Cytochalasin D modulates CD4 crosslinking sensitive mitogenic signal in T lymphocytes

Blocking action of cytochalasin D on protein kinase A stimulation of a stretch-activated cation channel in renal epithelial A6 cells

We have shown that the apical membrane of renal epithelial A6 cells has a 29-pS stretch-activated nonselective cation (NSC) channel, which is activated by cytosolic cyclic AMP (cAMP) (J Gen Physiol 1997;110:327-36). In general, downstream signalings of cAMP are mediated through a cAMP-activated protein kinase (protein kinase A, PKA)-dependent pathway. Therefore, to study if the channel is activated by a PKA-dependent pathway, we applied a PKA catalytic subunit directly to the channel from the cytosolic surface in cytosol-free excised inside-out patches, using the single channel recording (patch clamp) technique. Application of PKA catalytic subunit with 2 mM ATP increased the open probability (P(o)) of the channel from 0.11 +/- 0.04 to 0.58 +/- 0.10 (mean +/- SD, N = 11, P < 0.001). The channel has a gating kinetics "C(L) <--> C(S) <--> O, " where C(L,) C(S,) and O are the long closed state, the short closed state, and the open state, respectively. PKA influenced the communication of the channel between C(L) and C(S) without affecting the communication between C(S) and O, leading the channel to only stay in C(S) and O. The PKA-induced increase in P(o) was attributable to the interruption of communication between C(L) and C(S) or to the reduction of time the channel stays in C(L.) Pretreatment with cytochalasin D (Cyt-D), an inhibitor of the polymerization of actin filaments, blocked the stimulatory effect of PKA on the channel. These observations suggest that phosphorylation of polymerized actin filaments regulates the gating kinetics of a stretch-activated channel in renal epithelium.

Anti-psychotic drugs reverse multidrug resistance of tumor cell lines and human AML cells ex-vivo

Anti-psychotic drugs are used in cancer patients undergoing chemotherapy frequently and the concomitantly used drugs may alter the pharmacokinetics of each other. One reason for the alteration of pharmacokinetics may be the modulation of the function of P-glycoprotein, whose efflux pump occurs in resistant cancer cells, in human intestine and in the blood-brain barrier. For this reason we tested the effect of several anti-psychotic drugs on the multidrug-resistant pump, P-glycoprotein. We found that in the MDR gene transfected L121C MDR, L5178 MDR and in the KB-V-1 cells selected for resistance some antipsychotic drugs block the function of P-glycoprotein. Blood cells of two treatment-resistant leukemic patients also showed increased uptake of daunorubicin if treated ex vivo with the anti-psychotic drugs. Our results suggest that pharmacokinetic studies should be performed prior to concomitant clinical use of such drugs which block P-glycoprotein function.

A semiautomated fluorescence-based cell-to-cell fusion assay for gp120-gp41 and CD4 expressing cells

A novel fluorescence-based method was developed to measure HIV envelope glycoprotein (env)-CD4-mediated cell fusion. This method measures the spread of a fluorescent dye as the cytosolic compartments of adjacent cells become contiguous upon cell-to-cell fusion. Calcein-labeled CD4+ Sup-T1 cells were seeded onto a monolayer of unlabeled TF228.1.16 cells, which stably express env, the gp120-gp41 complex. Changes in the following parameters were measured using a stage-scanning laser microscope: total fluorescent area, average fluorescent area, and average shape factor. Anti-CD4 monoclonal antibodies, anti-Leu3a, and OKT4E were shown to block fusion in a dose-dependent manner, while OKT4 had no effect. Aurin tricarboxylic acid, a compound that interferes with the binding of anti-Leu3a mAb and gp120 to CD4+ human peripheral blood lymphocytes, T20, a peptide that interferes with gp41, and cytochalasin D, a microfilament disrupter, all blocked fusion in a dose-dependent manner. This semiautomated assay can be used to quickly assess the effectiveness of compounds acting at different sites to block CD4 and env initiated cell-to-cell fusion.

Regulation of parathyroid hormone-related protein secretion and mRNA expression in normal human keratinocytes and a squamous carcinoma cell line

Parathyroid hormone-related protein (PTHrP) has been identified as a causative factor in the pathogenesis of humoral hypercalcemia of malignancy (HHM). The regulation and mechanisms of PTHrP secretion in most normal and malignant cells are unknown. PTHrP secretion, mRNA expression, and transcription were measured in neoplastic human squamous carcinoma cells (A253) and normal human foreskin keratinocytes (NHFK) by radioimmunoassay, RNase protection assay, and transient transfections of the 5'-flanking region of human PTHrP in a luciferase expression vector. Mechanisms of PTHrP secretion were investigated using chemicals (monensin, colchicine, cytochalasin B, guanosine 5'-[gamma-thio]triphosphate (GTPgammaS)) that interfere with or facilitate intracellular transport. Monensin inhibited PTHrP secretion in both NHFK and A253 cells. Ultrastructurally, monensin caused dilatation of rough endoplasmic reticulum and the formation of numerous cytoplasmic secretory vacuoles in both cell lines. Colchicine decreased PTHrP production in NHFK cells and stimulated PTHrP production and mRNA levels in A253 cells. Colchicine also stimulated transcription of the PTHrP-luciferase reporter gene. Cytochalasin B stimulated PTHrP secretion and mRNA expression in A253 cells, but had no effect in NHFK cells. GTPgammaS had no effect on PTHrP secretion in either cell line. It was concluded that PTHrP secretion is dependent on the constitutive movement of secretory vesicles to the cytoplasmic membrane and regulation of PTHrP secretion and mRNA expression are altered in squamous carcinoma cells compared to normal human keratinocytes in vitro.