Clonal response of K562 leukemic cells to exogenous p21WAF1

STNM1 in human cancers: role, function and potential therapy sensitizer

STMN1 belongs to the stathmin gene family, it encodes a cytoplasmic phosphorylated protein, stathmin1, which is commonly observed in vertebrate cells. STMN1 is a structural microtubule-associated protein (MAP) that binds to microtubule protein dimers rather than microtubules, with each STMN1 binding two microtubule protein dimers and preventing their aggregation, leading to microtubule instability. STMN1 expression is elevated in a number of malignancies, and inhibition of its expression can interfere with tumor cell division. Its expression can change the division of tumor cells, thereby arresting cell growth in the G2/M phase. Moreover, STMN1 expression affects tumor cell sensitivity to anti-microtubule drug analogs, including vincristine and paclitaxel. The research on MAPs is limited, and new insights on the mechanism of STMN1 in different cancers are emerging. The effective application of STMN1 in cancer prognosis and treatment requires further understanding of this protein. Here, we summarize the general characteristics of STMN1 and outline how STMN1 plays a role in cancer development, targeting multiple signaling networks and acting as a downstream target for multiple microRNAs, circRNAs, and lincRNAs. We also summarize recent findings on the function role of STMN1 in tumor resistance and as a therapeutic target for cancer.

LRRC4 inhibits the proliferation of human glioma cells by modulating the expression of STMN1 and microtubule polymerization

LRRC4 is a tumor suppressor of glioma, and it is epigenetically inactivated commonly in glioma. Our previous study has shown that induction of LRRC4 expression inhibits the proliferation of glioma cells. However, little is known about the mechanisms underlying the action of LRRC4 in glioma cells. We employed two-dimensional fluorescence differential gel electrophoresis (2-D DIGE) and MALDI -TOF/TOF-MS/MS to identify 11 differentially expressed proteins, including the significantly down-regulated STMN1 expression in the LRRC4-expressing U251 glioma cells. The levels of STMN1 expression appeared to be positively associated with the pathogenic degrees of human glioma. Furthermore, induction of LRRC4 over-expression inhibited the STMN1 expression and U251 cell proliferation in vitro, and the glioma growth in vivo. In addition, induction of LRRC4 or knockdown of STMN1 expression induced cell cycle arrest in U251 cells, which was associated with modulating the p21, cyclin D1, and cyclin B expression, and the ERK phosphorylation, and inhibiting the CDK5 and cdc2 kinase activities, but increasing the microtubulin polymerization in U251 cells. LRRC4, at least partially by down-regulating the STMN1expression, acts as a major glioma suppressor, induces cell cycle arrest and modulates the dynamic process of microtubulin, leading to the inhibition of glioma cell proliferation and growth. Potentially, modulation of LRRC4 or STMN1 expression may be useful for design of new therapies for the intervention of glioma.

A retroviral mutagenesis screen reveals strong cooperation between Bcl11a overexpression and loss of the Nf1 tumor suppressor gene

NF1 inactivation occurs in specific human cancers, including juvenile myelomonocytic leukemia, an aggressive myeloproliferative disorder of childhood. However, evidence suggests that Nf1 loss alone does not cause leukemia. We therefore hypothesized that inactivation of the Nf1 tumor suppressor gene requires cooperating mutations to cause acute leukemia. To search for candidate genes that cooperate with Nf1 deficiency in leukemogenesis, we performed a forward genetic screen using retroviral insertion mutagenesis in Nf1 mutant mice. We identified 43 common proviral insertion sites that contain candidate genes involved in leukemogenesis. One of these genes, Bcl11a, confers a growth advantage in cultured Nf1 mutant hematopoietic cells and causes early onset of leukemia of either myeloid or lymphoid lineage in mice when expressed in Nf1-deficient bone marrow. Bcl11a-expressing cells display compromised p21(Cip1) induction, suggesting that Bcl11a's oncogenic effects are mediated, in part, through suppression of p21(Cip1). Importantly, Bcl11a is expressed in human chronic myelomonocytic leukemia and juvenile myelomonocytic leukemia samples. A subset of AML patients, who had poor outcomes, of 16 clusters, displayed high levels of BCL11A in leukemic cells. These findings suggest that deregulated Bcl11a cooperates with Nf1 in leukemogenesis, and a therapeutic strategy targeting the BCL11A pathway may prove beneficial in the treatment of leukemia.

