AKT(PKB), Cyclin D1, p90RSK2(RPS6KA3), p21, PtdIns(4,5)P2, c-MPL, MEK1(MAP2K1), PI3K cat class IA, ATF-1, GRB2, PKC-zeta, SOS, Cyclin A2, c-Fos, CrkL, IRS-2, PI3K reg class IA, VAV-1, c-Jun, GSK3 beta, PDK (PDPK1), FKHR, Erk (MAPK1/3), FKHR, Survivin, Elk-1, C3G, c-Cbl, CREB1, 188.8.131.52, Fyn, c-Raf-1, c-Myc, H-Ras, GSK3 alpha/beta, Thrombopoietin, RAP-1A, Shc, p70 S6 kinase1, BAD, JAK2, Rac1, B-Raf, PtdIns(3,4,5)P3, PKC-alpha, p27KIP1
Thrombopoietin-regulated cell processes
Thrombopoietin is a hormone involved in biological effects on a broad spectrum of hematopoietic progenitor cells, including stem cells. It supports stem cell survival and expansion. It is primarily a key physiological regulator of steady-state megakaryocytopoiesis, the process of megakaryocyte production and maturation that ultimately results in formation of platelets. Thrombopoietin is a 332-amino acid glycoprotein constitutively produced by the liver, kidney, marrow stroma and other tissues. Circulating concentration is thought to be controlled by receptor mediated internalization and degradation of Thrombopoietin by megakaryocytes and platelets , , .
Binding of Thrombopoietin with its receptor Myeloproliferative leukemia virus oncogene (c-Mpl ) leads to receptor homodimerization and subsequent activation of Janus kinase 2 ( JAK2 ). Activated JAK2 carries out tyrosine phosphorylation of multiple cellular proteins, by inducing phosphorylation of Myeloproliferative leukemia virus oncogene ( c-Mpl ) itself and recruitment of signaling proteins to the receptor via their SH2 domains. One of such proteins is SHC transforming protein ( Shc ), which in turn gets phosphorylated and recruits phosphorylated Growth factor receptor-bound protein 2 ( GRB2 ) and Son of sevenless homolog (SOS ), thereby activating small GTPase Harvey rat sarcoma viral oncogene homolog ( H-Ras ). GRB2 can also be associated with Vav guanine nucleotide exchange factor ( VAV ), that are guanine nucleotide exchange factors for Ras-related C3 botulinum toxin substrate 1 ( Rac1 ) . Activation of H-Ras is followed by recruitment of V-raf-1 murine leukemia viral oncogene homolog 1 ( c-Raf-1 ) and activation of Mitogen-activated protein kinase kinase (MEK) and Mitogen-activated protein kinase ( Erk ). Moreover, Thrombopoietin can activate Erk via Rap1, which activates v-raf murine sarcoma viral oncogene homolog B1 ( B-Raf ) . Recently it has been shown that kinase FYN oncogene related to SRC, FGR, YES ( Fyn ) appears to be activated upon Thrombopoietin stimulation .
Activated Erk phosphorylates ribosomal protein S6 kinases 90kD ( p90RSK ). Both kinases are involved in activation of a number of transcription factors, such as cAMP responsive element binding protein 1 ( CREB1 ), Activating transcription factor 1 ( ATF-1 ), ELK1 member of ETS oncogene family ( Elk-1), V-myc myelocytomatosis viral oncogene homolog (c-Myc ), Jun oncogene ( c-Jun ) , and V-fos FBJ murine osteosarcoma viral oncogene homolog ( c-Fos ). All of them regulate transcription of cell cycle proteins, for example, Cyclin A2 and Cyclin D1, Cyclin-dependent kinase inhibitor 1B ( p27KIP1 ) and Cyclin-dependent kinase inhibitor 1A ( p21 ) , , , , , . Besides, p90RSK phosphorylates and inactivates pro-apoptotic factor BCL2-associated agonist of cell death ( BAD ) , , .
Furthermore, Thrombopoietin stimulation leads to an activation of Phosphoinositide-3-kinase ( PI3K ) pathway . JAK2 activates the regulatory subunit of PI3K ( PI3K reg class 1A ), for example, through Insulin receptor substrate 2 ( IRS-2 ) phosphorylation . De-repression of PI3K catalytic subunit facilitates its association with membrane-bound H-Ras. Activated catalytic subunit then phosphorylates membrane posphatidylinositol, and phosphorylated lipids create membrane targets for proteins containing PH domains, such as 3-phosphoinositide dependent protein kinase-1 ( PDK (PDPK1) ) and AKT(PKB) kinase activated by PDK .
AKT(PKB) has a number of downstream targets that are involved in cell survival and cell cycle. For example, AKT(PKB) inhibits transcription factors Forkhead box O3 and O1 ( FOXO3A and FKHR ), that modulate transcription of p27KIP1 and survival, respectively , , , . Activation CREB1 by AKT(PKB) is shown . AKT(PKB) can also modulate the activity of p27KIP1 and p21 by phosporylating them , , . Moreover, AKT(PKB) phosphorylates and inhibits BAD . AKT(PKB) also inhibits the activity of Glycogen synthase kinase 3 beta ( GSK3 beta ) and its phosphorylation of Cyclin D1 and c-Myc , .
In addition to its effects on AKT, PDK is also an activating kinase for Protein kinase C ( PKC-zeta ) and Ribosomal protein S6 kinase 70kDa ( p70 S6 kinase1 ) . Accumulation of atypical PKC-zeta in the nucleus during megakaryocytopoiesis  allows the assumption that this kinase is one of the main down-stream targets of PDK (PDPK1). Both PKC-zeta and p70 S6 kinase1 are involved in the process of translation initiation. Another type of PKC that is implicated in Thrombopoietin -induced processes is PKC-alpha .