Evaluation of the cardiac isoform of alpha2-macroglobulin as a factor inducing cardiac hypertrophy.

Earlier studies from our laboratory showed that a 182-kDa high-molecular-weight protein appeared during early stages of development of cardiac hypertrophy in animals subjected to aortic constriction. Later it was confirmed that this protein is a cardiac isoform of alpha2-macroglobulin belonging to the macroglobulin family. Furthermore, it has been demonstrated that direct injection of the purified 182-kDa protein intravenously (through the tail vein) into the normal animals led to the development of cardiac hypertrophy. It was accompanied by enlargement of cardiac myocytes and induction of beta-myosin heavy chain (MHC) and MLC-2 gene expression. Multiple injections of 182-kDa protein-specific polyclonal antibody into the circulation of aorta-constricted animals completely abolished the development of hypertrophy and downregulated the expression of beta-MHC and myosin light chain (MLC)-2. The full-length cDNA of the 182-kDa protein cloned in eukaryotic expression vector, namely, pcDNA 3.1(-) could induce cardiac hypertrophy upon direct injection into rat heart. Hypertrophy was monitored by determining the heart weight/body weight ratio and also by Northern blot analysis of muscle-specific marker genes such as beta-MHC, MLC-2, and antrial natriuretic factor. Also, induction of promoter activity of beta-MHC and c-fos genes analyzed by chloramphenicol acetyl transferase assay confirmed the induction of cardiac hypertrophy upon direct injection of the full-length cDNA of the 182-kDa protein.