and insulin exert a synergistic mitogenic effect on Swiss 3T3 cells. Here, we showed that the activity of cyclin A-dependent kinase was elevated 3-fold at 24 h after cotreatment of cells with dibutyryl cAMP and insulin. The cotreatment elevated cyclin A protein levels 12-fold higher than those of insulin-treated cells without altering the levels of CDK2 and p27Kip1 proteins. Interestingly, the half-life of cyclin A protein increased from 7 h in the insulin-treated cells to 22 h in the cotreated cells. Levels of cyclin A mRNA were also elevated 9-fold. Cotreatment synergistically increased the binding activities of transcription factors to the NF-Y and the E2F-like sites of the cyclin A promoter. However, neither NF-Y nor E2F was involved in the binding. These results suggest that the synergistic elevation of cyclin A protein may be achieved by both the stabilization of cyclin A protein and the recruitment of transcription factors to the NF-Y and E2F-like sites of the cyclin A promoter.

Phosphatidylinositol 3-kinase inhibitors block aortic smooth muscle cell proliferation in mid-late G1 phase: effect on cyclin-dependent kinase 2 and the inhibitory protein p27KIP1

In the present study, we investigated the involvement of phosphatidylinositol 3-kinase (PI 3-kinase) activity in the progression of vascular smooth muscle cells (VSMCs) throughout the G1 phase of cell cycle. Addition of two selective inhibitors of PI 3-kinase, LY 294002 or wortmannin, to quiescent VSMCs prevented serum-induced DNA synthesis in a dose-dependent manner with IC50 of 8.7 +/- 2.0 microM and 53.9 +/- 8.5 nM, respectively. Time course studies revealed that the two PI 3-kinase inhibitors blocked VSMC proliferation in mid-late G1 phase, about 6 h before the G1/S transition. This G1 growth arrest was due, at least in part, to the reduction of the CDK2 associated kinase activity resulting mainly from the upregulation of the inhibitory protein p27KIP1.

Defects in the ubiquitin pathway induce caspase-independent apoptosis blocked by Bcl-2

Apoptosis requires the activation of caspases (formerly interleukin 1beta-converting enzyme-like proteases), in particular those related to the caspase-3/7/6 subfamily. Recent data, however, revealed that, although caspase-specific inhibitors delay apoptosis, they are often incapable of preventing it. To obtain evidence for caspase-independent steps of apoptosis, we artificially created a high amount of short-lived or aberrant proteins by blocking the ubiquitin degradation pathway. A temperature-sensitive defect in the ubiquitin-activating enzyme E1 induced apoptosis independent of the activation of caspase-3 and -6 and the cleavage of their respective substrates poly(ADP-ribose) polymerase and lamin A. In addition, neither the caspase 3/7-specific inhibitor N-benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone nor the general caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone were capable of blocking this type of cell death. By contrast, Bcl-2 overexpression effectively protected cells from apoptosis induced by a defect in the E1 enzyme at the nonpermissive temperature. Bcl-2 acted downstream of the accumulation of short-lived or aberrant proteins because it did not prevent the overexpression of the short-lived proteins p53, p27(kip1), and cyclins D1 and B1 under conditions of decreased ubiquitination. These results suggest the existence of short-lived proteins that may serve the role of caspase-independent effectors of apoptosis and attractive targets of the death-protective action of Bcl-2.

Association of activating transcription factor 2 (ATF2) with the ubiquitin-conjugating enzyme hUBC9. Implication of the ubiquitin/proteasome pathway in regulation of ATF2 in T cells

