Angiotensin II-induced inositol phosphate production in isolated rat zona glomerulosa and fasciculata/reticularis cells

Signaling Interactions in the Adrenal Cortex

The major physiological stimuli of aldosterone secretion are angiotensin II (AII) and extracellular K(+), whereas cortisol production is primarily regulated by corticotropin (ACTH) in fasciculata cells. AII triggers Ca(2+) release from internal stores that is followed by store-operated and voltage-dependent Ca(2+) entry, whereas K(+)-evoked depolarization activates voltage-dependent Ca(2+) channels. ACTH acts primarily through the formation of cAMP and subsequent protein phosphorylation by protein kinase A. Both Ca(2+) and cAMP facilitate the transfer of cholesterol to mitochondrial inner membrane. The cytosolic Ca(2+) signal is transferred into the mitochondrial matrix and enhances pyridine nucleotide reduction. Increased formation of NADH results in increased ATP production, whereas that of NADPH supports steroid production. In reality, the control of adrenocortical function is a lot more sophisticated with second messengers crosstalking and mutually modifying each other's pathways. Cytosolic Ca(2+) and cGMP are both capable of modifying cAMP metabolism, while cAMP may enhance Ca(2+) release and voltage-activated Ca(2+) channel activity. Besides, mitochondrial Ca(2+) signal brings about cAMP formation within the organelle and this further enhances aldosterone production. Maintained aldosterone and cortisol secretion are optimized by the concurrent actions of Ca(2+) and cAMP, as exemplified by the apparent synergism of Ca(2+) influx (inducing cAMP formation) and Ca(2+) release during response to AII. Thus, cross-actions of parallel signal transducing pathways are not mere intracellular curiosities but rather substantial phenomena, which fine-tune the biological response. Our review focuses on these functionally relevant interactions between the Ca(2+) and the cyclic nucleotide signal transducing pathways hitherto described in the adrenal cortex.

Control of aldosterone secretion: a model for convergence in cellular signaling pathways

Aldosterone secretion by glomerulosa cells is stimulated by angiotensin II (ANG II), extracellular K(+), corticotrophin, and several paracrine factors. Electrophysiological, fluorimetric, and molecular biological techniques have significantly clarified the molecular action of these stimuli. The steroidogenic effect of corticotrophin is mediated by adenylyl cyclase, whereas potassium activates voltage-operated Ca(2+) channels. ANG II, bound to AT(1) receptors, acts through the inositol 1,4,5-trisphosphate (IP(3))-Ca(2+)/calmodulin system. All three types of IP(3) receptors are coexpressed, rendering a complex control of Ca(2+) release possible. Ca(2+) release is followed by both capacitative and voltage-activated Ca(2+) influx. ANG II inhibits the background K(+) channel TASK and Na(+)-K(+)-ATPase, and the ensuing depolarization activates T-type (Ca(v)3.2) Ca(2+) channels. Activation of protein kinase C by diacylglycerol (DAG) inhibits aldosterone production, whereas the arachidonate released from DAG in ANG II-stimulated cells is converted by lipoxygenase to 12-hydroxyeicosatetraenoic acid, which may also induce Ca(2+) signaling. Feedback effects and cross-talk of signal-transducing pathways sensitize glomerulosa cells to low-intensity stimuli, such as physiological elevations of [K(+)] (< or =1 mM), ANG II, and ACTH. Ca(2+) signaling is also modified by cell swelling, as well as receptor desensitization, resensitization, and downregulation. Long-term regulation of glomerulosa cells involves cell growth and proliferation and induction of steroidogenic enzymes. Ca(2+), receptor, and nonreceptor tyrosine kinases and mitogen-activated kinases participate in these processes. Ca(2+)- and cAMP-dependent phosphorylation induce the transfer of the steroid precursor cholesterol from the cytoplasm to the inner mitochondrial membrane. Ca(2+) signaling, transferred into the mitochondria, stimulates the reduction of pyridine nucleotides.

Role of cAMP in the effects of K+ on the steroidogenesis of zona glomerulosa cells

1. Investigations of the role of cAMP in the stimulation of the steroidogenesis of zona glomerulosa (ZG) cells by increased extracellular K+ concentration are reviewed. 2. Possible reasons for discrepancies in the results of different investigators on whether K+ increases the cAMP content or output of ZG tissue or dispersed cells are discussed. 3. The concentration of cAMP in the incubating media of ZG tissue or cells, rather than their cAMP content, seems to respond more sensitively to stimulation by extracellular K+, as was also found for adrenocorticotropic hormone stimulation of zona fasciculata-reticularis cells. 4. The addition of the phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine (IBMX) to incubations with the aim of increasing the sensitivity of the response in cAMP to extracellular K+ in ZG cells may give rise to effects, probably nonspecific, which actually inhibit the measured response. 5. The immediate stimulation in the steroidogenesis of ZG cells with raised extracellular K+ is probably mostly due to the direct effect of increases in cytoplasmic Ca2+ (arising from increases in Ca2+ influx) on mitochondrial processes. However, increases in cAMP may prolong the stimulation of steroidogenesis by increased extracellular K+. This increased cAMP is probably due to stimulation of adenylyl cyclase activity. 6. It has been concluded that the increase in Ca2+ influx output after rises in the extracellular K+ concentration of ZG cells is responsible for most of the increase in cAMP. 7. According to one group of investigators, there is weak stimulation of phospholipase C (PLC) activity after increasing the extracellular K+ concentration of rat ZG cells. 8. If there is such a stimulation of PLC activity, it seems that the action of increased extracellular K+ can potentially involve all known mechanisms for the stimulation of steroidogenesis in endocrine cells. The common primary event is probably the increase in Ca2+ influx. The relative importance of these various potential mechanisms may depend on the particular in vitro conditions used.

