Regulation of Bcl2 phosphorylation and potential significance for leukemic cell chemoresistance

Although considered tightly linked, the linkage effectors for proliferation and antiapoptotic signaling pathways are not clear. Phosphorylation of Bcl2 at serine 70 is required for suppression of apoptosis in interleukin 3 (IL-3)-dependent myeloid cells deprived of IL-3 or treated with antileukemic drugs and can result from agonist activation of mitochondrial protein kinase C alpha (PKCalpha). However, we have recently found that high concentrations of staurosporine up to 1 microM: can only partially inhibit IL-3-stimulated Bcl2 phosphorylation but completely block PKCalpha-mediated Bcl2 phosphorylation in vitro, indicating the existence of a non-PKC, staurosporine-resistant Bcl2 kinase (SRK). Although the RAF-1MEK-1-mitogen-activated protein kinase (MAPK) cascade is required for factor-dependent mitogenic signaling, a direct role in antiapoptosis signaling is not clear. In particular, the role of phosphorylation in the regulation of death substrates is not yet clear. Our findings indicate a potential role for the MEK/MAPK pathway in addition to PKC in antiapoptosis signaling, involving Bcl2 phosphorylation that features a role for extracellular signal-regulated kinase (ERK)1 and 2 as SRKs. These findings indicate a novel role for ERK1 and 2 as molecular links between proliferative and survival signaling and may, at least in part, explain the apparent paradox by which Bcl2 may suppress staurosporine-induced apoptosis. Although the effect of phosphorylation on Bcl2 function is not clear, effector molecules that regulate Bcl2 phosphorylation may have clinical significance in patients with acute myelogenous leukemia (AML) who express detectable levels of Bcl2. Preliminary findings suggest that expression of PKCalpha, ERK2, and Bax in leukemic blast cells from patients with AML, although individually not prognostic, appears to have potential clinical value in predicting chemoresistance and survival outcomes.

Treatment of refractory acute leukemia with timed sequential chemotherapy using topotecan followed by etoposide + mitoxantrone (T-EM) and correlation with topoisomerase II levels

A phase I/II clinical study evaluated 17 patients with refractory/recurrent acute leukemia treated with 1.5 mg/m2/day topotecan on days 1-3 followed by etoposide (100 mg/m2/day)+mitoxantrone (10 mg/m2/day) on days 4, 5 and 9, 10. Timed sequential chemotherapy using the topoisomerase I-inhibitor topotecan before the topoisomerase II-inhibitors, etoposide+mitoxantrone (T-EM) treatment is proposed to induce topoisomerase II protein levels and potentiate the cytotoxic activity of the topoisomerase II-directed drugs. Fourteen patients had refractory and three had recurrent acute leukemia. The majority of patients were heavily pre-treated with greater than three re-induction chemotherapy regimens. Ten patients responded to T-EM treatment (59%). Four of seventeen (24%) had a complete remission and one had a partial remission. Four additional patients (24%) who scored complete leukemia clearance had no evidence of disease with complete white and red blood cell recovery but with platelet counts less than 100,000. The lack of platelet recovery in one patient having a partial response was scored as a partial leukemia clearance. The toxicity profile included major non-hematological toxicity including grade 3 mucositis (29%) and neutropenic fever (65%). Paired measurements of intracellular levels of topoisomerase II isoforms alpha and beta in leukemia blast cells (bone marrow) collected before (day 0) and after topotecan treatment (day 4) showed that a relative increase of topoisomerase IIalpha (Topo IIalpha) > or = 40% strongly correlated with response after T-EM treatment. Increased Topo IIalpha levels also corresponded to increased DNA fragmentation. Two patients who had an increase of Topo IIalpha of 20-25% had either a PR or PLC while patients with a < 10% increase showed no response to T-EM treatment. We conclude that timed sequential chemotherapy using topotecan followed by etoposide+mitoxantrone is an effective regimen for patients with refractory acute leukemia, and demonstrate Topo IIalpha protein level increases after topotecan treatment.

Novel role for JNK as a stress-activated Bcl2 kinase

Interleukin (IL)-3-induced Bcl2 phosphorylation at Ser(70) may be required for its full and potent antiapoptotic activity. However, in the absence of IL-3, increased expression of Bcl2 can also prolong cell survival. To determine how Bcl2 may be functionally phosphorylated following IL-3 withdrawal, a stress-activated Bcl2 kinase (SAK) was sought. Results indicate that anisomycin, a potent activator of the stress kinase JNK/SAPK, can induce Bcl2 phosphorylation at Ser(70) and that JNK1 can be latently activated following IL-3 withdrawal to mediate Bcl2 phosphorylation. JNK1 directly phosphorylates Bcl2 in vitro, co-localizes with Bcl2, and collaborates with Bcl-2 to mediate prolonged cell survival in the absence of IL-3 or following various stress applications. Dominant-negative (DN)-JNK1 can block both anisomycin and latent IL-3 withdrawal-induced Bcl2 phosphorylation (>90%) and potently enhances cell death. Furthermore, low dose okadaic acid (OA), a potent protein phosphatase 1 and 2A inhibitor, can activate the mitogen-activated protein kinases JNK1 and ERK1/2, but not p38 kinase, to induce Bcl2 phosphorylation and prolong cell survival in factor-deprived cells. Since PD98059, a specific MEK inhibitor, can only partially inhibit OA-induced Bcl2 phosphorylation but completely blocks OA-induced Bcl2 phosphorylation in cells expressing DN-JNK1, this supports the conclusion that OA may stimulate Bcl2 phosphorylation via a mechanism involving both JNK1 and ERK1/2. Collectively, these findings indicate a novel role for JNK1 as a SAK and may explain, at least in part, how functional phosphorylation of Bc12 can occur in the absence of growth factor.

In vitro effects of hyperbaric oxygen on sickle cell morphology

STUDY OBJECTIVE

To determine in vitro whether hyperbaric oxygen has any effect on the morphology of sickle cells.

DESIGN

Prospective, in vitro, study, with each patient sample serving as its own control.

SETTING

University medical center.

PATIENTS

10 children known to be homozygous for hemoglobin S.

INTERVENTIONS

Blood samples were obtained from 10 children during routine visits to the University sickle cell clinic. Blood samples were exposed to room air to achieve maximal sickling. Each sample was divided into control and study aliquots, and the study portions placed in a research hyperbaric chamber with 100% oxygen at 3 atmospheres absolute pressure for 15 min. Then smears were prepared from all samples at regular intervals and examined by technicians in the sickle cell clinic who were blinded as to the details of this study.

MEASUREMENTS

Percentages of normal cells, sickle cells and sickle forms were reported. Data were interpreted using t-tests.

MAIN RESULTS

Hyperbaric oxygen appeared to have no effect on sickle cell morphology. Percentages of each cell type were unaffected by hyperbaric oxygen exposure.

CONCLUSIONS

Hyperbaric oxygen appears to have no effect on the morphology of sickle cells in vitro. Other mechanisms may account for the beneficial clinical effects of hyperbaric oxygen in sickle cell crisis, although in vivo studies are warranted.

Phosphorylation of Bcl2 and regulation of apoptosis

Members of the Bcl2 family of proteins are important regulators of programmed cell death pathways with individual members that can suppress (eg Bcl2, Bcl-XL) or promote (eg Bax, Bad) apoptosis. While the mechanism(s) of Bcl2's anti-apoptotic function is not yet clear, introduction of Bcl2 into most eukaryotic cell types will protect the recipient cell from a wide variety of stress applications that lead to cell death. There are, however, physiologic situations in which Bcl2 expression apparently fails to protect cells from apoptosis (eg negative selection of thymocytes). Further, Bcl2 expression in patient tumor samples does not consistently correlate with a worse outcome or resistance to anticancer therapies. For example, patient response and survival following chemotherapy is independent of Bcl2 expression at least for pediatric patients with ALL. These findings indicate that simple expression of Bcl2 may not be enough to functionally protect cells from apoptosis. The finding that Bcl2 is post-translationally modified by phosphorylation suggests another level of regulation of function. Recent studies have shown that agonist-activated phosphorylation of Bcl2 at serine 70 (single site phosphorylation), a site within the flexible loop domain (FLD), is required for Bcl2's full and potent anti-apoptotic function, at least in murine IL-3-dependent myeloid cell lines. Several protein kinases have now been demonstrated to be physiologic Bcl2 kinases indicating the importance of this post-translational modification. Since Bcl2 phosphorylation has been found to be a dynamic process involving both a Bcl2 kinase(s) and phosphatase(s), a mechanism exists to rapidly and reversibly regulate Bcl2's activity and affect cell viability. In addition, multisite Bcl2 phosphorylation induced by anti-mitotic drugs like paclitaxel may inhibit Bcl2 indicating the potential wide range of functional consequences that this post-translational modification may have on function. While post-translational mechanisms other than phosphorylation may also regulate Bcl2's function (eg ubiquitination), this review will focus on the regulatory role for phosphorylation and discuss its potential clinical ramifications.

