Caspase-9 and effector caspases have sequential and distinct effects on mitochondria.

Proapoptotic Bcl-2 family members alter mitochondrial permeability resulting in the release of apoptogenic factors that initiate a caspase cascade. These changes are well described; however, the effects of caspases on mitochondrial function are less well characterized. Here we describe the consequence of caspase-9 and effector caspase inhibition on mitochondrial physiology during intrinsic cell death. Caspase inhibition prevents the complete loss of mitochondrial membrane potential without affecting cytochrome c release. When effector caspases are inhibited, mitochondria become uncoupled and produce reactive oxygen species. Interestingly, the effector caspase-mediated depolarization of the mitochondria occurs independent of the activity of complexes I-IV of the electron transport chain. In contrast, caspase-9 inhibition prevents mitochondrial uncoupling and ROS production and allows for continued electron transport despite the release of cytochrome c. Taken together, these data suggest that activated caspase-9 prevents the accessibility of cytochrome c to complex III, resulting in the production of reactive oxygen species, and that effector caspases may depolarize mitochondria to terminate ROS production and preserve an apoptotic phenotype.

Pre-B cell loss in senescence coincides with preferential development of immature B cells characterized by partial activation and altered Vh repertoire.

Senescent mice show decline in B lymphopoiesis marked by reduced pre-B cells. Analysis of bone marrow from aged (approximately 2 years old) BALB/c mice indicates that, in senescence, an increased proportion of immature B cells exhibit a CD43/S7+ surface phenotype. This results from continued production of new CD43/S7+ B cells in aged mice from their limited pre-B cell pool while production of CD43/S7- immature B cells is highly reduced. CD43/S7 is ordinarily observed on a minor subset of immature B cells in young mice and is indicative of their partial activation. Senescent immature B cells, both ex vivo and derived in vitro, also demonstrate increased expression of VhS107 concomitant with CD43/S7. These alterations in the phenotype and Vh repertoire among senescent immature B cells likely originate prior to surface Ig expression. In aged mice with depleted pre-B and immature B cells in vivo, pre-B and immature B cells exhibited increased apoptosis in vitro. Dexamethasone-induced apoptosis among B lineage cells in young adult mice also resulted in pre-B cell loss and increased expression of CD43/S7 and VhS107 among immature B cells similar to that observed spontaneously in aged mice. These results suggest that old age, possibly due to increased apoptosis, results in loss of pre-B cells and alterations in the phenotype and Vh repertoire of newly derived B cells.

Levels of E2A protein expression in B cell precursors are stage-dependent and inhibited by stem cell factor (c-kit ligand).

OBJECTIVE: The E2A-encoded proteins E47 and E12 are crucial to the development of pro-B and pre-B cells. The expression of E2A protein and mRNA during early B lymphopoiesis was determined and effects of stem cell factor (SCF; Steel factor; c-kit ligand) on E2A expression were evaluated. MATERIALS AND METHODS: Ex vivo murine pro-B cells and pre-B cells were isolated and in vitro B cell precursors were derived after culture of bone marrow with rmIL-7. Levels of E2A proteins were determined by Western analysis and mRNA by RT-PCR. E2A expression in vitro was also assessed in cultures supplemented with IL-7 +/- recombinant murine SCF (rmSCF), insulin-like growth factor-1 (rhIGF-1), or Flt3-ligand (rhFlt3-L). Turnover of E2A proteins was assessed following cycloheximide treatment. RESULTS: Ex vivo, pro-B cells had lower E47 protein levels than did pre-B cells but had comparable E2A mRNA levels. E2A protein, but not mRNA, levels were reduced in pro-B cells upon culture in vitro with IL-7 + rmSCF. This was associated with increased turnover of E2A proteins. In contrast, culture with IL-7 + IGF-1 or Flt3-L had minimal effects on E2A protein levels. CONCLUSION: Pre-B cells expressed higher levels of E2A protein than did pro-B cells and this mainly resulted from posttranscriptional regulation. Exogenous SCF inhibited E2A protein, but not mRNA, expression in cultured B cell precursors, possibly by increasing E2A protein turnover. The capacity to respond to SCF may influence the levels of E2A during B-cell development.

Enhanced Targeting of the EGFR Network with MM-151, an Oligoclonal Anti-EGFR Antibody Therapeutic.

