The α1-adrenergic transduction system in hamster brown adipocytes. Release of arachidonic acid accompanies activation of phospholipase C

Abstract

Previous studies of brown adipocytes identified an increased breakdown of phosphoinositides after selective alpha 1-adrenergic-receptor activation. The present paper reports that this response, elicited with phenylephrine in the presence of propranolol and measured as the accumulation of [3H]inositol phosphates, is accompanied by increased release of [3H]arachidonic acid from cells prelabelled with [3H]arachidonic acid. Differences between stimulated arachidonic acid release and formation of inositol phosphates included a requirement for extracellular Ca2+ for stimulated release of arachidonic acid but not for the formation of inositol phosphates and the preferential inhibition of inositol phosphate formation by phorbol 12-myristate 13-acetate. The release of arachidonic acid in response to phenylephrine was associated with an accumulation of [3H]arachidonic acid-labelled diacylglycerol, and this response was not dependent on extracellular Ca2+ but was partially prevented by treatment with the phorbol ester. The release of arachidonic acid was also stimulated by melittin, which increases the activity of phospholipase A2, by ionophore A23187, by lipolytic stimulation with forskolin and by exogenous phospholipase C. The arachidonic acid response to phospholipase C was completely blocked by RHC 80267, an inhibitor of diacylglycerol lipase, but this inhibitor had no effect on release stimulated with melittin or A23187 and inhibited phenylephrine-stimulated release by only 40%. The arachidonate response to forskolin was additive with the responses to either phenylephrine or exogenous phospholipase C. These data indicate that brown adipocytes are capable of releasing arachidonic acid from neutral lipids via triacylglycerol lipolysis, and from phospholipids via phospholipase A2 or by the sequential activities of phospholipase C and diacylglycerol lipase. Our findings also suggest that the action of phenylephrine to promote the liberation of arachidonic acid utilizes both of these reactions.

Protein Kinase C and Phospholipase C Mediate α‐ and β‐Adrenoceptor Intercommunication in Rat Hypothalamic Slices

“…Although our data indicate a role for PKC in the mechanism of &,-enhancement of the P-response, uncertainty remains concerning the effector enzyme involved in the stimulation of PKC. Both PLC (Smith et al, 1979;Schimmel, 1988) and PLA2 (Duman et al, 1986;Enna and Karbon, 1987;Schaad et al, 1987Schaad et al, , 1989Schimmel, 1988) have been implicated in alaugmentation of agonist-stimulated cyclic AMP formation. The y-isoform of PKC, the major form in brain, is sensitive to activation by low levels of arachidonic acid or its metabolites (Sekiguchi et al, 1987;Shearman et al, 1989) as well as by diacylglycerol (Nishizuka, 1984).…”

“…The y-isoform of PKC, the major form in brain, is sensitive to activation by low levels of arachidonic acid or its metabolites (Sekiguchi et al, 1987;Shearman et al, 1989) as well as by diacylglycerol (Nishizuka, 1984). PLA2 and PLC can be stimulated by a I -receptors simultaneously, separately, or sequentially to generate arachidonic acid (Duman et al, 1986;Slivka and Insel, 1987;Schimmel, 1988;Burch, 1989). To examine which effector enzyme is involved in the enhancement of ,&stimulated cyclic AMP synthesis, we tested the effects of PLC and PLAz inhibitors on PHE augmentation of cyclic AMP formation.…”

“…To date it is unclear which is the mechanism by which al-recep-tors enhance cyclic AMP generation. Some studies have implicated PLC (Smith et al, 1979;Schimmel, 1988), PLA2 (Duman et al, 1986;Enna and Karbon, 1987;Schaad et al, 1987Schaad et al, , 1989Schimmel, 1988), or neither (Robinson and Kendall, 1989) in the mechanism of Equilibrated slices were incubated for 5 rnin with staurospodne or H7 or for 15 rnin with neomycin or quinacnne. All slices were then exposed to IS0 or vehicle for 20 min.…”

Arachidonic acid stimulates DNA synthesis in brown preadipocytes through the activation of protein kinase C and MAPK

“…In hamster brown adipocytes AA is released in response to NE, the main regulator of brown adipocyte function, via 1 adrenergic receptors [19]. NE-induced AA release can be produced by the action of PLA2 or by the sequential action of PLC and diacylglycerol lipase.…”

“…Triglycerides accumulated in the lipid droplets of brown adipocytes constitute another important source of AA in the adipose tissues. It has been described that the lipolytic stimulation elicited by intracellular cAMP increases, promotes the release of AA from the lipid droplets in brown adipocytes [19] .…”