Activating transcription factor 2 (ATF2) is regulated by phosphorylation via the Jun N-terminal kinase, and its binding activity is markedly induced at late stages of T and B lymphocyte activation (Feuerstein, N., Firestein, R., Aiyer, N., Xiao, H., Murasko, D., and Cristofalo, V. (1996) J. Immunol. 156, 4582-4593). To identify proteins that interact specifically with ATF2 in lymphocytes, the yeast two-hybrid interaction system was employed using ATF2 cDNA as a "bait." In two separate screenings, a clone was identified that revealed a novel sequence with homology to several members of the ubiquitin-conjugating enzyme family. An identical sequence was recently reported as the human homolog of the yeast UBC9, hUBC9. Northern blot analysis revealed a 1.3-kilobase RNA transcript, which showed differential levels of expression in various human tissues and a moderate induction after a 48-h stimulation of peripheral blood T lymphocytes. An antibody that was generated against the bacterially expressed glutathione S-transferase-hUBC9 detected a approximately 19-kDa protein, which localizes predominantly in the nuclei of T cells. Further quantitative assays using the yeast two-hybrid system confirmed a high and specific level of interaction of hUBC9 with ATF2 and lack of interaction with lamin or control vectors. Two other cyclic AMP-responsive element-binding transcription factors, CREB and ATF1, also showed significant levels of interaction with hUBC9. However, this interaction was severalfold lower as compared with ATF2. Far Western blot analysis confirmed the specific binding of ATF2 and hUBC9 also in vitro. Evidence is presented that indicates a physiological significance for the interaction of hUBC9 with ATF2. (a) We show that ATF2 is ubiquitinated in vivo and in vitro, and (b) ATF2 ubiquitination in vitro is facilitated by addition of purified hUBC9. (c) ATF2 is shown to undergo a proteolytic process, which is rapidly regulated upon T cell activation concomitant with induction of ATF2 phosphorylation. (d) A proteasome inhibitor delays the down-regulation of ATF2 phophorylation after T cell activation. Taken collectively, these results implicate a role for hUBC9 and the ubiquitin/proteasome pathway in regulation of ATF2 in T cells.

Inhibition of Cell Cycle Progression by a Synthetic Peptide Corresponding to Residues 65–79 of an HLA Class II Sequence: Functional Similarities but Mechanistic Differences with the Immunosuppressive Drug Rapamycin

A synthetic peptide corresponding to a region of the α1 α-helix of DQA03011 (DQ 65–79) inhibits the proliferation of human PBL and T cells in an allele-nonspecific manner. It blocks proliferation stimulated by anti-CD3 mAb, PHA-P, and alloantigen, but not by PMA and ionomycin. Substitution of each amino acid with serine shows that residues 66, 68, 69, 71–73, and 75–79 are critical for function. Inhibition of proliferation is long lasting and is not reversible with exogenous IL-2. The peptide can be added 24 to 48 h after stimulation and still block proliferation. The DQ 65–79 peptide does not affect expression of IL-2 or IL-2R; however, IL-2-stimulated proliferation is inhibited. Cell cycle progression is blocked at the G1/S transition, and the activity of cdk2 (cyclin-dependent kinase 2) kinase is impaired by the continued presence of p27. Although these results suggest a mechanism similar to that of rapamycin, the peptide inhibition is not reversed with FK-506, which indicates a distinct mechanism.

Inhibition of cyclin D1 expression and induction of apoptosis by inostamycin in small cell lung carcinoma cells

Previously, we demonstrated that inostamycin, an inhibitor of phosphatidylinositol turnover, caused cell cycle arrest at the G1 phase, inhibiting the expression of cyclins D1 and E in normal cells. In the present study, we examined the effects of inostamycin on cell cycle progression and apoptosis in human small cell lung carcinoma Ms-1 cells. Treatment of exponentially proliferating Ms-1 cells with low concentrations of inostamycin caused cells to accumulate in the G1 phase. We found that inostamycin decreased cyclin D1, and increased cyclin-dependent kinase inhibitors such as p21WAF1 and p27KIP1 in Ms-1 cells. On the other hand, higher concentrations of inostamycin induced morphological apoptosis and DNA fragmentation in Ms-1 cells without affecting the expression of p53, Bcl-2 and Bax. Inostamycin-induced apoptosis was suppressed by an inhibitor of caspase-3, and a 17 kDa fragment of activated caspase-3 was detected following inostamycin treatment. Therefore, caspase-3(-like) would appear to be involved in inostamycin-induced apoptosis. On the other hand, an inhibitor of caspase-3(-like) proteases did not affect the inhibitory effect of inostamycin on cyclin D1 expression, suggesting that caspase-3(-like) proteases were not responsible for inostamycin-induced G1 arrest.

Inhibition of in vivo rat liver regeneration by 2-acetylaminofluorene affects the regulation of cell cycle-related proteins

The effects of dietary 2-acetylaminofluorene (2-AAF) on cell cycle-related proteins was studied in regenerating livers from male Wistar rats. The levels of cyclins, cyclin dependent kinases (cdks), and related proteins were studied at different times during the first cell cycle after partial hepatectomy (PH). The frequency of proliferation cell nuclear antigen (PCNA)-positive nuclei, a marker of S phase progression, was almost zero during the first 27 hours after PH in the mitoinhibited 2-AAF-treated rats, while about 50% of the nuclei were labeled 24 hours after PH in control animals. Accordingly, Western blot tests showed markedly elevated PCNA protein levels from 18 hours to the end of S phase in untreated animals but no upregulation in response to 2-AAF. Compared with control animals, animals treated with 2-AAF showed increased levels of cdk 4 and cyclin D3 from 12 and 15 hours after PH, respectively, and altered cyclicity in cyclin D3 expression. No effects on cyclin E were observed, while the increase in cdk 2 levels in control animals during late G1/S (15-27 hours) was abolished by 2-AAF. p53 was induced by 2-AAF treatment during the same period, with a peak at 24 hours. The protein detected with p21 antibodies was highly expressed in unstimulated hepatocytes in control animals, and further increased by 2-AAF. The expression was sustained until 15 hours after PH in control rats while 2-AAF-treated animals lacked detectable protein during this period; however, a transient increase was observed at 21 hours. Thus, 2-AAF affects several parameters of cell cycle regulation of possible relevance for its inhibitory effects on hepatocyte proliferation in vivo.

