Biomed & Pharmacother 1997;51:29-37 0 Elsevier. Paris Focus Enkephalins in hematopoiesis M Boranib, L Krizanac-Bengez, J Gabrilovac, T Marotti, D Breljak Department of Experimental Biology and Medicine, Rugjer Bofkovii Institute, POB 1016, Bijenirka Cesta 54, 41001 Zagreb, Croaria Summary - Recent data support the view that neuropeptide mediators, in particular opioid peptides, participate in the control of hematopoiesis. The main arguments are: neuropeptides modulate the functions of lymphoid cells, macrophages and mature granulocytes; they control cell proliferation and differentiation in many tissues, particularly during embryogenesis; lymphoid cells, macrophages, polymorphonuclear granulocytes and bone marrow stromal elements express neuropeptide receptors; bone marrow cells produce opioid-like neuropeptides; the CDlO/CALLA marker of lymphoid, myeloid and marrow stromal cells is an enzyme, endopeptidase, which cleaves - and thus activates/inactivates - opioid and other neuropeptides. We have shown that opioid peptides enkephalins, opioid antagonist naloxone, and the inhibitor of enkephalin-degrading endopeptidase, thiorphan, modulate the proliferation and differentiation of hematopoietic cells in clonal and long-term cultures of mouse bone marrow. The effects partly depended on the presence of the accessory hematopoietic elements, and followed a circadian pattern. The dose-responses were irregular, showed strain-dependent and individual variations, and apparently reflected the state of the activity of target cells, cellular interactions and simultaneous signals by other mediators. The enkephalins were shown to bind to specific (opioid) receptors on the target cells, and their signals to be transmitted to the cell interior by a cascade of secondary messengers including diacyl-glycerol (DAG), protein-kinase C (PKC) and Ca++ ions. Neuropeptide regulation of hematopoiesis might belong to a complex immuno-neuroendocrine network including melatonin. enkephalins / opioid peptides / hematopoiesis / immune response / CALLA/CDlO / secondary messengers INTRODUCTION The differentiation and proliferation of hemo- poietic cells are regulated by a complex network of growth factors and stromal elements in the hemopoietic microenvironment, which maintain a balance between stimulatory and inhibitory signals [8, 14, 33, 711. However, hematopoiesis is also susceptible to systemic neuroendocrine mo- dulations [6, 53, 75,76, 91,92, 971, and like the lymphatic tissue [l, 961, receives an autonomous nerve supply [9]. Opioid peptides have been shown to interfere with the effects of stress on hematopoiesis [18, 311. On the other hand, bone marrow cells produce short-chain neuropeptides with opioid-like analgesic properties [23, 76, 841. We have hypothesized that neuropeptide medi- ators, amply shown to modulate the functions of lymphoid cells and macrophages [22,43, 881, might also participate in the regulation of hema- topoiesis. The reasons are as follows: a), neu- ropeptides, including the opioid peptides, control cell proliferation and differentiation in many tis- sues [40,44,46,56, 57, 80,95, 1001; b), lym- phoid cells, macrophages, bone marrow cells and polymorphonuclear leucocytes express opioid re- ceptors [2, 10-13, 20, 58, 72, 811; c), bone mar- row cells produce opioid-like oligopeptides [78, 841; d), the CDlOKALLA marker of lym- phoid, myeloid and marrow stromal cells is an enzyme, endopeptidase (also called ‘enkephali- nase’), which cleaves - and thus activates/inacti- vates - opioid and other neuropeptides [15, 19, 38,44, 54, 87, 931. OPIOID PEPTIDES Opioid peptides belong to the family of neuropep- tides shown in table I [7, 17, 74, 79, 891. These oligo- and polypeptides mediate signal functions in the nervous system, gut, endocrine glands, bronchal tree, cardiovascular system, lymphatic and other tissues [17,47,74]. Originally, neuropeptides were considered to be specific products of the nerve and endocrine cells, but other cell types including the lymphoid cells and macrophages have recently been shown to synthesize and process these mediators