Myocardial α-thrombin receptor activation induces hypertrophy and increases atrial natriuretic factor gene expression

The protease, α-thrombin (αTh), affects myocardial cell contractility, a feature common among agents that induce hypertrophy. However, it is not known whether cardiac myocytes possess αTh receptors (αTh-R), or if long term treatment with αTh can enhance growth and gene expression. In the present study primary neonatal rat ventricular myocytes expressed a 3.6-kilobase mRNA species that hybridized with a rat αTh-R-specific probe. After 48 h, αTh induced hypertrophy, sarcomeric organization, and enhanced atrial natriuretic factor (ANF) expression, all of which were blocked by the αTh-selective protease inhibitor, D-Phe-Pro-Arg-chloromethyl ketone. The αTh-R agonist peptide, SFLLRNPND, was a potent activator of ANF expression, however, the non-agonist, FLLRNPND, was inactive. Transfection experiments showed the enhancement of ANF expression by αTh to be transcriptional. The abilities of αTh to induce myocyte hypertrophy and to augment ANF transcription and peptide production were inhibited by the protein kinase C inhibitor, chelerythrine, and by the tyrosine kinase inhibitor, tyrphostin. Thus, myocardial cell αTh-Rs are stimulated by the specific proteolytic actions of αTh, and pathways involving both protein kinase C and protein tyrosine kinases are required for subsequent hypertrophy and ANF expression. Further, these findings suggest a new role for extracellular proteases as regulators of myocardial cell gene expression and growth.