Ceramide regulates protein synthesis by a novel mechanism involving the cellular PKR activator RAX

The sphingolipid ceramide is an important second signal molecule and potent apoptotic agent. The production of ceramide is associated with virtually every known stress stimulus, and thus, generation of this sphingolipid has been suggested as a universal feature of apoptosis. Recent studies suggest that an important component of cell death following diverse stress stimuli (e.g. interleukin-3 withdrawal, sodium arsenite treatment, and peroxide treatment) is the activation of the double-stranded RNA-activable protein kinase, PKR, resulting in the inhibition of protein synthesis (Ito, T., Jagus, R., and May, W. S. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 7455-7459). The recently discovered cellular PKR activator, RAX, is phosphorylated in association with PKR activation (Ito, T., Yang, M., and May, W. S. (1999) J. Biol. Chem. 274, 15427-15432). Since RAX is phosphorylated by an as yet undetermined SAPK and ceramide is a potent activator of SAPKs such as JNK, a role for ceramide in the activation of RAX might be possible. Results indicate that overexpression of exogenous RAX potentiates ceramide-induced killing. Furthermore, ceramide can potently inhibit protein synthesis. Since ceramide potently promotes RAX and eukaryotic initiation factor-2alpha phosphorylation, a possible role for ceramide in this process may involve the activation of PKR by RAX. Since 2-aminopurine, a serine/threonine kinase inhibitor that has previously been shown to inhibit PKR, blocks both the potentiation of ceramide killing by RAX and ceramide-induced inhibition of protein synthesis, ceramide appears to promote PKR activation, at least indirectly. Collectively, these findings suggest a novel role for ceramide in the regulation of protein synthesis and apoptosis.

BAX and PKCalpha modulate the prognostic impact of BCL2 expression in acute myelogenous leukemia

Previously, we demonstrated that the level of BCL2 expression is prognostic in acute myelogenous leukemia (AML). High levels of BCL2 correlate with an adverse outcome when associated with favorable and intermediate prognosis cytogenetics (FIPC), whereas low levels portend an adverse outcome when associated with unfavorable cytogenetics (UC). Because BCL2 function can be modulated by dimerization with family members, like BAX, or by phosphorylation by protein kinase C alpha (PKCalpha), we hypothesize that the relative expression of these proteins in primary leukemic cells might alter the prognostic impact of BCL2 expression. We therefore measured BAX and PKCalpha protein levels in peripheral blood mononuclear cell lysates from 165 newly diagnosed AML patients and correlated the expression of these proteins with BCL2 expression, patient survival, and remission induction success. Expression levels of BAX and PKCalpha were normalized against a control cell line, K562. BAX and PKCalpha expression levels were heterogeneous and did not correlate with the percentage of blasts in the sample (R2 = 0.01 and <0.01). The median expression of both was similar across FAB groups but the range was greater for M4. A similar distribution of expression was observed in all cytogenetic groups, except that patients with inversion 16 demonstrated lower levels of BAX. Individually, neither PKCalpha nor BAX expression was prognostic of response to induction therapy or survival. A similar outcome was obtained when patients were stratified by cytogenetics into FIPC and UC groups. However, the ratio of either BCL2:BAX (B2:BX) or PKCalpha*B2:BX (PK*B2:BX) was highly prognostic. Patients with FIPC and a lower ratio (less than median) of either B2:BX or PK*B2:BX had a significantly higher remission induction rate (88 versus 69%, P = 0.04) and longer survival (median: 141 versus 80.5 weeks, P = 0.007) compared with those with ratios more than median. For patients with UC, values of either B2:BX or PK*B2:BX below the median had an inferior response rate to induction therapy (35 versus 78%, P = 0.0006) and inferior survival outcomes (median survival: 11 versus 53 weeks, P = 0.00002). Interestingly, FIPC and UC patients with antiapoptotic ratios (defined as B2:BX or PK*B2:BX more than median) had identical response rates and survival outcomes. In multivariate analyses, the compound variables of cytogenetics and B2:BX, or PK*B2:BX were independent predictors of survival. These results suggest that expression levels of proteins that affect the functional status of BCL2 modify the prognostic impact of BCL2 and suggest that the role of apoptosis in different cases of AML varies independently in the different cytogenetic subgroups.

Survival function of ERK1/2 as IL-3-activated, staurosporine-resistant Bcl2 kinases

Bcl2 phosphorylation at Ser-70 may be required for the full and potent suppression of apoptosis in IL-3-dependent myeloid cells and can result from agonist activation of mitochondrial protein kinase C (PKC). Paradoxically, expression of exogenous Bcl2 can protect parental cells from apoptosis induced by the potent PKC inhibitor, staurosporine (stauro). High concentrations of stauro of up to 1 microM only partially inhibit IL-3-stimulated Bcl2 phosphorylation but completely block PKC-mediated Bcl2 phosphorylation in vitro. These data indicate a role for a stauro-resistant Bcl2 kinase (SRK). We show that aurintricarboxylic acid (ATA), a nonpeptide activator of cellular MEK/mitogen-activated protein kinase (MAPK) kinase, can induce Ser-70 phosphorylation of Bcl2 and support survival of cells expressing wild-type but not the phosphorylation-incompetent S70A mutant Bcl2. A role for a MEK/MAPK as a responsible SRK was implicated because the highly specific MEK/MAPK inhibitor, PD98059, also can only partially inhibit IL-3-induced Bcl2 phosphorylation, whereas the combination of PD98059 and stauro completely blocks phosphorylation and synergistically enhances apoptosis. p44MAPK/extracellular signal-regulated kinase 1 (ERK1) and p42 MAPK/ERK2 are activated by IL-3, colocalize with mitochondrial Bcl2, and can directly phosphorylate Bcl2 on Ser-70 in a stauro-resistant manner both in vitro and in vivo. These findings suggest a role for the ERK1/2 kinases as SRKs. Thus, the SRKs can serve to functionally link the IL-3-stimulated proliferative and survival signaling pathways and, in a novel capacity, may explain how Bcl2 can suppress stauro-induced apoptosis. In addition, although the mechanism of regulation of Bcl2 by phosphorylation is not yet clear, our results indicate that phosphorylation may functionally stabilize the Bcl2-Bax heterodimerization.

Serine/threonine phosphorylation in cytokine signal transduction

Over the past decade, the involvement of tyrosine kinases in signal transduction pathways evoked by cytokines has been intensively investigated. Only relatively recently have the roles of serine/threonine kinases in cytokine-induced signal transduction and anti-apoptotic pathways been examined. Cytokine receptors without intrinsic kinase activity such as interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF) and the interferons were thought to transmit their regulatory signals primarily by the receptor-associated Jak family of tyrosine kinases. This family of tyrosine kinases activates STAT transcription factors, which subsequently transduced their signals into the nucleus to modulate gene expression. Cytokine receptors with intrinsic tyrosine kinase activity such as c-Kit were initially thought to transduce their signals independently of serine/threonine kinase cascades. Recently, both of these types of receptor signaling pathways have been shown to interact with serine/threonine kinase pathways as maximal activation of these tyrosine kinase regulated cascades involve serine/threonine phosphorylation modulated by, for example MAP kinases. A common intermediate pathway initiating from cytokine receptors is the Ras/Raf/MEK/ERK (MAPK) cascade, which can result in the phosphorylation and activation of additional downstream kinases and transcription factors such as p90Rsk, CREB, Elk and Egr-1. Serine/threonine phosphorylation is also involved in the regulation of the apoptosis-controlling Bcl-2 protein, as certain phosphorylation events induced by cytokines such as IL-3 are anti-apoptotic, whereas other phosphorylation events triggered by chemotherapeutic drugs such as Paclitaxel are associated with cell death. Serine/threonine phosphorylation is implicated in the etiology of certain human cancers as constitutive serine phosphorylation of STATs 1 and 3 is observed in chronic lymphocytic leukemia and can be inhibited by the chemotherapeutic drug fludarabine. Serine/threonine phosphorylation also plays a role in the etiology of immunodeficiencies. Activated STAT5 proteins are detected in reduced levels in lymphocytes recovered from HIV-infected individuals and immunocompromised mice. Serine/threonine phosphorylation may be an important target of certain chemotherapeutic drugs which recognize the activated proteins. This meeting report and mini-review will discuss the interactions of serine/threonine kinases with signal transduction and apoptotic molecules and how some of these pathways can be controlled by chemotherapeutic drugs. Leukemia (2000) 14, 9-21.

