SIK (Salt-inducible kinase): regulation of ACTH-mediated steroidogenic gene expression and nuclear/cytosol redistribution.
Possible involvement of salt-inducible kinase ( SIK), a serine/threonine protein kinase first cloned from high K+-diet treated rat adrenal glands, in the regulation of steroidogenesis was investigated. Y-1 cells, when treated with ACTH, underwent a rapid change in SIK's mRNA content. It reached the maximum within a few hours and returned to the base after 8 h. In contrast, the levels of mRNAs for CYP11A and StAR protein reached the maxima after 8 h. The SIK's mRNA induction failed to occur in ACTH-, forskolin- or 8-Br-cAMP-treated Kin-7 cells, a mutant cell line of Y-1 with defective cAMP-dependent protein kinase A (PKA). Y-1 cells that overexpress SIK, when treated with ACTH, had significantly repressed levels of mRNAs for CYP11A and StAR protein. Therefore, SIK might have a negative effect on the CYP11A- and StAR protein-gene expression in the early phase of ACTH-mediated steroidogenesis. To further explore the mechanisms underlying this phenomenon, we examined intracellular distribution of the green fluorescence protein (GFP)-tagged SIK. When GFP- SIK was introduced into HeLa cells, the fluorescent signals were detected in the nucleus. In Y-1 cells GFP- SIK was detected both in the nucleus and cytosol, and the signal in the former moved to the latter after ACTH-treatment. The nuclear/cytosol re-distribution of GFP- SIK was also observed in forskolin- or 8-Br-cAMP-treated Y-1 cells, but not in Kin-7 cells. These results suggest that the intracellular re-distribution of SIK in Y-1 cells may depend on the cAMP/PKA signaling pathway and has an important regulatory role in the ACTH-mediated steroidogenic gene expression.[1]References
- SIK (Salt-inducible kinase): regulation of ACTH-mediated steroidogenic gene expression and nuclear/cytosol redistribution. Lin, X., Takemori, H., Doi, J., Katoh, Y., Okamoto, M. Endocr. Res. (2000) [Pubmed]
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