Suppression of cell transformation by the cyclin-dependent kinase inhibitor p57KIP2 requires binding to proliferating cell nuclear antigen

Proper control of the mammalian cell cycle requires the function of cyclin-dependent kinase (CDK) inhibitors. The p21 family currently includes three distinct genes, p21, p27(Kip1), and p57(Kip2), that share a common N-terminal domain for binding to and inhibiting the kinase activity of CDK-cyclin complexes. The p21 protein also binds to proliferating cell nuclear antigen (PCNA) through a separate C-terminal domain affecting DNA replication and repair. The p27 and p57 proteins also each contain unique C-terminal domains whose functions are unknown. Here we show that the human p57 protein, like p21, contains a PCNA-binding domain within its C terminus that, when separated from its N-terminal CDK-cyclin binding domain, can prevent DNA replication in vitro and S phase entry in vivo. Disruption of either CDK/cyclin or PCNA binding partially reduced p57's ability to suppress myc/RAS-mediated transformation in primary cells, while loss of both inhibitory functions completely eliminated p57's suppressive activity. Thus, control of cell cycle and suppression of cell transformation by p57 require both CDK and PCNA inhibitory activity, and disruption of either or both functions may lead to uncontrolled cell growth.

The von Hippel-Lindau tumor suppressor gene is required for cell cycle exit upon serum withdrawal

The inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene predisposes affected individuals to the human VHL cancer syndrome and is associated with sporadic renal cell carcinomas (RCC) and brain hemangioblastomas. VHL-negative 786-0 RCC cells are tumorigenic in nude mice which is suppressed by the reintroduction of VHL. Remarkably, this occurs without affecting the growth rate and cell cycle profile of these cells in culture. The 786-0 cell line, like many cancer cells, fails to exit the cell cycle upon serum withdrawal. Here, it is shown that reintroduction of the wild-type VHL gene restores the ability of VHL-negative RCC cancer cells to exit the cell cycle and enter G0/quiescence in low serum. Both VHL-positive and VHL-negative RCC cells exit the cell cycle by contact inhibition. The cyclin-dependent kinase inhibitor, p27, accumulates upon serum withdrawal, only in the presence of VHL, as a result of the stabilization of the protein. We propose that the loss of wild-type VHL gene results in a specific cellular defect in serum-dependent growth control, which may initiate tumor formation. This is corrected by the reintroduction of wild-type VHL, implicating VHL as the first tumor suppressor involved in the regulation of cell cycle exit, which is consistent with its gatekeeper function in the kidney.

Assembly of cyclin D-dependent kinase and titration of p27Kip1 regulated by mitogen-activated protein kinase kinase (MEK1)

A constitutively active form of mitogen-activated protein kinase kinase (MEK1) was synthesized under control of a zinc-inducible promoter in NIH 3T3 fibroblasts. Zinc treatment of serum-starved cells activated extracellular signal-regulated protein kinases (ERKs) and induced expression of cyclin D1. Newly synthesized cyclin D1 assembled with cyclin-dependent kinase-4 (CDK4) to form holoenzyme complexes that phosphorylated the retinoblastoma protein inefficiently. Activation of the MEK1/ERK pathway neither triggered degradation of the CDK inhibitor kinase inhibitory protein-1 (p27(Kip1)) nor led to activation of cyclin E- and A-dependent CDK2, and such cells did not enter the DNA synthetic (S) phase of the cell division cycle. In contrast, zinc induction of active MEK1 in cells also engineered to ectopically overexpress cyclin D1 and CDK4 subunits generated levels of cyclin D-dependent retinoblastoma protein kinase activity approximating those achieved in cells stimulated by serum. In this setting, p27(Kip1) was mobilized into complexes containing cyclin D1; cyclin E- and A-dependent CDK2 complexes were activated; and serum-starved cells entered S phase. Thus, although the activity of p27(Kip1) normally is canceled through a serum-dependent degradative process, overexpressed cyclin D1-CDK complexes sequestered p27(Kip1) and reduced the effective inhibitory threshold through a stoichiometric mechanism. A fraction of these cells completed S phase and divided, but they were unable to continuously proliferate, indicating that other serum-responsive factors ultimately became rate limiting for cell cycle progression. Therefore, the MEK/ERK pathway not only acts transcriptionally to induce the cyclin D1 gene but functions posttranslationally to regulate cyclin D1 assembly with CDK4 and to thereby help cancel p27(Kip1)-mediated inhibition.

