Effects of IGF-1 on IK and IK1 Channels via PI3K/Akt Signaling in Neonatal Cardiac Myocytes

Previous studies suggest that sarcolemmal potassium currents play important roles in cardiac hypertrophy. IGF-1 contributes to cardiac hypertrophy via activation of PI3K/Akt signaling. However, the relationships between IGF-1, PI3K/Akt signaling and sarcolemmal potassium currents remain unknown. Therefore, we tested the hypothesis that IGF-1 and PI3K/Akt signaling, independently, decrease sarcolemmal potassium currents in cardiac myocytes of neonatal rats. We compared the delayed outward rectifier (IK) and the inward rectifier (IK) current densities resulting from IGF-1 treatments to those resulting from simulation of PI3K/Akt signaling using adenoviral (Ad) BD110 and wild-type Akt and to those resulting from inhibition of PI3K signaling by LY294002. Ad.BD110 and Ad.Akt decreased IK and these decrements were attenuated by LY 294002. The IGF-1 treatments decreased both IK and IK1 but only the IK decrement was attenuated by LY294002. These findings demonstrate that IGF-1 may contribute to cardiac hypertrophy by PI3K/Akt signal transduction mechanisms in neonatal rat cardiomyocytes. Failure of LY294002 to effectively antagonize IGF-1 induced decrements in IK1 suggests that a signal pathway adjunct to PI3K/Akt contributes to IGF-1 protection against arrhythmogenesis in these myocytes. Our findings imply that sarcolemmal outward and inward rectifier potassium channels are substrates for IGF-1/PI3K/Akt signal transduction molecules

Cross-Talk Between MAPKs and P-3K Pathways Alters the Functional Density of I\u3csub\u3eK\u3c/sub\u3e Channels in Hypertrophied Hearts

Mitogen activated protein kinases (MAPK), such as ERK1/2 and p38 MAPK and phosphatidylinositol-3 phosphate kinase (PI-3K) play a major role in the development of cardiac hypertrophy. Recently, we have shown their crucial role in the regulation of the myocardial function through their effects on crucial ion channels. It is the focus of this study to resolve the interaction between these pathways and its implication on the function of the normal and hypertrophied cardiomyocytes. To that end, we created arteriovenous fistula in the adult rat that developed volume-overload eccentric cardiac hypertrophy over a 3-week period. We measured the relative activity of ERK1/2, p38 MAPKs and Akt through western blot analysis and assessed the functional density of the outward rectifier potassium current (IK) using the patch-clamp technique. The results showed a mutual negative autoregulation between ERK1/2 and p38 in normal cardiomyocytes, which disappears during cardiac hypertrophy. In addition, PI-3K seems to assume a greater role in mediating IGF-1 effects on the MAPKs during cardiac hypertrophy. This was also relevant to IK functional density which was reduced by activation of both MAPKs and Akt by angiotensin II (ANG II) and insulin-like growth factor-1 (IGF-1), respectively; however, this reduction was reversed by inhibition of PI-3K alone in hypertrophied myocytes but not in normal ones. This raises an important implication relative to the role of IGF-1-dependent activation of PI-3K, which may translate into a differential prognostic for cardiac hypertrophy among ethnic groups. This is true in African Americans, having higher circulating IGF-1 levels, and especially true for the athletes among them

Basal and IGF-I-dependent regulation of potassium channels by MAP kinases and PI3-kinase during eccentric cardiac hypertrophy

The potassium channels IK and IK1, responsible for the action potential repolarization and resting potential respectively, are altered during cardiac hypertrophy. The activation of insulin-like growth factor-I (IGF-I) during hypertrophy may affect channel activity. The aim was to examine the modulatory effects of IGF-I on IK and IK1 through mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) pathways during hypertrophy. With the use of specific inhibitors for ERK1/2 (PD98059), p38 MAPK (SB203580) and PI3K/Akt (LY294002), Western blot and whole cell patch-clamp were conducted on sham and aorto-caval shunt-induced hypertrophy adult rat myocytes. Basal activation levels of MAPKs and Akt were increased during hypertrophy. Acute IGF-I (10−8 M) enhanced basal activation levels of these kinases in normal hearts but only those of Akt in hypertrophied ones. IK and IK1 activities were lowered by IGF-I. Inhibition of ERK1/2, p38 MAPK, or Akt reduced basal IK activity by 70, 32, or 50%, respectively, in normal cardiomyocytes vs. 53, 34, or 52% in hypertrophied ones. However, basal activity of IK1 was reduced by 45, 48, or 45% in the former vs. 63, 43, or 24% in the latter. The inhibition of either MAPKs or Akt alleviated IGF-I effects on IK and IK1. We conclude that basal IK and IK1 are positively maintained by steady-state Akt and ERK activities. K+ channels seem to be regulated in a dichotomic manner by acutely stimulated MAPKs and Akt. Eccentric cardiac hypertrophy may be associated with a change in the regulation of the steady-state basal activities of K+ channels towards MAPKs, while that of the acute IGF-I-stimulated ones toward Akt

Reduced Cardiac Hypertrophy in Toll-Like Receptor 4-Deficient Mice Following Pressure Overload

