Doxorubicin (DOX) is a clinically used anti-tumor drug that induces cardiotoxicity. HDL protects against DOX induced apoptosis in isolated cardiomyocytes, though the pathways of protection are not well known. The scavenger receptor class B type I (SRBI) is a high affinity HDL receptor, and its absence in atherogenic mice leads to occlusive CAD and MI, though a direct role for SRBI in cardiomyocytes has yet to be studied. Here we utilized both in vitro cell culture and in vivo mouse models to assess whether SRBI is required for HDL induced protection against DOX induced cardiotoxicity.
Mouse neonatal cardiomyocytes were isolated from C57Bl6 and SRBI KO hearts. SRBI was knocked down in SV40 immortalized human ventricular cardiomyocytes using siRNA. All cells were treated as follows: 24h lipoprotein starvation, 24h ±100μg/ml HDL, and 6h ±1μM DOX. Cells were then assessed for apoptosis. Secondly, male C57Bl6 or SRBI KO mice were injected i.p. with 5mg/kg DOX or saline by 5 weekly. Plasma was assessed for cardiac troponin-T. Heart sections were assessed for apoptosis and examined histologically.
HDL protected against DOX-induced apoptosis in both C57Bl6 mouse cardiomyocytes (7.7±5.0 vs 23.7±1.8% apoptotic cardiomyocytes n=3 p<.05), and in immortalized human ventricular cardiomyocytes (6.7±2.1 vs 24.5±4.8% apoptotic cardiomyocytes n=3 p<.05). Conversely, HDL was unable to protect against DOX induced apoptosis in cardiomyocytes from SRBI KO mice (37.4±6.8 vs 35.5±8.3% apoptotic cardiomyocytes, n=3 p<.05), or immortalized human cardiomyocytes transfected with siSRBI (27.2±8.9 vs 31.3±3.7% apoptotic cardiomyocytes n=3 p<.05).
In vivo studies showed that SR-BI KO hearts are more sensitive to DOX as shown by increased apoptosis (6.9±0.6 vs 3.9±0.7% apoptotic cardiomyocytes n=5/group p<.05) and greater plasma cardiac troponin T levels (1.7±0.3 vs 1.3±0.04ng/ml n=5/group p<.05). Hearts from DOX treated SRBI KO mice also exhibited greater myofibrillar loss, vacuolization, and inflammatory cell infiltration compared to C57Bl6 hearts.
Our results indicate that SRBI mediates HDL induced protection in both mouse and human cardiomyocytes. We show here that SRBI is an important factor for the protection of the heart as a whole against DOX induced cardiotoxicity.