Statin-dependent activation of protein kinase Cδ in acute promyelocytic leukemia cells and induction of leukemic cell differentiation

All-trans retinoic acid and protein kinase C α/β1 inhibitor combined treatment targets cancer stem cells and impairs breast tumor progression

Breast cancer is the leading cause of cancer death among women worldwide. Blocking a single signaling pathway is often an ineffective therapy, especially in the case of aggressive or drug-resistant tumors. Since we have previously described the mechanism involved in the crosstalk between Retinoic Acid system and protein kinase C (PKC) pathway, the rationale of our study was to evaluate the effect of combining all-trans-retinoic acid (ATRA) with a classical PCK inhibitor (Gö6976) in preclinical settings. Employing hormone-independent mammary cancer models, Gö6976 and ATRA combined treatment induced a synergistic reduction in proliferative potential that correlated with an increased apoptosis and RARs modulation towards an anti-oncogenic profile. Combined treatment also impairs growth, self-renewal and clonogenicity potential of cancer stem cells and reduced tumor growth, metastatic spread and cancer stem cells frequency in vivo. An in-silico analysis of “Kaplan–Meier plotter” database indicated that low PKCα together with high RARα mRNA expression is a favorable prognosis factor for hormone-independent breast cancer patients. Here we demonstrate that a classical PKC inhibitor potentiates ATRA antitumor effects also targeting cancer stem cells growth, self-renewal and frequency.

Current Understandings of Myeloid Differentiation Inducers in Leukemia Therapy

Differentiation therapy using all-trans retinoic acid for acute promyelocytic leukemia (APL) is well established. Several attempts have been made to treat non-APL, AML patients by employing differentiation inducers, such as hypomethylating agents (HMAs), and low-dose cytarabine (Ara-C) (LDAC), with encouraging results. Other than HMAs and LDAC, various inducers of myeloid cell differentiation have been identified. This review describes and categorizes these inducers, which include glycosylation modifiers, epigenetic modifiers, vitamin derivatives, cytokines, and chemotherapeutic agents. Some of these inducers are currently being used in clinical trials. I highlight the potential applications of glycosylation modifiers and epigenetic modifiers, which are attracting increasing attention in their use as differentiation therapy against AML. Among the agents described in this review, epigenomic modifiers seem particularly promising, and particular attention should also be paid to glycosylation modifiers. These drugs may signal a new era for AML differentiation therapy.

Statin-specific inhibition of Rab-GTPase regulates cPKC-mediated IKs internalization

Statins are prescribed for prevention and treatment of coronary artery disease. Statins have different cholesterol lowering abilities, with rosuvastatin and atorvastatin being the most effective, while statins like simvastatin and fluvastatin having lower effectiveness. Statins, in addition to their cholesterol lowering effects, can prevent isoprenylation of Rab-GTPase proteins, a protein family important for the regulation of membrane-bound protein trafficking. Here we show that endosomal localization of Rab-GTPases (Rab5, Rab7 and Rab11) was inhibited in a statin-specific manner, with stronger effects by fluvastatin, followed by simvastatin and atorvastatin, and with a limited effect by rosuvastatin. Fluvastatin inhibition of Rab5 has been shown to mediate cPKC-dependent trafficking regulation of the cardiac delayed rectifier KCNQ1/KCNE1 channels. We observed statin-specific inhibition of channel regulation consistent with statin-specific Rab-GTPase inhibition both in heterologous systems and cardiomyocytes. Our results uncover a non-cholesterol-reducing statin-specific effect of statins. Because Rab-GTPases are important regulators of membrane trafficking they may underlie statin specific pleiotropic effects. Therefore, statin-specificity may allow better treatment tailoring.

Atorvastatin Inhibited ROS Generation and Increased IL-1β And IL-6 Release by Mononuclear Cells from Diabetic Patients

BACKGROUND

Atorvastatin (ATV) inhibits the conversion of 3-Hydroxy-3-Methylglutaryl Coenzyme A (HMG-CoA) to mevalonate formation and promotes lowering of the LDL cholesterol fraction. However, ATV exhibits pleiotropic metabolic actions beyond cholesterol-lowering properties.

