Mir-130a-Mediated Downregulation of SMAD4 Contributes to Reduced Sensitivity to TGE beta Stimulation in Promyelocytic Cells

MicroRNAs (miRNA) are noncoding RNA molecules that regulate the synthesis of proteins and, if dysregulated, can result in development of various form of cancers. We have found that miR-130a is highly expressed in immature proliferating granulocytic precursors. In more mature granulocytes the miR-130a expression is significant lower.

In acute myeloid leukemia the granulocyte precursors have lost the ability to undergo terminal maturation, leading to accumulation of non-functional, immature granulocytes (myeloblasts). We hypothesize that a sustained high expression of miR-130a during granulopoiesis may sustain continuous cell proliferation.

TGF-beta is a strong inhibitor of cell proliferation and lack of TGF-beta expression is associated with various form of cancer. Smad4 is an essential compound in the TGF-beta signaling pathway. Using microRNA target-prediction software, we identified Smad4 as a putative target for miR-130a. This was confirmed experimentally by demonstrating that transient overexpression of miR-130a results in reduction in the amount of Smad4 protein.

Luciferase reporter constructs with the 3'-UTR of Stnad4 also respond to miR-130a - an effect that is abolished by point mutations in the miRNA-binding site. In agreement, we observed that stable overexpression of miR-130a in a granulocytic cell line reduces the level of Smad4 protein, and render the cells less sensitive to TGF-beta-induced growth inhibition.

This was also confirmed with cell cycles analysis. Furthermore, the effect was diminished when transfecting the same clones with SMAD4 lacking the 3-UTR. In line with our hypothesis, the most immature granulocyte precursors demonstrate the highest expression of miR-130a is highest, and the lowest. expression of Smad4 protein.

As the granulocyte precursors mature, the expression of miR-130a decreases dramatically whereas the level of Smad4 protein expression increases demonstrating inverse correlation between miR-130a and Smad4 protein. The level of Stnad4 mRNA is comparable at all stages of granulopoiesis. High miR-130a levels and low or no expression of Smad4 was found in primary cells from patients with acute myeloid leukemia and in a cell line (Kasumi-1) with the t(8:21)(q22;q22) chromosomal translocation.

The level of Smad4 increased in Kasumi-1 cells when the endogenous level of miR-130a was inhibited by anti-miR-130a LNA. Our data indicate that miR-130a is involved in cell cycle regulation of normal and malignant granulocytic cells through engagement of Smad4 in the TGF-beta-pathway. Grant acknowledgment: Lundbeck foundation, Carlsberg foundation, Swedish Research Council.