Lysophosphatidic acid and adenylyl cyclase inhibitor increase proliferation of senescent human diploid fibroblasts by inhibiting adenosine monophosphate-activated protein kinase

This study was designed to elucidate the molecular mechanism underlying lysophosphatidic acid (LPA) and adenylyl cyclase inhibitor SQ22536 (ACI)-induced senescent human diploid fibroblast (HDF) proliferation. Because adenosine monophosphate (AMP)-activated protein kinase (AMPK) is known to inhibit cell proliferation, we examined the phosphorylation status of AMPK and p53 and the expression level of p21(waf1/cip1) after treating HDFs with LPA and ACT. Phosphorylation of AMPK alpha on threonine-172 (p-Thr172-AMPK alpha) increases its catalytic activity but phosphorylation on serine-485/491 (p-Ser485/491-AMPK alpha) reduces the accessibility of the Thr172 phosphorylation site thereby inhibiting its catalytic activity. LPA increased p-Ser485/491-AMPKa, presumably by activating cAMP-dependent protein kinase (PKA). However, ACT reduced p-Thr172-AMPK alpha by inhibiting the LKB signaling. Our data demonstrated that both LPA and ACT inhibit the catalytic activity of AMPKa and p53 by differentially regulating phosphorylation of AMPKa, causing increased senescent cell proliferation. These findings suggest that the proliferation potential of senescent HDFs can be modulated through the regulation of the AMPK signaling pathway.