Programmed cell death is a critical process in B lymphocyte development. Premature apoptosis in developing B cells could affect the repertoire and number of mature B cells produced. Of particular concern is the ability of environmentally ubiquitous polycyclic aromatic hydrocarbons (PAH) to induce B cell apoptosis within the bone marrow microenvironment in a clonally nonspecific way. Here, models of bone marrow B cell development were used to assess the role of the "extrinsic" apoptosis pathway in PAH-induced apoptosis and to compare PAH-induced apoptosis with that induced during clonal deletion. As

The common commercial use of phthalate esters has resulted in significant human exposure to these bioactive compounds. The facts that phthalate ester metabolites, like endogenous PGs, are peroxisome proliferator-activated receptor (PPAR) agonists, and that PPARγ agonists induce lymphocyte apoptosis suggest that phthalate esters are immunosuppressants that could act together with PGs to modulate early B cell development. In this study we examined the effects of a metabolite of one environmental phthalate, mono(2-ethylhexyl)phthalate (MEHP), and 15-deoxy-Δ12,14-PGJ2 (15d-PGJ2), on developing B cells. MEHP inhibited [3H]thymidine incorporation by primary murine bone marrow B cells and a nontransformed murine pro/pre-B cell line (BU-11). Cotreatment with a retinoid X receptor α ligand, 9-cis-retinoic acid, decreased [3H]thymidine incorporation synergistically, thereby implicating activation of a PPARγ-retinoid X receptor α complex. These results were similar to those obtained with the natural PPARγ ligand 15d-PGJ2. At moderate MEHP concentrations (25 or 100 μM for primary pro-B cells and a pro/pre-B cell line, respectively), inhibition of [3H]thymidine incorporation resulted primarily from apoptosis induction, whereas at lower concentrations, the inhibition probably reflected growth arrest without apoptosis. Cotreatment of bone marrow B cells with 15d-PGJ2 and MEHP significantly enhanced the inhibition of [3H]thymidine incorporation seen with MEHP alone, potentially mimicking exposure in the bone marrow microenvironment where PG concentrations are high. Finally, MEHP- and 15d-PGJ2-induced death does not result from a decrease in NF-κB activation. These data demonstrate that environmental phthalates can cooperate with an endogenous ligand, 15d-PGJ2, to inhibit proliferation of and induce apoptosis in developing bone marrow B cells, potentially via PPARγ activation.

There is growing interest in using peroxisome proliferator-activated receptor (PPAR) gamma agonists as chemotherapeutic agents in hematologic malignancies. PPARgamma agonists of diverse chemical structure induce apoptosis in several malignant B cell lines. However, PPARgamma agonists also induce apoptosis in normal B cells. One such agonist, GW7845, rapidly induces apoptosis in early B cells. Understanding the mechanisms of PPARgamma agonist-induced death is essential to minimizing loss of normal cells during chemotherapy. PPARgamma agonists influence mitogen-activated protein kinase (MAPK) cascades in other systems, and MAPKs can be associated with apoptosis. Therefore, we investigated the activation of MAPKs in primary pro-B cells and cultured pro/pre-B cells and their role in GW7845-induced apoptosis. Treatment of a nontransformed murine pro/pre-B-cell line with GW7845 transiently induced the phosphorylation of extracellular signal-related protein kinase (ERK) 1/2, but strongly and persistently induced the activation of p38 MAPK and c-Jun NH(2)-terminal kinase (JNK). In primary pro-B-cells, p38 MAPK and JNK were activated following treatment with GW7845. Phosphorylation of activating transcription factor-2 (ATF-2) was induced strongly in both B-cell types. In pro/pre-B cells, pretreatment with the p38 MAPK/JNK inhibitor PD169316 potently suppressed multiple facets of GW7845-induced apoptosis signaling. However, when a series of p38 MAPK and JNK inhibitors were used, only SB202190, also a dual inhibitor, completely suppressed GW7845-induced apoptosis. Inhibitors specific for p38 MAPK and JNK were only partially effective, suggesting that suppression of a single MAPK is not sufficient to inhibit death. The results support the hypothesis that GW7845 initiates an apoptotic pathway in early B cells through the activation of a kinase cascade that includes at least p38 MAPK and JNK.

Apoptosis is a critical event in the deletion of B lymphocytes prior to their migration to the periphery. Synthetic peroxisome proliferator activated receptor gamma (PPARgamma) agonists, including the drug GW7845 and the environmental contaminant mono-(2-ethylhexyl) phthalate, as well as an endogenous ligand, 15-deoxy-Delta(12,14)-prostaglandin J(2), induce clonally unrestricted apoptosis in pro/pre-B cells. Considering that PPARgamma agonists are used clinically for the treatment of diabetes and postulated to be useful as chemotherapeutics, we used GW7845 as a model PPARgamma agonist to examine the mechanism of cell death that may contribute to tumor killing as well as normal bone marrow B lymphocyte toxicity. GW7845 induced rapid mitochondrial membrane depolarization and release of cytochrome c, along with nearly concurrent activation of capases-2, -3, -8, and -9 in primary pro-B cells and BU-11 cells, a nontransformed pro/pre-B cell line. GW7845-induced apoptosis was reduced significantly in Bax-deficient and Apaf-1 mutant primary pro-B cells, supporting the conclusion that GW7845-induced apoptosis is mitochondria- and apoptosome-dependent. Using benzyloxycarbonyl-VAD-fluoromethyl ketone (VAD-FMK) as a pan-caspase inhibitor, we demonstrated that an initial cytochrome c release occurred independently of caspase activation and that only caspase-9 activation was partially caspase independent. The attenuation of GW7845-induced apoptosis by multiple FMK-labeled peptide sequences suggests that multiple caspase pathways are responsible for initiating and executing apoptosis. The strong activation of Bid provides a mechanism by which caspases-2, -3, and -8 may amplify the apoptotic signal. These data support the hypothesis that pharmacologic concentrations of PPARgamma agonists induce an intrinsic apoptotic pathway that is driven in normal bone marrow B cells by multiple amplification loops.