MKL1 Promotes Megakaryocytic Differentiation Via Stimulation of Serum Response Factor Target Genes.

Our studies demonstrate a critical role for MKL1 (megakaryoblastic leukemia 1) in the molecular regulation of megakaryocytopoiesis. MKL1 is part of the fusion protein formed by the t (1; 22) translocation, which is found uniquely in Acute Megakaryoblastic Leukemia (AMKL). The translocation fuses the RBM15 (also known as OTT) gene on chromosome 1 with the MKL1 (also known as MAL) gene on chromosome 22. Previous studies in muscle cells show that MKL1 is a positive cofactor for the transcription factor serum response factor (SRF), and works via the Rho-A pathway to turn on immediate early genes and muscle specific genes. Using qRT-PCR we show that MKL1 mRNA is markedly up-regulated during megakaryocyte (MK) differentiation of primary murine bone marrow and fetal liver cells. When we overexpress MKL1 in the human erythroleukemia (HEL) cell line and differentiate the cells to become MK by phorbol ester (TPA), there is far greater MK differentiation than in control HEL cells. Via analysis of Wright-Geimsa stained cytospins, MKL1 overexpression increases the average percentage of mature MK from 26% to 48%. Using flow cytometry, platelet glycoprotein V positive cells increase from 14% to 43% on average. The percentage of cells with greater than 4N ploidy increases from 13% to 34%. In order to assess the mechanisms by which MKL1 promotes MK differentiation, we tested whether SRF expression was required for the effects of MKL1. The stimulatory effects of MKL1 are strongly abrogated when cells are transfected with siRNA against SRF, proving that MKL1 acts via SRF to stimulate MK differentiation. Interestingly, SRF siRNA also causes a statistically significant decrease in ploidy in control cells stimulated with TPA. By microarray analyses using both Affymetrix and Illumina platforms, enforced MKL1 expression upregulates many cytoskeletal genes and adhesion molecules, enhances the expression of platelet specific genes such as glycoprotein V (consistent with the FACS data), and accelerates the loss of expression of genes associated with erythropoiesis, such as erythrocyte membrane protein band 4.2. These data indicate that MKL1 enhances MK differentiation by promoting endoduplication as well as increasing expression of platelet-specific genes and of multiple cytoskeletal proteins required for proplatelet formation.