Disruption of astrocytic glutamine turnover by manganese is mediated by the protein kinase C pathway

Abstract

Manganese (Mn) is a trace element essential for normal human development and is required for the proper functioning of a variety of physiological processes. Chronic exposure to Mn can cause manganism, a neurodegenerative disorder resembling idiopathic Parkinson's disease (PD). Mn(II) neurotoxicity is characterized by astrocytic impairment both in the expression and activity of glutamine (Gln) transporters. Because protein kinase C (PKC) activation leads to the downregulation of a number of neurotransmitter transporters and Mn(II) increases PKC activity, we hypothesized that the PKC signaling pathway contributes to the Mn(II)-mediated disruption of Gln turnover. Our results have shown that Mn exposure increases the phosphorylation of both the PKCα and PKCδ isoforms. PKC activity was also shown to be increased in response to Mn(II) treatment. Corroborating our earlier observations, Mn(II) also caused a decrease in Gln uptake. This effect was blocked by PKC inhibitors. Notably, PKC activation caused a decrease in Gln uptake mediated by systems ASC and N, but had no effect on the activities of systems A and L. Exposure to α-phorbol 12-myristate 13-acetate significantly decreased SNAT3 (system N) and ASCT2 (system ASC) protein levels. Additionally, a co-immunoprecipitation study demonstrated the association of SNAT3 and ASCT2 with the PKCδ isoform, and Western blotting revealed the Mn(II)-mediated activation of PKCδ by proteolytic cleavage. PKC activation was also found to increase SNAT3 and ubiquitin ligase Nedd4-2 binding and to induce hyperubiquitination. Taken together, these findings demonstrate that the Mn(II)-induced dysregulation of Gln homeostasis in astrocytes involves PKCδ signaling accompanied by an increase in ubiquitin-mediated proteolysis.

Expression of glutamine transporter isoforms in cerebral cortex of rats with chronic hepatic encephalopathy

“…For that reason it should be considered that these transporters may be subject to post translational regulation, a mechanism that has been broadly described for SN1 (Karinch et al, 2002;Nissen-Meyer and Chaudhry, 2013). Accordingly, different studies observed that SN1 is directly phosphorylated by the enzymes PKCα, PKCγ, and PKCδ (Nissen-Meyer et al, 2011;Sidoryk-Wegrzynowicz et al, 2011). More specifically, PKC phosphorylates SN1 in astrocytes at one single serine residue at the N-terminal of the transporter, which consequently targets the transporter for intracellular reservoirs and therefore regulates the transporter activity (Nissen-Meyer et al, 2011).…”

Mechanism of Manganese-Induced Impairment of Astrocytic Glutamate Transporters

“…110 Recent studies revealed that phorbol ester-induced activation of PKC decreased glutamate uptake, and inhibition of PKC reversed the Mn-induced decrease of glutamate uptake as well as GLT-1 and GLAST protein levels. 109 Furthermore, Mn enhanced the interaction of GLT-1 with PKC-d, indicating that PKC-d isoform plays a critical role in glutamate turnover in astrocytes.…”

Manganese and Oxidative Stress

“…132 Furthermore, Mn decreases astrocytic glutamate uptake and expression of the astrocytic glutamate transporter, GLAST 132 via disruption of intracellular signaling. 133 Of potential clinical significance, estrogen and tamoxifen have been reported to increase the expression of glutamate transporters (GLAST and GLT-1) in astrocytes, decreasing Mn toxicity. [134][135][136][137] …”