Phosphoprotein enriched in astrocytes-15 kDa expression inhibits astrocyte migration by a protein kinase C delta-dependent mechanism.

Phosphoprotein enriched in astrocytes-15 kDa (PEA-15), a phosphoprotein enriched in astrocytes, inhibits both apoptosis and proliferation in normal and cancerous cells. Here, analysis of PEA-15 expression in glioblastoma organotypic cultures revealed low levels of PEA-15 in tumor cells migrating away from the explants, regardless of the expression levels in the originating explants. Because glioblastomas are highly invasive primary brain tumors that can originate from astrocytes, we explored the involvement of PEA-15 in the control of astrocyte migration. PEA-15-/- astrocytes presented an enhanced motility in vitro compared with their wild-type counterparts. Accordingly, NIH-3T3 cells transfected by green fluorescent protein-PEA-15 displayed a reduced migration. Reexpression of PEA-15 restored PEA-15-/- astrocyte motility to wild-type levels. Pharmacological manipulations excluded a participation of extracellular signal-regulated kinase/mitogen-activated protein kinase, phosphatidylinositol 3-kinase/Akt, and calcium/calmodulin-dependent protein kinase II in this effect of PEA-15. In contrast, treatment by bisindolylmaleimide, Gö6976, and rottlerin, and chronic application of phorbol 12-myristate 13-acetate and/or bryostatin-1 indicated that PKC delta mediated PEA-15 inhibition of astrocyte migration. PEA-15-/- astrocytes constitutively expressed a 40-kDa form of PKC delta that was down-regulated upon PEA-15 reexpression. Together, these data reveal a new function for PEA-15 in the inhibitory control of astrocyte motility through a PKC delta-dependent pathway involving the constitutive expression of a catalytic fragment of PKC delta.

Akt down-regulates ERK1/2 nuclear localization and angiotensin II-induced cell proliferation through PEA-15.

Angiotensin II (AngII) type 1 receptors (AT1) regulate cell growth through the extracellular signal-regulated kinase (ERK)1/2 and phosphatidylinositol 3-kinase (PI3K) pathways. ERK1/2 and Akt/protein kinase B, downstream of PI3K, are independently activated but both required for mediating AngII-induced proliferation when expressed at endogenous levels. We investigate the effect of an increase in the expression of wild-type Akt1 by using Chinese hamster ovary (CHO)-AT1 cells. Unexpectedly, Akt overexpression inhibits the AT1-mediated proliferation. This effect could be generated by a cross-talk between the PI3K and ERK1/2 pathways. A functional partner is the phosphoprotein enriched in astrocytes of 15 kDa (PEA-15), an Akt substrate known to bind ERK1/2 and to regulate their nuclear translocation. We report that Akt binds to PEA-15 and that Akt activation leads to PEA-15 stabilization, independently of PEA-15 interaction with ERK1/2. Akt cross-talk with PEA-15 does not affect ERK1/2 activation but decreases their nuclear activity as a result of the blockade of ERK1/2 nuclear accumulation. In response to AngII, PEA-15 overexpression displays the same functional consequences on ERK1/2 signaling as Akt overactivation. Thus, Akt overactivation prevents the nuclear translocation of ERK1/2 and the AngII-induced proliferation through interaction with and stabilization of endogenous PEA-15.

PEA-15 modulates TNFalpha intracellular signaling in astrocytes.

PEA-15 is a small protein (15 kDa) that was first identified as an abundant phosphoprotein in brain astrocytes and subsequently shown to be widely expressed in different tissues and highly conserved among mammals. It is composed of an N-terminal death effector domain (DED) and a C-terminal tail of irregular structure. PEA-15 is regulated by multiple calcium-dependent phosphorylation pathways. PEA-15 is ideally positioned to play a major role in signal integration. Accordingly, it has been demonstrated that PEA-15 diverts astrocytes from TNFalpha-triggered apoptosis and regulates the actions of the ERK MAP kinase cascade by binding to ERK and altering its subcellular localization. Expression of PEA-15 directs TNFalpha outcomes toward survival, whereas its absence allows the development of the cytokine-induced cell death.