Contrasting effects of A1 and A2b adenosine receptors on adipogenesis

Abstract

Background: Adenosine mediates its actions through four G protein-coupled receptors, A1, A2a, A2b and A3. The A1 receptor (A1R) is dominant in adipocytes where it mediates many actions that include inhibition of lipolysis, stimulation of leptin secretion and protection against obesity-related insulin resistance. Objective: The objective of this study is to investigate whether induced expression of A1Rs stimulates adipogenesis, or whether A1R expression is a consequence of cells having an adipocyte phenotype. Methodology: Human A1R and A2b receptors (A2bRs) were stably transfected into a murine osteoblast precursor cell line, 7F2. Adipogenesis was determined by lipid accumulation and expression of adipocyte and osteoblast marker molecules. Adenosine receptor expression and activation of associated signal molecules were also evaluated as 7F2 cells were induced to differentiate to adipocytes. Results: 7F2 cells transfected with the A1R showed increased adipocyte marker mRNA expression; lipoprotein lipase and glycerol-3-phosphate dehydrogenase were both upregulated, whereas the osteoblast marker alkaline phosphatase (ALP) was downregulated. When cultured in adipocyte differentiating media, such cells also showed increased adipogenesis as judged by lipid accumulation. Conversely, A2bR transfection stimulated osteocalcin and ALP expression, and in addition, adipogenesis was inhibited in the presence of adipocyte differentiation media. Adipogenic differentiation of naive 7F2 cells also resulted in increased expression of the A1R and reduced or modified expression of the A2a and A2bR. The loss of A2 receptors after adipogenic differentiation was accompanied by a loss of cyclic adenosine monophosphate and ERK1/2 signalling. Conclusion: These data show that expression of A1Rs induced adipocyte differentiation, whereas A2bR expression inhibited adipogenesis and stimulated an osteoblastic phenotype. These data suggest that targeting A1 and A2bR could be considered in the management of obesity and diabetes. Targeting adenosine signal pathways may be useful in treatment strategies for diseases in which there is an imbalance between osteoblasts and adipocytes.

Lack of effect of adenosine on the function of rodent osteoblasts and osteoclasts in vitro

“…However, our results are at variance with several studies which found that adenosine or adenosine analogues, acting via the A 2A or A 2B receptors, stimulate the differentiation and function of human and rodent bone marrow osteoblasts and promote bone regeneration [24,30,31,40]. Our data also do not concur with the reported inhibitory effects of adenosine analogues, acting via A 1 or A 2A receptors, on the differentiation of rodent osteoblast-like cells [41] or human osteoblasts [40]. We did observe small stimulatory effects of 2-chloroadenosine on rat bone marrow osteoblasts.…”

Adenosine A1 receptor regulates osteoclast formation by altering TRAF6/TAK1 signaling

“…However, the effects described by Teramachi and colleagues are most likely mediated by A 2b receptors on osteoblasts since, as noted in their work, the primary reason for inhibition of osteoclast formation in these bone marrow cultures was inhibition of RANKL expression without diminution of osteoprotegerin expression. Since RANKL, the principal stimulus for osteoclast differentiation, is primarily expressed on osteoblasts, which have been previously shown to express A 2b receptors which affect their function [50][51][52], the most likely explanation for the observed effects of A 2b receptor ligation on osteoclastogenesis is inhibition of osteoblast-mediated stimulation of osteoclastogenesis. It is also interesting to note that it is likely that the inhibitory effects of methotrexate on osteoclast-mediated bone destruction are mediated by the higher levels of adenosine released by methotrexate-treated cells and tissues which interact with A 2A and A 3 receptors [53].…”

Direct or indirect stimulation of adenosine A 2A receptors enhances bone regeneration as well as bone morphogenetic protein‐2

“…Thus, Gharibi et al (35) reported that, although A1R was expressed in osteoblast precursors, it was involved in induction of adipocyte differentiation rather than osteoblast differentiation. Nonetheless, Katebi et al (36) reported that adenosine, acting via A2AR, plays a critical role in promoting the proliferation of mouse bone marrowderived fibroblast-like mesenchymal stem cells, and Gharibi et al (22) reported that A2AR is up-regulated in later osteoblast differentiation stages, but stimulation of A2AR more strongly promotes adipocyte differentiation.…”