Mdm20 Modulates Actin Remodeling through the mTORC2 Pathway via Its Effect on Rictor Expression

Abstract

NatB is an N-terminal acetyltransferase consisting of a catalytic Nat5 subunit and an auxiliary Mdm20 subunit. In yeast, NatB acetylates N-terminal methionines of proteins during de novo protein synthesis and also regulates actin remodeling through N-terminal acetylation of tropomyosin (Trpm), which stabilizes the actin cytoskeleton by interacting with actin. However, in mammalian cells, the biological functions of the Mdm20 and Nat5 subunits are not well understood. In the present study, we show for the first time that Mdm20-knockdown (KD), but not Nat5-KD, in HEK293 and HeLa cells suppresses not only cell growth, but also cellular motility. Although stress fibers were formed in Mdm20-KD cells, and not in control or Nat5-KD cells, the localization of Trpm did not coincide with the formation of stress fibers in Mdm20-KD cells. Notably, knockdown of Mdm20 reduced the expression of Rictor, an mTORC2 complex component, through post-translational regulation. Additionally, PKCαS657 phosphorylation, which regulates the organization of the actin cytoskeleton, was also reduced in Mdm20-KD cells. Our data also suggest that FoxO1 phosphorylation is regulated by the Mdm20-mTORC2-Akt pathway in response to serum starvation and insulin stimulation. Taken together, the present findings suggest that Mdm20 acts as a novel regulator of Rictor, thereby controlling mTORC2 activity, and leading to the activation of PKCαS657 and FoxO1.

mTORC2/AKT/HSF1/HuR constitute a feed-forward loop regulating Rictor expression and tumor growth in glioblastoma

“…Indeed, our analysis identified several HuR binding sites (see figure 2A) and additional regulatory domains are likely to be present in such a large 3' UTR, possibly controlling mRNA stability under particular conditions. Our data supporting the ability of the Rictor transcript to be subject to translational control is also reinforced by recent data from Yasuda and colleagues, who investigated the role of Mdm20 in actin remodeling via Rictor-mediated mTORC2 activity 20 . Mdm20 was suggested to regulate the expression of Rictor at the level of de novo protein synthesis.…”

“…Rictor is overexpressed in several cancers leading to hyperactive mTORC2 and has been shown to play a causal role in glioma formation 12,[14][15][16][17] . Rictor expression has been demonstrated to be regulated transcriptionally and via protein degradation 18,19 , however recent studies have suggested that Rictor expression may also be regulated post-transcriptionally 20 .…”

mTORC2/AKT/HSF1/HuR constitute a feed-forward loop regulating Rictor expression and tumor growth in glioblastoma

“…Our analysis identified several HuR binding sites (see figure 2A ) and additional regulatory domains are likely to be present in such a large 3′ UTR, possibly controlling mRNA stability under particular conditions. Our data supporting the ability of the Rictor transcript to be subject to translational control is reinforced by recent data from Yasuda and colleagues, who investigated the role of Mdm20 in actin remodeling via Rictor-mediated mTORC2 activity 20 . Mdm20 was suggested to regulate the expression of Rictor at the level of de novo protein synthesis.…”

“…Rictor is overexpressed in several cancers leading to hyperactive mTORC2 and has been shown to play a causal role in glioma formation 12 , 14 - 17 . Rictor expression has been demonstrated to be regulated transcriptionally and via protein degradation 18 , 19 , however recent studies have suggested that Rictor expression may also be regulated post-transcriptionally 20 .…”

Naa20, the catalytic subunit of NatB complex, contributes to hepatocellular carcinoma by regulating the LKB1–AMPK–mTOR axis

“…In addition, Nt-acetylation of nicotinamide mononucleotide adenyltransferases (Nma1 and Nma2) was reported to be essential for maintaining NAD + homeostasis in yeast 9,10 . In mammals, NatB has been shown to participate in several cellular processes, including growth 11,12 , autophagy [13][14][15] , and viral infection 16 , by altering the levels of cell cycle-related genes, mammalian target of rapamycin (mTOR) C2 signaling, and the hippo/YAP and ERK1/2 pathways [11][12][13][14][15][16][17][18] , respectively, but the underlying mechanisms connecting Nt-acetylation with proteins are still unknown. Regarding tumorigenesis, several studies revealed that both subunits of NatB are upregulated in hepatocellular carcinoma (HCC) tumor tissues compared with nontumor tissues 11,12 , suggesting that NatB may be implicated in promoting tumorigenesis.…”