Alterations in myocardial energy metabolism induced by the anti-cancer drug doxorubicin

Doxorubicin and other anthracyclines are among the most potent chemotherapeutic drugs for the treatment of acute leukaemia, lymphomas and different types of solid tumours such as breast, liver and lung cancers. Their clinical use is, however, limited by the risk of severe cardiotoxicity, which can lead to irreversible congestive heart failure. There is increasing evidence that essential components of myocardial energy metabolism are among the highly sensitive and early targets of doxorubicin-induced damage. Here we review doxorubicin-induced detrimental changes in cardiac energetics, with an emphasis on the emerging importance of defects in energy-transferring and -signalling systems, like creatine kinase and AMP-activated protein kinase.     

PD98059 and U0126 activate AMP-activated protein kinase by increasing the cellular AMP:ATP ratio and not via inhibition of the MAP kinase pathway

The MAP kinase pathway inhibitor U0126 caused phosphorylation and activation of AMP-activated protein kinase (AMPK) and increased phosphorylation of its downstream target acetyl-CoA carboxylase, in HEK293 cells. This effect only occurred in cells expressing the upstream kinase, LKB1. Of two other widely used MAP kinase pathway inhibitors not closely related in structure to U0126, PD98059 also activated AMPK but PD184352 did not. U0126 and PD98059, but not PD184352, also increased the cellular ADP:ATP and AMP:ATP ratios, accounting for their ability to activate AMPK. These results suggest the need for caution in interpreting experiments conducted using U0126 and PD98059.     

Skeletal muscle: Energy metabolism, fiber types, fatigue and adaptability

Skeletal muscles cope with a large range of activities, from being able to support the body weight during long periods of upright standing to perform explosive movements in response to an unexpected threat. This requires systems for energy metabolism that can provide energy during long periods of moderately increased energy consumption as well as being able to rapidly increasing the rate of energy production more than 100-fold in response to explosive contractions. In this short review we discuss how muscles can deal with these divergent demands. We first outline the major energy metabolism pathways in skeletal muscle. Next we describe metabolic differences between different muscle fiber types. Contractile performance declines during intense activation, i.e. fatigue develops, and we discuss likely underlying mechanisms. Finally, we discuss the ability of muscle fibers to adapt to altered demands, and mechanisms behind these adaptations. The accumulated experimental evidence forces us to conclude that most aspects of energy metabolism involve multiple and overlapping signaling pathways, which indicates that the control of energy metabolism is too important to depend on one single molecule or mechanism.     

Conjugated linoleic acid-induced apoptosis in mouse mammary tumor cells is mediated by both G protein coupled receptor-dependent activation of the AMP-activated protein kinase pathway and by oxidative stress

Conjugated linoleic acid (CLA) has shown chemopreventive activity in several tumorigenesis models, in part through induction of apoptosis. We previously demonstrated that the t10,c12 isomer of CLA induced apoptosis of TM4t mouse mammary tumor cells through both mitochondrial and endoplasmic reticulum (ER) stress pathways, and that the AMP-activated protein kinase (AMPK) played a critical role in the apoptotic effect. In the current study, we focused on the upstream pathways by which AMPK was activated, and additionally evaluated the contributing role of oxidative stress to apoptosis. CLA-induced activation of AMPK and/or induction of apoptosis were inhibited by infection of TM4t cells with an adenovirus expressing a peptide which blocks the interaction between the G protein coupled receptor (GPCR) and Gα_q, by the phospholipase C (PLC) inhibitor U73122, by the inositol trisphosphate (IP₃) receptor inhibitor 2-APB, by the calcium/calmodulin-dependent protein kinase kinase α (CaMKK) inhibitor STO-609 and by the intracellular Ca²⁺ chelator BAPTA-AM. This suggests that t10,c12-CLA may exert its apoptotic effect by stimulating GPCR through Gα_q signaling, activation of phosphatidylinositol-PLC, followed by binding of the PLC-generated IP₃ to its receptor on the ER, triggering Ca²⁺ release from the ER and finally stimulating the CaMKK–AMPK pathway. t10,c12-CLA also increased oxidative stress and lipid peroxidation, and antioxidants blocked its apoptotic effect, as well as the CLA-induced activation of p38 MAPK, a downstream effector of AMPK. Together these data elucidate two major pathways by which t10,c12-CLA induces apoptosis, and suggest a point of intersection of the two pathways both upstream and downstream of AMPK.     

R-subunit isoform specificity in protein kinase A: distinct features of protein interfaces in PKA types I and II by amide H/2H exchange mass spectrometry

The two isoforms (RI and RII) of the regulatory (R) subunit of cAMP-dependent protein kinase or protein kinase A (PKA) are similar in sequence yet have different biochemical properties and physiological functions. To further understand the molecular basis for R-isoform-specificity, the interactions of the RIIβ isoform with the PKA catalytic (C) subunit were analyzed by amide H/²H exchange mass spectrometry to compare solvent accessibility of RIIβ and the C subunit in their free and complexed states. Direct mapping of the RIIβ-C interface revealed important differences between the intersubunit interfaces in the type I and type II holoenzyme complexes. These differences are seen in both the R-subunits as well as the C-subunit. Unlike the type I isoform, the type II isoform complexes require both cAMP-binding domains, and ATP is not obligatory for high affinity interactions with the C-subunit. Surprisingly, the C-subunit mediates distinct, overlapping surfaces of interaction with the two R-isoforms despite a strong homology in sequence and similarity in domain organization. Identification of a remote allosteric site on the C-subunit that is essential for interactions with RII, but not RI subunits, further highlights the considerable diversity in interfaces found in higher order protein complexes mediated by the C-subunit of PKA.