Protein synthesis inhibition by flavonoids: roles of eukaryotic initiation factor 2alpha kinases

Flavonoids such as genistein and quercetin suppress tumor cell growth in vitro and in vivo. Many metabolic enzymes, including protein kinases, are known to be inhibited by flavonoids, yet the molecular targets and biochemical mechanisms of the tumor growth suppression remain unclear. Here, we find that flavonoids inhibit protein synthesis in both mouse and human leukemia cells. This inhibition is associated with phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF2alpha), a key regulatory mechanism of protein translation. Three mammalian eIF2alpha kinases have been identified: the interferon-inducible double-stranded RNA-dependent kinase (PKR), the heme-regulated inhibitor (HRI), and the very recently discovered PERK/PEK. We find that all of these eIF2alpha kinases can be activated by quercetin and genistein, indicating redundant roles of the eIF2alpha kinases. Thus, activation of eIF2alpha kinases appears to be a mechanism by which flavonoids can inhibit the growth of tumor and leukemia cells.

JAZ requires the double-stranded RNA-binding zinc finger motifs for nuclear localization

We have cloned and characterized a novel zinc finger protein, termed JAZ. JAZ contains four C(2)H(2)-type zinc finger motifs that are connected by long (28-38) amino acid linker sequences. JAZ is expressed in all tissues tested and localizes in the nucleus, primarily the nucleolus. JAZ preferentially binds to double-stranded (ds) RNA or RNA/DNA hybrids rather than DNA. Mutation of individual zinc finger motifs reveals that the zinc finger domains are not only essential for dsRNA binding but are also required for its nucleolar localization, which demonstrates a complex trafficking mechanism dependent on the nucleic acid-binding capability of the protein. Furthermore, forced expression of JAZ potently induces apoptosis in murine fibroblast cells. Thus, JAZ may belong to a class of zinc finger proteins that features dsRNA binding and may regulate cell growth via the unique dsRNA binding properties.

Ceramide induces Bcl2 dephosphorylation via a mechanism involving mitochondrial PP2A

Phosphorylation of Bcl2 at serine 70 is required for its potent anti-apoptotic function. We have recently shown that Bcl2 phosphorylation is a dynamic process that involves the protein kinase C alpha and protein phosphatase 2A (PP2A) (Ruvolo, P. P., Deng, X., Carr, B. K., and May, W. S. (1998) J. Biol. Chem. 273, 25436-25442; and Deng, X., Ito, T., Carr, B. K., Mumby, M. C., and May, W. S. (1998) J. Biol. Chem. 273, 34157-34163). The potent apoptotic agent ceramide can activate a PP2A, suggesting that one potential component of the ceramide-induced death signal may involve the inactivation of Bcl2. Results indicate that C2-ceramide but not inactive C2-dihydroceramide, was found to specifically activate a mitochondrial PP2A, which rapidly and completely induced Bcl2 dephosphorylation and correlated closely with ceramide-induced cell death. Using a genetic approach, the gain-of-function S70E Bcl2 mutation, which mimics phosphorylation, fails to undergo apoptosis even with the addition of high doses of ceramide (IC50 > 50 microM). In contrast, cells overexpressing exogenous wild-type Bcl2 were sensitive to ceramide at dosages where PP2A is fully active and Bcl2 would be expected to be dephosphorylated (IC50 = 14 microM). These findings indicate that in cells expressing functional Bcl2, the mechanism of death action for ceramide may involve, at least in part, a mitochondrial PP2A that dephosphorylates and inactivates Bcl2.

RAX, a cellular activator for double-stranded RNA-dependent protein kinase during stress signaling

The double-stranded (ds) RNA-dependent protein kinase (PKR) regulates protein synthesis by phosphorylating the alpha subunit of eukaryotic initiation factor-2. PKR is activated by viral induced dsRNA and thought to be involved in the host antiviral defense mechanism. PKR is also activated by various nonviral stresses such as growth factor deprivation, although the mechanism is unknown. By screening a mouse cDNA expression library, we have identified an ubiquitously expressed PKR-associated protein, RAX. RAX has a high sequence homology to human PACT, which activates PKR in the absence of dsRNA. Although RAX also can directly activate PKR in vitro, overexpression of RAX does not induce PKR activation or inhibit growth of interleukin-3 (IL-3)-dependent cells in the presence of IL-3. However, IL-3 deprivation as well as diverse cell stress treatments including arsenite, thapsigargin, and H2O2, which are known to inhibit protein synthesis, induce the rapid phosphorylation of RAX followed by RAX-PKR association and activation of PKR. Therefore, cellular RAX may be a stress-activated, physiologic activator of PKR that couples transmembrane stress signals and protein synthesis.

Reversible phosphorylation of Bcl2 following interleukin 3 or bryostatin 1 is mediated by direct interaction with protein phosphatase 2A

Interleukin 3 (IL-3) stimulates the net growth of murine factor-dependent NSF/N1.H7 and FDC-P1/ER myeloid cells by stimulating proliferation and suppressing apoptosis. Recently, we discovered that Bcl2 is phosphorylated at an evolutionarily conserved serine residue (Ser70) after treatment with the survival agonists IL-3 or bryostatin 1, a potent activator of protein kinase (Ito, T., Deng, X., Carr, B., and May, W. S. (1997) J. Biol. Chem. 272, 11671-11673). In addition, an intact Ser70 was found to be required for Bcl2's ability to suppress apoptosis after IL-3 withdrawal or toxic chemotherapy. We now show that phosphorylation of Bcl2 occurs rapidly after the addition of agonist to IL-3-deprived cells and can be reversed by the action of an okadaic acid (OA)-sensitive phosphatase. A role for protein phosphatase (PP) 2A as the Bcl2 regulatory phosphatase is supported by several observations: 1) dephosphorylation of Bcl2 is blocked by OA, a potent PP1 and PP2A inhibitor; 2) intracellular PP2A, but not PP1, co-localizes with Bcl2; 3) the purified PP2Ac catalytic subunit directly dephosphorylates Bcl2 in vitro in an OA-sensitive manner; 4) the purified PP2Ac catalytic subunit preferentially dephosphorylates Bcl2 in vitro compared with PP1 and PP2B; 5) reciprocal immunoprecipitation studies indicate a direct interaction between PP2A and hemagglutinin (HA)-Bcl2; and 6) treatment of factor-deprived cells with bryostatin 1 dramatically increases the association between PP2A and Bcl2. Increased association between Bcl2 and PP2A occurs 15 min after agonist stimulation when Bcl2 phosphorylation has peaked and immediately before dephosphorylation. An agonist-induced increased association of PP2A and Bcl2 fails to occur in cells expressing the inactive, phosphorylation-negative S70A Bcl2 mutant, which indicates that an intact Ser70 site is necessary and sufficient for the interaction to occur. Functional phosphorylation of Bcl2 at Ser70 is proposed to be a dynamic process regulated by the sequential action of an agonist-activated Bcl2 kinase and PP2A.

