Throughout adult life, all blood cells are constantly regenerated from a small pool of hematopoietic stem cells. Genes encoding secreted growth factors, signal transduction molecules and nuclear proteins regulate this process. Their cognate receptors are single transmembrane proteins that constitute the so-called cytokine receptor superfamily. One such ligand-receptor pair is Tpo, also known as the c-Mpl ligand or MGDF and its receptor, Tpo-R. Tpo is a hormone constitutively produced mainly by the liver and kidney and mediates biological effects on a broad spectrum of hematopoietic progenitor cells, including stem cells. Plasma Tpo levels are regulated by the platelet and megakaryocyte mass through c-Mpl receptor. Upon ligand-induced Mpl homodimerization, the major signaling events for proliferation are mediated through the JAK-STAT pathway, while differentiation might occur through a prolonged activation of the MAPK pathway.The unique contribution of Tpo to hematopoiesis is manifest in its ability to exert a highly positive influence on expanding the megakaryocytic pool and facilitating its maturation into platelet shedding cells. Binding of Tpo to c-Mpl induces receptor homodimerization and subsequently activation and tyrosine phosphorylation of JAK2. Once activated, JAK2 phosphorylate a number of substrates, including the Mpl receptor itself, STAT1, STAT3, STAT5, and a number of adapter proteins including SHC, Raf1, MAPK and PI3K. Phosphorylated STAT1 and STAT3 homodimerizes to form a complex and translocates to the nucleus to bind to the Sis Inducible Element (SIE). Several essential Mpl-induced signaling molecules can be activated by more than one pathway. ERK is activated by both SHC-dependent and SHC-independent pathways by the MEK of Raf family. Once activated, ERKs are translocated to the nucleus, where they regulate gene expression by phosphorylating TRE (Transcription Release Elements) such as Elk1, c-Fos and c-Jun. PI3K can be activated by either GAB2 or IRS2 complex formation. In addition, phosphorylation of distal tyrosine residues on Mpl creates docking sites for SHC, which undergoes phosphorylation and then recruits GRB2-SOS, thus activating the Ras-Raf-MAPK pathway. Tpo also potentiates activation of PLC-γ, DAG and elevation of intracellular Ca2+, providing a molecular explanation for potentiation of functional responses.
As a therapeutic agent Tpo has been shown to speed platelet recovery following Myelosuppressive Therapy in cancer patients receiving Chemotherapy. In addition, the biological effects of Tpo are not limited to the megakaryocytic lineage; the growth of erythroid and myeloid colony-forming cells is also expanded by Tpo in vitro, and its use in normal and myelosuppressed mice and non-human primates leads to enhanced recovery of multiple hematopoietic lineages. Incongenital amegakaryocytic Thrombocytopenia (a rare disorder in newborns with isolated thrombocytopenia and characterized by severely reduced bone marrow megakaryocytes) and in chemotherapy-induced myelosuppression absence of c-Mpl mRNA expression in bone marrow cells and their failure to respond to exogenous Tpo in vitro have recently been discovered. In TAR (Thrombocytopenia with Absent Radii) syndrome, negative c-Mpl gene mutation screening combined with normal Mpl density, but raised Tpo levels suggest that Mpl signal transduction defects may be responsible for the characteristic HIV-associated thrombocytopenia, Myelodysplastic Syndrome and ITP (Immune Thrombocytopenia).