Although EGFR is a validated therapeutic target across multiple cancer indications, the often modest clinical responses to current anti-EGFR agents suggest the need for improved therapeutics. Here, we demonstrate that signal amplification driven by high-affinity EGFR ligands limits the capacity of monoclonal anti-EGFR antibodies to block pathway signaling and cell proliferation and that these ligands are commonly coexpressed with low-affinity EGFR ligands in epithelial tumors. To develop an improved antibody therapeutic capable of overcoming high-affinity ligand-mediated signal amplification, we used a network biology approach comprised of signaling studies and computational modeling of receptor-antagonist interactions. Model simulations suggested that an oligoclonal antibody combination may overcome signal amplification within the EGFR:ERK pathway driven by all EGFR ligands. Based on this, we designed MM-151, a combination of three fully human IgG1 monoclonal antibodies that can simultaneously engage distinct, nonoverlapping epitopes on EGFR with subnanomolar affinities. In signaling studies, MM-151 antagonized high-affinity EGFR ligands more effectively than cetuximab, leading to an approximately 65-fold greater decrease in signal amplification to ERK. In cell viability studies, MM-151 demonstrated antiproliferative activity against high-affinity EGFR ligands, either singly or in combination, while cetuximab activity was largely abrogated under these conditions. We confirmed this finding both in vitro and in vivo in a cell line model of autocrine high-affinity ligand expression. Together, these preclinical studies provide rationale for the clinical study of MM-151 and suggest that high-affinity EGFR ligand expression may be a predictive response marker that distinguishes MM-151 from other anti-EGFR therapeutics.

NK cells in the CD19- B220+ bone marrow fraction are increased in senescence and reduce E2A and surrogate light chain proteins in B cell precursors.

E2A encoded proteins, key transcriptional regulators in B lineage specification and commitment, have been shown to decrease in B cell precursors in old age. E2A regulates genes encoding the surrogate light chain proteins lambda5 and VpreB. In old age, B cell precursors express less surrogate light chain and this results in compromised pre-B cell receptor function and diminished expansion of new pre-B cells in senescence. Herein, we show that aged bone marrow has increased Hardy Fraction A (CD19(-) B220(+)) cells, including NK cells, that can inhibit both E47 (E2A) protein and surrogate light chain protein expression in B cell precursors. In vitro, NK-associated inhibition of E47 protein is contact-independent and partially reversed by neutralization of TNFalpha. In vivo, depletion of NK cells in aged mice by treatment with anti-asialo GM1 antibody led to restoration of surrogate light chain protein levels to that typical of young B cell precursors. These studies suggest that NK cells, within the CD19(-) B220(+) bone marrow cell fraction, may contribute to a bone marrow microenvironment that has the potential to negatively regulate E47 (E2A) as well as surrogate light chain levels in B cell precursors in old age.

Accelerated Notch-dependent degradation of E47 proteins in aged B cell precursors is associated with increased ERK MAPK activation.

The transcriptional regulator E47, encoded by the E2A gene, is crucial to B lymphopoiesis. In BALB/c senescent mice (approximately 2 years old), the incidence of E47-expressing pro-B cells in vivo and E47 protein steady state levels in B cell precursors in vitro were reduced. Poor expression of E47 protein was a consequence of accelerated proteasome-mediated turnover and was associated with heightened ubiquitin modification of E2A-encoded proteins in aged B cell precursors. Both MAPK and Notch activity have been previously associated with E2A-encoded protein stability in lymphocytes. Aged B cell precursors exhibited heightened levels of MAPK activity reflected in increased levels of phospho-ERK proteins. Phosphorylation of E2A-encoded proteins was also increased in aged B cell precursors and pharmacologic inhibition of MEK-1 resulted in a partial restoration of their E47 protein. Both Notch proteins and their Delta-like ligands were detected comparably in young and aged B cell precursors. Either inhibition of Notch activation via gamma-secretase or Ab blockade of Notch-Delta-like ligand interactions partially restored E47 expression in aged B cell precursors. We hypothesize that increased MAPK activity promotes phosphorylation of E2A-encoded protein in aged B cell precursors. Subsequently, E2A-encoded proteins undergo ubiquitination and accelerated degradation in a Notch-dependent process. The dysregulation of E2A-encoded protein expression may contribute to the reductions seen in early B lymphopoiesis during murine senescence.

CD28-mediated regulation of multiple myeloma cell proliferation and survival.

