The cellular membrane phospholipid phosphatidylinositol (PtdIns) and its metabolites are critical signaling molecules. PtdIns is synthesized at the endoplasmic reticulum (ER) membrane (1
) and can then be phosphorylated at the 3, 4, and 5 positions of the inositol ring, generating a variety of monophosphorylated metabolites. These metabolites serve as precursors for additional phosphorylation events that result in the generation of PtdInsP2
is in turn hydrolyzed by phospholipase C (PLC), generating diacylglycerol (DAG) and inositol-1,4,5-triphosphate, resulting in the formation of additional molecules capable of intracellular signaling (38
) is generated by the PtdIns3-kinase (PI3K) superfamily of lipid kinases (16
). PI3K activity and PIP3
production are regulated by growth factors and chemokines, leading to the activation of Akt, one of the key growth and survival pathways in the cell. Additionally, generation of phosphatidic acid via the mitogen-stimulated activation of phospholipase D (PLD) provides another signal promoting Akt activation due to the phosphatidic acid-dependent assembly of the mTORC2 complex (18
PI3K class IA
is composed of a 110-kDa catalytic subunit (p110) and an 85-kDa adaptor/regulatory subunit (p85). Mammalian cells have three p110 isoforms (p110α, -β, and -δ), encoded by three separate genes and at least seven adaptor proteins that are generated through alternative splicing of transcripts encoded by three distinct genes, p85α, p85β, and p55γ. The p85 subunit has two Src homology 2 (SH2) domains that dock with phosphorylated tyrosine residues generated by activated tyrosine kinases (3
). The p85 SH2 domain mediates recruitment of the cytosolic PI3Ks to cellular membranes where their lipid substrates reside. The p110 subunit-binding site within p85 is located between the two SH2 domains (inter-SH2 domain) (13
The ER transmembrane serine/threonine kinase PERK, or EIF2AK3 (26
), is activated under conditions of physiological ER stress such as low carbon source (glucose deprivation), low oxygen (hypoxia), or increased synthetic demand in secretory tissues, as well as by chemical inducers of ER stress (tunicamycin, thapsigargin). PERK-dependent phosphorylation of eIF2α triggers decreased protein synthesis to alleviate ER client protein load while simultaneously increasing production of key protein substrates necessary for cell adaptation (26
). PERK also maintains cellular redox homeostasis via direct phosphorylation of the Nrf2 transcription factor (12
). Moreover, PERK has been linked to the activation of PI3K signaling and Akt during conditions of ER stress (25
). PKR, another eIF2α kinase, can also stimulate Akt phosphorylation (31
). Since these two kinases share eIF2α as their substrate, it was proposed that PI3K activation is dependent on the effect of PERK/PKR on translation attenuation.
Here, we present evidence that PERK possesses an intrinsic lipid kinase activity toward diacylglycerol (DAG), generating phosphatidic acid (PA) as a major product. Additional evidence is provided demonstrating that PERK lipid kinase activity is regulated in a PI3K regulatory subunit p85α-dependent manner. The lipid kinase function of PERK mediates mTOR, Akt, and Erk1/2 activation during ER stress. Critically, the activity of PERK is necessary for the activation of anabolic pathways downstream of Akt in a physiologically relevant setting.