Protein kinase C is not necessary for transforming growth factor beta-induced growth-arrest in leukemia cell lines

Inhibition of Cot1/Tlp2 oncogene in AML cells reduces ERK5 activation and up-regulates p27Kip1 concomitant with enhancement of differentiation and cell cycle arrest induced by silibinin and 1,25-dihydroxyvitamin D(3)

Acute myelogenous leukemia (AML) is a disease characterized by dysregulated cell proliferation associated with impaired cell differentiation, and current treatment regimens rarely save the patient. Thus, new mechanism-based approaches are needed to improve prognosis of this disease. We have investigated in preclinical studies the potential anti-leukemia use of the plant-derived polyphenol Silibinin (SIL) in combination with 1,25-dihydroxyvitamin D3 (1,25D). Although most of the leukemic blasts ex vivo responded by differentiation to treatment with this combination, the reasons for the absence of SIL-1,25D synergy in some cases were unclear. Here we report that failure of SIL to enhance the action of 1,25D is likely due to the SIL-induced increase in the activity of differentiation-antagonizing cell components, such as ERK5. This kinase is under the control of Cot1/Tlp2, and inhibition of Cot1 activity by a specific pharmacological inhibitor 4-(3-chloro-4-fluorophenylamino)-6-(pyridin-3-yl-methylamino-3-cyano-[1-7]-naphthyridine, or by Cot1 siRNA, increases the differentiation by SIL/1,25D combinations. Conversely, over-expression of a Cot1 construct increases the cellular levels of P-ERK5, and SIL/1,25D-induced differentiation and cell cycle arrest are diminished. It appears that reduction in ERK5 activity by inhibition of Cot1 allows SIL to augment the expression of 1,25D-induced differentiation promoting factors and cell cycle regulators such as p27 (Kip1) , which leads to cell cycle arrest. This study shows that in some cell contexts SIL/1,25D can promote expression of both differentiation-promoting and differentiation-inhibiting genes, and that the latter can be neutralized by a highly specific pharmacological inhibitor, suggesting a potential for supplementing treatment of AML with this combination of agents.

The role of protein kinase C-alpha in hematologic malignancies

In recent years advances in histopathological and molecular understanding of hematologic malignancies have led to the development of drugs which selectively target proteins associated with hematologic tumorigenesis. One such targeted agent is the antisense oligonucleotide aprinocarsen, which specifically inhibits the signaling protein, protein kinase C-alpha (PKC-alpha). Although PKC-alpha has been associated with tumorigenesis, its role and expression levels in patients with hematologic malignancies are not well understood. We here review studies investigating the expression and role of PKC-alpha in hematologic malignancies. Such a review may offer new insights on how to develop strategies in identifying patients that might best benefit from PKC-alpha inhibition.

CAL72: a human osteosarcoma cell line with unique effects on hematopoietic cells

Permanent osteoblastic cell lines are potential tools to study the interactions between osteoblastic and hematopoietic cells in the bone marrow cavity. In a recent work we have shown that the osteosarcoma cell line CAL72 may be more closely related to normal osteoblasts than the osteosarcoma cells previously described. In the present work we continued the characterisation of the CAL72 cell line with regard to its effects on various hematopoietic cells, in coculture experiments. We show here that CAL72 cells, in contrast to MG-63 or SaOS-2 osteosarcoma cell lines, do not inhibit hematopoietic colony formation and sustain the limited expansion of hematopoietic progenitors in a similar way to that described for normal osteoblasts. We also demonstrate that CAL72 cells induce the monocytic differentiation of the promyelocytic HL-60 cell line like MG-63 and SaOS-2, but support a better maturation and a longer survival of the differentiated cells than the two other osteosarcoma cell lines. In order to better understand the differential effects observed between CAL72 and MG-63 or SaOS-2, we analysed the cytokine and chemokine mRNA expression of these cells using the RNase protection quantitative assay. We show here that the expression profile of CAL72 is clearly different from that of MG-63 or SaOS-2 and may explain, at least in part, its specific effects on hematopoietic cells. Taken together these experiments confirm that CAL72 has particular properties and is an interesting tool to study the role of osteoblastic cells in hematopoietic cell growth and differentiation.

Regulation of Smad signaling by protein kinase C

Cross talk between transforming growth factor b(TGF-b) serine/threonine kinase receptor signaling and tyrosine kinase receptor signaling modulates cell responsiveness to polypeptide growth factors regulating cell proliferation, differentiation, and apoptosis. Here we provide a mechanism through which Smad-dependent TGF-b signaling is modulated by protein kinase C (PKC). PKC, for example, is activated downstream of tyrosine kinase receptors. We show that PKC directly phosphorylates receptor-regulated Smad proteins. This phosphorylation abrogates the ability of Smad3 to bind directly to DNA, which leads to subsequent inability to mediate transcriptional responses dependent on the direct binding of Smad3 to DNA. Interference with PKC regulation of Smad functions increased cell sensitivity to transformation by the tumor promoter phorbol 12-myristate 13-acetate (PMA). PKC-dependent phosphorylation of Smad3 was found also to be a key event in the PMA-dependent inactivation of TGF-b-stimulated cell death. Thus, PKC-dependent phosphorylation of Smad3 leads to down-regulation of the growth inhibitory and apoptotic action of TGF-b.