Dipeptidyl peptidase III in malignant and non-malignant gynaecological tissue

Exopeptidases, in contrast to endopeptidases (proteinases) have been much less studied in relation to cancer. The aim of this study was to investigate one such enzyme, dipeptidyl peptidase III (DPP III), in gynaecological tissues, by measuring both the enzyme activity and enzyme content. DPP III activity was assessed in normal (n = 65), benign (n = 9) and malignant (n = 51) gynaecological tissues. A statistically significant higher DPP III activity was observed in endometrial (n = 40, P = 4.6 x 10(-7)) and ovarian (n = 11, P = 8.1 x 10(-4)) malignant tumours, whereas no significant difference was detected for leiomyomas (n = 8), if compared to the activity in normal tissue. A matched pair analysis of normal and cancerous endometrial tissue confirmed the significance of the DPP III activity increase in the transformed tissue (n = 7, P = 0.022). Western blot analysis revealed a significantly (P = 0.014) increased level of DPP III in endometrial cancer. Further, regression analysis showed a positive correlation between the activity and the content of DPP III in normal tissue (r = 0.637, P = 0.047) and in endometrial cancer (r = 0.574, P < 0.007). The increase of the DPP III activity was observed in the endometrial carcinomas of various histological types, grade or the depth of myometrial invasion. The easy-to-perform determination of this exopeptidase activity may serve as a potential indicator of endometrial and ovarian malignancies.

Proteolysis and the G1-S transition: the SCF connection

Temporal control of ubiquitin-proteasome mediated protein degradation is critical for normal G1 and S phase progression. Recent work has shown that central to the temporal control mechanism is a relationship between newly identified E3 ubiquitin protein ligases, designated SCFs (Skp1-cullin-F-box protein ligase complexes), which confer substrate specificity on ubiquitination reactions and the activities of protein kinases that phosphorylate substrates destined for destruction at specific sites, thereby converting them into preferred targets for ubiquitin modification catalyzed by SCFs. The constituents of SCFs are members of evolutionary conserved protein families. SCF-based ubiquitination pathways may play a key role in diverse biological processes, such as cell proliferation, differentiation and development.

Activation of protein kinase C triggers its ubiquitination and degradation

Treatment of cells with tumor-promoting phorbol esters results in the activation but then depletion of phorbol ester-responsive protein kinase C (PKC) isoforms. The ubiquitin-proteasome pathway has been implicated in regulating the levels of many cellular proteins, including those involved in cell cycle control. We report here that in 3Y1 rat fibroblasts, proteasome inhibitors prevent the depletion of PKC isoforms alpha, delta, and epsilon in response to the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Proteasome inhibitors also blocked the tumor-promoting effects of TPA on 3Y1 cells overexpressing c-Src, which results from the depletion of PKC delta. Consistent with the involvement of the ubiquitin-proteasome pathway in the degradation of PKC isoforms, ubiquitinated PKC alpha, delta, and epsilon were detected within 30 min of TPA treatment. Diacylglycerol, the physiological activator of PKC, also stimulated ubiquitination and degradation of PKC, suggesting that ubiquitination is a physiological response to PKC activation. Compounds that inhibit activation of PKC prevented both TPA- and diacylglycerol-induced PKC depletion and ubiquitination. Moreover, a kinase-dead ATP-binding mutant of PKC alpha could not be depleted by TPA treatment. These data are consistent with a suicide model whereby activation of PKC triggers its own degradation via the ubiquitin-proteasome pathway.

Cyclin-dependent kinases: engines, clocks, and microprocessors

Cyclin-dependent kinases (Cdks) play a well-established role in the regulation of the eukaryotic cell division cycle and have also been implicated in the control of gene transcription and other processes. Cdk activity is governed by a complex network of regulatory subunits and phosphorylation events whose precise effects on Cdk conformation have been revealed by recent crystallographic studies. In the cell, these regulatory mechanisms generate an interlinked series of Cdk oscillators that trigger the events of cell division.