Proteomic analysis of acute promyelocytic leukemia: PML-RARalpha leads to decreased phosphorylation of OP18 at serine 63

In the present study, we employed 2-DE to characterize the effect of the acute promyelocytic leukemia (APL)-specific PML-RARalpha fusion protein on the proteome. Differentially expressed proteins, a number of which are related to the cell cycle function, including oncoprotein18 (OP18), heat shock protein70, glucose-regulated protein75, and peptidyl-prolyl isomerase, were identified by MS. Subsequent bioinformatic pathway discovery revealed an integrated network constituting SMARCB1, MYC, and TP53-regulated pathways. The data from the DNA microarray and proteomic experiments demonstrated the correlation between the translocation and higher expression of OP18 at mRNA and protein levels. Transient cotransfection assay revealed that PML-RARalpha is a potent activator of OP18 promoter and this transcriptional activation is retinoic acid sensitive. PML-RARalpha induction also leads to decreased phosphorylation on Ser63 residue of OP18, which is okadaic acid sensitive suggesting the involvement of a phosphatase pathway. Overexpression of a constitutively phosphorylated Ser63 mutant of OP18 in PML-RARalpha expressing APL patient, PR9, and NB4 cells led to a G2/M-phase arrest in contrast to a phosphorylation-deficient Ser63 mutant and untransfected control. Taken together, our results demonstrate the significance of decreased Ser63 phosphorylation of OP18 in PML-RARalpha-mediated effects on cell cycle.

Expression of the Op18 gene is maintained by the CCAAT-binding transcription factor NF-Y

Op18 (Oncoprotein 18, Stathmin) is a mitotic regulator that is highly expressed in many cancers. We have characterized four functional CCAAT boxes in the Op18 gene located at positions: -980, -745, -599 and -65, relative to the transcriptional start site. NF-Y is a ubiquitously expressed CCAAT-binding transcription factor that regulates a number of cell cycle controlled genes. We have used promoter-reporter assays and mobility shift assays to functionally examine these CCAAT boxes. All sites contribute to the basal expression of Op18, with the sites at -980 and -599 being repressive and the sites at -745 and -65 being stimulatory. Mobility shift assays indicate that all CCAAT box sites bind factors in nuclear extracts from Hek293. However, only the repressive site at -599 and the stimulatory site at -65 are competent to bind NF-Y, suggesting that NF-Y may play a role in promoting both activation and repression of Op18 expression. The NF-Y site at -65 accounts for greater than 60% of the Op18 gene expression. EMSA competition studies indicate that NF-Y binds with a much higher affinity to the -65 site than to the -599 site, suggesting that in asynchronously growing cells NF-Y functions only to stimulate expression through the -65 binding site. These data suggest that NF-Y is a major transcription factor promoting expression of Op18.

E2F sites in the Op18 promoter are required for high level of expression in the human prostate carcinoma cell line PC-3-M

Op18 (Oncoprotein 18, Stathmin) is an important mitotic regulator that is highly expressed in many cancers including the metastatic prostate carcinoma cell line PC-3-M. Recent studies indicate that antisense-mediated down-regulation of Op18 can inhibit cellular proliferation. However, the transcriptional mechanisms responsible for its normal regulation and for its high level of expression in proliferating cells remain poorly understood. In the study presented here, we have characterized transcription factor binding sites that together account for nearly 80% of the Op18 expression in PC-3-M cells. The 5' flanking region of the Op18 gene contains four putative E2F sites located at -700 (site 1), -28 (site 2), -19 (site 3), and +720 (site 4) relative to the transcriptional start site. E2F has been implicated in both the c-Jun-mediated up-regulation and the doxorubicin-induced repression of Op18 expression. We have used promoter-reporter assays and mobility shift assays to functionally examine each of these E2F sites. Mutagenesis studies indicate that all sites contribute to the basal expression of Op18. Mutagenesis of either site 1 or 4 reduced the reporter activity by 40%, mutagenesis of site 2 reduced reporter activity by 20%, and mutations in site 3 had no effect on reporter activity. Combinatorial mutagenesis indicates that site 1 and 4 function independently, whereas site 2 functions synergistically with either site 3 or 4. Mobility shift assays indicate that all elements bind factors in the nuclear extracts of PC-3-M cells. Characterization of the sites show that site 1, though a positive element, is not E2F specific; sites 2 and 3 may contain an overlapping binding site for E2F and NF1; and site 4, which resides in intron 1, is the only site shown to interact exclusively with E2F. These studies suggest that the overexpression of Op18 in PC-3-M cells is mediated predominantly through the E2F family of transcription factors.

