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• PM 2.5 caused inflammation, oxidative stress, and pathological damage in the hearts. • PM 2.5 induced heart histopathological and cardiac hypertrophy in rats. • PM 2.5 increased β-MHC and GATA4 expression and ratio of β-MHC/α-MHC expression. • MiR-208a/b associated with β-MHC/GATA4 expression in rats' hearts exposed to PM 2.5. Ambient air fine particulate matter (PM 2.5) may increase cardiovascular disease risks. In this study, we investigated the miR-208/GATA4/myosin heavy chain (MHC) regulation mechanisms on cardiac injury in rats after PM 2.5 exposure via an animal inhalation device. The results showed that PM 2.5 exposure for 2 months caused pathological heart injury, reduced nucleus-cytoplasm ratio, and increased the levels of CK-MB and cTnI, showing cardiac hypertrophy. Oxidative stress and inflammatory responses were also observed in rats' hearts exposed to PM 2.5. Of note, PM 2.5 exposure for 2-month significantly elevated GATA4 and β-MHC mRNA and protein expression compared with the corresponding controls, along with the high-expression of miR-208b. The ratios of β-MHC/α-MHC expression induced by PM 2.5 were remarkably raised in comparison to their controls. It suggested that the up-regulation of miR-208b/β-MHC and GATA4 and the conversion from α-MHC to β-MHC may be the important causes of cardiac hypertrophy in rats incurred by PM 2.5. [ABSTRACT FROM AUTHOR]