CD34 expression by embryonic hematopoietic and endothelial cells does not require c-Myb

CD34 is a cell-surface glycoprotein expressed in a developmental, stage-specific manner by bone marrow stem and progenitor cells. In this study we explored a possible role for c-Myb in CD34 regulation during developmental hematopoiesis. The results indicate that c-Myb can induce CD34 expression in hematopoietic and nonhematopoietic cells, and that murine CD34 promoter activity is enhanced in myeloid cells transgenic for c-Myb. To test whether c-Myb is necessary for CD34 expression during developmental hematopoiesis in vitro, c-Myb-null D3 embryonic stem (ES) cells were analyzed for their ability to develop CD34+ hematopoietic cells in vitro. CD34 promoter activity in transient transfections and CD34 upregulation during ES cell differentiation into embryoid bodies was identical in wild-type and c-Myb-null ES cells, indicating that c-Myb is not required for CD34 expression. CD34 protein is expressed on both hematopoietic and endothelial cells of the E8.5 blood islands during the development of c-Myb-null embryos, and expression is nearly identical in wild-type and c-Myb-null embryos. However, in E12.5 c-Myb-null embryos, the majority of identifiable CD34+ cells in the developing liver are endothelial rather than hematopoietic, which is consistent with the absence of colony-forming units in c-Myb-null embryos and developing ES cells. These data indicate that c-Myb is not required for CD34 expression in endothelial or primitive hematopoietic cells in the yolk sac, but is necessary for definitive hematopoiesis.

A functional role for mitochondrial protein kinase Calpha in Bcl2 phosphorylation and suppression of apoptosis

Phosphorylation of Bcl2 at serine 70 may result from activation of a classic protein kinase C (PKC) isoform and is required for functional suppression of apoptosis by Bcl2 in murine growth factor-dependent cell lines (Ito, T., Deng, X., Carr, B., and May, W. S. (1997) J. Biol. Chem. 272, 11671-11673). Human pre-B REH cells express high levels of Bcl2 yet remain sensitive to the chemotherapeutic agents etoposide, cytosine arabinoside, and Adriamycin. In contrast, myeloid leukemia-derived HL60 cells express less than half the level of Bcl-2 but are >10-fold more resistant to apoptosis induced by these drugs. The mechanism responsible for this apparent dichotomy appears to involve a deficiency of mitochondrial PKCalpha since 1) HL60 but not REH cells contain highly phosphorylated Bcl2; 2) PKCalpha is the only classical isoform co-localized with Bcl2 in HL60 but not REH mitochondrial membranes; 3) the natural product and potent PKC activator bryostatin-1 induces mitochondrial localization of PKCalpha in association with Bcl2 phosphorylation and increased REH cell resistance to drug-induced apoptosis; 4) PKCalpha can directly phosphorylate wild-type but not phosphorylation-negative and loss of function S70A Bcl2 in vitro; 5) stable, forced expression of exogenous PKCalpha induces mitochondrial localization of PKCalpha, increased Bcl2 phosphorylation and a >10-fold increase in resistance to drug-induced cell death; and () PKCalpha-transduced cells remain highly sensitive to staurosporine, a potent PKC inhibitor. Furthermore, treatment of the PKCalpha transformants with bryostatin-1 leads to even higher levels of mitochondrial PKCalpha, Bcl2 phosphorylation, and REH cell survival following chemotherapy. While these findings strongly support a role for PKCalpha as a functional Bcl2 kinase that can enhance cell resistance to antileukemic chemotherapy, they do not exclude the possibility that another Bcl2 kinase(s) may also exist. Collectively, these findings identify a functional role for PKCalpha in Bcl2 phosphorylation and in resistance to chemotherapy and suggest a novel target for antileukemic strategies.

Regulation of CD34 expression in differentiating M1 cells

CD34 is a cell surface glycoprotein expressed on hematopoietic stem and progenitor cells, but not on mature blood cells. In the present study we found that CD34 downregulation during hematopoiesis occured at the level of transcriptional initiation. Two transcription initiation sites (TISs) were identified in each of three different CD34+ cell lines; these TISs were located at 120 and 80 bp 5' of the translation start site, respectively. The promoter lacks TATA elements and, like other TATA-less promoters, the TISs conform to the consensus sequence for an INR (PyPyCAPyPyPyPy). An additional 3000 bp of upstream genomic DNA were sequenced and found to contain consensus sites for transcription factors, suggesting their potential role in gene regulation. Transient transfection assays using CD34 promoter-luciferase reporter constructs, containing sequences up to 3 kb upstream and inclusive of the TIS, indicate that this promoter drives transcription in hematopoietic CD34+ cells but not CD34+ nonhematopoietic cells. Both cell type specific expression and full promoter activity are maintained in constructs that contain as little as 454 bp upstream of the TISs. Optimal promoter activity requires the 5' untranslated region of exon 1, which contains a 51-bp element that has the potential to form an extensive secondary structure. In the plasmid DNA, however, this secondary structure was not detectable by P1 nuclease digestion. At least three proteins present in uninduced M1 nuclear extracts bind to this element. Two of the three proteins were identified as Sp 1 and Sp 3 based on supershift experiments. These data suggest that CD34 expression by hematopoietic stem and progenitor cells involves hematopoietic cell-specific factors that interact with regulatory elements within the first 230 bp of the promoter and that optimal expression requires a 60-bp segment of the 5' untranslated region.

Bcl-2 phosphorylation required for anti-apoptosis function

The protooncogene Bcl-2 functions as a suppressor of apoptosis in growth factor-dependent cells, but a post-receptor signaling mechanism is not known. We recently reported that interleukin 3 (IL-3) and erythropoietin, or the protein kinase C activator bryostatin-1 (Bryo), not only suppresses apoptosis but also stimulates the phosphorylation of Bcl-2 (May, W. S., Tyler, P. G., Ito, T., Armstrong, D. K., Qatsha, K. A., and Davidson, N. E. (1994) J. Biol. Chem. 269, 26865-26870). To test whether phosphorylation is required for Bcl-2 function, conservative serine --> alanine mutations were produced at the seven putative protein kinase C phosphorylation sites in Bcl-2. Results indicate that the S70A Bcl-2 mutant fails to be phosphorylated after IL-3 or Bryo stimulation and is unable to support prolonged cell survival either upon IL-3 deprivation or etoposide treatment when compared with wild-type Bcl-2. In contrast, a Ser --> Glu mutant, S70E, which may mimic a potential phosphate charge, more potently suppressed the etoposide-induced apoptosis than wild type in the absence of IL-3. Since the loss of function S70A mutant can heterodimerize with its partner protein and death effector Bax, these findings demonstrate that Bcl-2:Bax heterodimerization is not sufficient and Bcl-2 phosphorylation is required for full Bcl-2 death suppressor signaling activity.

Characterization of mouse lymphohematopoietic stem cells lacking spleen colony-forming activity

The classical definition of lymphohematopoietic stem cells (LHSC), the most primitive progenitors of all blood cells, requires that they have the capacity for self-renewal and for the long-term production of all blood cell lineages. However, other characteristics of LHSC have been debated. Our previous data suggested that mouse LHSC are very slowly proliferating cells that generate delayed multilineage engraftment, while "radioprotection" (rapid engraftment that will prevent early death from radiation-induced marrow aplasia) results from more committed progenitors. Alternatively, some groups have reported that mouse LHSC are responsible for both radioprotection and long-term repopulation of all blood cell lineages. A possible explanation for this difference is that cells with the capacity for long-term production of all blood cell lineages are biologically heterogeneous. We now show that 10 LHSC can generate all blood cell lineages for the lifetime of the animal. However, these cells lacked radioprotection and spleen colony-forming activity. LHSC were identified and isolated by their small size, their lack of expression of antigens characteristic of mature blood cell lineages, and their high expression of aldehyde dehydrogenase. In addition, these cells were found to express undetectable or low levels of many antigens presumed to mark LHSC, including Thy-1, Ly-6A/E (Sca-1), c-kit, and CD34. There appears to be at least two classes of LHSC with the capacity for long-term production of all blood cell lineages: one that generates both radioprotection and long-term engraftment and one that produces delayed but durable engraftment. Our data suggest that this latter class may represent a very primitive class of LHSC.

