Access Type

Open Access Dissertation

Date of Award

January 2014

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Physiology

First Advisor

Robert D. Lasley

Abstract

Adenosine A2B receptor (A2BR) appear to contribute to chronic inflammation. This receptor is highly expressed in macrophages and cardiac fibroblasts, cells which play key roles in inflammation and healing following myocardial infarction (MI). A2BR have been shown to induce collagen production and promote organ fibrosis, although the reports of A2BR role on MI are limited and conflicting. The results of cardiac fibroblast (CF) studies however suggest that non-selective A2BR stimulation inhibits collagen expression. The hypothesis of the present study was that deletion of A2BR reduces adverse remodeling in post-MI, and selective activation of A2BR increases WT murine CF collagen and pro-inflammatory cytokines production.

In our in vivo studies, MI was induced by permanent coronary artery occlusion in male WT and A2BR KO mice. Hearts were harvested at 5 or 28-day post-MI for semi quantitative RT-PCR assay or sectioned and stained for morphological and histological studies. In CF experments, effects of selective A2BR (BAY 60-6583, BAY) and A2AR (CGS-21680, CGS) agonists on signaling (10 min) and collagen expression (24 h) were assessed by western blots. Adenosine receptor expression and agonist effects (24 h) on pro-inflammatory cytokine expression were analyzed with semi-quantitative real time PCR.

Mortality of total animals and 28-day infarct size did not differ between genotypes. A2BR expression was 2.7 fold greater in WT scar zone compared to remote zone. TNF-α and MMP-9 expressions in the scar zone and collagen 1 and 3 mRNA levels in the remote zone were much greater in 5-day WT group without any change in macrophage infiltration. A2BR KO hearts had greater scar thickness and lower infarct expansion. Our results indicated that ablation of A2BR significantly decreased collagen deposition in 28-day post-MI remote zone. We also found that this effect was not due to downregulation of myofibroblasts but related to decreased macrophage infiltration at 28-day. Our results from CF studies indicated that A2BR gene expression was two-fold greater than A2AR, which was comparable to angiotensin AT1R. The A2BR agonist BAY and the A2AR agonist CGS both increased ERK and CREB phosphorylation. BAY and CGS effects on signaling were blocked by deletion of A2BR and A2AR, respectively. TGFβ-induced increases in collagen-1 expression were not altered by adenosine receptor agonists; in contrast, selective A2BR and A2AR stimulation increased collagen-1 expression, effects which were blunted by MEK (U0126) and PKA (H89) inhibitors. BAY, but not CGS, increased IL-6 gene expression, but neither agonist showed an effect on IL-1β mRNA levels.

Our findings that a large increase in expression of the pro-inflammatory A2BR occurs in the WT scar zone, and that A2BR deletion reducing adverse post-MI heart remodeling in both scar and remote zones, suggest a detrimental role for A2BR in this chronic pathological process, which could be, at least partially, through the effects on CF collagen and pro-inflammatory cytokine production.

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