The in vitro culture of porcine bone marrow-derived mesenchymal stem cells (MSCs) was used for the investigation of adult stem cell biology. Isolated porcine MSCs possessed the ability to proliferate extensively in an antioxidants-rich medium containing 5% fetal bovine serum (FBS). Greater than 40 serial MSC passages and 100 cell population doublings have been recorded for some MSC batches. Early and late passage MSCs were defined here as those cultures receiving less than 5 trypsin passages and more than 15 trypsin passages, respectively. Consistent with their robust ability to proliferate, both the early and late passage MSCs expressed the cell-cycle promoting enzyme p34cdc2 kinase. Late MSCs, however, exhibited certain features reminiscent of cellular aging such as actin accumulation, reduced substrate adherence, and increased activity of lysosomal acid beta-galactosidase. Early MSCs retained the multipotentiality capable of chondrogenic, osteogenic, and adipogenic differentiation upon induction in vitro. In contrast, late MSCs were only capable of adipogenic differentiation, which was greatly enhanced at the expense of the osteochondrogenic potential. Along with these changes in multipotentiality, late MSCs expressed decreased levels of the bone morphogenic protein (BMP-7) and reduced activity of alkaline phosphatase. Late MSCs also exhibited attenuated synthesis of the hematopoietic cytokines granulocyte colony-stimulating factor (G-CSF), leukemia inhibitory factor (LIF), and stem cell factor (SCF). The long-term porcine MSC culture, thus, provides a model system to study the molecular interplay between multiple MSC differentiation cascades in the context of cellular aging.

We previously proposed that the calpain-mediated proteolytic pathway is activated in cultured cardiomyocytes following exposure to hypoxia (Mol Cell Biochem 214: 47, 2000). The potential role of calpastatin, the endogenous specific inhibitor of calpain, and its expression in the hypoxic state were investigated here. Hypoxia induced the expression of two calpastatin and multiple VEGF splice variants. Although cardiomyocytes and fibroblasts responded to hypoxia differentially, both cell types exhibited hypoxia-induced calpastatin transcription. The two functional calpastatin splice variants encoding the 593- and 654-amino acid calpastatin isoforms differed only in their N-terminal leader domain sequences. In spite of the increased mRNA expression, levels of the calpastatin protein doublet were not increased, but rather slightly decreased under the hypoxic condition. Cardiac hypoxia was accompanied by preferential proteolytic cleavage of troponin I (TnI), one of the major myofibrillar proteins. Forced expression of calpastatin through an adenoviral vector effectively prevented the hypoxia- and calpain-mediated TnI proteolysis. Our results highlight the discordant expression pattern of cardiac calpastatin mRNA and protein in the hypoxic state. We suggest that although induction of calpastatin gene transcription may constitute a compensatory mechanism coping with the hypoxic stress, a sustained high calpastatin protein level appears to be essential in the intervention of the activated calpain proteolytic cascade.