Induction of LFA-1-mediated homotypic adhesions in promonocytic U-937 cells occurs independently of cell differentiation

Activation of protein kinase C relays distinct signaling pathways in the same cell type: differentiation and caspase-mediated apoptosis

Activation of PKC with 5 nM 12-O-tetradecanoylphorbol-13-acetate (TPA) for 72 h in human U937 myeloid leukemia cells is associated with induction of adherence, followed by monocytic differentiation and G0/G1 cell cycle arrest. In this study, we demonstrate that in addition to these effects about 25% of U937 cells accumulated in an apoptotic subG1 phase after TPA treatment. The appearance of these apoptotic suspension cells was detectable throughout the time course of the culture and was independent of TPA concentrations between 0.5 and 500 nM. Experiments with cells synchronized by centrifugal elutriation revealed dominant susceptibility of G1-phase cells to TPA-mediated apoptosis. While adherent cells expressed differentiation markers including the integrin CD11c, this effect was less pronounced in the TPA-treated suspension fraction. Moreover, previous work has demonstrated cell cycle arrest in differentiating U937 cells. Accordingly, PKC activation by TPA treatment was associated with a significant expression of the cdk/cyclin inhibitor p21WAF/CIP/sdi-1 in the adherent population and subsequent G0/G1 cell cycle arrest. In contrast, suspension cells failed to induce significant levels of p21WAF/CIP/sdi-1 after TPA stimulation. Immunoblotting experiments demonstrated no difference in the expression of the pro-apoptotic factors Bax, Bad, and Bak in either control U937 and TPA-treated adherent or suspension cells, respectively. However, anti-apoptotic factors including Bcl-2, Bcl-xL, and Mcl-1 were significantly induced in the adherent population whereas no induction was detectable in the suspension cells. In this context, incubation with the caspase-3/caspase-7 specific tetrapeptide inhibitor DEVD prior to TPA treatment prevented an accumulation of cells in subG1, respectively, demonstrating an involvement of these caspases. Taken together, these data suggest that PKC activation can relay distinct signaling pathways such as induction of adherence coupled with monocytic differentiation and growth arrest, or induction of caspase-mediated apoptosis coupled with the failure to adhere and to differentiate.

Regulation of the expression of leukocyte function-associated antigen-1 (LFA-1) and intercellular adhesion molecule-1 (ICAM-1) on a human eosinophilic leukemia cell line EoL-3

The effects of several cytokines and phorbol myristate acetate (PMA) on LFA-1 and ICAM-1 expression on a human eosinophilic leukemia cell line, EoL-3, were investigated and compared with those of a human monocytic leukemia cell line, U937. EoL-3 cells expressed large amounts of LFA-1 and small amounts of ICAM-1, and their expression was regulated similarly in EoL-3 cells and U937 cells. Interferon-gamma (IFN-gamma) enhanced ICAM-1 expression but not LFA-1 expression, and PMA augmented both LFA-1 and ICAM-1 expression. IFN-gamma and PMA showed an additive effect on ICAM-1 expression. These results collectively suggest that expression of LFA-1 and ICAM-1 is regulated differently and that IFN-gamma and PMA regulate the expression through different mechanisms. PMA but not IFN-gamma induced homotypic adhesion of EoL-3 and U937 cells, suggesting that PMA but not IFN-gamma activated the adhesive function of these cells. Staurosporin, an inhibitor of protein kinases (PKs), partly suppressed IFN-gamma- and PMA-augmented expression of ICAM-1 on EoL-3 and U937 cells, but did not affect PMA-augmented LFA-1 expression, suggesting that staurosporin-sensitive PKs are involved in IFN-gamma- and PMA-augmented ICAM-1 expression but not in PMA-augmented LFA-1 expression. The role of protein kinase C (PK-C) in these mechanisms was not revealed because a PK-C inhibitor, H-7, did not show any definitive effect on IFN-gamma- and PMA-induced expression of LFA-1 and ICAM-1. Moreover, cyclic AMP (cAMP)- and cGMP-dependent pathways were not shown to be involved in the augmentation of the expression of these molecules.

Monoclonal antibodies targeting murine LFA-1 induce LFA-1/ ICAM-1-independent homotypic lymphocyte aggregation

We have identified three anti-murine LFA-1 alpha monoclonal antibodies (M17/4.2, G-48, and FD441.8) which are capable of inducing homotypic aggregation of murine T cell lines (3A9 EL-4 cells). The LFA-1-induced aggregation is temperature-dependent, necessitates metabolic energy, and requires an intact cytoskeleton, but is independent of transcription and protein synthesis. The aggregation is inhibited in Ca2+ and Mg2+ free media and is also blocked with EDTA and EGTA. The aggregation does not involve protein kinase A or C or changes in intracellular calcium. The LFA-1 alpha-induced homotypic aggregation is inhibited with LFA-1 beta antibodies, but not with antibodies targeting ICAM-1, VCAM-1, VLA-4, or CD2. 3A9 cells do not express the LFA-1 ligand ICAM-1, whereas EL-4 cells express moderate amounts of ICAM-1. Thus, targeting LFA-1 alpha with mAb results in homotypic aggregation of T cell lines which is independent of ICAM-1/LFA-1 interactions, but may involve other LFA-1 ligands such as ICAM-2 or ICAM-3. Alternatively, LFA-1 may function as a signaling molecule, triggering other yet to be defined adhesion molecules to interact.

Homotypic cell aggregations of human myeloma cells with ICAM-1 and LFA-1 molecules

Some myeloma cells freshly isolated from bone marrow aspirates in human myelomas and some myeloma cell lines formed spontaneous cell aggregations in vitro (homotypic cell aggregations). In order to clarify the surface molecules involved in homotypic cell aggregations and physiological roles of these cell aggregations, we investigated the expressions of intercellular adhesion molecule 1 (ICAM-1) and lymphocyte function-associated antigen 1 (LFA-1) on 20 samples of freshly isolated myeloma cells and three myeloma cell lines and the effect of anti-ICAM-1 and anti-LFA-1 alpha antibodies on myeloma cell proliferation in vitro. All myeloma cells that we tested expressed ICAM-1 on their surface. Among them, myeloma cells that strongly coexpressed LFA-1 alpha, formed homotypic cell aggregates in vitro. These spontaneous cell aggregations were completely released by adding either anti-ICAM-1 or anti-LFA-1 alpha antibody. During short-term culture, spontaneous proliferation of myeloma cells in vitro and their proliferative responses to recombinant interleukin-6 (rIL-6) were not affected by pretreatment of myeloma cells with anti-ICAM-1 or anti-LFA-1 alpha antibody. Therefore these data suggest that homotypic cell aggregation of myeloma cells is mediated by ICAM-1 and LFA-1 molecules, but myeloma cell proliferation may not be modulated by these adhesion molecules during short-term cultures.

Lymphocyte-fibroblast adhesion. A useful model for analysis of the interaction of the leucocyte integrin LFA-1 with ICAM-1

The adhesion of human T lymphoblasts to ICAM-1-expressing normal dermal fibroblasts has been assessed as a sensitive model system for the analysis of the interaction of the leucocyte integrin LFA-1 with its counter-receptor ICAM-1. Using this model system, the effects of factors known to regulate the activity of LFA-1 have been quantitated: temperature; concentration of divalent cations; and exposure to phorbol esters. We show here that under the appropriate assay conditions, this model system represents a useful and simple alternative to the detection of leucocyte binding to purified ICAM-1 and also has the additional advantage of permitting more sensitive quantification than is possible using the homotypic adhesion assay.