MEK/ERK signaling controls osmoregulation of nucleus pulposus cells of the intervertebral disc by transactivation of TonEBP/OREBP.
Earlier studies have shown that intervertebral disc cells express TonEBP, a transcription factor that permits adaptation to osmotic stress and regulates aggrecan gene expression. However, the mechanism of hyperosmotic activation of TonEBP in disc cells is not known. Results of this study show that hypertonic activation of ERK signaling regulates transactivation activity of TonEBP, modulating its function.


BACKGROUND
In an earlier report, we showed that tonicity enhancer binding protein (TonEBP) positively regulates aggrecan gene expression in disc cells, thereby autoregulating its osmotic environment. Although these studies indicated that the cells of the nucleus pulposus were optimally adapted to a hyperosmotic state, the mechanism by which the cells transduce the osmotic stress was not delineated. The primary goal of this study was to test the hypothesis that, in a hyperosmotic medium, the extracellular signal-regulated kinase (ERK) signaling pathway regulated TonEBP activity.


METHODS
Nucleus pulposus cells were maintained in isotonic or hypertonic media, and MAPK activation and TonEBP expression were analyzed. To study the role of MAPK in regulation of TonEBP function, gel shift and luciferase reporter assays were performed. ERK expression in cells was modulated by using expression plasmids or siRNA, and transactivation domain (TAD)-TonEBP activity was studied.


RESULTS
We found that hypertonicity resulted in phosphorylation and activation of ERK1/2 proteins and concomitant activation of C terminus TAD activity of ELK-1, a downstream transcription factor. In hypertonic media, treatment with ERK and p38 inhibitors resulted in downregulation of TonE promoter activity of TauT and HSP-70 and decreased binding of TonEBP to TonE motif. Similarly, forced expression of DN-ERK and DN-p38 in nucleus pulposus cells suppressed TauT and HSP-70 reporter gene activity. Finally, we noted that ERK was needed for transactivation of TonEBP. Expression of DN-ERK significantly suppressed, whereas, WT-ERK and CA-MEK1 enhanced, TAD activity of TonEBP. Experiments performed with HeLa cells indicated that the ERK signaling pathway also served a major role in regulating the osmotic response in nondiscal cells.


CONCLUSIONS
Together, these studies showed that adaptation of the nucleus pulposus cells to their hyperosmotic milieu is dependent on activation of the ERK and p38- MAPK pathways acting through TonEBP and its target genes.
Tsai
Tsung-Ting
TT
Department of Orthopaedic Surgery, Thomas Jefferson University, Philadelphia, Pennsylvania19107, USA.
Guttapalli
Asha
A
Agrawal
Amit
A
Albert
Todd J
TJ
Shapiro
Irving M
IM
Risbud
Makarand V
MV
D000818:Animals; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism*; D017930:Genes, Reporter; D006367:HeLa Cells; D006801:Humans; D006982:Hypertonic Solutions; D007403:Intervertebral Disc / Q000166:cytology / Q000201:enzymology*; D020935:MAP Kinase Signaling System; D008562:Membrane Glycoproteins / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000378:metabolism; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism; D048052:Mitogen-Activated Protein Kinase 3 / Q000378:metabolism; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism*; D051381:Rats; D014157:Transcription Factors / Q000235:genetics*; D015533:Transcriptional Activation / Q000235:genetics*; D014882:Water-Electrolyte Balance; D000099247:Taurine Transporters
D048049:Extracellular Signal-Regulated MAP Kinases; D006982:Hypertonic Solutions; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020929:Mitogen-Activated Protein Kinase Kinases; D014157:Transcription Factors; D000099247:Taurine Transporters; C492199:Nfat5 protein, rat