An obligatory requirement for the heterotrimeric G protein G _i3 in the antiautophagic action of insulin in the liver

Abstract

Heterotrimeric G proteins of the Gi class have been implicated in signaling pathways regulating growth and metabolism under physiological and pathophysiological conditions. Knockout mice carrying inactivating mutations in both of the widely expressed G␣ i class genes, G␣i2 and G␣i3, demonstrate shared as well as genespecific functions. The presence of a single active allele of G␣i3 is sufficient for embryonic development, whereas at least one allele of G␣ i2 is required for extrauterine life. Mice lacking both G␣i2 and G␣ i3 are massively growth-retarded and die in utero. We have used biochemical and cell biological methods together with in situ liver perfusion experiments to study G␣ i isoform-specific functions in G␣i2-and G␣i3-deficient mice. The subcellular localization of G␣i3 in isolated mouse hepatocytes depends on the cellular metabolic status. G␣ i3 localizes to autophagosomes upon starvation-induced autophagy and distributes to the plasma membrane upon insulin stimulation. Analysis of autophagic proteolysis in perfused mouse livers showed that mice lacking G␣ i3 are deficient in the inhibitory action of insulin. These data indicate that G␣ i3 is crucial for the antiautophagic action of insulin and suggest an as-yet-unrecognized function for G␣ i3 on autophagosomal membranes.anticatabolic actions ͉ autophagy ͉ mouse knockout ͉ pertussis toxin-sensitive G proteins

Defective Macrophage Migration in Gαi2- but Not Gαi3-Deficient Mice

“…These findings serve as an important basis for the identification of the underlying mechanisms of redundancy versus isoform specificity of Ga i2 and Ga i3 . Such mechanisms could be based on a different subcellular localization of these isoforms (16,47), structural differences in regions different from the receptor-or effector-binding domains, as well as the total amount of Ga i proteins in the cells, with the latter obviously not playing a decisive role in our model.…”

“…Macrophages, like most other cell types (16), express Ga i2 in much higher concentrations than Ga i3 (23). Importantly, however, low Ga i3 can compensate for the lack of high Ga i2 in LPS-induced macrophage activation and C5aR-regulated erythrophagocytosis (Fig.…”

“…Previous studies in mice lacking Ga i2 or Ga i3 suggested key roles of these two G i isoforms in distinct cellular responses (13)(14)(15)(16). For instance, Ga i3 deficiency leads to an impaired response of insulin-regulated autophagy in liver cells (16,17), whereas Ga i2 2/2 mice show defects in the migration of T and B cells to lymph nodes (18,19) and of eosinophils to sites of allergic tissue injury (20). Moreover, GPCR-induced activation of platelets is affected in Ga i2 2/2 mice (21).…”

Autophagic disposal of the aggregation-prone protein that causes liver inflammation and carcinogenesis in α-1-antitrypsin deficiency

“…RGS16 RNA levels were also significantly elevated in the liver of ATdeficient patients when compared with liver from normal and disease controls. Although we did not originally understand the functional importance of these specific changes in gene expression, a recent study by Gohla et al 35 has led to the hypothesis that upregulation of RGS16 plays a role in Figure 3 A hypothetical model for the role of RGS16 in activating hepatic autophagy in AT deficiency. The intermediates of the insulin-signaling pathway (Akt, TSC1 and 2, Rheb and mTOR) are shown downstream of the insulin receptor as well as the effect of amino acids.…”

Autophagy in the liver

“…Recently it has been found that both insulin and amino acids, such as phenylalanine, could also work through a pathway related to G␣i3 because deletion of G␣i3 eliminates their protective effects against protein degradation in perfused liver. 38 G␣i3 is a G␣ subunit of heterotrimeric G proteins, and its positive role (in guanosine diphosphate [GDP]-binding status) in autophagy induction was shown previously in a colon cancer cell line. 39 Consistently, an early study implicated the involvement of guanosine triphosphatase (GTPase) activity in autophagy induction in permeabilized hepatocytes.…”