Nuclear Factor-κB modulates cellular glutathione and prevents oxidative stress in cancer cells.

The NF-κB is best known for its role in inflammation. Here we show that constitutive NF-κB activity in cancer cells promotes the biosynthesis of redox scavenger glutathione (GSH), which in turn confers resistance to oxidative stress. Inhibition of NF-κB significantly decreases GSH in several lines of human leukemia and prostate cancer cells possessing high or moderate NF-κB activities. Concomitantly, NF-κB inhibition by pharmacological and molecular means sensitizes "NF-κB positive" cancer cells to chemically-induced oxidative stress and death. We propose that inhibition of NF-κB can reduce intracellular GSH in "NF-κB-positive" cancers thereby improving the efficacy of oxidative stress-based anti-cancer therapy.

Methyl-beta-cyclodextrin suppresses hyaluronan synthesis by down-regulation of hyaluronan synthase 2 through inhibition of Akt.

Hyaluronan synthases (HAS1-3) are integral plasma membrane proteins that synthesize hyaluronan, a cell surface and extracellular matrix polysaccharide necessary for many biological processes. It has been shown that HAS is partly localized in cholesterol-rich lipid rafts of MCF-7 cells, and cholesterol depletion with methyl-beta-cyclodextrin (MbetaCD) suppresses hyaluronan secretion in smooth muscle cells. However, the mechanism by which cholesterol depletion inhibits hyaluronan production has remained unknown. We found that cholesterol depletion from MCF-7 cells by MbetaCD inhibits synthesis but does not decrease the molecular mass of hyaluronan, suggesting no major influence on HAS stability in the membrane. The inhibition of hyaluronan synthesis was not due to the availability of HAS substrates UDP-GlcUA and UDP-GlcNAc. Instead, MbetaCD specifically down-regulated the expression of HAS2 but not HAS1 or HAS3. Screening of signaling proteins after MbetaCD treatment revealed that phosphorylation of Akt and its downstream target p70S6 kinase, both members of phosphoinositide 3-kinase-Akt pathway, were inhibited. Inhibitors of this pathway suppressed hyaluronan synthesis and HAS2 expression in MCF-7 cells, suggesting that the reduced hyaluronan synthesis by MbetaCD is due to down-regulation of HAS2, mediated by the phosphoinositide 3-kinase-Akt-mTOR-p70S6K pathway.

Activated Ca2+/calmodulin-dependent protein kinase IIgamma is a critical regulator of myeloid leukemia cell proliferation.

Ca(2+) signaling is an important component of signal transduction pathways regulating B and T lymphocyte proliferation, but the functional role of Ca(2+) signaling in regulating myeloid leukemia cell proliferation has been largely unexplored. We observe that the activated (autophosphorylated) Ca(2+)/calmodulin-dependent protein kinase IIgamma (CaMKIIgamma) is invariably present in myeloid leukemia cell lines as well as in the majority of primary acute myelogenous leukemia patient samples. In contrast, myeloid leukemia cells induced to terminally differentiate or undergo growth arrest display a marked reduction in this CaMKIIgamma autophosphorylation. In cells harboring the bcr-abl oncogene, the activation (autophosphorylation) of CaMKIIgamma is regulated by this oncogene. Moreover, inhibition of CaMKIIgamma activity with pharmacologic agents, dominant-negative constructs, or short hairpin RNAs inhibits the proliferation of myeloid leukemia cells, and this is associated with the inactivation/down-regulation of multiple critical signal transduction networks involving the mitogen-activated protein kinase, Janus-activated kinase/signal transducers and activators of transcription (Jak/Stat), and glycogen synthase kinase (GSK3beta)/beta-catenin pathways. In myeloid leukemia cells, CaMKIIgamma directly phosphorylates Stat3 and enhances its transcriptional activity. Thus, CaMKIIgamma is a critical regulator of multiple signaling networks regulating the proliferation of myeloid leukemia cells. Inhibiting CaMKIIgamma may represent a novel approach in the targeted therapy of myeloid leukemia.

The retinoic acid receptor/CaMKII interaction: pharmacologic inhibition of CaMKII enhances the differentiation of myeloid leukemia cells.

Certain myeloid leukemia cells, particularly the acute promyelocytic leukemia (APL) subset, undergo terminal granulocytic differentiation in response to retinoic acid (RA). RA mediates its biologic effects through specific retinoic acid receptors (RARs) which serve as ligand-activated nuclear transcription factors. The Ca(++)/calmodulin-dependent protein kinases (CaMKs) are multifunctional serine/threonine kinases that are regulated by Ca(++) signaling. We have observed significant cross-talk between these Ca(++) and RA signaling pathways that regulates the differentiation of myeloid leukemia cells. We observe that CaMKIIgamma is the CaMK that is predominantly expressed in myeloid cells. This enzyme localizes to the promoter of RAR target genes, physically interacts with and phosphorylates RARalpha and inhibits RAR transcriptional activity. KN-62, a pharmacological inhibitor of the CaMKs, enhances both retinoic acid receptor transcriptional activity as well as the terminal in vitro differentiation of certain myeloid leukemia cell lines including HL-60. However, this compound, as well as related synthetic analogs that enhance HL-60 terminal differentiation, fails to inhibit the growth of HL-60 xenografts in NOD-SCID mice likely because of the unfavorable pharmacokinetics displayed by these compounds. Nevertheless, our observations suggest that CaMKIIgamma may provide a new therapeutic target for the treatment of the RA-responsive human myeloid leukemias.

