LMO7 Mediates Cell-Specific Activation of the Rho-Myocardin-Related Transcription Factor-Serum Response Factor Pathway and Plays an Important Role in Breast Cancer Cell Migration

Abstract

Serum response factor (SRF) is a ubiquitously expressed transcription factor that regulates cell-specific functions such as muscle development and breast cancer metastasis. The myocardin-related transcription factors (MRTFs), which are transcriptional coactivators mediating cell-specific functions of SRF, are also ubiquitously expressed. How MRTFs and SRF drive cell-specific transcription is still not fully understood. Here we show that LIM domain only 7 (LMO7) is a cell-specific regulator of MRTFs and plays an important role in breast cancer cell migration. LMO7 activates MRTFs by relieving actin-mediated inhibition in a manner that requires, and is synergistic with, Rho GTPase. Whereas Rho is required for LMO7 to activate full-length MRTFs that have three RPEL actin-binding motifs, the disruption of individual actin-RPEL interactions is sufficient to eliminate the Rho dependency and to allow the strong Rho-independent function of LMO7. Mechanistically, we show that LMO7 colocalizes with F-actin and reduces the G-actin/F-actin ratio via a Rho-independent mechanism. The knockdown of LMO7 in HeLa and MDA-MB-231 cells compromises both basal and Rho-stimulated MRTF activities and impairs the migration of MDA-MB-231 breast cancer cells. We also show that LMO7 is upregulated in the stroma of invasive breast carcinoma in a manner that correlates with the increased expression of SRF target genes that regulate muscle and actin cytoskeleton functions. Together, this study reveals a novel cell-specific mechanism regulating Rho-MRTF-SRF signaling and breast cancer cell migration and identifies a role for actin-RPEL interactions in integrating Rho and cell-specific signals to achieve both the synergistic and Rho-dependent activation of MRTFs.

ACKNOWLEDGMENTS

This work was supported by funding from the University of Cincinnati College of Medicine and the Department of Cancer and Cell Biology (to J.Z.) and National Institutes of Health grants R01HL093195-01A1 (to J.Z.) and U54RR025216 (to B.J.A.). FACS data were acquired using equipment maintained by the Research Flow Cytometry Core in the Division of Rheumatology at Cincinnati Children's Hospital Medical Center, supported in part by NIH grant AR-47363.

We thank Ron Prywes, Deniz Toksoz, and Andrew Sharrocks for plasmids; Sohaib Khan, Jerry Lingrel, and Peter Stambrook for critical reading and helpful comments; and Birgit Ehmer for technical help.