Although interactions with bone marrow stromal cells are essential for multiple myeloma (MM) cell survival, the specific molecular and cellular elements involved are largely unknown, due in large part to the complexity of the bone marrow microenvironment itself. The T-cell costimulatory receptor CD28 is also expressed on normal and malignant plasma cells, and CD28 expression in MM correlates significantly with poor prognosis and disease progression. In contrast to T cells, activation and function of CD28 in myeloma cells is largely undefined. We have found that direct activation of myeloma cell CD28 by anti-CD28 mAb alone induces activation of PI3K and NFkappaB, suppresses MM cell proliferation, and protects against serum starvation and dexamethasone (dex)-induced cell death. Coculture with dendritic cells (DCs) expressing the CD28 ligands CD80 and CD86 also elicits CD28-mediated effects on MM survival and proliferation, and DCs appear to preferentially localize within myeloma infiltrates in primary patient samples. Our findings suggest a previously undescribed myeloma/DC cell-cell interaction involving CD28 that may play an important role in myeloma cell survival within the bone marrow stroma. These data also point to CD28 as a potential therapeutic target in the treatment of MM.

Deficient B lymphopoiesis in murine senescence: potential roles for dysregulation of E2A, Pax-5, and STAT5.

B lymphopoiesis in senescent mice is typically diminished and characterized by low pre-B cell numbers. The transcription factors E2A, Pax-5, and STAT5 have been implicated in the differentiation, proliferation, and survival of B cell precursors. In this review, we discuss the impairment of B lymphopoiesis during old age in the context of mechanisms at the molecular level responsible for the handling and turnover of these key transcriptional proteins. Alterations in the expression of E2A, Pax-5, and STAT5 may affect multiple stages of B cell development, contribute to reduced B lymphopoiesis, and preface changes in the "read-out" of the BCR repertoire during murine senescence.

A phenotypically distinct subset of immature B cells exhibits partial activation, increased survival, and preferential expression of VhS107.

We have observed that immature B cells (IgM(low)IgD(-)) in the bone marrow of adult BALB/c mice exhibit heterogeneity, with a distinct subpopulation ( approximately 4-10%) expressing the CD43/S7 surface protein. These CD43/S7(+) immature B cells often express other surface antigens associated with B cell activation (CD5, CD11b, PD-1). Generation of optimal numbers of CD43/S7(+) immature B cells requires expression of a functional Btk protein, consistent with activation as a requisite for the CD43/S7(+) immature B cell phenotype. Like typical CD43/S7(-) immature B cells, the CD43/S7(+) immature B cells are predominantly resting cells, which are derived from cycling bone marrow B cell precursors. The CD43/S7(+) immature B cell population exhibits enhanced survival in vivo upon administration of the apoptosis-inducing corticosteroid, dexamethasone. Finally, CD43/S7(+) immature B cells show a fourfold increase in incidence of VhS107 micro heavy chain expression compared to the CD43/S7(-) immature B cells. Therefore, in adult murine bone marrow, the presence of a phenotypically distinct immature B cell population can be demonstrated which has undergone partial activation leading to increased survival and BCR-dependent Vh repertoire selection.

Decreased E47 in senescent B cell precursors is stage specific and regulated posttranslationally by protein turnover.

The E2A-encoded transcription factor E47 is crucial to B lymphopoiesis. Senescent BALB/c mice ( approximately 2 years old) had reduced pre-B cells ex vivo. Pro-B/early pre-B cells from these aged mice, both ex vivo and in vitro, were deficient in E47 protein. In vitro, IL-7 expanded pro-B/early pre-B cells from young BALB/c mice expressed E47 protein that was relatively stable over a 5-h period. Cultured senescent pro-B/early pre-B cells exhibited reduced E47 protein stability with approximately 50-90% loss of E47 over the same time period. Degradation of E47 was effectively blocked by the proteasome inhibitor lactacystin as well as calpain I and II inhibitors; E2A proteins were also shown to undergo ubiquitination. Although senescent B cell precursors expressed less E47 protein, E47 mRNA levels and turnover were normal. Therefore, E47 protein levels are reduced relatively early in B lineage differentiation in senescence and the decline in E47 protein occurs via increased protein degradation by proteasome and, possibly, calpain pathways. In contrast, normal E47 protein levels were observed within the highly reduced pre-B cell pool in aged mice. This suggests that pre-B cells in senescence undergo selection based on E47 expression. Increased degradation rates and lower steady-state levels were also observed for the transcription factors Pax-5/BSAP, Bob-1, and Ikaros, but this was not a general property of all proteins in aged B cell precursors. Therefore, altered turnover of multiple, select proteins crucial to B cell development may contribute to diminished B lymphopoiesis in old age.