Phospholipase C activation and diacylglycerol kinase inactivation lead to an increase in diacylglycerol content in spontaneously hypertensive rat

Diacylglycerol kinases: at the hub of cell signalling

DGKs (diacylglycerol kinases) are members of a unique and conserved family of intracellular lipid kinases that phosphorylate DAG (diacylglycerol), catalysing its conversion into PA (phosphatidic acid). This reaction leads to attenuation of DAG levels in the cell membrane, regulating a host of intracellular signalling proteins that have evolved the ability to bind this lipid. The product of the DGK reaction, PA, is also linked to the regulation of diverse functions, including cell growth, membrane trafficking, differentiation and migration. In multicellular eukaryotes, DGKs provide a link between lipid metabolism and signalling. Genetic experiments in Caenorhabditis elegans, Drosophila melanogaster and mice have started to unveil the role of members of this protein family as modulators of receptor-dependent responses in processes such as synaptic transmission and photoreceptor transduction, as well as acquired and innate immune responses. Recent discoveries provide new insights into the complex mechanisms controlling DGK activation and their participation in receptor-regulated processes. After more than 50 years of intense research, the DGK pathway emerges as a key player in the regulation of cell responses, offering new possibilities of therapeutic intervention in human pathologies, including cancer, heart disease, diabetes, brain afflictions and immune dysfunctions.

Differential regulation of diacylglycerol kinase isozymes in cardiac hypertrophy

To examine the involvement of diacylglycerol kinase (DGK) and phosphatidic acid phosphatase (PAP) in pressure overloaded cardiac hypertrophy, rats were subjected to either ascending aortic banding for 3, 7, and 28 days or sham operation. In comparison with sham-operated rats, the left ventricular (LV) weight of the aortic-banded rats increased progressively. At 28 days after surgery, the expression of DGKepsilon mRNA but not DGKzeta or PAP2b mRNA in the LV myocardium significantly decreased in the aortic-banded rats compared with the sham-operated rats. DGKzeta protein in the LV myocardium translocated from the particulate to the cytosolic compartment in the aortic-banded rats. Furthermore, the myocardial content of 1,2-diacylglycerol and PKCdelta protein expression in the particulate fraction of the LV myocardium significantly increased in aortic-banded rats compared with sham-operated rats. These results suggest that DGKepsilon and DGKzeta play distinct roles in the development of pressure overloaded cardiac hypertrophy and that the two isozymes are differentially regulated.

Differentiation of HL-60 cells to granulocytes involves regulation of select diacylglycerol kinases (DGKs)

Diacylglycerol Kinases (DGKs) are a family of enzymes that regulate the levels of different pools of diacylglycerol (DAG), affecting DAG-mediated signal transduction. Since DAG is known to play several important regulatory roles in granulocyte physiology, we investigated the expression pattern of DGK isoforms throughout differentiation of HL-60 cells to granulocytes. HL-60 cells were incubated with 1.25% dimethyl-sulfoxide (DMSO) to initiate differentiation and total RNA isolated at different time points. DGK expression was assessed through Northern blot, end-point PCR, and real-time PCR. The non-selective inhibitors R59022 and R59949 were used to block DGK at different time points throughout differentiation. CD11b and GPI-80, reactive oxygen species (ROS) generation, changes in the cell cycle, and apoptosis were used as markers of differentiation. Of the nine isoforms of DGK evaluated (alpha, delta, epsilon, gamma, zeta, beta, theta;, iota, eta), only five (alpha, delta, epsilon, gamma, and zeta) were expressed in HL-60 cells. DGKalpha was virtually absent in non-differentiated cells, but was markedly upregulated throughout differentiation. The other isoforms (delta, epsilon, gamma, and zeta) were expressed in undifferentiated HL-60 cells but were substantially decreased throughout differentiation. Non-selective blocking of DGK with R59022 and R59949 led to acceleration of differentiation, reducing the time necessary to observe upregulation of CD11b, GPI-80 and generation of ROS by 50%. Likewise, the cell cycle was disrupted when DGK isoforms were inhibited. These results provide evidence that DGK levels are dynamically regulated throughout differentiation and that expression of DGKs play an important regulatory function during the differentiation of neutrophils.

Adrenoceptors in SHR: alterations in binding characteristics and intracellular signal transduction pathways

There is much data on altered adrenoceptor function in the heart, blood vessel and kidney from spontaneously hypertensive rats (SHR). The enhancement of vascular and renal alpha-adrenoceptor function, i.e. vasoconstriction and retention of water and sodium, may contribute to the development and maintenance of the hypertension, whereas cardiac alpha1-adrenoceptor may be of minor physiological significance. Alpha1-adrenoceptor-mediated signal transduction as a whole is increased in SHR vascular tissues, but the intracellular signaling per receptor in the kidney seems to be decreased despite increased alpha1-adrenoceptor density. On the other hand, cardiac and vascular beta-adrenoceptor responsiveness is attenuated in SHR. Reduced vasorelaxation mediated by beta-adrenoceptors may also contribute to high blood pressure. The impaired cardiovascular beta-adrenoceptor function in SHR does not appear to be necessarily explained by alterations observed at receptor levels. Alterations in signal transduction should be also considered. Limited data on renal beta-adrenoceptor density and its signaling suggest decreased or unaltered cyclic AMP formation per receptor in SHR. We will review alterations in both binding characteristics and each component of intracellular signal transduction pathways in cardiovascular and renal adrenoceptors of SHR.

Guanine nucleotide regulatory protein levels and function in spontaneously hypertensive rat vascular smooth-muscle cells

We compared G-protein levels and function in membranes from vascular smooth-muscle cells (VSMC) derived from mesenteric arteries from SHR, WKY and Wistar rats. Basal adenylyl cyclase activity was significantly reduced in SHR membranes compared with Wistar, but was similar to WKY. Isoproterenol stimulation (10(-4) M) was significantly lower in SHR membranes compared to WKY, but was similar to that in Wistar, which was also significantly lower than WKY. Forskolin (10(-4) M) and NaF (10(-2) M), resulted in a higher stimulatory response in SHR membranes. Biphasic effects of GTP on isoproterenol-stimulated membranes demonstrated unaltered Gi function in SHR membranes. No significant differences were seen in the levels of Gs alpha (44- and 42-kDa forms), Gi2 alpha and the beta-subunit in immunoblotting studies of the membranes. Amounts of Gq alpha/G11 alpha and Gi3 alpha were also unchanged. In conclusion, there are differences in adenylyl cyclase responses in SHR VSMC membranes which are not a consequence of altered levels of G-proteins, but may reflect genetic differences rather than effects of hypertension.

Many membrane abnormalities in hypertension result from one primary defect

Evidence has been presented that: 1.) Changes in lipid bilayer alter the function of integral membrane proteins. 2.) There is less calcium bound to the plasma membrane in hypertension. 3.) Structural and functional abnormalities of the lipid bilayer have been reported in genetic hypertension. We hypothesize that multiple abnormalities of membrane transport systems in hypertension are secondary to an inherent abnormality of the lipid bilayer in which these transport proteins reside.

Vasopressin stimulates phospholipase D activity against phosphatidylcholine in vascular smooth muscle cells

