Brad J. Niles

The yeast AGC kinase orthologs Ypk1 and Ypk2 control several important cellular processes, including actin polarization, endocytosis, and sphingolipid metabolism. Activation of Ypk1/2 requires phosphorylation by kinases localized at the plasma membrane (PM), including the 3-phosphoinositide-dependent kinase 1 orthologs Pkh1/Pkh2 and the target of rapamycin complex 2 (TORC2). Unlike their mammalian counterparts SGK and Akt, Ypk1 and Ypk2 lack an identifiable lipid-targeting motif; therefore, how these proteins are recruited to the PM has remained elusive. To explore Ypk1/2 function, we constructed ATP analog-sensitive alleles of both kinases and monitored global changes in gene expression following their inhibition, where we observed increased expression of stress-responsive target genes controlled by Ca(2+)-dependent phosphatase calcineurin. TORC2 has been shown previously to negatively regulate calcineurin in part by phosphorylating two related proteins, Slm1 and Slm2, which associate with the PM via plextrin homology domains. We therefore investigated the relationship between Slm1 and Ypk1 and discovered that these proteins interact physically and that Slm1 recruits Ypk1 to the PM for phosphorylation by TORC2. We observed further that these steps facilitate subsequent phosphorylation of Ypk1 by Pkh1/2. Remarkably, a requirement for Slm1, can be bypassed by fusing the plextrin homology domain of Slm1 alone onto Ypk1, demonstrating that the essential function of Slm1 is largely attributable to its role in Ypk1 activation. These findings both extend the scope of cellular processes regulated by Ypk1/2 to include negative regulation of calcineurin and broaden the repertoire of mechanisms for membrane recruitment and activation of a protein kinase.

Zinc deficiency is characterized by an attenuation of growth factor signaling pathways and an amplification of p53 pathways. This outcome is facilitated by hypo-phosphorylation of AKT and ERK secondary to zinc deficiency, which are permissive events to the activation of the intrinsic cell death pathway. Low zinc concentrations provide an environment that is also conducive to the production of reactive oxygen/reactive nitrogen species (ROS/RNS) and caspase activation. Additionally, during zinc deficiency endogenous survival pathways such as NF-kappaB are inhibited in their transactivation potential. The above factors contribute to the irreversible commitment of the zinc deficient cell to death.

Reactive oxygen species (ROS) are produced during normal metabolism and can function as signaling molecules. However, ROS at elevated levels can damage cells. Here, we identify the conserved target of rapamycin complex 2 (TORC2)/Ypk1 signaling module as an important regulator of ROS in the model eukaryotic organism, S. cerevisiae. We show that TORC2/Ypk1 suppresses ROS produced both by mitochondria as well as by nonmitochondrial sources, including changes in acidification of the vacuole. Furthermore, we link vacuole-related ROS to sphingolipids, essential components of cellular membranes, whose synthesis is also controlled by TORC2/Ypk1 signaling. In total, our data reveal that TORC2/Ypk1 act within a homeostatic feedback loop to maintain sphingolipid levels and that ROS are a critical regulatory signal within this system. Thus, ROS sensing and signaling by TORC2/Ypk1 play a central physiological role in sphingolipid biosynthesis and in the maintenance of cell growth and viability.

The evolutionarily conserved mTOR complex 2 (mTORC2) signaling pathway is an important regulator of actin cytoskeletal architecture and, as such, is a candidate target for preventing cancer cell motility and invasion. Remarkably, the precise mechanism(s) by which mTORC2 regulates the actin cytoskeleton have remained elusive. Here we show that in budding yeast, TORC2 and its downstream kinase Ypk1 regulate actin polarization by controlling reactive oxygen species (ROS) accumulation. Specifically, we find that TORC2-Ypk1 regulates actin polarization both by vacuole-related ROS, controlled by the phospholipid flippase kinase Fpk1 and sphingolipids, and by mitochondria-mediated ROS, controlled by the PKA subunit Tpk3. In addition, we find that the protein kinase C (Pkc1)/MAPK cascade, a well-established regulator of actin, acts downstream of Ypk1 to regulate ROS, in part by promoting degradation of the oxidative stress responsive repressor, cyclin C. Furthermore, we show that Ypk1 regulates Pkc1 activity through proper localization of Rom2 at the plasma membrane, which is also dependent on Fpk1 and sphingolipids. Together these findings demonstrate important links between TORC2/Ypk1 signaling, Fpk1, sphingolipids, Pkc1, and ROS as regulators of actin and suggest that ROS may play an important role in mTORC2-dependent dysregulation of the actin cytoskeleton in cancer cells.

One consequence of zinc deficiency is an elevation in cell and tissue iron concentrations. To examine the mechanism(s) underlying this phenomenon, Swiss 3T3 cells were cultured in zinc-deficient (D, 0.5 μm zinc), zinc-supplemented (S, 50 μm zinc), or control (C, 4 μm zinc) media. After 24 h of culture, cells in the D group were characterized by a 50% decrease in intracellular zinc and a 35% increase in intracellular iron relative cells in the and increase in iron and and zinc iron of zinc-deficient cells of iron and of a in zinc-deficient cells an of of of zinc intracellular by the of the or the D the zinc deficiency in in iron and iron One consequence of zinc deficiency is an elevation in cell and tissue iron concentrations. To examine the mechanism(s) underlying this phenomenon, Swiss 3T3 cells were cultured in zinc-deficient (D, 0.5 μm zinc), zinc-supplemented (S, 50 μm zinc), or control (C, 4 μm zinc) media. 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