Effects of hypertonic sucrose and potassium depletion on the binding properties of beta and alpha 1 adrenergic receptors measured on intact cells.

The effects of hypertonic sucrose and of intracellular potassium depletion on the intact-cell-binding properties of beta (beta AR) and alpha 1 (alpha 1AR) adrenergic receptors of DDT1 MF-2 hamster smooth muscle cells were investigated. These treatments are known to block clathrin assembly into coated pits, an early step in the pathway of receptor endocytosis. Conducting intact-cell-binding assays in the presence of 0.4 M sucrose markedly decreased the fraction of both beta ARs and alpha 1ARs converted during the assay to a form exhibiting low apparent affinity for agonists. Intracellular potassium depletion also decreased the fraction of both beta ARs and alpha 1ARs converted to this low-affinity form. In contrast the intact-cell-binding properties of antagonists were unaltered by these treatments. These results suggest a role for receptor internalization in conversion of both beta ARs and alpha 1ARs to their low-affinity forms. A model is proposed in which either an initial agonist-induced receptor sequestration within the plasma membrane or the subsequent endocytosis of receptors into intracellular vesicles allows conversion of these receptors to the form exhibiting low affinity for agonists in binding assays with intact cells.

Lysophosphatidic acid regulation of cyclic AMP accumulation in cultured human airway smooth muscle cells.

The effects of the simple bioactive lipid mediator lysophosphatidic acid (LPA) on cAMP accumulation were investigated in cultured human airway smooth muscle cells (ASMC). Pretreatment of cells with LPA induced an increase in subsequent stimulation of cAMP accumulation by forskolin and by isoproterenol. When included during the assay of cAMP accumulation rather than as a pretreatment, LPA inhibited forskolin stimulation but enhanced isoproterenol stimulation. Both effects of LPA on forskolin stimulation were completely blocked by pertussis toxin treatment, whereas the effects on isoproterenol stimulation appeared relatively insensitive to pertussis toxin. The protein kinase C activator phorbol-12-myristate-13-acetate (PMA) sensitized forskolin stimulation to a similar extent as did LPA, and the combination of LPA plus PMA caused markedly more sensitization than either agent alone. In contrast, PMA inhibited isoproterenol stimulation and markedly decreased the sensitization induced by LPA. Serum also induced sensitization, and sensitization by LPA plus serum was no greater than that with LPA alone. LPA-induced sensitization appeared to be independent of protein kinase C activation because it was unchanged in cells treated to down-regulate protein kinase C. LPA also stimulated polyphosphoinositide hydrolysis, and this stimulation was partially inhibited by pertussis toxin treatment. These results suggest that LPA activates receptors coupled to both the pertussis toxin-sensitive G protein Gi and the pertussis toxin-insensitive G protein Gq. The complex effects of LPA, PMA, and pertussis toxin on cAMP accumulation in these cells are consistent with the expression of the type 2 isozyme of adenylyl cyclase in these cells.

Human placental tissue factor: protease susceptibility of extracellular and cytoplasmic domains.

We have examined the effects of seven proteases on human placental tissue factor in Triton X-100, focusing on extracellular and cytoplasmic domains recognized by monoclonal antibodies HTF1, C28 1.1, and C28 2.1. Plasmin produced peptides recognized on Western blots by C281.1 but not HTF1. None of the other proteases destroyed the extracellular epitope without also removing the cytoplasmic epitope, and both trypsin and chymotrypsin removed the cytoplasmic epitope with little effect on the extracellular domain. Proteinase K destroyed both epitopes, as did neutrophil elastase when used at a relatively high concentration. When digests were sampled over time and reconstituted with lipids for determination of tissue factor activity, only proteinase K consistently produced a loss in tissue factor activity at four hours. After 24 hr, other enzymes also decreased the recovered activity, with the order of effectiveness elastase > trypsin > chymotrypsin.

Lysophosphatidic acid mimics serum-induced sensitization of cyclic AMP accumulation.

Pretreatment of 1321N1 human astrocytoma cells with serum induces a pronounced increase in subsequent stimulation by forskolin and other agents of intracellular cyclic AMP accumulation, a phenomenon referred to as sensitization (Mol. Pharmacol. 39, 399-406, 1991). Pretreatment of these cells with lysophosphatidic acid induced sensitization to a similar extent as that with serum (approximately fivefold for forskolin stimulation and twofold for isoproterenol and prostaglandin E1 stimulation), with half-maximal effects at approximately 30 nM lysophosphatidic acid. Phosphatidic acid was effective but less potent whereas other lipids were ineffective. Sensitization by serum and by lysophosphatidic acid were almost completely inhibited by pertussis toxin pretreatment and partially inhibited by prolonged phorbol ester exposure to induce protein kinase C down-regulation. Among nine cell lines tested, those that exhibited sensitization with serum showed comparable sensitization with lysophosphatidic acid. The effects of both lysophosphatidic acid and serum were markedly inhibited by treatment with phospholipase B but only minimally altered with phospholipases A2, D, and C. Exposure of cells to phospholipase C alone induced approximately threefold sensitization, but both serum and lysophosphatidic acid were able to induce further three- to fourfold sensitization above that induced by phospholipase C alone. In contrast, the effects of serum and lysophosphatidic acid were not additive with each other. Together these results suggest that lysophosphatidic acid or a closely related compound present in serum is the factor responsible for sensitization of the cyclic AMP pathway.