As these GEITP pages have discussed a number of times, gene-environment interactions are best studied (in vivo) in the intact animal (or in clinical studies, i.e. in humans). Many labs study a biological model system in an isolated organ or tissue (ex vivo), or fresh cells or subcellular fractions in a test tube (in vitro), or in cell culture lines. Back in the 1970s, some of us noticed that –– when tissues are minced and trypsinized and then plated as individual cells in culture –– they often rapidly lose the function that had been present in the intact laboratory animal. [In the early 1970s, before cloning methods were available, “the function” was usually “an enzyme activity”, which geneticists would regard as a trait, or phenotype.]
The [attached; left] study represents a landmark discovery of why this discrepancy –– first observed more than 45 years ago (between the intact animal and cell culture lines) –– is likely to occur. During the past four decades, we have now learned that lipid mediators (LMs), derived from polyunsaturated fatty acids (PUFAs), comprise >150 chemicals, many of which have potent bioactivity. These “potent bioactivities” include virtually every critical life process. Rather than list them here, the interested Reader is referred to this review [J Biol Chem 2oo8; 283: 36061–36065]; the informative Supplementary Data online file from this review is [attached; right] to this email.
The w-6 fatty acid is converted to arachidonic acid, which is then further converted to 11 known classes of LMs: prostaglandins, prostacyclins, thromboxanes, leukotrienes, epoxyeicosatrienoic acids (EETs), hydroxyeicosatetraenoic acids (HETEs), dihydroxyeicosatetraenoic acids (DHETEs), hydroperoxyeicosatetraenoic acids (HPETEs), w- and w-1 alcohols, lipoxins, and hepoxilins. The w–3 fatty acid is converted to eicosapentaenoic acid and docosahexaneoic acid –– which are then further converted to four additional classes of LMs: resolvins, docosatrienes, neuroprotectins, and eoxins. The attached (left) paper shows that addition of several of these LM metabolites to cells in culture restores a phenotype (response to inflammation) that had been lost. “RAW 264.7” cells were stimulated to produce specific LMs, indicating that addition of LMs restored many of the phospholipid functions that had been lost when cells from the intact animal are cultured in a dish. It is anticipated that future lipidomics studies of this nature will follow up on this exciting breakthrough publication.
J Lipid Res 2o18; 59: 542–549
Dan, this is probably such a naive question that I ask you to please keep “the sender” anonymous.
Are lipid mediators classified as hormones? Or are they in some other category? I can see where “hormones” and other molecules that diffuse throughout the entire body of the intact animal, might easily not be present when cells are trypsinized and then individually plated in a dish.
Lipid mediators (LMs) –– discussed in the article below –– are considered to be “second-messengers”. Common examples of second-messengers include cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), calcium ion (Ca2+), inositol trisphosphate (IP3), and diacylglycerol (DAG). Second-messengers are intracellular substances that mediate cell activity by relaying a signal from “first-messengers”, which are extracellular substances (e.g. the hormone epinephrine or estrogen, or neurotransmitter serotonin) that typically bind to cell-surface receptors and initiate intracellular activity (i.e. initiate the signaling pathway).
This discussion brings to mind the four basic categories of chemical signaling found in human, mouse and, in fact, in all multicellular organisms: paracrine signaling, autocrine signaling, endocrine signaling, and signaling by direct contact. The main difference between the different categories of signaling –– is the distance that the signal travels through the organism to reach the target cell. “Autocrine” denotes the mode of hormone action by which they bind to receptors on (or in) the cell and affect the same cell that has produced it. “Paracrine” indicates hormone action in which hormones are released from one cell and bind to a receptor on nearby cells and affects their function. “Endocrine” specifies hormone action by being secreted into the blood and acting at long distance (i.e. no ducts). “Exocrine” refers to glands that secrete their products through ducts –– that open onto an epithelium (e.g. skin, GI tract), rather than directly into the bloodstream.
And now we all know more than we would REALLY like to know about “initiation of signaling pathways.” 🙂