OBJECTIVE

We aimed to evaluate the effect of ATV on oxidizing species generation and cytokine secretion in Peripheral Blood Mononuclear Cells (PBMNC) of Type 2 Diabetes Mellitus (T2DM) patients in comparison to healthy control.

METHODS

Both NADPH-oxidase-dependent and mitochondrial ROS generation were assessed by chemoluminescence luminol-dependent assay and fluorometric experiment, using Dichlorofluorescein Assay (DCFH-DA), respectively. IL-1β and IL-6 were quantified by classical ELISA.

RESULTS

ATV inhibited NADPH-oxidase dependent ROS generation, but showed no effect on mitochondrial ROS generation and activated IL-1β and IL-6 secretions in PBMNC from control and T2DM patients. ROS generation and cytokine secretion in the presence of an inhibitor of Protein Kinase Cβ (iPKCβ) and ATV led to similar results. The secretion of IL-1β, PDB-induced in the presence of iPKCβ, but not ATV, was increased. ATV and iPKCβ exacerbated PDB-induced IL-6 secretion. LPS activated the secretion of IL-1β and IL-6 which was potentiated by ATV.

CONCLUSION

ATV inhibited ROS generation and activated IL-1 β/IL-6 secretion in PBMNC of diabetes patients. Its effect was not affected by the hyperglycemia.

The Caspase-3/PKCδ/Akt/VEGF-A Signaling Pathway Mediates Tumor Repopulation during Radiotherapy

PURPOSE

Tumor repopulation is known as a major cause of treatment failure and/or tumor recurrence after radiotherapy. The underlying mechanism remains unclear. Our previous study demonstrated that irradiated apoptotic cells mediated tumor repopulation, in which caspase-3 played an important role. Herein, we investigated downstream effectors of caspase-3 involved in this process.

EXPERIMENTAL DESIGN

A dominant-negative protein kinase Cδ (DN_PKCδ) mutant that could not be cleaved by caspase-3 and therefore could not be activated by irradiation-induced apoptosis was constructed. DN_PKCδ stably transduced tumor cells were compared with wild-type tumor cells for their growth stimulation effects in in vitro and in vivo tumor repopulation models. Downstream effectors of caspase-3 and PKCδ were investigated. The role of PKCδ was further verified in human colorectal tumor specimens.

RESULTS

Inactivation of caspase-3 or caspase-7 attenuated tumor repopulation and weakened PKCδ cleavage. Both DN_PKCδ and PKCδ inhibitors restrained tumor repopulation both in vitro and in vivo. Phosphorylated Akt was attenuated in caspase-3-, caspase-7-, or PKCδ-inactivated tumor cells. Furthermore, expression of vascular endothelial growth factor (VEGF)-A but not hypoxia-inducible factor 1α (HIF1α) was decreased in PKCδ- or Akt-inactivated tumor cells. In addition, inhibition of p-Akt, HIF1α, VEGF-A, or VEGF-A receptor reduced tumor repopulation significantly. Finally, increased nuclear translocation of PKCδ in colorectal tumor specimens was associated with worse patient prognosis.

CONCLUSIONS

The caspase-3/PKCδ/Akt/VEGF-A axis is involved in tumor repopulation and could be exploited as a potential target to enhance the efficacy of radiotherapy.

Simvastatin Enhances Activity and Trafficking of α7 Nicotinic Acetylcholine Receptor in Hippocampal Neurons Through PKC and CaMKII Signaling Pathways