Bryostatins trigger human polymorphonuclear neutrophil and monocyte oxidative metabolism: association with in vitro antineoplastic activity

Bryostatin-1-but not bryostatin-13-a macrocyclic lactone isolated from the marine bryozoan Bugula neritina, triggered human polymorphonuclear neutrophil (PMN) and monocyte release of reactive oxygen radicals, as measured by the generation of lucigenin chemiluminescence and by the ferricytochrome c reduction assay. The release of oxygen radicals by bryostatins was sensitive to inhibitors of protein kinases, but resistant to the inhibition of phospholipase A2 activity and arachidonic acid metabolism (prior treatment with mepacrine or indomethacin). Comparison of the effect of protein kinase (PK) inhibitors H-8, H-7 and staurosporine on bryostatin-1-induced neutrophil oxygen radical release further suggested a requirement for activation of phospholipid-dependent PKC, but not for cGMP- or cAMP-dependent PK. In cytostatic assays, PMNs treated with bryostatin-1 inhibited the growth of the erythroleukaemic cell line K562 in a concentration-dependent manner. These findings suggest that the reported antineoplastic effect of bryostatins may result at least in part from activation of PMNs and monocytes.

Full-length but not truncated CD34 inhibits hematopoietic cell differentiation of M1 cells

CD34 is expressed on human and murine hematopoietic stem and progenitor cells and its clinical usefulness for isolation of stem/progenitor cells has been well established. Although expression of CD34 is regulated in a developmental stage-specific manner, the function of CD34 is not known. Recently we have shown that both a full-length and truncated form of CD34 protein is expressed by hematopoietic cells (Blood 84:691, 1994). To test whether failure to suppress either form of CD34 could affect terminal myeloid differentiation, we constitutively expressed these CD34 proteins in murine M1 myeloid leukemia cells, which can be terminally differentiated to macrophages by treatment with interleukin-6 of leukemia inhibitory factor. Surprisingly our results show that forced expression of the full-length but not the truncated form of CD34 impedes terminal differentiation by these agents. Because the difference between the two forms of CD34 protein resides in the length of their respective cytoplasmic tail domains, our findings strongly suggest that the cytoplasmic domain region of full-length CD34 is responsible for the observed maturation arrest phenotype. These findings suggest a potential negative regulatory role for full-length CD34 in hematopoietic cell differentiation and may explain, at least in part, the block in maturation observed in CD34+ acute myeloid leukemia.

Interleukin-3 and bryostatin-1 mediate hyperphosphorylation of BCL2 alpha in association with suppression of apoptosis

Using murine myeloid factor-dependent FDC-P1/ER cells, we demonstrate that the hematopoietic growth factors interleukin-3 and erythropoietin and bryostatin-1, a macrocyclic lactone natural product and potent activator of protein kinase C (PKC), suppress apoptosis and induce the rapid serine phosphorylation of Bc12 alpha. Expression of recombinant wild type Bc12 alpha in NFS/N1.H-7 cells confirms that murine Bc12 alpha is phosphorylated following PKC activation. The PKC inhibitors H-7 and staurosporine, but not the protein kinase A inhibitor HA1004, block not only interleukin-3- and bryostatin-1-induced hyperphosphorylation of Bc12 alpha but also their anti-apoptotic effect on growth factor-dependent cells, suggesting a role for activated PKC in both processes. A potential direct role for a classic isoform of PKC is indicated by the Ca(2+)-dependent nature of phosphorylation of Bc12 alpha mediated by purified PKC in vitro. Comparative phosphopeptide maps confirm that Bc12 alpha phosphorylation occurs on identical serine site(s) whether phosphorylation occurs in cells following agonist treatment or directly by PKC in vitro. These findings strongly support a role for activated PKC in growth factor-induced Bc12 alpha phosphorylation as well as suppression of apoptosis.

Interleukin 3 stimulates protein synthesis by regulating double-stranded RNA-dependent protein kinase

In a murine interleukin 3 (IL-3)-dependent cell line, IL-3 deprivation resulted in increased autophosphorylation of double-stranded RNA-dependent protein kinase (PKR) that has been reported to inhibit protein synthesis by phosphorylating the alpha subunit of eukaryotic initiation factor 2 (eIF-2 alpha). Autophosphorylation was characterized by a shift up in mobility of PKR on SDS/PAGE gels from a 60- to a 64-kDa form. In vitro kinase studies comparing the autophosphorylated 64-kDa PKR with the nonphosphorylated 60-kDa PKR confirmed that only the 64-kDa form was active for eIF-2 alpha phosphorylation. PKR activation in vivo was associated with phosphorylation of eIF-2 alpha and inhibition of protein synthesis. Addition of IL-3 to deprived cells elicited a reciprocal response characterized by the rapid dephosphorylation of PKR and eIF-2 alpha, indicating inactivation of PKR. This was rapidly followed by the full recovery of protein synthesis. Furthermore, upon IL-3 addition, a 97-kDa phosphotyrosine-containing protein becomes rapidly and transiently associated with PKR prior to dephosphorylation of PKR and eIF-2 alpha. Genistein, a tyrosine kinase inhibitor, blocks both phosphorylation of the 97-kDa phosphoprotein and protein synthesis after IL-3 addition, suggesting a role for the 97-kDa phosphoprotein in the mechanism of inactivation of PKR and stimulation of protein synthesis. Thus, IL-3 appears to positively regulate protein synthesis by inducing the inactivation of PKR in a growth factor signaling pathway.

Characterization of murine CD34, a marker for hematopoietic progenitor and stem cells

CD34 is expressed on human hematopoietic stem and progenitor cells, and its clinical usefulness for the purification of stem cells has been well established. However, a similar pattern of expression for murine CD34 (mCD34) has not yet been determined. Two polyclonal anti-mCD34 antibodies that specifically recognize both endogenous and recombinant murine CD34 were developed to characterize the mCD34 protein and to determine its pattern of expression on murine cell lines and hematopoietic progenitor cells. Fluorescence-activated cell sorter analysis showed that mCD34 is expressed on NIH/3T3 embryonic fibroblasts, PA6 stromal cells, embryonic stem cells, M1 leukemia cells, and a subpopulation of normal bone marrow cells. Murine CD34 was found to be a glycoprotein expressed on the cell surface as either a full-length (approximately 100 kD) or truncated (approximately 90 kD) protein in NIH/3T3 and PA6 cells. Recombinant full-length CD34, when expressed in the CHO-K1 cell line, had a molecular weight of approximately 105 kD. Full-length CD34 expressed on M1 leukemia cells, had a higher apparent molecular weight (110 kD). These results suggest that there are glycosylation differences between CD34 expressed by different cell types. The full-length form, but not the truncated form, is a phosphoprotein that is hyperphosphorylated in response to 12-0-Tetradecanoyl phorbol 13-acetate treatment, suggesting potential functional differences between the two forms. Selection of the 3% highest-expressing CD34+ bone marrow cells enriched for the hematopoietic precursors that form colony-forming unit-spleen (CFU-S), CFU-granulocyte-macrophage, and burst-forming unit-erythroid. Transplantation of lethally irradiated mice with these cells demonstrated both short- and long-term repopulating ability, indicating that this population contains both functional hematopoietic progenitors and the putative stem cell. These antibodies should be useful to select for murine hematopoietic stem cells.

Protein kinase C-mediated serine phosphorylation directly activates Raf-1 in murine hematopoietic cells

We have previously found that Raf-1, which is activated by hematopoietic growth factors in association with phosphorylation, is required for hematopoietic cell proliferation. Recently, 12-O-tetradecanoylphorbol 13-acetate has been found to mediate Raf-1 phosphorylation, suggesting that protein kinase C (PKC) may be involved in the Raf-1 activation mechanism(s). Since PKC can be activated by hematopoietic growth factors, it was investigated as a potential "Raf-1 kinase-kinase." Results demonstrate that bryostatin 1, a pharmacologic activator of PKC, induces activation of Raf-1 in FDC-P1 cells. PKC inhibitors H7 and staurosporine block both bryostatin 1- and interleukin-3-mediated Raf-1 phosphorylation and FDC-P1 cell proliferation. Additionally, an antisense c-raf oligodeoxyribonucleotide specifically inhibits bryostatin 1-mediated proliferation, indicating a necessary role for Raf-1 in PKC signaling. Purified PKC can phosphorylate Raf-1 serine residues to high stoichiometry in vitro. Comparative phosphopeptide maps localize two PKC phosphorylation sites to Raf-1 phosphopeptides isolated from hematopoietic growth factor- or bryostatin 1-stimulated cells. The sites of PKC-mediated Raf-1 phosphorylation are deduced to be Ser497 and Ser619. Furthermore, PKC-mediated serine phosphorylation is sufficient to activate the enzymatic function of Raf-1 in vitro. These findings demonstrate that activated PKC can promote hematopoietic cell growth by regulating the enzymatic activity of Raf-1 through direct serine phosphorylation.

BMT for severe aplastic anemia using cyclosporine

Between 1984 and 1991 24 patients with severe aplastic anemia (SAA) were transplanted with HLA identical sibling donor BM. The overall long-term survival was 79 +/- 8%. The average age was 21 years (range 4-53 years) and the median pre-transplant disease duration was 35 days (range 12-2998 days). Over one-half (15 of 24) of the patients had received > 10 units of blood product transfusions prior to BMT. The pre-transplant conditioning regimen consisted of 200 mg/kg cyclophosphamide (CY). Cyclosporine (CYA) was administered from 2 days prior to BMT and continued for 6-12 months. Two of the 24 patients failed to achieve primary engraftment (FTE). One of these patients had autologous recovery of BM function and is alive and well. Five of the 22 patients who engrafted failed to sustain engraftment (FTSE). Of these, three are alive and well following a second BMT or marrow boost. Only 1 of the 22 patients who engrafted had clinically significant (i.e. Stage II-IV) acute GVHD. No patient developed chronic GVHD. Our results indicate that BMT following a regimen consisting of CY with the continuous use of CYA in the post-transplant period is well tolerated and associated with excellent long-term survival. The high incidence of secondary graft instability (i.e. FTSE), however, suggests that future studies should focus on post-transplantation immunomodulation.

Gö 6976, a selective inhibitor of protein kinase C, is a potent antagonist of human immunodeficiency virus 1 induction from latent/low-level-producing reservoir cells in vitro

Human immunodeficiency virus (HIV-1) infection is followed by a period of latency or a low-level-persistent (LLP) state that results in an asymptomatic infection of the host. Productive viral expression may be triggered by a variety of activators including mitogens, antigens, and cytokines. Protein kinase C (PKC) has been shown to be important in the intracellular cascade of signals induced by such activators. With U1 and ACH-2 cell lines representative of an HIV-1 postintegration state, the effect of Gö 6976, a synthetic inhibitor of PKC was tested. Gö 6976 is a nonglycosidic indolocarbazole found to potently inhibit HIV-1 induction by Bryostatin 1, tumor necrosis factor alpha, and interleukin 6. Gö 6976 effectively blocks viral transcription induced by Bryostatin 1 or tumor necrosis factor alpha that leads to the inhibition of intracellular viral protein synthesis and viral shedding. Gö 6976 also blocks interleukin 6-mediated posttranscriptional induction of viral proteins. The IC50 of Gö 6976 shows a 12- to 60-fold more potent effect than for H-7, another PKC inhibitor with a similar mechanism. The inhibitory effect is reduced when Gö 6976 is not added before or within 1 hr of induction by the potent PKC activator Bryostatin 1. However, U1 cells can be grown for long periods in a nontoxic concentration of Gö 6976 (300 nM), which confers virtual inhibition of HIV-1 induction without the development of resistance. Results indicate that inhibition of HIV-1 proviral induction from latent/low-level-producing infectious states with potent PKC inhibitors like Gö 6976 may represent an additional and promising antiviral approach.

Activated protein kinase C directly phosphorylates the CD34 antigen in acute lymphoblastic leukemia cells

The precursors of all blood cell lineages are contained within the 1-3% of bone marrow cells which express the CD34 antigen, and this population can reconstitute the hematopoietic system of lethally irradiated animals and humans. A potential regulatory role for the CD34 antigen in progenitor cell function and differentiation was indicated by our recent findings that the CD34 antigen can be phosphorylated in vivo to high stoichiometry in primitive CD34+ cell-lines by activated protein kinase C. To exclude the possibility that these effects were restricted to cell-lines, we have performed similar experiments on fresh cells from a patient with drug-resistant acute lymphoblastic leukemia. Similar to our previous findings, we found the CD34 antigen to be hyperphosphorylated in lymphoblasts labeled in the presence of active phorbols. The same peptides which were hyperphosphorylated in phorbol-stimulated cell-lines were also phosphorylated in phorbol-stimulated lymphoblasts. These data indicate that CD34 is a substrate molecule for PKC in fresh CD34+ lymphoblasts and underline the role of modulators of PKC activity in the biology of primitive leucocytes.

Four interleukin-2 surface binding proteins detected in rat spleen cells

Four specific interleukin-2 (IL-2) surface binding proteins can be detected by covalent cross-linking of [125I]IL-2 to rat spleen cells that have been activated with various stimuli including concanavalin A (Con A), phytohaemagglutinin (PHA), calcium ionophore, and phorbol dibutyrate (PDB) with or without calcium ionophore. These four cross-linked proteins could not be demonstrated in either unstimulated T cells or in activated T cells when binding was performed in the presence of a 20-100-fold excess of unlabelled IL-2. The molecular weights of the four cross-linked proteins, after subtraction of the molecular weight contribution of IL-2 are: 53,000, 70,000, 90,000 and 118,000. The 53,000 MW protein was identified as the rat IL-2 receptor (IL-2R) alpha-chain by immune precipitation. Additionally, results suggest that the rat IL-2R alpha-chain is tightly complexed to both the 118,000 and 90,000 MW IL-2 binding proteins. Purification of surface labelled proteins from activated cells using IL-2 affinity chromatography yields four proteins with similar molecular weight to those identified by cross-linking plus an additional non-ligand cross-linked protein of 46,000 MW. The 46,000 MW band may be a non-binding associated protein since it was not seen following [125I]IL-2 binding cross-linking. Tryptic digests and two-dimensional separation of the affinity-isolated proteins indicate that unique peptide maps are generated for the 46,000, 53,000 and 70,000 MW proteins and excludes the possibility that the bands identified by cross-linking represents cross-linking of multiple ligands to the 53,000 MW subunit. However, the 90,000 and 118,000 MW bands yield peptide maps that closely resemble each other suggesting that these binding proteins may be related. These results suggest that at least four IL-2 surface binding proteins may constitute the rat IL-2R system.

Up-regulation of surface CD34 is associated with protein kinase C-mediated hyperphosphorylation of CD34

CD34 is a transmembrane sialoglycoprotein expressed by early hematopoietic progenitor cells as well as endothelial cells. Previously we found that CD34 is rapidly and stoichiometrically phosphorylated by activated protein kinase C (PKC) (Fackler, M.J., Civin, C.I., Sutherland, D.R., Baker, M.A., and May, W.S. (1990) J. Biol. Chem. 265, 11056-11061). In the present study, we find dose-dependent up-regulation of CD34 surface expression following treatment of normal human CD34+ bone marrow progenitor cells, cord blood-derived KMT-2, or KG1 a myeloid leukemia cells with the PKC activator 12-O-tetradecanoylphorbol-13-acetate. Up-regulation begins within 1 min of treatment, is maximal by 30 min, is maintained for at least 3 h, and is associated with CD34 hyperphosphorylation. A specific inhibitor of PKC, 2,6-diamino-N-(1[1-(1-oxotridecyl)-2-piperadinyl]methyl)h exan-amide (NPC 15437), blocks both up-regulation and hyperphosphorylation of CD34. CD34 up-regulation is independent of transcription and/or translation and results from the recruitment of preformed intracellular CD34. The endocytosis rate of surface CD34 is unaltered by 12-O-tetradecanoylphorbol-13-acetate. Thus, activation of PKC mediates increased surface expression of the CD34 molecule possibly as a result of phosphorylation of CD34.

Differential effects of bryostatin 1 and phorbol ester on human breast cancer cell lines

The effects of the protein kinase C (PKC) activators, phorbol ester 12-O-tetradecanoyl-13-phorbol acetate (TPA) and the marine natural product, bryostatin 1, on the growth and morphology of human breast cancer cell lines were examined. TPA (1 to 100 nM) inhibited growth of four of six cell lines by up to 75% in 5-day cultures. Bryostatin 1 inhibited growth of only MCF-7 cells and only at a high dose (100 nM). However, bryostatin 1 completely antagonized the growth inhibition and morphological changes induced by TPA in MCF-7 cells. The divergent effects of these two agents are associated with differing effects on PKC activity and isoform expression in MCF-7 cells. TPA induced rapid translocation of the PKC-alpha isozyme and PKC activity to the membrane fraction of MCF-7 cells. In contrast, bryostatin 1 treatment resulted in the loss of the PKC-alpha isozyme and PKC activity from both cytosolic and membrane compartments within 10 min of treatment. In coincubation assays the bryostatin 1 effect was dominant over that of TPA. Similar effects on PKC-alpha isozyme and PKC activity were seen in a second cell line whose growth was inhibited by TPA but not by bryostatin 1, MDA-MB-468. In contrast, in the T47D cell line, where TPA was not growth inhibitory, TPA failed to induce translocation of PKC-alpha to the cell membrane. Bryostatin, however, still caused loss of PKC-alpha isozyme and PKC activity from cytosolic and membrane fractions. Thus, differential actions of bryostatin 1 and TPA on PKC activity and alpha-isoform level in the membrane-associated fraction of MCF-7 and MDA-MB-468 cells may account for the divergent effects of these two agents on cell growth and morphology. These results suggest that the PKC-alpha isoform may specifically play a role in inhibiting growth of human breast cancer cells.

AIC2A is a component of the purified high affinity mouse IL-3 receptor: temperature-dependent modulation of AIC2A structure

IL-3, a potent hemopoietic growth factor, interacts with distinct classes of receptor, one of high affinity and the other of low affinity. The gene for a 115 kDa, low affinity IL-3 binding protein (AIC2A) was recently cloned. Ligand affinity purification was used to show that the AIC2A gene product participates in the formation of a high affinity IL-3 receptor (IL-3R). Cells were incubated with biotin-IL-3 at 4 degrees C and IL-3 bound to the low affinity site was removed by washing, cells were detergent extracted, and then streptavidin - agarose was used to purify proteins bound to biotin-IL-3. A 115 kDa phosphotyrosine (Ptyr)-containing protein was specifically purified and its identity as AIC2A was shown in Western assays using polyclonal anti-AIC2A antibodies. A brief temperature shift of the intact, biotin-IL-3-treated cells from 4 to 37 degrees C, prior to receptor purification, results in structural and compositional changes in the IL-3R, including: (i) a 10-20 kDa increase in the apparent Mr of both the AIC2A and the Ptyr antigens, and (ii) the association of a serine/threonine kinase. These observations indicate that in its native environment, the low affinity IL-3 binding protein, AIC2A, participates to form the high affinity IL-3R and is a substrate for a tyrosine kinase. Moreover, a ligand-induced, temperature-regulatable structural change in the IL-3R may be of importance in the transduction of information through the receptor, as suggested by the enhanced association of the IL-3R with a serine/threonine kinase.

Response of Jurkat T cells to phorbol ester and bryostatin. Development of sublines with distinct functional responses and changes in protein kinase C activity

T lymphocyte activation is initiated as a result of the interaction between the TCR complex and Ag as seen in the framework of a membrane-bound MHC molecule. Receptor stimulation results in a rise in free intracellular Ca2+ and the activation of protein kinase C (PKC). Bryostatin (Bryo) and phorbol esters (e.g., 12-O-tetradecanoylphorbol 13-acetate (TPA] are PKC activators with somewhat different immunologic effects. We compared the effect of Bryo and TPA on the T cell tumor line Jurkat and derivatives of Jurkat cells grown in media supplemented with 100 nM Bryo ("BR100" cells) or 100 nM TPA ("TP100" cells). In untreated Jurkat cells, there is a dose- and time-dependent decrease in proliferation, compared to media controls, after the administration of as little as 10 nM TPA. This can be reversed in a dose- and time-dependent manner by Bryo. Interestingly, the expression of the transferrin receptor parallelled this effect on proliferation. Furthermore, Jurkat cells grown continuously in 100 nM TPA regained full proliferative capacity after several weeks in culture and transferrin receptor expression returned to near the level seen in untreated Jurkat cells. The chromatographic separation of PKC activity in these three cell lines showed that total PKC activity was dramatically decreased in both the TP100 and BR100 cells when compared to untreated Jurkat cells. However, in the TP100 cells there exists a peak of activity that is activated by Bryo, but not TPA. Western blots of whole cell lysates of the three cell lines showed that PKC-alpha and PKC-beta II were both down-regulated in BR100 and TP100 cells compared to untreated Jurkat cells. PKC-gamma was not detected in any of the cell lines. Therefore, the Bryo-specific peak seen in TP100 cells may be PKC-delta, -epsilon, -zeta, -eta, or a novel PKC isoform. This could provide the basis for a molecular characterization of the differences in PKC activation between phorbol esters and Bryo.

Response of Jurkat T cells to phorbol ester and bryostatin. Development of sublines with distinct functional responses and changes in protein kinase C activity

T lymphocyte activation is initiated as a result of the interaction between the TCR complex and Ag as seen in the framework of a membrane-bound MHC molecule. Receptor stimulation results in a rise in free intracellular Ca2+ and the activation of protein kinase C (PKC). Bryostatin (Bryo) and phorbol esters (e.g., 12-O-tetradecanoylphorbol 13-acetate (TPA] are PKC activators with somewhat different immunologic effects. We compared the effect of Bryo and TPA on the T cell tumor line Jurkat and derivatives of Jurkat cells grown in media supplemented with 100 nM Bryo ("BR100" cells) or 100 nM TPA ("TP100" cells). In untreated Jurkat cells, there is a dose- and time-dependent decrease in proliferation, compared to media controls, after the administration of as little as 10 nM TPA. This can be reversed in a dose- and time-dependent manner by Bryo. Interestingly, the expression of the transferrin receptor parallelled this effect on proliferation. Furthermore, Jurkat cells grown continuously in 100 nM TPA regained full proliferative capacity after several weeks in culture and transferrin receptor expression returned to near the level seen in untreated Jurkat cells. The chromatographic separation of PKC activity in these three cell lines showed that total PKC activity was dramatically decreased in both the TP100 and BR100 cells when compared to untreated Jurkat cells. However, in the TP100 cells there exists a peak of activity that is activated by Bryo, but not TPA. Western blots of whole cell lysates of the three cell lines showed that PKC-alpha and PKC-beta II were both down-regulated in BR100 and TP100 cells compared to untreated Jurkat cells. PKC-gamma was not detected in any of the cell lines. Therefore, the Bryo-specific peak seen in TP100 cells may be PKC-delta, -epsilon, -zeta, -eta, or a novel PKC isoform. This could provide the basis for a molecular characterization of the differences in PKC activation between phorbol esters and Bryo.

c-myc gene-induced alterations in protein kinase C expression: a possible mechanism facilitating myc-ras gene complementation

The mechanism(s) by which the c-myc nuclear protein and the membrane-associated ras protein interact to mediate phenotypic changes is unknown. We now find that c-mcy gene expression is associated with alterations in the principal signal transduction pathway through which the ras protein is thought to function. We studied the transcript and protein expression of protein kinase C (PKC) isoforms in a culture line of human small cell lung cancer cells (NCI H209) in which expression of inserted c-myc and Ha-ras genes together, but not alone, causes a transition to a large cell phenotype. In control H209 cells, at the transcript and cell membrane protein levels, PKC-alpha is the dominant PKC species. In this cell line, the expression of an exogenous c-myc gene, but not of a viral Ha-ras gene, causes a 5- to 10-fold increase in the PKC-beta isoform transcript and protein. The insertion of ras into the exogenous myc-expressing 209 cells, in addition to causing phenotypic transition, results in the translocation of the PKC-beta protein from the cytosol to the membrane fraction and a decrease in membrane-associated PKC-alpha. Concomitant with these changes, the increased PKC isoform transcript levels induced by myc alone are completely reversed. These observations suggest that a complex set of PKC transcript and protein alterations, most prominently involving an increased PKC-beta protein level in the cell membrane, a decrease in PKC-alpha protein, and a decrease in all PKC isoform transcripts, may represent a fundamental event(s) for c-myc collaboration with Ha-ras to alter cell phenotype.

