To establish an appropriate in vitro model for the local environment of cardiomyocytes, three-dimensional (3D) spheroids derived from H9c2 cardiomyoblasts were prepared, and their morphological, biophysical phase contrast and biochemical characteristics were evaluated. The 3D H9c2 spheroids were successfully obtained, the sizes of the spheroids decreased, and they became stiffer during 3-4 days. In contrast to the cell multiplication that occurs in conventional 2D planar cell cultures, the 3D H9c2 spheroids developed into a more mature form without any cell multiplication being detected. qPCR analyses of the 3D H9c2 spheroids indicated that the production of collagen4 (COL4) and fibronectin (FN), connexin43 (CX43), β-catenin, N-cadherin, STAT3, and HIF1 molecules had increased and that the production of COL6 and α-smooth muscle actin (α-SMA) molecules had decreased as compared to 2D cultured cells. In addition, treatment with rapamycin (Rapa), an mTOR complex (mTORC) 1 inhibitor, and Torin 1, an mTORC1/2 inhibitor, resulted in significantly decreased cell densities of the 2D cultured H9c2 cells, but the size and stiffness of the H9c2 cells within the 3D spheroids were reduced with the gene expressions of several of the above several factors being reduced. The metabolic responses to mTOR modulators were also different between the 2D and 3D cultures. These results suggest that as unique aspects of the local environments of the 3D spheroids, the spontaneous expression of GJ-related molecules and hypoxia within the core may be associated with their maturation, suggesting that this may become a useful in vitro model that replicates the local environment of cardiomyocytes.
Keywords: 2D culture; 3D spheroid culture; H9c2; Seahorse Bioanalyzer; mTOR; rapamycin.