Abstract 952: Activation of PKN Causes Cardiac Hypertrophy and Resistance to Ischemia-Reperfusion.
PKN is a serine/threonine kinase whose catalytic domain is homologous to that of protein kinase C. PKN upregulates ANF, a fetal type gene, expression in cardiac myocytes in vitro. However, the function of PKN is poorly understood in the heart. In yeast, Pkc1, whose N-terminal regulatory region is highly homologous to PKN, is activated by hypotonic stress (HS). Ischemia-reperfusion (I/R) induces hypotonic cell swelling, which is known to affect survival/ death of cardiac myocytes. We examined whether PKN is activated by HS in cardiac myocytes in vitro, and by I/R in the heart in vivo. We constructed transgenic mice with cardiac specific overexpression of constitutively active (CA) PKN (Tg-CAPKN) and examined whether activation of PKN has a protective effect in the heart. In neonatal rat ventricular myocytes, PKN was activated by HS (67% osmolarity) within 15 min. Activation of PKN by HS reached a peak within 60 min, which was accompanied by Thr774 phosphorylation. Phosphorylation of PKN was also observed in the heart subjected to I/R in vivo. In Tg-CAPKN, left ventricular (LV) weight to body weight ratio and LV wall thickness were greater than in non-transgenic mice (NTg), indicating that PKN stimulates cardiac hypertrophy. Echocardiographic analysis indicated that cardiac function was comparable between Tg-CAPKN and NTg. In NTg hearts, 45 min ischemia followed by 24 hr reperfusion caused myocardial infarction (MI), while the size of MI was significantly smaller in Tg-CAPKN hearts (NTg vs. Tg-CAPKN, 37.5±4.9 vs. 15.1±5.2%, p<0.01). The number of TUNEL positive nuclei in the I/R heart was also significantly smaller in Tg-CAPKN (0.96±0.24% vs. 0.30±0.15%, p<0.05). In Tg-CAPKN hearts, phosphorylation and membrane translocation of heat shock proteins, such as alphaB crystallin and HSP25, were greater than in NTg hearts. Adenovirus harboring a dominant negative form of PKN aggravated cell death induced by H2O2 in cardiac myocytes in vitro. These results suggest that PKN is activated by HS in cardiac myocytes and I/R in the mouse heart. Furthermore, activation of PKN causes well compensated LV hypertrophy and plays cell protective roles in hearts under stress in vivo. Finally, PKN is essential for survival of cardiac myocytes against stresses.