Extracellular ATP: a central player in the regulation of vascular smooth muscle phenotype. Focus on “Dual role of PKA in phenotype modulation of vascular smooth muscle cells by extracellular ATP”

Open Access
Publisher Website
Google Scholar
Abstract
The extracellular nucleotides ATP, ADP, UTP, and UDP are potent growth factors for vascular smooth muscle cells (VSMC) by actions on P2Y receptors ([7][1], [9][2], [14][3], [20][4]). The growth-stimulating effects may be of importance in the development of atherosclerosis, neointima after balloon angioplasty, chronic vascular rejection after transplantation, and hypertension ([3][5]). In these processes a central mechanism is the change of the VSMC from a highly differentiated contractile phenotype into a proliferative, more fibroblast-like synthetic phenotype ([18][6]). Extracellular nucleotides have different effects depending on phenotype. In the contractile phenotype, ATP acts as an important stimulator of VSMC contraction. For example, knockout experiments have shown that the ATP receptor P2X1 contributes to 50% of the sympathetic vasoconstriction in mice ([19][7]). By contrast, in the synthetic phenotype, extracellular nucleotides contribute to the mitogenic response. In the current article in focus (Ref. [8][8], see p. [C449][9] in this issue), Hogarth et al. show that ATP has a dual concentration-dependent effect on the VSMC phenotype. Low ATP concentrations stimulate expression of genes specific for the contractile phenotype. High ATP concentrations cause a phenotypic shift from the contractile to the synthetic phenotype, and this shift is dependent on a transient activation of protein kinase A (PKA), which inhibits activation of a serum response factor (SRF). This previously unrecognized mechanism also appears to be responsible for the profound mitogenic effect of ATP. ### Mitogenic Effects In contrast to the previously documented inhibitory effect of the intracellular second messenger cAMP on VSMC mitogenesis ([13][10]), Hogarth et al. ([8][8]) report that ATP-mediated activation of PKA is necessary for a maximum stimulation of DNA synthesis. If the adenylyl cyclase activator forskolin and the β-adrenergic agonist isoproterenol were used to induce a prolonged PKA stimulation, mitogenesis was inhibited. By contrast, transient PKA stimulation evoked by ATP not only stimulated mitogenesis but also appeared to be necessary for the maximum mitogenic effect. However, when the authors tried to mimic the transient PKA stimulation by brief stimulation with forskolin, this treatment resulted in loss of the inhibitory effect but did not stimulate DNA synthesis. Thus the positive role of PKA in ATP-induced DNA synthesis cannot be explained solely by the transient duration of PKA activity induced by the agonist. Hogarth et al. suggest that an ATP stimulation of the PKA substrate vasodilator-stimulated phosphoprotein (VASP) could be crucial, but further studies are needed to confirm this possibility. Hogarth et al. ([8][8]) also found that ERK1/2 was stimulated at both low and high concentrations of ATP. Interestingly, ERK1/2 activation was not inhibited by a high-efficiency adenovirus-mediated expression of the PKA inhibitor protein kinase I (PKI), even though DNA synthesis in response to high ATP was inhibited. Thus ATP stimulation of ERK1/2 is not dependent on PKA activation. In contrast, both ERK1/2 and transient PKA stimulation were found to be crucial for the major DNA stimulation by high-dose ATP, but on its own, ERK1/2 phosphorylation was not sufficient. On the other hand, low-dose ATP stimulation of DNA synthesis was shown to be ERK1/2 dependent but PKA independent. Thus ERK1/2 is required, but not sufficient, for profound DNA synthesis. ### VSMC Phenotype The phenotypic shift is a central mechanism for smooth muscle cell participation in the development of vascular disease. The cells lose contractile proteins, start to produce growth factors and matrix, become less differentiated and more fibroblast-like, begin to migrate, and proliferate ([18][6]). In the article in focus, Hogarth et al. report that low concentrations of ATP stimulated the activity of SRF. SRF is implicated in the transcription of VSMC-specific genes. Low concentrations of ATP also stimulated expression of SM22 and SM-α-actin, which are proteins specific for the contractile phenotype. By using an adenovirus encoding a DNA binding-deficient SRF mutant with dominant negative properties, the authors confirm that stimulation of SM22 and SM-α-actin expression by low ATP concentration is mediated by SRF activation. High concentrations of ATP, on the other hand, inhibited both SRF activity and the expression of VSMC-specific proteins, indicating a conversion into a synthetic phenotype. This was dependent on the transient PKA activation described above. Interestingly, the change from the contractile to the synthetic phenotype is also characterized by downregulation of the contractile P2X1 receptor and upregulation of mitogenic P2Y receptors ([6][11], [17][12]). The P2X1 receptor is specific for the contractile VSMC ([1][13], [21][14]). Among P2 receptors, that subtype has the highest expression in VSMC in the intact vessel wall, and its expression is lost during cell culture ([6][11]). Shear stress, which is known to release ATP ([2][15]), induces changes in VSMC that are similar to a shift toward a more synthetic phenotype ([22][16]). Furthermore, high concentrations of ATP upregulate the expression of the mitogenic P2Y receptors P2Y2 and P2Y6 ([9][2], [11][17]). Hogarth et al. ([8][8]) provide a possible mechanism for the observed ATP-induced upregulation of P2Y receptors via transient stimulation of cAMP and PKA with a subsequent decrease in SRF activity. This could explain the observed forskolin-induced upregulation of P2Y2 receptors ([10][18]). It will be of interest to test for direct evidence for the link between SRF and P2 receptor regulation. ### P2 Receptor Pharmacology Nucleotide receptor pharmacology is complex because of the large number of P2 receptor subtypes and the limited availability of selective antagonists ([16][19]). Hogarth et al. ([8][8]) thus used various agonists to help...

This publication has 22 references indexed in Scilit: