A Synaptosomal Protein Kinase Is Regulated by Phosphatidylinositol 4-Phosphate and Mg2+

Ligation of low-density lipoprotein receptor-related protein with antibodies elevates intracellular calcium and inositol 1,4, 5-trisphosphate in macrophages

We have probed the signaling characteristics of the macrophage low-density lipoprotein receptor-related protein (LRP) with monoclonal antibody 8G1, its Fab and F(ab')(2) fragments directed against the ligand binding heavy chain, and monoclonal antibody 5A6 directed against the membrane-spanning light chain of LRP. Ligation of LRP with 8G1, its Fab and F(ab')(2) fragments, or 5A6 increased intracellular Ca(2+) levels two- to threefold. Prior ligation of LRP with 8G1 did not affect the increase in [Ca(2+)](i) observed on subsequent ligation of LRP with lactoferrin, P. exotoxin A, or lipoprotein lipase. Binding to LRP by 8G1, its Fab and F(ab')(2) fragments, or 5A6 increased inositol 1,4,5-trisphosphate (IP(3)) levels by 50 to 100%. Incubation of macrophages with guanosine 5', 3'-O(thio)-triphosphate (GTP-gamma-S) before treatment with antibody potentiated and sustained the 8G1-induced increase in IP(3) levels. Treatment of macrophages with guanyl-5'-yl thiophosphate prior to GTP-gamma-S treatment abolished the GTP-gamma-S-potentiated increase in IP(3) levels in 8G1-treated macrophages. Antibody-induced increases in IP(3) and [Ca(2+)](i) in macrophages on ligation of LRP were pertussis toxin sensitive. Binding of 8G1 or its Fab or F(ab')(2) fragments to LRP stimulated macrophage protein kinase C (PKC) activity as evaluated by histone IIIs phosphorylation by about two- to sevenfold. Staurosporin inhibited the anti-LRP antibody-induced increase in PKC activity. Ligation of LRP with 8G1 increased cellular cAMP levels about twofold. Preincubation of macrophage with the LRP-binding protein receptor-associated protein suppressed the 8G1-induced increase in cAMP levels. Thus, binding of antibodies directed against either chain of LRP triggers complex signaling cascades.

Binding of rat alpha 1-inhibitor-3-methylamine to the alpha 2-macroglobulin signaling receptor induces second messengers

Binding of receptor-recognized forms of tetrameric human alpha 2-macroglobulin (alpha 2M*) to a macrophage signaling receptor induces cAMP synthesis, increases in inositol 1,4,5-triphosphate (IP3) synthesis, and a concomitant rise in cytosolic free calcium ([Ca2+]i). The alpha 2M* signaling receptor is coupled to a pertussis-toxin insensitive G protein. Binding of alpha 2M* also occurs to the low density lipoprotein receptor-related protein/alpha 2M receptor (LRP/alpha 2MR), but this binding does not induce signal transduction. Rat alpha 1-inhibitor-3 (alpha 1I3) is a monomeric member of the alpha-macroglobulin/complement superfamily. Like alpha 2M, it can react with proteinases or methylamine which induces a conformational change causing activated alpha 1I3 to bind to LRP/alpha 2MR. We now report that alpha 1I3-methylamine binds to the macrophage alpha 2M* signaling receptor inducing a rapid rise in the synthesis of IP3 with a subsequent 1.5- to 3-fold rise in [Ca2+]i. alpha 1I3-methylamine binding to macrophages also caused a statistically significant elevation in cAMP. Native alpha 1I3, like alpha 2M, was unable to induce signal transduction. alpha 1I3 forms a complex with alpha 1-microglobulin, which has a distinct conformation from alpha 1I3 and is recognized by LRP/alpha 2MR. This complex also induces an increase in [Ca2+]i comparable to the effect of alpha 1I3-methylamine on macrophages. It is concluded that activation of alpha 1I3 by methylamine or binding of alpha 1-microglobulin causes similar conformational changes in the inhibitor, exposing the receptor recognition site for the alpha 2M* signaling receptor, as well as for LRP/alpha 2MR.

Stimulation of purified phosphatidylinositol 4-kinase by cobra venom cardiotoxin

Cobra venom cardiotoxin was found to stimulate the phosphatidylinositol kinase activity present in A431 cell membranes and in detergent extracts of these membranes. Incubation of highly purified phosphatidylinositol 4-kinase from this source with cardiotoxin resulted in a 2- to 3-fold stimulation of phosphatidylinositol kinase activity. The activation of the purified phosphatidylinositol 4-kinase by cardiotoxin was time- and dose-dependent and appeared to be associated with a decrease in the Km apparent of the enzyme for phosphatidylinositol with no change in the Vmax apparent of the enzyme. The data suggest that the phosphatidylinositol 4-kinase is activated by direct interaction of the enzyme with cobra venom cardiotoxin.

Synthesis, transfer, and phosphorylation of phosphoinositides in cardiac membranes

Compartmentation of phosphoinositide synthesis and transfer of endogenous phosphatidylinositol (PI) were characterized in membrane fractions prepared from rabbit myocardium. De novo synthesis of PI was highly enriched in free sarcoplasmic reticulum (551 pmol.mg-1. min-1) compared with that in sarcolemma (26.8 pmol.mg-1. min-1) and junctional sarcoplasmic reticulum (178 pmol.mg-1. min-1). In contrast, PI phosphorylation was highly enriched in sarcolemma (2.69 nmol.mg-1.min-1) compared with that in free sarcoplasmic reticulum (0.22 nmol.mg-1.min-1) and junctional sarcoplasmic reticulum (0.38 nmol.mg-1.min-1). Phosphorylation of endogenous phosphatidylinositol 4-phosphate to phosphatidylinositol 4,5-bisphosphate was also enriched in sarcolemma (38.5 pmol.mg-1.min-1) compared with that in free sarcoplasmic reticulum (less than 5.0 pmol.mg-1.min-1) and junctional sarcoplasmic reticulum (6.5 pmol.mg-1.min-1). Transfer of endogenous PI was characterized as a mechanism to overcome compartmentation of PI synthesis in cardiac membranes. A 29-kDa PI transfer protein was purified 1,500-fold from cytosol of rabbit myocardium. Both cytosol and purified PI transfer protein catalyzed the transfer of endogenous PI from microsomal sites of synthesis to sarcolemma. In conclusion, synthesis of PI is highly enriched in free sarcoplasmic reticulum, whereas phosphorylation of phosphoinositides is highly enriched in sarcolemma. A 29-kDa PI transfer protein in myocardial cytosol can mediate in vitro transfer of de novo-synthesized PI to the sarcolemma.