ABSTRCT In mammals protein tyrosine phosphorylation plays an important role in the activation of programmed cell death. However, tyrosine phosphorylation involved in cell death has not been examined in plants. These studies demonstrated that D-mannose induced cell death and DNA fragmentation in rice suspension cells. In the presence of mannose for 24 hours, tyrosine phosphorylation of two proteins, 20 kDa and 43 kDa markedly increased. After incubating 3 days, the level of phosphotyrosine accumulation declined in bands of 16 and 20 kDa. In addition, the occurrence of DNA fragmentation and two tyrosine-phosphorylated proteins, 26 kDa and 40 kDa, were detected in aged suspension-cultured cells. The expression of genes that encode mitogen-activated protein kinase (MAPK) kinase (MEK)/MAPK signalling pathway, OsMEK and OsMAPK2, are up regulated during D-mannose treatment. The results provide evidence that protein tyrosine phosphorylation as well as MEK/MAPK signalling pathway is associated with cell death in rice.

Heart failure induced by cardiac hypertrophy is a cause of high mortality in the world and has been the fastest growing cardiovascular disease over the past decade. Cardiac hypertrophy is characterised as a reactive increase in cardiac mass growth with a complex of ventricular remodelling. It occurs initially as a compensatory response to an increased workload but eventually leads to cardiac dysfunction. An in-depth understanding of cardiac hypertrophy and the capacity to regulate it has profound clinical implications. The MAPK pathways provide an important connection between external stimuli and intracellular signals for cardiac hypertrophic response. At least four MAPK subfamilies have been identified: extracellular-regulated protein kinases 1 and 2 (ERK1/2), ERK5, c-Jun NH2-terminal protein kinases (JNKs) and p38 MAPKs. Mitogen-activated protein kinase kinase 4 (MKK4), a vital activator of JNK and p38 is implicated as an important mediator of hypertrophy. ERK5, an atypical MAPK, is also involved in both hypertrophic growth and cardiomyocyte survival. However, conflicting data have been yielded from previously-published studies, since the results are based entirely on experiments conducted in cultured cardiomyocytes or transgenic and conventional knockout mouse models. To elucidate their biological roles and underlying signalling mechanisms in hypertrophy, mice with a cardiomyocyte-specific deletion of MKK4 or ERK5 (MKK4cko and ERK5cko mice) were generated in the present study. In response to pathological hypertrophic stresses including pressure overload or isoprenaline stimulation, MKK4cko mice developed exacerbated pathological hypertrophy with increased cardiomyocyte apoptosis, impaired cardiac function and remarkably upregulated NFAT (nuclear factor of T-cell) transcriptional activity. However, MKK4cko mice exhibited a similar extent of swimming exercise-induced physiological hypertrophy compared with the controls. In response to pathological hypertrophic stimuli, ERK5cko mice were resistant to hypertrophic growth, foetal gene induction and ventricular fibrosis, which is associated with repressed activation of MEF2 (myocyte enhancer factor 2). ERK5 deficiency also caused a profound increase in cardiomyocyte apoptosis which accounted for the impaired cardiac function. In conclusion, the present study provides biological evidence that clarifies in vivo functions of MKK4 and ERK5 in hypertrophy. MKK4 acts a protective role against pathological hypertrophy through inhibiting NFAT signalling, but it is not necessary for the regulation of physiological hypertrophy. ERK5 is essential for pathological hypertrophic remodelling and cardiomyocyte survival and its function in hypertrophic remodelling is mediated through regulation of MEF2 activity. Taken together, these data presented in my thesis advances knowledge about biological functions of MAPK pathways in the heart.