Mechanisms of Increased NGF Production in Vascular Smooth Muscle of the Spontaneously Hypertensive Rat

Abstract

The spontaneously hypertensive rat (SHR) was developed as a genetic model of essential hypertension.In vivoandin vitroevidence demonstrates that vascular smooth muscle cells (VSMCs) from the SHR produce more nerve growth factor (NGF) than the normotensive Wistar-Kyoto (WKY) control strain. This increased NGF production is accompanied by excessive innervation of target tissues in the SHR. In the present study, a sensitive, competitive, quantitative, reverse-transcriptase polymerase chain reaction (C Q RT-PCR) assay is characterized and used to analyze levels of NGF mRNA in cultured VSMCs derived from the SHR and WKY strains as well as bladder tissue. Differences in NGF secretion rates between SHR and WKY VSMCs were partially due to an increased stability of NGF mRNA in SHR VSMCs. Following treatment with platelet-derived growth factor (PDGF) and transforming growth factor-β (TGF-β1) to elevate NGF production, the half-life of the NGF mRNA was 104.5 ± 18.0 min in SHR VSMCs, compared to only 36.5 ± 11.6 min in WKY VSMCs. Sequence analysis of the 3′ untranslated region (UTR) revealed no strain differences incis-acting sequences potentially involved in determining mRNA stability. Thus, it seems unlikely to be a 3′UTR mutation that prolongs mRNA lifetime. Rather, differential regulation of an RNA-binding protein may play a role in the abnormal NGF mRNA stability in SHR VSMCs. SHR VSMCs also demonstrate an increased translational efficiency of NGF protein; more NGF protein is synthesized per unit of NGF mRNA. The use of a C Q RT-PCR assay has allowed the determination that abnormal NGF mRNA stabilization as well as altered translational efficiency may contribute to excess NGF synthesis and progressive hypertension in the SHR.