It is now clear that various hormones and agonists can stimulate the production of lipid mediators from non-phosphoinositide phospholipids. We have investigated the production of diacylglycerol from nonphosphoinositide sources, and we demonstrated that vasopressin and other vasoactive agents stimulate hydrolysis of phosphatidylcholine in a variety of cultured vascular smooth muscle cells of rat and human origin. We used vasopressin to characterize this response and found that vasopressin stimulates phospholipase D activity against phosphatidylcholine in A-10 vascular smooth muscle cells. The vasopressin-stimulated phosphatidylcholine hydrolysis is both time- and concentration-dependent. The half-maximal dose of vasopressin required for phosphatidylcholine hydrolysis (ED50 approximately 1 nM) correlates well with vasopressin binding to A-10 cells (Kd approximately 2 nM). The phosphatidylcholine in A-10 cells can be preferentially radiolabeled with [3H]myristic acid; subsequent treatment with vasopressin stimulates a rapid increase in 3H-labeled phosphatidate (approximately 4 X control values at 3 min), and after a short lag, 3H-labeled diacylglycerol rises and reaches maximal levels at 10 min (approximately 2 X control values). Similar temporal elevations of phosphatidate and diacylglycerol occur in A-10 cells labeled with [3H] glycerol. In A-10 cells radiolabeled with [3H] choline, the elevation of cellular phosphatidate and diacylglycerol is concomitant with the release of [3H] choline metabolites (predominantly choline) to the culture medium. The temporal production of phosphatidate and diacylglycerol as well as the release of choline to the culture medium are consistent with vasopressin activating phospholipase D. In addition, vasopressin stimulates a transphosphatidylation reaction that is characteristic of phospholipase D. The transphosphatidylation reaction is detected by the production of phosphatidylethanol that occurs when A-10 cells are incubated with ethanol and stimulated with vasopressin. The phospholipase D is active in the absence of extracellular Ca++ whereas the vasopressin-stimulated mobilization of arachidonic acid is dependent on extracellular Ca++. The data indicate that vasopressin stimulates phospholipase D which hydrolyzes phosphatidylcholine to phosphatidate. The phosphatidate is then metabolized, presumably by a phosphatidate phosphohydrolase, to produce sustained levels of cellular diacylglycerol. These sustained levels of diacylglycerol may activate protein kinase C and thereby function in the "sustained phase" of cellular responses.

Alterations of phosphoinositide-specific phospholipase C and protein kinase C in the myocardium of spontaneously hypertensive rats

In order to determine whether phosphoinositide metabolism is altered in hypertensive cardiac hypertrophy, phospholipase C (PLC) and protein kinase C activities were measured in hearts from 4- and 20-week-old spontaneously hypertensive rats (SHR) and age-matched, normotensive Wistar-Kyoto rats (WKY). PLC activities were assayed using phosphatidylinositol (PI) and phosphatidylinositol-4,5-bisphosphate (PIP2) as substrates to assess the substrate specificity. PI-hydrolyzing PLC activity (PI-PLC) was predominantly located in the cytosol, and its activity was similar in both strains. Membrane-bound PIP2-hydrolyzing PLC activity (PIP2-PLC) was significantly lower in 20-week-old SHR than in WKY, but there was no significant difference in soluble PIP2-PLC. Protein kinase C activity was significantly elevated in 20-week-old SHR and Ca2(+)-phospholipid-dependent phosphorylation was observed in the proteins of molecular weight 26, 32, 43, and 95 KDa. In 4-week-old prehypertensive SHR, there were no significant differences in PI-PLC, PIP2-PLC, or protein kinase C activities as compared with age-matched WKY. These data demonstrated that protein kinase C and membrane-bound PIP2-PLC are altered during the period of hypertension development. These alterations may have important roles in the development or maintenance of hypertensive cardiac hypertrophy in SHR.

1,2-diacylglycerol content in thoracic aorta of spontaneously hypertensive rats

Phosphoinositide metabolism participates in the control of cell calcium homeostasis. Because a notable neutral lipid (1,2-diacylglycerol) is generated from phosphoinositide hydrolysis and is assumed to be a secondary messenger, we determined 1,2-diacylglycerol content and its fatty acid profiles in the thoracic aorta of spontaneously hypertensive rats (SHR) and compared it with those of normotensive Wistar-Kyoto (WKY) rats. After the aorta was exposed to 10(-5) M norepinephrine as a stimulant, 1,2-diacylglycerol content in SHR was significantly higher by 33% than in WKY rats at 4 weeks of age, whereas there was no difference in 1,2-diacylglycerol content between the two strains at 20 weeks of age. Before norepinephrine stimulation, there was no significant difference in 1,2-diacylglycerol level between the two strains at 4 weeks of age. Analysis on a gas chromatograph showed that 1,2-diacylglycerol was composed of similar molecular species of fatty acids in aortas obtained from SHR and WKY rats. On the other hand, the cholesterol content of aortas was higher in SHR than in WKY rats at 20 weeks of age, whereas the difference at 4 weeks was not significant. Phosphatidylcholine, phosphatidylethanolamine, and triglyceride showed no significant difference between the two strains. It is concluded that norepinephrine-induced 1,2-diacylglycerol production increases in the thoracic aorta of SHR before the development of hypertension.

