Thrombopoietin (TPO) is a hematopoietic
cytokine that regulates megakaryocytosis and
thrombocytosis by binding to its receptor (c-Mpl). The signaling pathways downstream of c-Mpl include the Ras/Raf/MAP
kinase and JAK/STAT pathway and are transduced into the regulation of immediate early-, early- and delayed-response genes. How these genes couple c-Mpl activation to the biochemical machinery of cell growth and cell cycle progression in hematopoietic cells is still unclear. UT-7/TPO is a recently characterized TPO-dependent cell line. Using
RNA fingerprinting with arbitrarily primed PCR (RAP-PCR) to identify the TPO-regulated genes in this cell line, we found that the
mRNA expression of
nucleolin was upregulated in the UT-7/TPO cells in response to TPO. Concomitantly, the TPO-stimulated cells expressed an increased amount of full length
nucleolin as determined by immunoblot analysis. The TPO-induced upregulation of
nucleolin mRNA was not inhibited by the MEK1/2 inhibitor
PD98059, suggesting that ERK/MAPK activation is not necessary for elevation of
nucleolin gene expression in response to TPO in UT-7/TPO.
Nucleolin is a multifunctional
nucleolar protein thought to be involved in many cellular processes, including ribosome biogenesis, the processing of
ribosomal RNA (rRNA), mRNA stability, transcriptional regulation, and cell proliferation. Thus, these results indicate that the upregulation of
nucleolin mRNA and
protein may be important for the TPO-induced effects of hematopoietic cells.