Glucocorticoids inhibit induced and non-induced mRNA accumulation of genes encoding hyaluronan synthases (HAS): hydrocortisone inhibits HAS1 activation by blocking the p38 mitogen-activated protein kinase signalling pathway.

OBJECTIVE: Glucocorticoids are still a mainstay in the treatment of rheumatoid arthritis (RA). Unfettered hyaluronan release is a hallmark of RA. The discovery of three genes encoding hyaluronan synthase (HAS) led us to investigate the effect of hydrocortisone and dexamethasone on the activation of these genes at the molecular level and, at least in part, the mode of action of these drugs. METHODS: Reverse transcription-polymerase chain reaction (RT-PCR) was used to monitor levels of HAS1, HAS2, and HAS3 mRNAs in cultured fibroblast-like synoviocytes (FS) and in leucocytes isolated from synovial fluid of RA patients. Western blot experiments were used to investigate the effect of hydrocortisone on transforming growth factor beta (TGF-beta)-induced activation of the p38 mitogen-activated protein kinase (MAPK) pathway. RESULTS: Hydrocortisone and dexamethasone suppressed HAS2 and HAS3 mRNAs accumulation concentration-dependently. Contrary to HAS2 and HAS3, HAS1 in FS was not constitutively activated. When cells were stimulated with TGF-beta, a potent activator of HAS1 mRNA transcription, treating them with hydrocortisone suppressed induced activation of HAS1 in a concentration- and time-dependent manner. Similar suppressive effects of hydrocortisone were observed when leucocytes isolated from synovial fluid of inflamed joints were used instead of cultured FS. Furthermore, western blot experiments confirmed that hydrocortisone blocked TGF-beta-induced phosphorylation of p38 MAPK, a kinase essential for TGF-beta-induced HAS activation. CONCLUSION: Our data demonstrate that glucocorticoids suppress all genes encoding hyaluronan. We speculate that inhibition of HAS genes might account for the beneficial effect of glucocorticoid treatment, and also for the detrimental effects of long-term use.

A MAP kinase is activated late in plant mitosis and becomes localized to the plane of cell division.

In eukaryotes, mitogen-activated protein kinases (MAPKs) are part of signaling modules that transmit diverse stimuli, such as mitogens, developmental cues, or various stresses. Here, we report a novel alfalfa MAPK, Medicago MAP kinase 3 (MMK3). Using an MMK3-specific antibody, we detected the MMK3 protein and its associated activity only in dividing cells. The MMK3 protein could be found during all stages of the cell cycle, but its protein kinase activity was transient in mitosis and correlated with the timing of phragmoplast formation. Depolymerization of microtubules by short treatments with the drug amiprophosmethyl during anaphase and telophase abolished MMK3 activity, indicating that intact microtubules are required for MMK3 activation. During anaphase, MMK3 was found to be concentrated in between the segregating chromosomes; later, it localized at the midplane of cell division in the phragmoplast. As the phragmoplast microtubules were redistributed from the center to the periphery during telophase, MMK3 still localized to the whole plane of division; thus, phragmoplast microtubules are not required to keep MMK3 at this location. Together, these data strongly support a role for MMK3 in the regulation of plant cytokinesis.