Maturation of murine erythroleukemia cells committed to differentiation requires protein kinase C

Regulation of the ErbB3 binding protein Ebp1 by protein kinase C

Ebp1, a 47 kDa ubiquituously expressed protein, binds the ErbB3 receptor in human serum starved breast cancer cell lines and dissociates from ErbB3 on treatment with the ErbB3 ligand, Heregulin (HRG). However, the mechanism of Ebp1-ErbB3 association/dissociation is not understood. Since Ebp1 contains six putative Protein Kinase C serine/threonine phosphorylation sites, we examined the ability of PKC to phosphorylate Ebp1 and to regulate Ebp1-ErbB3 binding. We found that Ebp1 was basally phosphorylated in AU565 breast cancer cells on serine/threonine residues and that this phosphorylation was enhanced by heregulin treatment. Both serine and threonine residues of a GST-Ebp1 fusion protein were phosphorylated by PKC in vitro. In vivo, we demonstrated that basal Ebp1 phosphorylation was dependent upon PKC. However, HRG-induced phosphorylation of Ebp1 occurred predominantly in a PKC-independent manner. The ability of Ebp1 to associate with ErbB3 in serum-starved NIH3T3 cells overexpresssing ErbB3 was abrogated by treating cells with a PKC inhibitor. These findings suggest that PKC plays a role in regulating phosphorylation and function of Ebp1 in vivo.

Protein kinase C-α isoform is involved in erythropoietin-induced erythroid differentiation of CD34+ progenitor cells from human bone marrow

Protein kinase C (PKC) is a family of serine/threonine protein kinases involved in many cellular responses. Although the analysis of PKC activity in many systems has provided crucial insights to its biologic function, the precise role of different isoforms on the differentiation of normal hematopoietic progenitor cells into the various lineages remains to be investigated. The authors have assessed the state of activation and protein expression of PKC isoforms after cytokine stimulation of CD34+ progenitor cells from human bone marrow. Freshly isolated CD34+ cells were found to express PKC-, PKC-β2, and PKC-ɛ, whereas PKC-δ, PKC-γ, and PKC-ζ were not detected. Treatment with erythropoietin (EPO) or with EPO and stem cell factor (SCF) induced a predominantly erythroid differentiation of CD34+ cells that was accompanied by the up-regulation of PKC- and PKC-β2 protein levels (11.8- and 2.5-fold, respectively) compared with cells cultured in medium. Stimulation with EPO also resulted in the nuclear translocation of PKC- and PKC-β2 isoforms. Notably, none of the PKC isoforms tested were detectable in CD34+ cells induced to myeloid differentiation by G-CSF and SCF stimulation. The PKC inhibitors staurosporine and calphostin C prevented EPO-induced erythroid differentiation. Down-regulation of the PKC-, PKC-β2, and PKC-ɛ expression by TPA pretreatment, or the down-regulation of PKC- with a specific ribozyme, also inhibited the EPO-induced erythroid differentiation of CD34+ cells. No effect was seen with PKC-β2–specific ribozymes. Taken together, these findings point to a novel role for the PKC- isoform in mediating EPO-induced erythroid differentiation of the CD34+ progenitor cells from human bone marrow.

Effects of PKC alpha activation on Ca2+ pump and K(Ca) channel in deoxygenated sickle cells

We have previously shown that a pretreatment with phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), reduced deoxygenation-induced K+ loss and Ca2+ uptake and prevented cell dehydration in sickle anemia red blood cells (SS cells) (H. Fathallah, E. Coezy, R.-S. De Neef, M.-D. Hardy-Dessources, and F. Giraud. Blood 86: 1999-2007, 1995). The present study explores the detailed mechanism of this PMA-induced inhibition. The main findings are, first, the detection of PKC alpha and PKC zeta in normal red blood cells and the demonstration that both isoforms are expressed at higher levels in SS cells. The alpha-isoform only is translocated to the membrane and activated by PMA and by elevation of cytosolic Ca2+. Second, PMA is demonstrated to activate Ca2+ efflux in deoxygenated SS cells by a direct stimulation of the Ca2+ pump. PMA, moreover, inhibits deoxygenation-induced, charybdotoxin-sensitive K+ efflux in SS cells. This inhibition is partly indirect and explained by the reduced deoxygenation-induced rise in cytosolic Ca2+ resulting from Ca2+ pump stimulation. However, a significant inhibition of the Ca2+-activated K+ channels (K(Ca) channels) by PMA can also be demonstrated when the channels are activated by Ca2+ plus ionophore, under conditions in which the Ca2+ pump is operating near its maximal extrusion rate, but swamped by Ca2+ plus ionophore. The data thus suggest a PKC alpha-mediated phosphorylation both of the Ca2+ pump and of the K(Ca) channel or an auxiliary protein.

