Correlation between myocardial malate/aspartate shuttle activity and EAAT1 protein expression in hyper- and hypothyroidism

Abstract

. Correlation between myocardial malate/aspartate shuttle activity and EAAT1 protein expression in hyper-and hypothyroidism. Am J Physiol Heart Circ Physiol 288: H2521-H2526, 2005. First published December 22, 2004; doi:10.1152/ajpheart.00991.2004.-In the heart, elevated thyroid hormone leads to upregulation of metabolic pathways associated with energy production and development of hypertrophy. The malate/aspartate shuttle, which transfers cytosolicreducing equivalents into the cardiac mitochondria, is increased 33% in hyperthyroid rats. Within the shuttle, the aspartate-glutamate carrier is rate limiting. The excitatory amino acid transporter type 1 (EAAT1) functions as a glutamate carrier in the malate/aspartate shuttle. In this study, we hypothesize that EAAT1 is regulated by thyroid hormone. Adult rats were injected with triiodothyronine (T 3) or saline over a period of 8 -9 days or provided with propylthiouracil (PTU) in their drinking water for 2 mo. Steady-state mRNA levels of EAAT1 and aralar1 and citrin (both cardiac mitochondrial aspartate-glutamate transporters) were determined by Northern blot analysis and normalized to 18S rRNA. A spectrophotometric assay of maximal malate/ aspartate shuttle activity was performed on isolated cardiac mitochondria from PTU-treated and control animals. Protein lysates from mitochondria were separated by SDS-PAGE and probed with a human anti-EAAT1 IgG. Compared with control, EAAT1 mRNA levels (arbitrary units) were increased nearly threefold in T3-treated (3.1 Ϯ 0.5 vs. 1.1 Ϯ 0.2; P Ͻ 0.05) and decreased in PTU-treated (2.0 Ϯ 0. 3 vs. 5.2 Ϯ 1; P Ͻ 0.05) rats. Aralar1 mRNA levels were unchanged in T3-treated and somewhat decreased in PTU-treated (7.1 Ϯ 1.0 vs. 9.3 Ϯ 0.1, P Ͻ 0.05) rats. Citrin mRNA levels were decreased in T3-treated and unchanged in PTU-treated rats. EAAT1 protein levels (arbitrary units) in T3-treated cardiac mitochondria were increased compared with controls (8.9 Ϯ 0.4 vs. 5.9 Ϯ 0.6; P Ͻ 0.005) and unchanged in PTU-treated mitochondria. No difference in malate/ aspartate shuttle capacity was found between PTU-treated and control cardiac mitochondria. Hyperthyroidism in rats is related to an increase in cardiac expression of EAAT1 mRNA and protein. The 49% increase in EAAT1 mitochondrial protein level shows that malate/ aspartate shuttle activity increased in hyperthyroid rat cardiac mitochondria. Although hypothyroidism resulted in a decrease in EAAT1 mRNA, neither the EAAT1 protein level nor shuttle activity was affected. EAAT1 regulation by thyroid hormone may facilitate increased metabolic demands of the cardiomyocyte during hyperthyroidism and impact cardiac function in hyperthyroidism. thyroid hormone; myocardial hypertrophy GIVEN THE CENTRAL ROLE OF the heart in responding to physiological changes and the high energy requirements of the myocardium, it is not surprising that cardiac myocytes are sensitive to the effects of thyroid hormone. Thyroid hormone acts on the heart through mechanisms that are both independent of and dependent on gene tra...

Free triiodothyronine plasma concentrations are positively associated with insulin secretion in euthyroid individuals

“…T 3 promotes expression of important proteins involved in both glucose and lipid metabolism that may influence insulin secretion. In particular, carnitine palmitoyl transferase 1 (CPT-1) expression and activity (35,36), mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH) activity (37), and levels of excitatory amino acid transporter type 1 (EAAT1) mRNA and protein (38) are increased by TH. EAAT1 and mGPDH are key components of the NADH shuttles which are essential for coupling glycolysis with mitochondrial ATP generation and triggering glucose-induced insulin secretion (39).…”

HORMONE-DEPENDENT AND HORMONE-INDEPENDENT CONTROL OF METABOLIC AND DE­VEL­OP­MEN­TAL FUNCTIONS OF MALATE DEHYDROGENASE – Review

“…The malate/aspartate as well as α-glycerophosphate shuttles show existence of sufficient capacity in cardiac mitochondria to accommodate increased shuttle flux even in hypertrophied myocardium that becomes more glycolytically active (RUPERT et al, 2000). However, elevated thyroid hormone leads to upregulation of metabolic pathways associated with energy production and development of heart hypertrophy, as observed by RALPHE et al (2005). Expression of excitatory amino acid transporter type 1 (EAAT1) mRNA and protein that functions as a glutamate carrier in the malate/aspartate shuttle was increased nearly threefold in T3-treated animals, whereas expression of aralar1 and citrin (both cardiac mitochondrial aspartate-glutamate transporters) mRNA and protein levels remain decreased and unchanged, respectively.…”

Ca2+ Activation Kinetics of the Two Aspartate-Glutamate Mitochondrial Carriers, Aralar and Citrin

“…However, the role of each isoform in MAS function remains unknown. Recent reports have suggested that the AGCs are only one of the possible players in the heart malate-aspartate shuttle at the step of glutamate uptake in mitochondria, and that the EAAT1, a plasma membrane glutamate carrier from brain, is localized to heart mitochondria and functions as a glutamate carrier within MAS (40,41). Therefore, it is necessary to clarify the contribution and role of each of the two AGC isoforms to understand MAS function in heart.…”

“…Moreover, recent reports have introduced added complexity to heart MAS activity. Ralphe et al (40,41) have proposed that an isoform of the excitatory amino acid transporter type 1 (EAAT1) is present in heart mitochondria where it acts as a glutamate carrier within MAS and is responsible for the up-regulation of MAS by thyroid hormone.…”