[Metabolism of bioactive lipids]

In addition to their well-known roles as structural elements (amphiphilic lipids of the cell membranes) and as energetic reserve (essentially triglycerides), the lipids function as "second messengers" is increasingly recognised. Indeed, they are also endogenous mediators with specific biological activities, synthesized "on request", often in response to a primary cellular signal. Most of the bio-active lipids originate from cleavage of the membrane structures under the action of phospholipases A2, C, D and of sphigomyelinases. Several products of lipolysis are involved in cell activation, signalling, apoptosis or proliferation. One of the major cascades of bio-active lipid production involves the release of arachidonic acid followed by the formation of prostanoids, e.g.: prostaglandins, thromboxanes, and of leukotrienes and related products. These molecules play major roles in such biological processes as inflammation or thrombosis. Arachidonic acid is also the substrate for cytochrome P450-dependent mono-oxygenases, producing molecules regulating ion fluxes in cell membranes. Action of sphingomyelinases leads to the release of ceramides, associated with apoptosis. Further hydrolysis of ceramides produces fatty acids and sphingosine; the latter being able, in its phosphorylated form, to induce cell proliferation and re-equilibrate the balance between cell death and cell renewal.