Expression of inositol 1,4,5-trisphosphate receptors in rat adrenocortical zones

Previously we demonstrated the presence of InsP3R-I, -II and -III subtypes in the zona glomerulosa. Now we have examined the expression of different subtypes of inositol 1,4,5-trisphosphate receptor (InsP3R) in the inner zones of rat adrenal cortex. RNA extracted from decapsulated adrenal tissue (zonae fasciculata-reticularis and the medulla) or from fasciculata-reticularis cells was reverse transcribed. Subsequent polymerase chain reaction revealed the presence of InsP3R-I, -II and -III subtypes in decapsulated tissue but failed to demonstrate the expression of any known subtypes of InsP3R in fasciculata-reticularis cells. Accordingly, InsP3 receptors expressed in the decapsulated tissue are of medullary origin.

Divergent differentiation of rat adrenocortical cells is associated with an interruption of angiotensin II-mediated signal transduction

The zones of the adrenal cortex contain distinct populations of cells which share a common developmental origin and steroidogenic template. In the rat, zona glomerulosa cells respond to angiotensin II (Ang II) with increased steroidogenesis while zona fasciculata/reticularis cells do not. We have examined Ang II-mediated signal transduction in homogeneous cellular sub-populations derived from either the zona glomerulosa (GLOM) or the zona fasciculata (FASC). In both of these sub-populations Ang II treatment significantly increased the levels of 3H-labelled inositol phosphates as well as the total mass of inositol 1,4,5-triphosphate. In contrast, the two cell types exhibited very different Ang II-mediated changes in free intracellular calcium ([Ca2+]i). Ang II (10 nM), induced [Ca2+]i increases of > 50 nM in 90% of individual GLOM cells (53/58), but in only 28% of FASC cells (11/39). These [Ca2+]i responses occurred after a transient Ang II stimulation ( < 1 min), in the presence of verapamil and in the absence of extracellular calcium, indicating an intracellular release. In small groups of 10-30 cells, stimulation with 1, 10 and 100 nM Ang II induced [Ca2+]i increases of 78, 178 and 215 nM respectively in GLOM cultures compared to only 35, 64, and 65 nM in FASC cultures. Thapsigargin treatment, which releases intracellular calcium in an inositol phosphate independent manner, elicited [Ca2+]i increases in both populations. Importantly, a calcium ionophore induced elevation of [Ca2+]i increased steroidogenesis in both cell types. These results suggest that an interruption of the signaling cascade at the level of intracellular calcium release contributes to the lack of a steroidogenic response to Ang II by the FASC cells. Therefore, in the rat adrenal cortex, divergent differentiation of related cell types may involve alterations within signal transduction pathways distal to initial receptor-mediated events (i.e. inositol phosphate production) and proximal to downstream effector events (i.e. steroidogenesis).

Identification and metabolism of phosphoinositol species formed on angiotensin II stimulation of zona fasciculata-reticularis cells from the bovine adrenal cortex

The identity of phosphoinositol isomers accumulating on stimulation of primary cultures of bovine adrenocortical zona fasciculata/reticularis cells with angiotensin II (AII), in the presence of Li+, has been established by chromatographic separation on a MonoQ HR5/5 column. The metabolism of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) in a broken cell preparation has also been studied in the absence or presence of added ATP. Our results show that Ins(1,4,5)P3 is formed within 5 s of stimulation of whole cells, but is rapidly converted to Ins(1,3,4)P3 through an Ins(1,3,4,5)P4 intermediate. All the phosphoinositol products accumulating on prolonged (15 min) stimulation of whole cells (Ins1P, Ins4P, Ins(1,3)P2, Ins(1,4)P2, Ins(1,3,4)P3, Ins(1,4,5)P3, Ins(1,3,4,5)P4) can be accounted for by the metabolism of Ins(1,4,5)P3 in broken cells, either through direct dephosphorylation in the absence of added ATP (Ins(1,4)P2, Ins4P) or through dephosphorylation of Ins(1,3,4,5)P4 formed in the presence of added ATP (Ins(1,3,4)P3, Ins(1,3)P2 and Ins1P). Our results provide further evidence to suggest that AII stimulates the rapid and sustained breakdown of phosphatidylinositol 4,5-diphosphate (PtdIns(4,5)P2) to form Ins(1,4,5)P3.

Density separation of rat adrenocortical cells: morphology, steroidogenesis, and P-450scc expression in primary culture

We have developed a method that separates rat adrenocortical cells by density into populations which retain zone specific properties in primary culture. Two different parenchymal populations were obtained and designated 2FASC (1.034 g/ml, 18.0 microns cell diameter) and 7GLOM (1.069 g/ml, 11.7 microns cell diameter). In freshly isolated cell suspensions the physical characteristics and differential steroidogenic responses to adrenocorticotropin and angiotensin II suggested that 2FASC cells originated predominantly from the zona fasciculata and 7GLOM cells from the zona glomerulosa. In primary culture (Dulbecco's Modified Eagle's Medium-F12 medium with 15% horse serum and 2.5% fetal bovine serum) the two populations exhibited different morphologies. 2FASC cells retained lipid and formed cohesive epithelial monolayers that remained stationary for 3 wk. 7GLOM cells were initially epithelial but rapidly lost lipid, spread, and assumed fibroblastic shapes. Both cell types were positive for the cholesterol side-chain cleavage cytochrome P-450 by immunofluorescence. Therefore, the morphologic changes seen in 7GLOM cultures were due to modulation, not fibroblastic overgrowth. This phenotypic plasticity may reflect the mesodermal origin of the adrenal cortex, and the subcapsular location of 7GLOM cells in vivo. In contrast, cells such as 2FASC which are located deeper in the cortex seem to have a more restricted, fully committed parenchymal phenotype.