Downstream molecular events in the altered profiles of lysophosphatidic acid-induced cAMP in senescent human diploid fibroblasts

Ik-Soon Jang,Eui-Ju Yeo,Sang Chul Park,Ji-Heon Rhim

doi:10.1038/emm.2006.17

Abstract

Lysophosphatidic acid (LPA) is a phospholipid growth factor that acts through G-protein-coupled receptors. Previously, we demonstrated an altered profile of LPA-dependent cAMP content during the aging process of human diploid fibroblasts (HDFs). In attempts to define the molecular events associated with the age-dependent changes in cAMP profiles, we determined the protein kinase A (PKA) activity, phosphorylation of cAMP-response element binding protein (CREB), and the protein expression of CRE-regulatory genes, c-fos and COX-2 in young and senescent HDFs. We observed in senescent cells, an increase in mRNA levels of the catalytic subunit a of PKA and of the major regulatory subunit Ialpha. Senescence-associated increase of cAMP after LPA treatment correlated well with increased CREB phosphorylation accompanying activation of PKA in senescent cells. In senescent cells, after LPA treatment, the expression of c-fos and COX-2 decreased initially, followed by an increase. In young HDFs, CREB phosphorylation decreased following LPA treatment, and both c-fos and COX-2 protein levels increased rapidly. CRE-luciferase assay revealed higher basal CRE-dependent gene expression in young HDFs compared to senescent HDFs. However, LPA-dependent slope of luciferase increased more rapidly in senescent cells than in young cells, presumably due to an increase of LPA-induced CREB phosphorylation. CRE-dependent luciferase activation was abrogated in the presence of inhibitors of PKC, MEK1, p38MAPK, and PKA, in both young and senescent HDFs. We conclude that these kinase are coactivators of the expression of CRE-responsive genes in LPA-induced HDFs and that their changed activities during the aging process contribute to the final expression level of CRE-responsive genes.

Highlights

Lysophosphatidic acid (LPA) is a phospholipid growth factor that elicits a variety of cellular responses, such as changes in cell shape, chemotaxis, proliferation, and differentiation, depending on the cell type (Moolenaar et al, 1997; An et al, 1998b; Moolenaar, 2000)
Altered expression of catalytic and regulatory subunits of protein kinase A (PKA) in senescent cells Previously, we reported that levels of cellular com ponents of cAMP signaling systems variously
We demonstrated an altered profile of LPA-induced cAMP accumulation in the senescent state (Jang et al, 2003)

Summary

Introduction

Lysophosphatidic acid (LPA) is a phospholipid growth factor that elicits a variety of cellular responses, such as changes in cell shape, chemotaxis, proliferation, and differentiation, depending on the cell type (Moolenaar et al, 1997; An et al, 1998b; Moolenaar, 2000). The cell surface G-protein-coupled receptors for LPA were identified as products of a family of endothelial cell differentiation genes (EDGs) (Contos et al, 2000; Fukushima et al, 2001). The major members of this EDG family of proteins were shown to be EDG-2 (Hecht et al, 1996), EDG-4 (An et al, 1998a), and EDG-7 (Bandoh et al, 1999), recently renamed as LPA1, LPA2, and LPA3, respectively. LPA1 and LPA2 inhibit all types of ACs through Gi activation, resulting in the reduction of cAMP (Taussig et al, 1993)

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