Maturation and secretion of rat hepatic lipase is inhibited by α1B-adrenergic stimulation through changes in Ca2+ homoeostasis: thapsigargin and EGTA both mimic the effect of adrenaline

Abstract

In rats, the daily changes in hepatic lipase (HL) activity in the liver follow the diurnal rhythm of the catecholamines. To study the underlying mechanism, the effect of adrenaline on maturation and secretion of HL was determined in freshly isolated rat hepatocytes. Adrenaline (10 microM) acutely inhibited the secretion of HL. This effect was abolished by 0.1 microM prazosin, but not by 1 microM propranolol, indicating the involvement of the alpha1-adrenergic pathway. Prazosin was at least 1000-fold more potent than WB4101, a selective alpha1A-antagonist. Adrenaline had no effect on HL secretion in hepatocytes pretreated with chloroethylclonidine, an irreversible alpha1B-selective antagonist. Inhibition of HL was not induced by 10 microM methoxamine, a alpha1A-selective agonist. Thus, adrenaline inhibited HL secretion through activation of the alpha1-adrenoceptors subtype B, which have been shown to signal through Ca2+ as well as cAMP. A similar reduction in HL secretion was induced by the Ca2+-mobilizing hormones angiotensin II (100 nM) and vasopressin (12 nM), the Ca2+ ionophore A23187 (2 microM), and by thapsigargin (1 microM), which inhibits the ER Ca2+-ATPase pump. HL secretion was unaffected by elevating cAMP with 10 microM forskolin or 1 microM 8-Br-cAMP. These results suggest that the alpha1B-adrenergic effects on HL expression are mainly mediated through elevation of intracellular Ca2+. Chelation of extracellular Ca2+ and subsequent lowering of intracellular Ca2+ with EGTA also inhibited HL secretion. In pulse-chase experiments, adrenaline was shown to inhibit the maturation of HL from the 53 kDa, Endo H-sensitive precursor to the Endo H-resistant, catalytically active protein of 58 kDa. In addition, adrenaline induced intracellular degradation of newly synthesized HL. Similar post-translational effects, both qualitatively and quantitatively, were observed with A23187, thapsigargin and EGTA. We conclude that the inhibition of HL maturation and increase in intracellular degradation induced by catecholamines, A23187, thapsigargin and EGTA is evoked by changes in Ca2+ homoeostasis, possibly through lowering ER Ca2+.

Mitochondria maintain maturation and secretion of lipoprotein lipase in the endoplasmic reticulum

“…However, these principles of Ca 2+ -dependent protein folding have been assessed mainly on recombinant proteins [7,8,10] or by artificially applying excessive ER stress [11][12][13], while a detailed analysis of its kinetics and correlation with physiological Ca 2+ fluctuations are lacking. While there are studies that address this issue [14], it still remains questionable whether the ER protein-folding machinery is influenced by spatial and temporal dynamics of [Ca 2+ ] ER in intact cells and, if so, how efficient protein folding is maintained under physiological cell stimulation. Moreover, since recent studies revealed a crucial role of mitochondria in the process of rescuing the Ca 2+ content of the ER during stimulation with IP 3 (inositol 1,4,5-trisphosphate)-elevating agonists [15], the importance of mitochondrial for Ca 2+ -dependent protein maturation in the ER awaits to be assessed.…”

Down-regulation of hepatic lipase expression by elevation of cAMP in human hepatoma but not adrenocortical cells

“…In rats, expression of HL has been shown to be lower in the fasting than in the well-fed state (Peinado-Onsurbe et al, 1991, 2000Stam et al, 1984), an effect that is shown to be mediated by adrenaline through α 1B adrenergic receptors (Neve et al, 1998). In freshly isolated rat hepatocytes, α 1B agonists acutely reduce hepatic lipase secretion (Neve et al, 1997;Peinado-Onsurbe et al, 1991; by inhibiting the intracellular maturation of newly synthesized hepatic lipase protein, an effect that is mediated by the increase in intracellular Ca 2+ -concentration (Neve et al, 1998). This acute, post-translational effect of adrenaline on HL secretion is paralleled by a similar fall in albumin secretion, and is therefore not considered to be specific for HL (Galan et al, 2002).…”

“…This effect has been attributed to the increased catecholamine levels during fasting (Peinado-Onsurbe et al, 1991). Indeed, in vitro studies with rat hepatocytes show an acute inhibition of HL secretion by α 1B -adrenergic agonists at the post-transcriptional level (Neve et al, 1998;A c c e p t e d M a n u s c r i p t 4 Onsurbe et al, 1991Onsurbe et al, , 2000, mainly through the mobilization of intracellular Ca 2+ (Neve et al, 1997(Neve et al, , 1998. However, α 1B -adrenergic agonists also increase the generation of cAMP in liver cells (Morgan et al, 1983;Nomura et al, 1993), and cAMP elevation in rat hepatocytes has been shown to inhibit secretion of HL activity (Klin et al, 1996).…”

Prazosin Stimulates the Release of Hepatic Triacylglycerole Lipase Caused from Primary-culture Rat Hepatocytes

“…According to the report, 25) the maturation and the secretion of HTGL is inhibited by adrenaline though changes in Ca 2+ homeostasis. 25) The intracellular Ca 2+ contents may be important for the stimulatory release of HTGL activity by prazosin, an alpha 1 adrenoceptor antagonist. The prazosin-stimulated release of HTGL activity is also suppressed by inhibitors of Ca 2+ /calmodulin inhibitor and CaMK-II (Fig.…”