STEM-CELL FACTOR REGULATES THE EXPRESSION OF CYCLIN-A AND RETINOBLASTOMA GENE-PRODUCT IN THE GROWTH AND DIFFERENTIATION PATHWAY OF HUMAN MEGAKARYOCYTIC CELLS

The biological effects of c-kit ligand (stem-cell factor: SCF) on an i mmortalized human megakaryocytic cell line (CMK) was evaluated using m ethods including the 3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyltetrazo lium bromide (MTT) assay, surface marker analysis, DNA cell-cycle anal ysis and immunoblotting. SCF stimulated the growth of CMK cells. Incub ation with SCF resulted in increased expression of IIb/IIIa platelet-r elated glycoprotein (gpIIb/IIIa), indicating enhanced differentiation of CMK cells. Treatment of CMK cells with SCF resulted in a decrease i n the subpopulation in the G1 phase, with a reciprocal increase in tho se in the S phase and the G2 + M phase. Moreover, SCF significantly in creased cellular expression of cyclin A, a regulatory subunit of cycli n-dependent protein kinase (CDK), and the ratio of phosphorylated/deph osphorylated retinoblastoma gene product (RB protein). These results s uggest that SCF stimulates the growth and differentiation of megakaryo cytic cells possibly through mechanisms related to the activation of c ell-cycle-dependent serine/threonine kinase and inactivation of the nu clear tumor-suppressor gene product.