Chronic antihypertensive drug treatment decreases protein kinase C activity in platelets from SHR

There is evidence that protein kinase C activity in platelets from adult SHR is significantly higher than this activity in age-matched WKY. In the present study, protein kinase C activity in the SHR was measured following antihypertensive drug treatment. Chronic administration of enalapril to SHR for 2 weeks decreased both systolic blood pressure and protein kinase C activity to the levels seen in the WKY. Similar results were obtained in case of chronic treatment of SHR with hydralazine or nifedipine. These results suggest that enhanced protein kinase C activity of SHR can be suppressed by lowering blood pressure by antihypertensive drugs.

The effect of relaxants working through different transduction mechanisms on the tonic contraction produced in rat aorta by 4 beta-phorbol dibutyrate

1. We have examined the effects of a range of smooth muscle relaxants on the maintained contractions produced in rat aortic rings by the protein kinase C activator, 4 beta-phorbol dibutyrate; these effects were compared with those on the contraction induced by the selective alpha 1-adrenoceptor agonist, methoxamine. The phorbol ester, at 0.3 microM, gave a sustained contraction which was, on average, of approximately the same magnitude as the maximum contraction produced by methoxamine, 10 microM. 2. The beta-adrenoceptor agonist, isoprenaline (0.01-1 microM) caused a dose-related relaxation of the methoxamine-induced contraction but had no effect on the contraction induced by the phorbol ester. 3. An activator of adenylate cyclase, forskolin (0.01-1 microM) produced a dose-related relaxation of the methoxamine-induced contraction and at 0.01-10 microM caused relaxation of the contraction induced by the phorbol ester. Similar results were obtained with the potassium channel activator, cromakalim (0.001-10 microM). 4. An activator of guanylate cyclase, sodium nitroprusside (0.001-100 microM) caused a dose-related relaxation of both the methoxamine-induced and the phorbol ester-induced contraction, being more effective on the former than on the latter. Similar results were obtained with enprofylline (1-1000 microM). 5. Methoxamine (10 nM-100 microM), given cumulatively, caused a dose-related contractile response. Pretreatment with isoprenaline (1 microM), enprofylline (10 microM) and nicorandil (1 microM) resulted in partial decrease of the subsequent response to methoxamine, while nicorandil (10 microM), forskolin (1 microM), sodium nitroprusside (10 microM) and cromakalim (1 microM) totally abolished it. 6. The phorbol ester, given cumulatively, caused increasing contraction in the concentration range 30 nM-10 microM. Pretreatment with forskolin (1 microM), sodium nitroprusside (10 microM), isoprenaline (1 microM), enprofylline (10 microM), nicorandil (1 microM or 10 microM), or cromakalin (1 microM or 10 microM), resulted in partial decrease of the subsequent response to 4 beta-phorbol dibutyrate. 7. These results are discussed in the light of the suggestion that protein kinase C may have a role in the 'latch-bridge' phase of smooth muscle contraction, and that inappropriate activation of protein kinase C may contribute to the pathogenesis of hypertension and other conditions involving vasospasm.

Phosphorylation of diacylglycerol kinase in vitro by protein kinase C

We investigated the effects of enzyme phosphorylation in vitro on the properties of diacylglycerol kinase. Diacylglycerol kinase and protein kinase C, both present as Mr-80,000 proteins, were highly purified from pig thymus cytosol. Protein kinase C phosphorylated diacylglycerol kinase (up to 1 mol of 32P/mol of enzyme) much more actively than did cyclic AMP-dependent protein kinase. Phosphorylated and non-phosphorylated diacylglycerol kinase showed a similar pI, approx. 6.8. Diacylglycerol kinase phosphorylated by either protein kinase C or cyclic AMP-dependent protein kinase was almost exclusively associated with phosphatidylserine membranes. In contrast, soluble kinase consisted of the non-phosphorylated form. The catalytic properties of the lipid kinase were not much affected by phosphorylation, although phosphorylation-linked binding with phosphatidylserine vesicles resulted in stabilization of the enzyme activity.