SummaryIn cytotoxic T cells (CTL), Akt, also known as protein kinase B, is activated by the T cell antigen receptor (TCR) and the cytokine interleukin 2 (IL-2). Akt can control cell metabolism in many cell types but whether this role is important for CTL function has not been determined. Here we have shown that Akt does not mediate IL-2- or TCR-induced cell metabolic responses; rather, this role is assumed by other Akt-related kinases. There is, however, a nonredundant role for sustained and strong activation of Akt in CTL to coordinate the TCR- and IL-2-induced transcriptional programs that control expression of key cytolytic effector molecules, adhesion molecules, and cytokine and chemokine receptors that distinguish effector versus memory and naive T cells. Akt is thus dispensable for metabolism, but the strength and duration of Akt activity dictates the CTL transcriptional program and determines CTL fate.

The adenosine monophosphate (AMP)–activated protein kinase (AMPK) has a crucial role in maintaining cellular energy homeostasis. This study shows that human and mouse T lymphocytes express AMPKα1 and that this is rapidly activated in response to triggering of the T cell antigen receptor (TCR). TCR stimulation of AMPK was dependent on the adaptors LAT and SLP76 and could be mimicked by the elevation of intracellular Ca2+ with Ca2+ ionophores or thapsigargin. AMPK activation was also induced by energy stress and depletion of cellular adenosine triphosphate (ATP). However, TCR and Ca2+ stimulation of AMPK required the activity of Ca2+–calmodulin-dependent protein kinase kinases (CaMKKs), whereas AMPK activation induced by increased AMP/ATP ratios did not. These experiments reveal two distinct pathways for the regulation of AMPK in T lymphocytes. The role of AMPK is to promote ATP conservation and production. The rapid activation of AMPK in response to Ca2+ signaling in T lymphocytes thus reveals that TCR triggering is linked to an evolutionally conserved serine kinase that regulates energy metabolism. Moreover, AMPK does not just react to cellular energy depletion but also anticipates it.

Several pleckstrin-homology (PH) domains with the ability to bind phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P3, PIP3] were expressed as green fluorescent protein (GFP) fusion proteins to determine their effects on various cellular responses known to be activated by PIP3. These proteins comprised the PH domains of Akt, ARNO, Btk or GRP1, and were found to show growth-factor-stimulated and wortmannin-sensitive translocation from the cytosol to the plasma membrane in several cell types, indicating their ability to recognize PIP3. Remarkably, although overexpressed Akt-PH–GFP and Btk-PH–GFP were quite potent in antagonizing the PIP3-mediated activation of the Akt protein kinase, such inhibition was not observed with the other PH domains. By contrast, expression of the PH domains of GRP1 and ARNO, but not of Akt or Btk, inhibited the attachment and spreading of freshly seeded cells to culture dishes. Activation of PLCγ by epidermal growth factor (EGF) was attenuated by the PH domains of GRP1, ARNO and Akt, but was significantly enhanced by the Btk PH domain. By following the kinetics of expression of the various GFP-fused PH domains for several days, only the PH domain of Akt showed a lipid-binding-dependent self-elimination, consistent with its interference with the anti-apoptotic Akt signaling pathway. Mutations of selective residues that do not directly participate in PIP3 binding in the GRP1-PH and Akt-PH domain were able to reduce the dominant-negative effects of these constructs yet retain their lipid binding. These data suggest that interaction with and sequestration of PIP3 may not be the sole mechanism by which PH domains interfere with cellular responses and that their interaction with other membrane components, most probably with proteins, allows a more specific participation in the regulation of specific signaling pathways.

The serine/threonine kinase LKB1 has a conserved role in Drosophila and nematodes to co-ordinate cell metabolism. During T lymphocyte development in the thymus, progenitors need to synchronize increased metabolism with the onset of proliferation and differentiation to ensure that they can meet the energy requirements for development. The present study explores the role of LKB1 in this process and shows that loss of LKB1 prevents thymocyte differentiation and the production of peripheral T lymphocytes. We find that LKB1 is required for several key metabolic processes in T-cell progenitors. For example, LKB1 controls expression of CD98, a key subunit of the l-system aa transporter and is also required for the pre-TCR to induce and sustain the regulated phosphorylation of the ribosomal S6 subunit, a key regulator of protein synthesis. In the absence of LKB1 TCR-β-selected thymocytes failed to proliferate and did not survive. LBK1 was also required for survival and proliferation of peripheral T cells. These data thus reveal a conserved and essential role for LKB1 in the proliferative responses of both thymocytes and mature T cells.

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