Steven J. Collins

Molecular Genetics of Leukemia:
Retinoic acid receptor activation in normal and malignant cells.

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 types 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 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. We have found that small molecule inhibitors of the CaM kinases will enhance RAR transcriptional activity and trigger the differentiation of myeloid leukemia cells in vitro. In addition we have observed that CaMKII is involved in regulating the proliferation of a wide variety of myeloid leukemia cells by regulating several important signal transduction pathways including the MAP kinase, JaK/Stat and GSK3beta/ beta catenin pathways. We are attempting to identify small molecules that will specifically inhibit the activated CaMKII which may prove useful in inhibiting the proliferation of myeloid leukemia cells.

In studies assessing how retinoic acid receptors may regulate normal hematopoietic differentiation we have made the unexpected observation that retinoic acid enhances the generation of both short and long term marrow repopulating cells in liquid suspension cultures of primitive hematopoietic precursors. This appears to be regulated through the RAR gamma isoform rather than the RARalpha isoform. We are determining whether this observation has specific clinical value either for the ex vivo expansion of hematopoietic stem cells or in retroviral vector-mediated gene therapy of hematopoietic stem cell disorders.