Sarcoplasmic Reticulum Genes are Selectively Down-regulated in Cardiomyopathy Produced by Doxorubicin in Rabbits

The clinical utility of doxorubicin, an antineoplastic agent, is limited by its cardiotoxicity. Our objective was to determine whether expression of genes encoding proteins that affect Ca2+homeostasis were altered in the hearts of rabbits chronically treated with doxorubicin. Twelve male New Zealand white rabbits received an injection of doxorubicin (2.5 mg/kg i.v.) once a week for 8 weeks. Eight rabbits were similarly injected with saline as controls. The cardiac function of both groups was evaluated 8 weeks after the final injection, as were the levels of expression of mRNA for Ca2+transport proteins in the sarcoplasmic reticulum and plasma membrane. The amount of the sarcoplasmic reticulum Ca2+-ATPase and the Ca2+uptake capacity of the protein were also quantitated. Cardiac output was significantly decreased in the doxorubicin-treated group (71±21 ml/min,P<0.05) compared with the control group (118±15 ml/min). The mRNA levels for the sarcoplasmic reticulum proteins were significantly diminished in the doxorubicin-treated hearts: ryanodine receptor-2 (relative expression level compared with controls, 0.35±0.13,P<0.01), sarcoplasmic reticulum Ca2+-ATPase (0.56±0.13,P<0.01), phospholamban (0.62±0.20,P<0.01) and cardiac calsequestrin (0.57±0.26,P<0.01). In addition, both relative amount of sarcoplasmic reticulum Ca2+-ATPase protein (doxorubicin-treated group, 69±17% of control,P<0.01) and the Ca2+uptake capacity (46.9±9.8 nmol Ca2+/mg protein-5 min in doxorubicin groupv63.2±10.4 in the control group,P<0.01) were concomitantly decreased with its mRNA expression level. Conversely, the mRNA levels for the plasma membrane proteins did not differ from those of control rabbits: the dihydropyridine receptor (relative expression level, 1.03±0.30, ), plasma membrane Ca2+-ATPase (0.93±0.33, ) and the Na+/Ca2+exchanger (0.87±0.34, ). These findings suggest that a selective decrease in mRNA expression for sarcoplasmic reticulum Ca2+transport proteins is responsible for the impaired Ca2+handling, and thus, for the reduced cardiac function seen in the cardiomyopathy induced in rabbits by the long-term treatment with doxorubicin.