Simvastatin (SV) enhances glutamate release and synaptic plasticity in hippocampal CA1 region upon activation of α7 nicotinic acetylcholine receptor (α7nAChR). In this study, we examined the effects of SV on the functional activity of α7nAChR on CA1 pyramidal cells using patch-clamp recording and explored the underlying mechanisms. We found that the treatment of hippocampal slices with SV for 2 h induced a dose-dependent increase in the amplitude of ACh-evoked inward currents (I_ACh) and the level of α7nAChR protein on the cell membrane without change in the level of α7nAChR phosphorylation. These SV-induced phenotypes were suppressed by addition of farnesol (FOH) that converts farnesyl pyrophosphate, but not geranylgeraniol. Similarly, the farnesyl transferase inhibitor FTI277 was able to increase the amplitude of I_ACh and enhance the trafficking of α7nAChR. The treatment with SV enhanced phosphorylation of CaMKII and PKC. The SV-enhanced phosphorylation of CaMKII rather than PKC was blocked by FOH, Src inhibitor PP2 or NMDA receptor antagonist MK801 and mimicked by FTI. The SV-enhanced phosphorylation of PKC was sensitive to the IP3R antagonist 2-APB. The SV-increased amplitude of I_ACh was suppressed by PKC inhibitor GF109203X and Go6983, or CaMKII inhibitor KN93. The SV- and FTI-enhanced trafficking of α7nAChR was sensitive to KN93, but not GF109203X or Go6983. The PKC activator PMA increased α7nAChR activity, but had no effect on trafficking of α7nAChR. Collectively, these results indicate that acute treatment with SV enhances the activity and trafficking of α7nAChR by increasing PKC phosphorylation and reducing farnesyl-pyrophosphate to trigger NMDA receptor-mediated CaMKII activation.

Human thrombopoiesis depends on Protein kinase Cδ/protein kinase Cε functional couple

A deeper understanding of the molecular events driving megakaryocytopoiesis and thrombopoiesis is essential to regulate in vitro and in vivo platelet production for clinical applications. We previously documented the crucial role of PKCε in the regulation of human and mouse megakaryocyte maturation and platelet release. However, since several data show that different PKC isoforms fulfill complementary functions, we targeted PKCε and PKCδ, which show functional and phenotypical reciprocity, at the same time as boosting platelet production in vitro. Results show that PKCδ, contrary to PKCε, is persistently expressed during megakaryocytic differentiation, and a forced PKCδ down-modulation impairs megakaryocyte maturation and platelet production. PKCδ and PKCε work as a functional couple with opposite roles on thrombopoiesis, and the modulation of their balance strongly impacts platelet production. Indeed, we show an imbalance of PKCδ/PKCε ratio both in primary myelofibrosis and essential thrombocythemia, featured by impaired megakaryocyte differentiation and increased platelet production, respectively. Finally, we demonstrate that concurrent molecular targeting of both PKCδ and PKCε represents a strategy for in vitro platelet factories.

[Lipid metabolism in patients with hematologic cancers]

It is considered that hypercholesterolemia is life-threatening and low cholesterol levels are a positive factor. However, taking into consideration the fact that cholesterol plays a key role in cell proliferation, it should be remembered that its low blood level may be linked to high cholesterol demands from neoplastic cells. The literature review analyzes the results of recent investigations of lipid metabolism in patients with hematologic cancers and their other types. All given investigations show a significant reduction in the serum levels of total cholesterol and high-density lipoproteins in patients with hematological disease at its onset. The data for other indicators of the lipid transport system are ambiguous. Such changes have been elucidated to be associated with the accumulation of cholesterol in the leukemia cells due to enhanced synthesis de novo, a more active absorption from circulation and blocked release of its surplus. If the disease runs a favorable course, lipid metabolic parameters become normalized and, in case of remission, correspond to those seen in healthy individuals. They continue to decline in patients with disease progression. This allows the consideration of cholesterol, its fractions, and apolipoproteins as biochemical prognostic markers in hematological cancer patients and as indicators for assessment of treatment results. In addition, there is evidence for the effect of chemotherapeutic agents on lipid metabolism. Recent attempts to elaborate new treatment strategies, by using the current knowledge on the role of lipid metabolism in cancers, are considered.

Statin-mediated inhibition of cholesterol synthesis induces cytoprotective autophagy in human leukemic cells

