Elevation of cyclic AMP levels in HL-60 cells accumulated in G1 or G2 by transmethylation inhibitors

Antiresorptive agents and osteoclast apoptosis

Antiresorptive agents have proven to be effective therapies for the treatment of bone diseases associated with excessive osteoclast activity. Decreased osteoclast formation, inhibition of osteoclast actions, and reduced osteoclast survival represent mechanisms by which antiresorptive agents could act. The goals of this article are to present the evidence that antiresorptive agents can decrease osteoclast survival through apoptosis, to review the mechanisms by which they are thought to activate the apoptotic process, and to consider whether the actions on apoptosis fully account for the antiresorptive effects. As background, the apoptotic process will be briefly summarized together with the evidence that factors that promote osteoclast survival affect steps in the process. Following this, therapeutic agents that are both antiresorptive and can stimulate osteoclast apoptosis will be discussed. Other bone therapeutic agents that are either antiresorptive or apoptotic, but not both, will be described. Finally, newer antiresorptive compounds that elicit apoptosis and could represent potential therapeutic agents will be noted.

Second-messenger regulation of receptor association with clathrin-coated pits: a novel and selective mechanism in the control of CD4 endocytosis

CD4, a member of the immunoglobulin superfamily, is not only expressed in T4 helper lymphocytes but also in myeloid cells. Receptor-mediated endocytosis plays a crucial role in the regulation of surface expression of adhesion molecules such as CD4. In T lymphocytes p56lck, a CD4-associated tyrosine kinase, prevents CD4 internalization, but in myeloid cells p56lck is not expressed and CD4 is constitutively internalized. In this study, we have investigated the role of cyclic AMP (cAMP) in the regulation of CD4 endocytosis in the myeloid cell line HL-60. Elevations of cellular cAMP were elicited by 1) cholera toxin, 2) pertussis toxin, 3) forskolin and IBMX, 4) NaF, or 5) the physiological receptor agonist prostaglandin E1. All five interventions led to an inhibition of CD4 internalization. Increased cAMP levels did not inhibit endocytosis per se, because internalization of insulin receptors and transferrin receptors and fluid phase endocytosis were either unchanged or slightly enhanced. The mechanism of cAMP inhibition was further analyzed at the ultrastructural level. CD4 internalization, followed either by quantitative electron microscopy autoradiography or by immunogold labeling, showed a rapid and temperature-dependent association of CD4 with clathrin-coated pits in control cells. This association was markedly inhibited in cells with elevated cAMP levels. Thus these findings suggest a second-messenger regulation of CD4 internalization through an inhibition of CD4 association with clathrin-coated pits in p56lck-negative cells.

Cell death initiated by 3-deazaadenosine in HL-60 cells is apoptosis and is partially inhibited by homocysteine

Cell death initiated by the adenosine analog 3-deazaadenosine (c3 Ado) was studied in human promyelocytic leukemia HL-60 cells. A rapid decrease in cell number was seen after 4-hr exposure to 50-100 microM c3 Ado. The dominating mode of cell death was apoptosis as demonstrated by condensation and fragmentation of the nucleus, formation of apoptotic bodies and endonucleolytic degradation of DNA. Four hour treatment with 100 microM c3 Ado resulted in a reduction of early S-phase cells, and appearance of cells with a lower DNA and protein content than that of the G1 population. Whereas 25 and 50 microM c3 Ado only initiated apoptosis in S-phase cells, 75 and 100 microM c3 Ado also initiated apoptosis in G1- and G2 + M-phase cells, suggesting different mechanisms for cell death at different concentrations. Apoptosis initiated by 100 microM c3 Ado was completely inhibited by 1 mM ZnCl2. Addition of homocysteine thiolactone (Hcy) partly inhibited cell death by c3 Ado. Light microscopic examination of cultures treated with 100 microM c3 Ado and 1 mM Hcy showed nuclear condensation and fragmentation consistent with the first stage in apoptosis, however, only a minor formation of apoptotic bodies took place in these cultures compared to that observed in cultures treated with 100 microM c3 Ado alone. The modifying action of Hcy on c3 Ado initiated apoptosis in HL-60 cells and this suggests that c3 Ado and 3-deazaadenosylhomocysteine (c3 AdoHcy) interact with different targets during initiation and progression of cell death in this cell line.