Erythropoietin induces Raf-1 activation and Raf-1 is required for erythropoietin-mediated proliferation

Erythropoietin mediates the rapid phosphorylation of Raf-1 in the murine cell lines HCD-57 and FDC-P1/ER, which proliferate in response to this cytokine. Phosphorylation occurs at both serine and tyrosine residues and as such is similar to the Raf-1 phosphorylation seen after interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor, and interleukin-2 stimulation in other murine cell lines. Such data suggest that these growth factors may share a common mechanism(s) of Raf-1 phosphorylation. Furthermore, in association with Raf-1 phosphorylation, erythropoietin induces a 2-3-fold increase in Raf-1 kinase activity as measured in immune complex kinase assays in vitro. Finally, a c-raf antisense oligodeoxyribonucleotide, which specifically decreases intracellular Raf-1 levels, also substantially inhibits both erythropoietin and IL-3-directed DNA synthesis. Together, these results provide evidence that activated Raf-1 is a necessary component of erythropoietin and IL-3 growth signaling pathways.

Interleukin-3 and granulocyte-macrophage colony-stimulating factor mediate rapid phosphorylation and activation of cytosolic c-raf

Interleukin-3 (IL-3) and granulocyte-macrophage colony-stimulating factor induce the rapid phosphorylation of the c-raf protein in the growth factor-dependent FDC-P1 and DA-3 murine myeloid cell lines. Furthermore, immunoprecipitates of c-raf isolated from growth factor-stimulated cells demonstrate a marked increase in intrinsic protein kinase activity as measured in vitro. IL-3 and granulocyte-macrophage colony-stimulating factor induce phosphorylation of c-raf at both serine and tyrosine residues. Antiphosphotyrosine immunoprecipitates from IL-3-stimulated cells demonstrate the rapid and coordinate phosphorylation of both c-raf and a protein co-migrating with the 140-kDa putative IL-3 receptor component. Collectively, the findings of rapid and coordinate ligand-induced phosphorylation of a potential IL-3 growth factor receptor component and cytoplasmic c-raf with concomitant c-raf activation provide a cogent sequential molecular model for linking external growth stimuli to intracellular signal transduction events.

The action of bryostatin on normal human hematopoietic progenitors is mediated by accessory cell release of growth factors

Since enrichment of human bone-marrow hematopoietic progenitors is becoming more feasible and since purified growth factors are now available, we sought to study the action of growth factors on CD34-positive enriched cultures of human bone-marrow cells. We tested the effect of recombinant human (rh) granulocyte-macrophage colony-stimulating factor (GM-CSF), rh interleukin-3 (IL-3), or a unique biologic response modifier, bryostatin 1, on the growth of purified CD34 cells obtained by limiting dilution in single-cell cultures. We have shown previously that bryostatin 1 stimulates both myeloid and erythroid progenitors of human origin in vitro. In this study both IL-3 and GM-CSF supported colony formation from 500, 100, or single-cell cultures at equivalent plating efficiences, suggesting a direct action of these factors on hematopoietic cell growth. Conversely, bryostatin 1 did not support the growth of CD34 cells in single-cell cultures, and the cloning efficiency increased with increasing the number of cells in the culture. To test whether the indirect action of bryostatin 1 might be mediated through the production of growth factors by accessory cells, studies were performed using antibodies directed against human IL-3 and GM-CSF in culture with bryostatin 1 and normal human bone-marrow cells. Results are consistent with the hypothesis that bryostatin 1 could have a stimulatory effect on the accessory cell populations to produce either IL-3 or GM-CSF. Further support for this notion was obtained by demonstrating that T cells, which are cells known to be able to produce IL-3 and GM-CSF, are stimulated by bryostatin 1 to express messenger RNA (mRNA) for specific growth factors, including GM-CSF. These results provide further support that bryostatin 1 may be a useful clinical agent to stimulate hematopoiesis in vivo.

Activated protein kinase C directly phosphorylates the CD34 antigen on hematopoietic cells

The CD34 antigen is a human leukocyte membrane protein expressed specifically by lymphohematopoietic progenitor cells. We found that CD34 is a phosphoprotein and therefore examined the regulation of its phosphorylation. Activation of protein kinase C (PKC) enhanced CD34 phosphorylation. The PKC activators, 12-O-tetradecanoylphorbol-13-acetate and bryostatin-1, induced rapid, stoichiometric hyperphosphorylation of CD34 protein in cells, resulting in a 5-fold increase in CD34 phosphorylation. In vitro kinase studies revealed that purified PKC could directly phosphorylate purified CD34. Only serine phosphorylation was detected in the CD34 molecule. Two-dimensional phosphopeptide mapping experiments indicated that PKC induces the phosphorylation of identical serine residue(s) in vitro and in vivo (in KG1 cells). These are newly phosphorylated serine residue(s), which are not detectably phosphorylated in CD34 from exponentially growing KG1 cells. These data indicate that the developmental stage-specific molecule, CD34, is a phosphorylation target for activated PKC. Furthermore, these findings raise the possibility that PKC activation and phosphorylation of the CD34 molecule may play a role in signal transduction during early lymphohematopoiesis.

Role of nuclear protein kinase C in the mitogenic response to platelet-derived growth factor

We have assessed the involvement of nuclear envelope protein phosphorylation in the mitogenic response to platelet-derived growth factor (PDGF) in NIH/3T3 fibroblasts. We find that stimulation of quiescent NIH/3T3 cells with PDGF or with the mitogenic protein kinase C (PKC) activators phorbol 12-myristate 13-acetate (PMA) or bryostatin 1 (bryo) leads to rapid, dose-dependent phosphorylation of several nuclear envelope polypeptides. The predominant nuclear envelope targets for mitogen-induced phosphorylation are immunologically identified as the nuclear envelope lamins. All three lamin species (A, B and C) are phosphorylated in response to PMA or bryo, while lamins A and C are preferentially phosphorylated in response to PDGF. Phosphopeptide mapping and phosphoamino acid analysis indicate that similar serine sites on the lamins are phosphorylated in response to PDGF, PMA and bryo. Both mitogenicity and lamina phosphorylation induced by these mitogens can be inhibited by the selective PKC inhibitor staurosporine at 2 nM. Treatment of quiescent NIH/3T3 cells with PDGF, PMA or bryo leads to rapid translocation of PKC to the nuclear envelope. These data indicate that rapid nuclear events, including translocation of cytosolic PKC to the nuclear membrane and lamina phosphorylation, may play a role in the transduction of the mitogenic signals of PDGF from the cytoplasm to the nucleus in NIH/3T3 fibroblasts.

Bryostatin 1, a unique biologic response modifier: anti-leukemic activity in vitro

Bryostatin 1, a macrocyclic lactone isolated from the marine bryozoan Bugula neritina, has demonstrated both antineoplastic activity against the murine P388 leukemia line in vivo and stimulatory activity against mouse and human hematopoietic progenitors. We studied the effects of bryostatin 1 on the growth of human leukemias in vitro. Bryostatin 1 inhibited 1 to 4 logs of clonogenic leukemia cell growth from three of four leukemia cell lines. Bryostatin 1 also inhibited, by at least 1 log, the proliferation of clonogenic acute nonlymphocytic leukemia (ANLL) cells from 10 to 12 patients with newly diagnosed or relapsed ANLL. Maximal inhibition of leukemic growth occurred at 10(-9) to 10(-7) mol/L bryostatin 1. Interestingly, bryostatin 1 also inhibited the growth of hematopoietic progenitors from eight patients with myelodysplastic syndromes (MDS). Leukemia cells exposed to bryostatin 1 for up to 96 hours and then washed, demonstrated no substantial inhibition of clonogenic growth, indicating that the anti-leukemic effect of bryostatin 1 is cytostatic. The phorbol ester 12-0-tetradecanoylphorbol-13-acetate (TPA) produced more potent inhibition of clonogenic leukemia growth, and this inhibition was blocked by bryostatin 1. Thus, the anti-leukemic activity of bryostatin 1 may be mediated through activation of protein kinase C. Bryostatin 1 inhibits clonogenic leukemia cells at concentrations that stimulate normal hematopoietic progenitors. The differential effects of bryostatin 1 on normal and abnormal hematopoiesis suggest that bryostatin 1 may have value in the treatment of leukemias and MDS.