Heat shock protein 27 (Hsp27) is a well-known stress response protein that becomes phosphorylated through extracellular signal-regulated kinase (ERK). Different drugs of abuse, such as morphine and/or its withdrawal, induce severe stress situations. In this study, we investigated Hsp27 and phospho-Hsp27 expression during morphine dependence and withdrawal and evaluated the involvement of ERK in the phosphorylation of Hsp27 in the rat right ventricle. Dependence on morphine was induced by a 7-day s.c. implantation of morphine pellets. Morphine withdrawal was precipitated on day 8 by injection of naloxone (2 mg/kg, s.c.). ERK1/2, Hsp27 and phospho-Hsp27 at Ser15 were determined by quantitative blot immunolabeling using specific antibodies. Hsp27 expression was increased 30, 60, 90 and 120 min (144.5±14.2%, P<0.0001; 128.9±4.6%, P=0.04; 177.4±12.7, P<0.0001; and 136.2±11.0%, P=0.042, respectively) after saline injection to rats dependent on morphine. Naloxone-precipitated morphine withdrawal also increased the phosphorylation of Hsp27 at Ser15 at those time points (146.8±19.8%, P=0.034; 143.9±17.9%, P=0.032; 161.2±33.3%, P=0.029; and 152.2±25.5%, P=0.008, respectively). However, there were no changes in Hsp27 phosphorylation in the morphine dependent group injected with saline. In addition, there was an increase in the phosphorylation of ERK 60 min after naloxone injection in morphine dependent rats (pERK1: 116.3±4.2%, P=0.015 and pERK2: 117.2±1.5%, P=0.05). Pretreatment with SL327, an inhibitor of ERK phosphorylation, decreased activation (phosphorylation) of both ERK and Hsp27 (pERK1: 4.5±3.6%, P<0.0001; pERK2: 42.3±3.3%, P<0.0001; and pHsp27: 97.6±1.5%, P=0.008), suggesting that ERK activation triggers Hsp27 phosphorylation. The present findings demonstrate that morphine withdrawal is capable of inducing the activation of Hsp27 in the heart and suggest that phosphorylation of Hsp27 is closely linked to and also dependent on the ERK pathway.
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