Inhibition by 1alpha,25-dihydroxyvitamin D3 of activin A-induced differentiation of murine erythroleukemic F5-5 cells

1alpha,25-Dihydroxyvitamin D3 (1alpha,25-(OH)2D3) and other vitamin D3 (VD3) analogs enhanced the inhibitory effect of Activin A on murine erythroleukemia (MEL) cell proliferation and differentiation in a dose-dependent manner. 1alpha,25-(OH)2D3 inhibited differentiation more potently than proliferation by one order of magnitude. The VD3 analog study demonstrated either effect of VD3 on MEL cells via vitamin D receptor (VDR), as evidenced from the close relationship with the reported affinities for VDR. The effects of 1alpha,25-(OH)2D3 were preceded by the suppression of ornithine decarboxylase (ODC) activity, a rate-limiting enzyme in polyamine metabolism. Difluoromethylornithine (DFMO), an inhibitor of ODC, inhibited MEL cell proliferation, which was reversed by the simultaneous addition of putrescine, a product of ODC, but did not affect differentiation. 1alpha,25-(OH)2D3 inhibited cell differentiation during the phenotype-expression stage as reflected by the inhibition of beta-globin gene expression, while it inhibited proliferation in the commitment stage. Furthermore, it seems unlikely that the different effects of VD3 on proliferation and differentiation may be a result of upregulation of VDR or nongenomic action. In summary, it was suggested that 1alpha,25-(OH)2D3 inhibited Activin A-induced MEL cell proliferation and differentiation by distinct mechanisms and inhibited the proliferation by inhibiting ODC activity. We demonstrated the presence of 1alpha,25-(OH)2D3 action on leukemic cells at physiological concentration, which was distinct from the pharmacological effect of VD3 reported thus far.

1,25-Dihydroxyvitamin D3 stimulates expression and translocation of protein kinase Calpha and Cdelta via a nongenomic mechanism and rapidly induces phosphorylation of a 33-kDa protein in acute promyelocytic NB4 cells

1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3) primes NB4 cells for 12-O-tetradecanoylphorbol-13-acetate-induced monocytic differentiation in a dose- and sequence-dependent fashion. Experiments utilizing 1,25-(OH)2D3 analogues and kinase/phosphatase inhibitors suggested that tyrosine kinase and serine/threonine phosphorylation cascades, rather than vitamin D3 receptor-mediated signals, were involved in 1,25-(OH)2D3 action. Here we show that NB4 cells express the alpha and delta (but not the beta, epsilon, and theta) isoforms of protein kinase C (PKC). Both authentic 1, 25-(OH)2D3 and the nongenomic analogue 1alpha,25-dihydroxyprevitamin D3 (HF) increased expression of PKCalpha and PKCdelta. PKCalpha and PKCdelta were translocated to the nucleus of the cell in response to 1,25-(OH)2D3 or HF. The effects of HF were attenuated by the nongenomic antagonist 1beta,25-dihydroxyvitamin D3, suggesting that changes in PKC expression are mediated by a nongenomic signaling pathway. Consistent with the involvement of serine, threonine, and tyrosine phosphorylation cascades mediating 1,25-(OH)2D3 action, enhanced phosphorylation of a variety of cellular proteins at serine and threonine residues and the specific enhanced phosphotyrosyl content of a 33-kDa protein (vdrp33) were observed immediately after 1,25-(OH)2D3 addition. We propose that 1,25-(OH)2D3 primes NB4 cells for 12-O-tetradecanoylphorbol-13-acetate-induced monocytic differentiation by increasing the expression of specific PKC isoforms and inducing the specific phosphorylation of key protein signaling intermediates.