CaMKII regulates retinoic acid receptor transcriptional activity and the differentiation of myeloid leukemia cells.

Retinoic acid receptors (RARs) are members of the nuclear hormone receptor family and regulate the proliferation and differentiation of multiple different cell types, including promyelocytic leukemia cells. Here we describe a biochemical/functional interaction between the Ca(2+)/calmodulin-dependent protein kinases (CaMKs) and RARs that modulates the differentiation of myeloid leukemia cells. We observe that CaMKIIgamma is the CaMK that is predominantly expressed in myeloid cells. CaMKII inhibits RAR transcriptional activity, and this enzyme directly interacts with RAR through a CaMKII LxxLL binding motif. CaMKIIgamma phosphorylates RARalpha both in vitro and in vivo, and this phosphorylation inhibits RARalpha activity by enhancing its interaction with transcriptional corepressors. In myeloid cell lines, CaMKIIgamma localizes to RAR target sites within myeloid gene promoters but dissociates from the promoter upon retinoic acid-induced myeloid cell differentiation. KN62, a pharmacological inhibitor of the CaMKs, enhances the terminal differentiation of myeloid leukemia cell lines, and this is associated with a reduction in activated (autophosphorylated) CaMKII in the terminally differentiating cells. These observations reveal a significant cross-talk between Ca(2+) and retinoic acid signaling pathways that regulates the differentiation of myeloid leukemia cells, and they suggest that CaMKIIgamma may provide a new therapeutic target for the treatment of certain human myeloid leukemias.

KN-62 analogues as potent differentiating agents of HL-60 cells.

KN-62, an inhibitor of the calmodulin-dependent protein kinases (CaMKs), enhances the terminal differentiation of retinoic acid sensitive human myeloid leukemia cell lines. In an effort to identify additional CaMK inhibitors that exhibit more potent activity in triggering leukemia cell differentiation, we synthesized 45 analogues of KN-62 and determined their ability to induce HL-60 cell differentiation. Sixteen of these novel analogues exhibited significant differentiation-inducing activity, and one analogue, AS-004, was five times more potent than KN-62 in inhibiting proliferation and inducing differentiation of HL-60 cells. Such KN-62 analogues and/or related compounds may prove useful in treating promyelocytic leukemia.

IL-3-induced enhancement of retinoic acid receptor activity is mediated through Stat5, which physically associates with retinoic acid receptors in an IL-3-dependent manner.

The regulation of hematopoiesis involves the interaction of specific hematopoietic cytokines with lineage-specific transcription factors, but little is known about how these cytokines might regulate the expression/activity of these different transcription factors. Here we identify the critical signal transduction pathways that mediate the interleukin 3 (IL-3)-induced enhancement of retinoic acid receptor (RAR) transcriptional activity that accompanies the IL-3-mediated commitment of the multipotent, stem cell factor (SCF)-dependent EML cell line to granulocyte/monocyte progenitors. We observe that the addition of IL-3 to EML cells induces activation of the phosphatidylinositol-3 kinase, mitogen-activated protein kinase, and Jak/Stat pathways and that Jak2 activation is the critical "proximal" mediator of the IL-3-induced enhancement of RAR activity. Constitutively active Stat5 constructs enhance both the transcriptional activity of RARs in EML cells and the commitment of these cells to granulocyte/monocyte progenitors, whereas dominant-negative Stat5 constructs inhibit this IL-3-induced enhancement of RAR transcriptional activity. We observe that the retinoic acid response element (RARE) used in our RA responsive reporter harbors overlapping Stat/RAR-binding sites. Moreover, coimmunoprecipitation studies indicate an interaction between Stat5 and RARs that is IL-3 dependent. Thus, Stat5 is an important mediator of the IL-3-induced enhancement of RAR transcriptional activity that accompanies the commitment of immature EML cells to the granulocyte/monocyte lineage. Cytokine-mediated physical and functional interactions between Stat5 and RARs may play critical roles in regulating different stages of hematopoiesis.

The cytokines IL-3 and GM-CSF regulate the transcriptional activity of retinoic acid receptors in different in vitro models of myeloid differentiation.

The disruption of retinoic acid receptor (RAR) activity that characterizes human acute promyelocytic leukemia (APL) is associated with a block to granulocytic differentiation indicating that RARs are critical regulators of normal myeloid differentiation. Nevertheless, how RAR activity might be regulated in the presumably homogenous concentration of retinoids in blood and bone marrow and how these receptors might interact with specific hematopoietic cytokines to regulate normal myeloid differentiation remain unclear. Here, using several cytokine-dependent in vitro models of myeloid development, it was observed that specific hematopoietic cytokines that normally regulate myeloid lineage commitment and differentiation (interleukin-3 and granulocyte-macrophage colony-stimulating factor) trigger the enhancement of both ligand-dependent and ligand-independent transcriptional activity of both endogenous and exogenous (transiently transfected) RARs. This cytokine-mediated enhancement of RAR activity is not associated with any observed changes in expression of the RARs or their respective coactivators/corepressors. These studies define a previously unknown cytokine-RAR interaction during myelopoiesis and suggest that RAR activation might be a critical downstream event following interleukin-3 and granulocyte-macrophage colony-stimulating factor signaling during myeloid differentiation. This observation of ligand-independent activation of RARs that is mediated by certain cytokines represents a new paradigm with respect to how RAR activity might be modulated during hematopoiesis and also suggests a molecular basis for the differential sensitivity of human acute myelogenous leukemia cells to retinoids.