Consistent with the selective effect of SP600125 on gene expression, global histone H3 Ser10 dephosphorylation certainly does not trigger global repression Bergenin of gene expression. The expression of other genes involved in cholesterol homeostasis did not show any decrease despite the global reduction in phospho histone H3 Ser10 following SP600125 treatment. Although a strong correlation between histone H3 dephosphorylation and selective transcriptional repression of the LDL receptor expression suggests that the two processes are mechanistically linked, the exact mechanisms involved for this type of selectivity are still unclear. The mechanism may involve different kinase lipid and or protein microenvironments resulting from differences in the transcription factors bound to the promoter, inherent nucleosome positioning, density, or stability, the accessibility of the promoter to the transcriptional machinery caused by the participation of chromatin remodeling machinery, or the nature of the local histone H3 Ser10 kinase.
We favor the last possibility for our results, as several kinases have been implicated as potential histone H3 Ser10 kinase in mammalian cells and are activated by distinct pathways in a cell type specific NVP-ADW742 manner. These kinases include the MAPKs, p90RSK2, MSK 1 2, cyclic AMP dependent protein kinase A, I B kinase, protein kinase C, mixed lineage triple kinase alpha, p21 activated kinase 1, and Fyn. The kinase mediating SP600125 dependent histone H3 Ser10 dephosphorylation remains to be characterized. It is interesting to note that there is precedence for the promoter specific effects of histone H3 Ser10 phosphorylation in both S. cerevisiae and Drosophila.
Future work will examine whether there are selective histone H3 Ser10 phosphorylation sites that are marked for transcriptional regulation and targeted by the SP600125 sensitive kinase in a promoter specific manner. In conclusion, this is the first report that demonstrates SP600125 as an effective inhibitor of histone H3 Ser10 phosphorylation. The ability to easily manipulate this modification in vivo by use of SP600125 provides an excellent strategy for elucidating the histone code and to assess the functional role of this modification in various cellular events. Our discovery that histone H3 Ser10 phosphorylation possibly regulates the basal expression of selective genes by modulating their accessibility to the transcription factor is unprecedented and points to this modification as a unique regulatory mechanism for transcription initiation.
A better understanding of the underlying mechanisms will help guide the development of more effective agents to treat hypercholesterolemia and cancer. Macrophages may be crucial in inflammatory diseases including chronic obstructive pulmonary disease , atherosclerosis, arthritis, and tubulointerstitial kidney disease via the release of pro inflammatory mediators. Macrophages additionally contribute to tissue destruction by the release of several proteases including matrix metalloproteinase 9. MMP 9 is a protease that degrades extracellular matrix proteins including gelatin, collagen, elastin, and laminin. In addition, MMP 9 modulates the activities of other proteases, growth factors, cytokines, and chemokines through proteolytic cleavage.