FOXC1 Transcriptional Regulatory Activity Is Impaired by PBX1 in a Filamin A-Mediated Manner

Abstract

FOXC1 mutations underlie Axenfeld-Rieger syndrome, an autosomal dominant disorder that is characterized by a spectrum of ocular and nonocular phenotypes and results in an increased susceptibility to glaucoma. Proteins interacting with FOXC1 were identified in human nonpigmented ciliary epithelial cells. Here we demonstrate that FOXC1 interacts with the actin-binding protein filamin A (FLNA). In A7 melanoma cells possessing elevated levels of nuclear FLNA, FOXC1 is unable to activate transcription and is partitioned to an HP1α, heterochromatin-rich region of the nucleus. This inhibition is mediated through an interaction between FOXC1 and the homeodomain protein PBX1a. In addition, we demonstrate that efficient nuclear and subnuclear localization of PBX1 is mediated by FLNA. Together, these data reveal a mechanism by which structural proteins such as FLNA can influence the activity of a developmentally and pathologically important transcription factor such as FOXC1. Given the resemblance of the skeletal phenotypes caused by FOXC1 loss-of-function mutations and FLNA gain-of-function mutations, this inhibitory activity of FLNA on FOXC1 may contribute to the pathogenesis of FLNA-linked skeletal disorders.

ACKNOWLEDGMENTS

We thank D. A. Underhill, D. M. Glerum, R. Rachubinski, and members of the Ocular Genetics Laboratory for critical reading of the manuscript. We thank T. Stossel and Y. Ohta for providing the M2 and A7 cells lines. We also thank May Yu for technical assistance.

This work was supported by operating grants awarded to M.A.W. from the Canadian Institutes for Health Research (CIHR). F.B.B. is supported by a CIHR postdoctoral fellowship. M.A.W. is an AHFMR senior scholar and a CIHR investigator.