Altered levels and regulation of stathmin in paclitaxel-resistant ovarian cancer cells

Two paclitaxel(Ptx)-resistant ovarian cancer cell lines, 1A9/Ptx-10 and 1A9/Ptx-22, isolated from the 1A9 cell line (a clone of the A2780 line) by continuous exposure to Ptx and verapamil, have point mutations in their major beta-tubulin gene and in one or both alleles of their TP53 gene. These cells were examined for alterations in cell cycle regulators and the tubulin-binding protein stathmin. Unlike parental cells, neither 1A9/Ptx-10 nor 1A9/Ptx-22 expressed detectable levels of p21(WAF1/Cip1), a putative transcriptional regulator of stathmin, but did overexpress stathmin and Bcl2. No differences were noted in the expression levels of proliferative cell nuclear antigen or tyrosine-phosphorylated p34Cdc2. Ptx treatment altered little the expression of stathmin in the parental cell line, although it increased p21(WAF1/Cip1) levels several-fold. Infection of Ptx-resistant lines with a wild-type TP53-bearing adenovirus (AdWTp53) changed cell cycle distribution and increased the levels of p21(WAF1/Cip1), but caused no changes in stathmin levels. Microtubule drug resistance in ovarian carcinoma may be associated with altered p53/21(WAF1/Cip1) regulatory pathways for stathmin expression and function.

Cell cycle regulators and hematopoiesis

The cell cycle behavior of hematopoietic cells varies from extended quiescence to spectacular proliferation. Cell cycle regulators choreograph these transitions through variation in the makeup of cyclin-dependent kinase (cdk)-containing complexes and through alteration in protein expression levels and subcellular localization. The mechanisms through which cell cycle regulators couple proliferation, differentiation and survival is coming into sharper focus. Cdk-inhibitors, once thought of solely in terms of a checkpoint function on cycling, are now known to interact directly with proteins and pathways central to differentiation and apoptosis. By shuttling between binding partners committed to discrete functional pathways, cell cycle regulators may directly coordinate proliferation with differentiation, migration and apoptosis.

Cell cycle-independent upregulation of p27Kip1 by p21Waf1 in K562 cells

Cellular differentiation frequently involves sequential peaks in the expression of cyclin-dependent kinase inhibitors (cdki's). For example, an increase in levels of the cdki p27Kip1 follows upregulation of p21Waf1 in several cell types induced to differentiate by diverse stimuli. In this study, we have investigated whether p21Waf1 expression itself, rather than the differentiating agent, could be increasing p27Kip1 protein levels. We used an inducible p21Waf1 expression vector in a K562 leukemic cell model which we had previously shown to initiate differentiation following p21Waf1 upregulation. The current study reports that p21Waf1 upregulated p27Kip1 protein without altering p27Kip1 mRNA levels. This effect did not depend on G1-phase arrest-the increase in p27Kip1 occurred at all phases of the cell cycle. p21Waf1-expressing extracts inhibited phosphorylation of p27Kip1 on threonine-187, leading to decreased ubiquitination and decreased proteasomal destruction of p27Kip1. In K562 cells, upregulation of p27Kip1 by p21Waf1 during differentiation facilitated an ordered transition between these two cdki's, each of which may distinctly influence the differentiation process.