Peroxisome Proliferator-Activated Receptor-g Coactivator-1a (PGC-1a) Regulates the Expression of B-Cell Lymphoma/Leukemia-2 (Bcl-2) and Promotes the Survival of Mesenchymal Stem Cells (MSCs) via PGC-1a/ERRa Interaction in the Absence of Serum, Hypoxia, and High Glucose Conditions.