Relationship between type II phosphatidylinositol 4-kinase activity and protein tyrosine phosphorylation in membranes from normal and sickle red cells

To assess the origin of the previously reported higher type II phosphatidylinositol 4-kinase (PtdIns 4-kinase) activity of sickle-red-cell membranes [Rhoda-Hardy-Dessources, M.D., de Neef, R.S., Mérault, G.& Giraud, F. (1993) Biochim. Biophs. Acta 1181, 90-96], we have investigated the possible involvement of protein kinase C and tyrosine kinases in the regulation of the lipid kinase activity. Both protein kinase activities were found to be markedly higher in membranes from the pathological cells. When isolated normal-red-cell or sickle-red-cell membranes were assayed, phosphatidylinositol phosphorylation activity was not significantly modified after phorbol ester modulation of protein kinase C. In contrast, stimulation (with sodium orthovanadate) or inhibiton (by tyrphostin) of tyrosine phosphorylation led respectively, to increased or decreased PtdIns 4-kinase activity in membranes from both cell types. Moreover, immunoprecipitations of membrane extracts from normal and sickle red cells types with anti-PtdIns 4-kinase antibody 4C5G, followed by immunoblotting with an anti-phosphotyrosine Ig, revealed a 56-kDa band migrating with PtdIns 4-kinase activity. Taken together, these findings indicate that PtdIns 4-Kinase in red blood cells is a phosphotyrosine-containing protein and could be regulated by a mechanism involving tyrosine phosphorylation, and the increase in PtdIns 4-Kinase activity of sickle-red-cell membranes is at least in part mediated by their intrinsic tyrosine kinase activity.

Dynamics of ubiquitin conjugation during erythroid differentiation in vitro

To gain insight into the role of ubiquitin-mediated proteolysis in erythroid differentiation, levels of ubiquitin conjugating enzymes (E2s) and ubiquitin conjugates were analyzed during in vitro differentiation of murine erythroleukemic (MEL) cells. After 4 days of culture in the presence of the inducer dimethyl sulfoxide, MEL cells expressed high levels of the erythroid-specific proteins, globin, and band 3. During the same interval, cellular contents (mol/cell) of E2-14K, E2-25K, and E2-35K decreased up to approximately 5-fold; as suggested by results obtained with E2-25K, this reflected a lower level of mRNA in differentiating cells. Concentrations of these E2s changed more modestly during in vitro differentiation, since cellular volume also decreased. Comparison of levels of the three E2s in undifferentiated MEL cells and reticulocytes suggests that their concentrations remain fairly constant during in vivo differentiation of proerythroblasts into reticulocytes. Thus, these components of the ubiquitin-mediated proteolytic pathway are likely to function constitutively during this interval. Two-dimensional Western blots showed a broad spectrum of ubiquitin conjugates, including free multiubiquitin chains, in undifferentiated MEL cells. As seen for several E2s, the concentration of ubiquitin conjugates (including free chains) decreased modestly during in vitro differentiation. E2-20K and E2-230K, which are abundant in reticulocytes, were low or absent in undifferentiated and differentiated MEL cells. In erythroid cells these two E2s are reticulocyte-specific; apparently MEL cells do not differentiate far enough to allow induction of their expression.

Integrins and adhesive receptors in normal and leukemic CD34+ progenitor cells: potential regulatory checkpoints for cellular traffic

Multiple adhesion receptors are involved in the interaction of hematopoietic cells with the marrow microenvironment. This work characterizes the expression of various adhesive receptors on normal early hematopoietic precursors and reviews how they might be altered in leukemic states. Early hematopoietic CD34+ cells express CD18, CD11a, CD49d, CD49e, CD44, ICAM-1, and ICAM-3. Likewise, most AML samples express CD49d, CD49e, and CD44. In addition to mediating the adherence of progenitors to the marrow, these multiple receptors and their respective ligands may serve to regulate in vivo leukemic cell agrees from marrow and the ability of certain leukemic phenotypes to selectively seek extramedullary sanctuary sites such as the skin and the central nervous system.