The mechanism of inhibition of phospholipase D (PLD) by ceramides was determined using granulocytes differentiated from human promyelocytic leukemic (HL-60) cells. In a cell-free system, hydrolysis of phosphatidylcholine by membrane-bound PLD depended upon phosphatidylinositol 4,5-bisphosphate, guanosine 5′-3-O-(thio)triphosphate) (GTPγS), and cytosolic factors including ADP-ribosylating factor (ARF) and RhoA. C₂- (N-acetyl-), C₈- (N-octanoyl-), and long-chain ceramides, but not dihydro-C₂-ceramide, inhibited PLD activity. Apyrase or okadaic acid did not modify the inhibition of PLD by ceramides, indicating that the effect in the cell-free system was unlikely to be dependent upon a ceramide-stimulated kinase or phosphoprotein phosphatases. C₂- and C₈-ceramides prevented the GTPγS-induced translocation of ARF1 and RhoA from the cytosol to the membrane fraction. In whole cells, C₂-ceramide, but not dihydro-C₂-ceramide, inhibited the stimulation of PLD by N-formylmethionylleucylphenylalanine and decreased the amounts of ARF1, RhoA, CDC42, Rab4, and protein kinase C-α and -β₁ that were associated with the membrane fraction, but did not alter the distribution of protein kinase C-ϵ and -ζ. It is concluded that one mechanism by which ceramides prevent the activation of PLD is inhibition of the translocation to membranes of G-proteins and protein kinase C isoforms that are required for PLD activity.

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.