Hyperosmotic stress regulates the distribution and stability of myocardin-related transcription factor, a key modulator of the cytoskeleton.
Hyperosmotic stress initiates several adaptive responses, including the remodeling of the cytoskeleton. Besides maintaining structural integrity, the cytoskeleton has emerged as an important regulator of gene transcription. Myocardin-related transcription factor (MRTF), an actin-regulated coactivator of serum response factor, is a major link between the actin skeleton and transcriptional control. We therefore investigated whether MRTF is regulated by hyperosmotic stress. Here we show that hypertonicity induces robust, rapid, and transient translocation of MRTF from the cytosol to the nucleus in kidney tubular cells. We found that the hyperosmolarity-triggered MRTF translocation is mediated by the RhoA/Rho kinase (ROK) pathway. Moreover, the Rho guanine nucleotide exchange factor GEF-H1 is activated by hyperosmotic stress, and it is a key contributor to the ensuing RhoA activation and MRTF translocation, since siRNA-mediated GEF-H1 downregulation suppresses these responses. While the osmotically induced RhoA activation promotes nuclear MRTF accumulation, the concomitant activation of p38 MAP kinase mitigates this effect. Moderate hyperosmotic stress (600 mosM) drives MRTF-dependent transcription through the cis-element CArG box. Silencing or pharmacological inhibition of MRTF prevents the osmotic stimulation of CArG-dependent transcription and renders the cells susceptible to osmotic shock-induced structural damage. Interestingly, strong hyperosmolarity promotes proteasomal degradation of MRTF, concomitant with apoptosis. Thus, MRTF is an osmosensitive and osmoprotective transcription factor, whose intracellular distribution is regulated by the GEF-H1/RhoA/ROK and p38 pathways. However, strong osmotic stress destabilizes MRTF, concomitant with apoptosis, implying that hyperosmotically induced cell death takes precedence over epithelial-myofibroblast transition, a potential consequence of MRTF-mediated phenotypic reprogramming.
Keenan Research Centre in the Li Ka Shing Knowledge Institute, St. Michael’s Hospital and Department of Surgery, University of Toronto, Ontario, Canada.
Donald L
DL
Ly
Faiza
F
Waheed
Monika
M
Lodyga
Pam
P
Speight
András
A
Masszi
Hiroyasu
H
Nakano
Maria
M
Hersom
Stine F
SF
Pedersen
Katalin
K
Szászi
András
A
Kapus
D021581:Active Transport, Cell Nucleus / Q000502:physiology*; D000818:Animals; D017209:Apoptosis / Q000502:physiology; D002460:Cell Line; D003599:Cytoskeleton / Q000502:physiology*; D005786:Gene Expression Regulation / Q000502:physiology; D020868:Gene Silencing / Q000502:physiology; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000502:physiology; D006982:Hypertonic Solutions; D007684:Kidney Tubules / Q000502:physiology; D020935:MAP Kinase Signaling System / Q000502:physiology; D009687:Nuclear Proteins / Q000378:metabolism*; D009997:Osmotic Pressure / Q000502:physiology*; D011401:Promoter Regions, Genetic; D046988:Proteasome Endopeptidase Complex / Q000502:physiology; D055550:Protein Stability; D013312:Stress, Physiological; D013552:Swine; D015534:Trans-Activators / Q000378:metabolism*; D014157:Transcription Factors / Q000378:metabolism*; D054460:rho-Associated Kinases / Q000502:physiology; D000099251:Myocardin
D020662:Guanine Nucleotide Exchange Factors; D006982:Hypertonic Solutions; D009687:Nuclear Proteins; D046988:Proteasome Endopeptidase Complex; D015534:Trans-Activators; D014157:Transcription Factors; D054460:rho-Associated Kinases; D000099251:Myocardin