Steven J. Collins

Molecular Genetics of Leukemia:
CaMKIIgamma activation in the pathogenesis of human myeloid leukemia

Retinoids and in particular retinoic acid (RA) influence the growth and differentiation of a wide variety of different cell types and are effective chemopreventive and therapeutic agents for certain subsets of human malignancies. The biologic effects of RA are mediated through specific nuclear receptors that function as ligand-dependent transcription factors. RA induces terminal granulocytic differentiation of human acute promyelocytic leukemia (APL) cells and is an effective therapeutic agent for this disease. One goal of our laboratory is to dissect the molecular mechanisms involved in this RA-induced differentiation of human leukemia cells and to define the role that RA and RA receptors play in normal hematopoiesis.

We are particularly interested in determining why only a subset of acute myeloid leukemias (i.e. the APLs) exhibit a dramatic differentiative response to retinoids while the great majority of AML patients do not. Utilizing different growth factor dependent hematopoietic cell lines representing distinct hematopoietic lineages we have found significant differences in the activation of RA receptors in these different hematopoietic cell types. In exploring the molecular basis for this phenomenon we have discovered several important coregulators of retinoic acid receptor activity including Stat family members (particularly Stat5), which bind to RA receptors in a cytokine-dependent manner and enhance their transcriptional activity, as well as the Calcium/calmodulin dependent protein kinases (CaM kinases), which directly phosphorylate the RA receptors and inhibit their transcriptional activity.

Our recent observations indicate that a particular CaMkinase (i.e. CaMKIIgamma) is a critical regulator of myeloid leukemia cell proliferation. The activated (autophosphorylated) CaMKII is present in 100% of myeloid leukemia cell lines and in approximately 50-60% of primary patient AML cells. Levels of the activated CaMKII are markedly diminished in leukemia cells undergoing terminal differentiation /apoptosis. Inhibiting CaMKII utilizing pharmacological agents, dominant negative constructs or shRNA vectors inhibits myeloid leukemia cell proliferation, and this is associated with the inactivation / downregulation of a number of important phosphoprotein signaling networks involving the MAP kinase, JAK/Stat and GSK3beta / beta-catenin pathways. Our current experimental efforts are focused on: (1) identifying the critical substrates of CaMKII in myeloid leukemia cells; (2) defining the molecular mechanisms that regulate the calcium-mediated CaMKII activation in myeloid leukemia; (3) determining the clinicopathological significance of activated CaMKII in primary human AML cells; and (4) identifying small molecule inhibitors of the activated CaMKII that potentially could be useful in the therapy of myeloid leukemia.