ALTERATIONS IN CARDIAC GENE-EXPRESSION DURING THE TRANSITION FROM STABLE HYPERTROPHY TO HEART-FAILURE - MARKED UP-REGULATION OF GENES ENCODING EXTRACELLULAR-MATRIX COMPONENTS

The failing heart is characterized by impaired cardiac muscle function and increased interstitial fibrosis. Our purpose was to determine whe ther the functional impairment of the failing heart is associated with changes in levels of mRNA encoding proteins that modulate parameters of contraction and relaxation and whether the increased fibrosis obser ved in the failing heart is related to elevated expression of genes en coding extracellular matrix components. We studied hearts of 18- to 24 -month-old spontaneously hypertensive rats with signs and symptoms of heart failure (SHR-F) or without evidence of failure (SHR-NF) and of a ge-matched normotensive Wistar-Kyoto (WKY) rats. Compared with WKY rat s, SHR-NF exhibited left ventricular (LV) hypertrophy (2.2-fold) and r ight ventricular (RV) hypertrophy (1.5-fold), whereas SHR-F were chara cterized by comparable LV hypertrophy (2.1-fold) and augmented RV hype rtrophy (2.4-fold; all P<.01). Total RNA was isolated from ventricles and subjected to Northern blot analysis. In SHR-F hearts, the level of alpha-myosin heavy chain mRNA was decreased in both ventricles to 1/3 and 1/5 of the SHR-NF and WKY values, respectively (both P<.01). Leve ls of beta-myosin heavy chain, alpha-cardiac actin, and myosin light c hain-2 mRNAs were not significantly altered in hearts of SM-NF or SHR- F. Levels of alpha-skeletal actin were twofold greater in SHR-NF heart s compared with WKY hearts and were intermediate in SHR-F hearts. Leve ls of atrial natriuretic factor (ANF) mRNA were elevated threefold in the LV of SHR-NF (P<.05) but were not significantly increased in the R V of SHR-NF compared with WKY rats. During the transition to failure ( SHR-F versus SHR-NF), ANF mRNA levels increased an additional 1.6-fold in the LV and were elevated 4.7-fold in the RV (both P<.05), Levels o f sarcoplasmic reticulum Ca2+ ATPase (SRCA) mRNA were maintained in th e LV of hypertensive and failing hearts at levels not significantly di fferent from WKY values. In contrast, the level of RV SRCA mRNA was 24 % less in SHR-NF compared with WKY rats, and during the transition to failure, this difference was not significantly exacerbated (29% less t han the WKY value). The levels of fibronectin and pro-alpha 1(I) and p ro-alpha 1(III) collagen mRNAs were not significantly elevated in eith er ventricle of the SHR-NF group but were fourfold to fivefold higher in both ventricles of SHR-F (all P<.05). The increase in fibronectin g ene expression was at least partially explained by an elevation in the level of the EIIIA-containing isoform, an alternatively spliced varia nt expressed during wound healing and pressure overload hypertrophy. T ransforming growth factor-beta(1) (TGF-beta(1)) mRNA abundance was not elevated in ventricles of SHR-NF but increased 1.3-fold in the LV and twofold in the RV during the transition to heart failure compared wit h SHR-NF values (both P<.05). The decrease in alpha-myosin heavy chain mRNA levels in SHR-F hearts represents a pretranslational basis for t he slowed contraction previously observed in cardiac muscle from these hearts. The survey of specific contractile protein mRNAs provides no evidence of a downregulation of these genes during the transition to h eart failure. The increase in fibronectin and collagen mRNA levels sug gests that the previously observed increase in interstitial fibrosis i n cardiac muscles of failing hearts is regulated at the level of gene expression. The increase in abundance of TGF-beta(1) mRNA in conjuncti on with the upregulation of fibronectin and collagen genes suggests th at activation of TGF-beta(1) gene expression may be a mechanism initia ting interstitial fibrosis during the transition from stable hypertrop hy to failure.