The microphthalmia transcription factor (MITF) is required for terminal differentiation of developmentally unrelated cell types including osteoclasts, mast cells, pigmented retinal epithelial cells and melanocytes and regulates distinct target genes in each of these cell types. During terminal differentiation of osteoclasts, MITF regulates a set of genes that are required for bone resorption, including Cathepsin K (Ctsk), the chloride channel Ccln7, Ostm1/grey lethal, and tartrate resistant acid phosphatase/acid phosphatase 5 (Trap/Acp5). Mutations in this set of genes are associated with osteoclast dysfunction and bone disorders in humans and mice, strongly arguing that MITF plays a central role in gene regulation during terminal osteoclast differentiation. Our work has demonstrated that interactions between MITF and the ETS-family factor PU.1 are necessary to selectively regulate this set of target genes in osteoclasts. MITF is also a direct target of CSF-1/RANKL signaling during osteoclast differentiation, activated directly by Mitogen Activated Protein Kinases (MAPK), Erk and p38. In the current grant period, our work has revealed interacting proteins that suggest potential mechanisms underlying ITF/PU.1 action in osteoclasts. Unexpectedly, in committed osteoclast progenitors treated with CSF-1 alone, the MITF/PU.1 complex interacts with the repressor Eos and co-repressor complexes to suppress the expression of osteoclast target genes. In the presence of both CSF-1 and RANKL, the Eos/co-repressor complexes are replaced by complexes that contain p38 MAPK, the co-activator CBP/p300, and the BRG-1 chromatin remodeling complex. Subsequent to these events, the transcription factor NFATc1 is recruited to the target promoters. This data leads to our overall hypothesis: MITF/PU.1 complexes act in committed myeloid progenitors as integrators of signals encountered in the bone microenvironment to effect changes in the expression of genes essential for osteoclast function.
This work will define mechanisms by which gene expression patterns are regulated in committed myeloid progenitors before signals from the bone microenvironment trigger differentiation into specific cell types. This is a key problem not only in osteoclast differentiation, but is a problem of general biological interest. In addition, these studies may have direct applications to significant human diseases. In particular, osteoporosis in post-menopausal women and the osteolytic bone destruction that occurs in patients with multiple myeloma, breast cancer and prostate cancer are examples of clinical conditions where this research may have potential impact. The identification of molecular targets in cancer cells and the development of pharmacological agents that selectively interfere with the action of these targets have been long term goals in cancer research that have finally started to pay dividends. A similar strategy in bone diseases should allow for the rational design of agents that interfere with specific molecular targets. The genetics and molecular biology of MITF suggests that the MITF pathway may provide molecular targets for certain bone disorders.
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