Statins exhibit anti-leukemic properties due to suppression of the mevalonate pathway by the inhibition of 3-hydroxy-3-methylglutaryl coenzyme A reductase, and subsequent depletion of cholesterol, farnesylpyrophosphate, and geranylgeranylpyrophosphate. We investigated the role of autophagy, a controlled intracellular self-digestion, in the anti-leukemic action of statins. Treatment with low concentrations (≤6 µM) of statins, cholesterol depletion, and specific inhibition of cholesterol synthesis and protein farnesylation or geranylgeranylation, all inhibited proliferation of leukemic cell lines and primary leukemic cells without inducing overt cell death. Statins and agents that selectively reduce intracellular cholesterol levels, but not the inhibition of protein farnesylation or geranylgeranylation, induced autophagy in leukemic cells. The observed autophagic response was associated with the reduction of phosphorylated Akt levels in the lipid rafts, accompanied by a decrease in the activation of the main autophagy suppressor mammalian target of rapamycin (mTOR) and its substrate ribosomal p70S6 kinase (p70S6K). No significant autophagy induction and downregulation of mTOR/p70S6K activation were observed in normal leukocytes. Autophagy suppression by bafilomycin A1 or RNA interference-mediated knockdown of beclin-1 and microtubule-associated protein 1 light chain 3B induced apoptotic death in statin-treated leukemic cells, an effect attenuated by the addition of mevalonate or squalene, but not farnesylpyrophosphate or geranylgeranylpyrophosphate. Therefore, while the inhibition of cholesterol synthesis, protein farnesylation, and geranylgeranylation all contributed to anti-leukemic effects of statins, the inhibition of cholesterol synthesis was solely responsible for the induction of cytoprotective autophagy. These data indicate that combined treatment with statins and autophagy inhibitors might be potentially useful in anti-leukemic therapy.

Protein Kinase C (PKC) Isozymes and Cancer

Protein kinase C (PKC) is a family of phospholipid-dependent serine/threonine kinases, which can be further classified into three PKC isozymes subfamilies: conventional or classic, novel or nonclassic, and atypical. PKC isozymes are known to be involved in cell proliferation, survival, invasion, migration, apoptosis, angiogenesis, and drug resistance. Because of their key roles in cell signaling, PKC isozymes also have the potential to be promising therapeutic targets for several diseases, such as cardiovascular diseases, immune and inflammatory diseases, neurological diseases, metabolic disorders, and multiple types of cancer. This review primarily focuses on the activation, mechanism, and function of PKC isozymes during cancer development and progression.

6-Formylindolo (3,2-b)carbazole (FICZ) enhances retinoic acid (RA)-induced differentiation of HL-60 myeloblastic leukemia cells

BACKGROUND

The aryl hydrocarbon receptor (AhR) ligand 6-Formylindolo(3,2-b)carbazole (FICZ) has received increasing attention since its identification as an endogenous AhR ligand and a photoproduct of tryptophan. FICZ and its metabolites have been detected in human fluids. We recently reported that AhR promotes retinoic acid (RA)-induced granulocytic differentiation of HL-60 myeloblastic leukemia cells by restricting the nuclear abundance of the stem cell associated transcription factor Oct4. The standard clinical management of acute promyelocytic leukemia (APL) is differentiation induction therapy using RA. But RA is not effective for other myeloid leukemias, making the mechanism of RA-induced differentiation observed in a non-APL myeloid leukemia of interest. To our knowledge, this is the first study regarding the influence of FICZ on RA-induced differentiation in any type of leukemic blasts.

METHODS

Using flow cytometry and Western blotting assays, we determined the effects of FICZ on RA-induced differentiation of HL-60 human leukemia cells. All experiments were performed in triplicate. The groups RA and FICZ + RA were compared using the Paired-Samples T-Test. Western blot figures present the typical blots.

RESULTS

We demonstrate that FICZ enhances RA-induced differentiation, assessed by the expression of the membrane differentiation marker CD11b; cell cycle arrest; and the functional differentiation marker, inducible-oxidative metabolism. FICZ causes changes in signaling events that are known to drive differentiation, and notably augments the RA-induced sustained activation of the RAF/MEK/ERK axis of the mitogen-activated protein kinase (MAPK) cascade. FICZ also augments expression of the known MAPK signaling regulatory molecules c-Cbl, VAV1, pY458 p85 PI3K, Src-family kinases (SFKs), and IRF-1, a transcription factor associated with this putative signalsome that promotes RA-induced differentiation. Moreover, FICZ in combination with RA also increases expression of AhR and even more so of both Cyp1A2 and p47phox, which are known to be transcriptionally regulated by AhR. pY1021 PDGFRβ, a marker associated with retinoic acid syndrome was also increased.

CONCLUSIONS

Our data suggest that FICZ modulates intracellular signaling pathways and enhances RA-induced differentiation.