Objective: We have previously demonstrated that nuclear factor kappa B (NFκB) activation is needed for the development of cardiac hypertrophy in vivo. NFκB is a downstream transcription factor in the Toll-like receptor (TLR)-mediated signaling pathway; therefore, we investigated a role of TLR4 in cardiac hypertrophy in vivo. Methods: TLR4-deficient mice (C.C3H-Tlr4 lps-d, n = 6), wild-type (WT) genetic background mice (BALB/c, n = 6), TLR4-deleted strain (C57BL/10ScCr, n = 8), and WT controls (C57BL/10ScSn, n = 8) were subjected to aortic banding for 2 weeks. Age-matched surgically operated mice served as controls. In a separate experiment, rapamycin (2 mg/kg, daily) was administered to TLR4-deficient mice and WT mice immediately following aortic banding. The ratio of heart weight/body weight (HW / BW) was calculated, and cardiac myocyte size was examined by FITC-labeled wheat germ agglutinin staining of membranes. NFκB binding activity and the levels of phospho-p70S6K in the myocardium were also examined. Results: Aortic banding significantly increased the ratio of HW / BW by 33.9% (0.601 ± 0.026 vs. 0.449 ± 0.004) and cell size by 68.4% in WT mice and by 10.00% (0.543 ± 0.011 vs. 0.495 ± 0.005) and by 11.8% in TLR4-deficient mice, respectively, compared with respective sham controls. NFκB binding activity and phospho-p70S6K levels were increased by 182.6% and 115.2% in aortic-banded WT mice and by 78.0% and 162.0% in aortic-banded TLR4-deficient mice compared with respective sham controls. In rapamycin-treated aortic-banded mice, the ratio of HW / BW was increased by 18.0% in WT mice and by 3.5% in TLR4-deficient mice compared with respective sham controls. Conclusion: Our results demonstrate that TLR4 is a novel receptor contributing to the development of cardiac hypertrophy in vivo and that both the TLR4-mediated pathway and PI3K/Akt/mTOR signaling are involved in the development of cardiac hypertrophy in vivo

Blockade of myd88 Attenuates Cardiac Hypertrophy and Decreases Cardiac Myocyte Apoptosis in Pressure Overload-Induced Cardiac Hypertrophy in Vivo

In this study, we evaluated whether blocking myeloid differentiation factor-88 (MyD88) could decrease cardiac myocyte apoptosis following pressure overload. Adenovirus expressing dominant negative MyD88 (Ad5-dnMyD88) or Ad5-green fluorescent protein (GFP) (Ad5-GFP) was transfected into rat hearts (n = 8/group) immediately followed by aortic banding for 3 wk. One group of rats (n = 8) was subjected to aortic banding for 3 wk without transfection. Sham surgical operation (n = 8) served as control. The ratios of heart weight to body weight (HW/BW) and heart weight to tibia length (HW/TL) were calculated. Cardiomyocyte size was examined by FITC-labeled wheat germ agglutinin staining of membranes. Cardiac myocyte apoptosis was determined by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assay, and myocardial interstitial fibrosis was examined by Masson\u27s Trichrome staining. Aortic banding significantly increased the HW/BW by 41.0% (0.44 ± 0.013 vs. 0.31 ± 0.008), HW/TL by 47.2% (42.7 ± 1.30 vs. 29.0 ± 0.69), cardiac myocyte size by 49.6%, and cardiac myocyte apoptosis by 11.5%, and myocardial fibrosis and decreased cardiac function compared with sham controls. Transfection of Ad5-dnMyD88 significantly reduced the HW/BW by 18.2% (0.36 ± 0.006 vs. 0.44 ± 0.013) and HW/TL by 22.3% (33.2 ± 0.95 vs. 42.7 ± 1.30) and decreased cardiomyocyte size by 56.8%, cardiac myocyte apoptosis by 76.2%, as well as fibrosis, and improved cardiac function compared with aortic-banded group. Our results suggest that MyD88 is an important component in the Toll-like receptor-4-mediated nuclear factor-κB activation pathway that contributes to the development of cardiac hypertrophy. Blockade of MyD88 significantly reduced cardiac hypertrophy, cardiac myocyte apoptosis, and improved cardiac function in vivo

Attenuation of Cardiac Hypertrophy by Inhibiting Both mTOR and NFκB Activation in Vivo

A role for the PI3K/Akt/mTOR pathway in cardiac hypertrophy has been well documented. We reported that NFκB activation is needed for cardiac hypertrophy in vivo. To investigate whether both NFκB activation and PI3K/Akt/mTOR signaling participate in the development of cardiac hypertrophy, two models of cardiac hypertrophy, namely, induction in caAkt-transgenic mice and by aortic banding in mice, were employed. Rapamycin (2 mg/kg/daily), an inhibitor of the mammalian target of rapamycin, and the antioxidant pyrrolidine dithiocarbamate (PDTC; 120 mg/kg/daily), which can inhibit NFκB activation, were administered to caAkt mice at 8 weeks of age for 2 weeks. Both rapamycin and PDTC were also administered to the mice immediately after aortic banding for 2 weeks. Administration of either rapamycin or PDTC separately or together to caAkt mice reduced the ratio of heart weight/body weight by 21.54, 32.68, and 42.07% compared with untreated caAkt mice. PDTC administration significantly reduced cardiac NFκB activation by 46.67% and rapamycin significantly decreased the levels of p70S6K by 34.20% compared with untreated caAkt mice. Similar results were observed in aortic-banding-induced cardiac hypertrophy in mice. Our results suggest that both NFκB activation and the PI3K/Akt signaling pathway participate in the development of cardiac hypertrophy in vivo