Abstract 1427: Differential Activation of Protein Kinase C (PKC) Isoforms and Mitogen-Activated Protein Kinases (MAPKs) in Diabetic Heart Lacking Osteopontin
We have shown that osteopontin (OPN) plays a deleterious role in streptozotocin-induced diabetic cardiomyopathy with effects on cardiac hypertrophy, fibrosis and cardiac myocyte apoptosis. The objective of this study is to analyze the molecular signals involved in OPN-mediated cardiac remodeling following induction of diabetes.
Methods and Results: Diabetes was induced in a group of wild type (WT) and OPN knockout (KO) mice by a single intraperitoneal injection of streptozotocin (STZ; 150 mg/kg). Phosphorylation (activation) of total PKC, PKC-β2, PKC-ζ and MAPKs (ERK and JNK) and expression of PPAR-γ, atrial natriuretic factor (ANF) and transforming growth factor β1 (TGF-β1) was studied in the heart after 60 days of STZ injection. Induction of diabetes increased phosphorylation of PKC in both diabetic (D) hearts. However, the phosphorylation of PKC was significantly higher in KO-D hearts as compared to WT-D hearts (Fold change vs. WT-sham, KO-sham, 2.1±0.5*; WT-D, 4.2±0.9*; KO-D, 6.6±1.2*#, *p<0.05 vs. WT-sham; #p<0.05 vs. WT-D; n=6). Increased phosphorylation of PKC-β2 (shown to induce hypertrophy and apoptosis) was only observed in WT-D (Fold change vs. WT-sham, KO-sham, 1.4± 0.3; WT-D, 2.2±0.5*; KO-D, 1.0 ± 0.2#, *p<0.05 vs. WT-sham; #p<0.05 vs. WT-D; n=6), while increased phosphorylation of PKC-ζ (anti-apoptotic molecule) was only observed in KO-D (p<0.05 vs. WT-D; n=6) hearts. Phosphorylation of JNK (pro-apoptotic molecule) was significantly lower, while phosphorylation of ERK1/2 (anti-apoptotic molecule) was significantly higher (p<0.05) in KO-D group. Levels of PPAR-γ (shown to attenuate cardiac hypertrophy, fibrosis and apoptosis) were significantly decreased in WT-D (p<0.05 vs WT-sham and KO-D; n=7), not in KO-D group. The expression of ANF (hypertrophy-related gene) and TGF-β1 (marker of fibrosis) was significantly higher in WT-D heart as compared to KO-D heart (p<0.05 vs. KO-D; n=4) as measured by RT-PCR.
Conclusion: Lack of OPN preserves function of the diabetic heart, at least in part, via the activation of PKC-ζ and ERK1/2, and maintaining levels of PPAR-γ.