The bifunctional role of TP53INP2 in transcription and autophagy

ABSTRACT

Cells integrate intracellular and extracellular cues to precisely control the balance of anabolic and catabolic processes, which is essential for cells to maintain homeostasis. The nuclear protein TP53INP2 (tumor protein p53 inducible nuclear protein 2) has emerged as one of the key players participating in both anabolic and catabolic processes. In the nucleus including the nucleolus, TP53INP2 binds to multiple transcription-related factors to modulate transcription, such as the transcription of thyroid hormone-related genes and ribosomal DNA. Interestingly, upon nutrient deprivation, TP53INP2 rapidly moves from the nucleus to the cytoplasm and participates in the regulation of macroautophagy/autophagy. By acting as a nutrient status sensor, TP53INP2 switches its role between transcription and autophagy by changing its subcellular localization and helps the cell to cope with environmental changes.

Abbreviations

Atg: autophagy related; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MTORC1: mechanistic target of rapamycin kinase complex 1; rDNA: ribosomal DNA; TP53INP2: tumor protein p53 inducible nuclear protein 2; UIM: ubiquitin-interacting motif

TP53INP2/DOR (tumor protein p53 inducible nuclear protein 2 was first characterized as a highly expressed gene in actively metabolizing tissues, such as skeletal muscle and heart [1]. Interestingly, the expression of TP53INP2 is dramatically repressed in the skeletal muscle of obese diabetic rats [1], and the reduction of TP53INP2 is to be protective for muscle wasting [2]. In addition, overexpression of TP53INP2 in muscle is recently reported to exacerbate cancer-induced muscle wasting [3]. As a nuclear protein, TP53INP2 is involved in transcription regulation by activating various nuclear hormone receptors [1,4]. Consistent with its role in transcription, TP53INP2 is able to be localized in the nucleolus [5,6], promoting ribosome biogenesis by stimulating ribosomal DNA (rDNA) transcription [6]. Upon nutrient starvation, nuclear TP53INP2 rapidly moves to the cytoplasm and targets to autophagic membranes to regulate autophagosome formation [7–9]. Moreover, TP53INP2 also plays a role in the autophagic degradation of ubiquitinated proteins [10]. Based on current evidence, TP53INP2 is proposed to play completely different roles in the nucleus and in the cytoplasm, and the change of its subcellular distribution seems to be the key switch, which is tightly controlled by nutrient signals (Figure 1).

Figure 1.

Overview of the role of TP53INP2 in transcription and autophagy. Under nutrient rich–conditions, TP53INP2 is mainly localized in the nucleus including the nucleolus, promoting nuclear hormone receptor-related transcription or rDNA transcription. Upon starvation, nuclear TP53INP2 rapidly moves to the cytoplasm to participate in autophagy regulation, by binding to different partners within associated autophagic membrane compartments. ATG, autophagy related; ER, endoplasmic reticulum; LC3, microtubule associated protein 1 light chain 3; rDNA, ribosomal DNA; TP53INP2, tumor protein p53 inducible nuclear protein 2; TR, thyroid hormone receptor; Ub, ubiquitin; UBTF, upstream binding transcription factor; VMP1, vacuole membrane protein 1.

The first identified nuclear hormone receptor targeted by TP53INP2 is THRA/TRα (thyroid hormone receptor alph/TRα (thyroid hormone receptor alpha) [1]. TP53INP2 physically binds to THRA to enhance its transcriptional activity [1]. Mechanistically, the N-terminal part of TP53INP2 is necessary and sufficient to modulate transcription, whereas the C-terminal part seems to play the inhibitory role [4]. In Drosophila, the homolog of TP53INP2 is also involved in transcription regulation through interacting with and activating the ecdysone receptor, whose activity is required for the degradation of the salivary gland during Drosophila development [11]. In addition, TP53INP2 enhances the transcriptional activity of many other nuclear hormone receptors, including NR3C1/glucocorticoid receptor (nuclear receptor subfamily 3 group C member 1), PPAR (peroxisome proliferator-activated receptor, and VDR (vitamin D receptor) [4]. However, TP53INP2 doesnot affect the transcriptional activity of TP53 or MYC/c-Myc [4], suggesting that TP53INP2 doesnot serve as a universal co-activator for all transcription factors. It needs further studies to explore the molecular mechanisms underlying the preference of TP53INP2 for nuclear hormone receptors, which would be helpful to elucidate the comprehensive role of TP53INP2 in transcription.

In addition to localization in the nucleoplasm, TP53INP2 is also reported to be partially distributed in the nucleolus [5,6], the major site for rDNA transcription in eukaryotic cells. Under nutrient rich conditions, TP53INP2 rapidly shuttles between the nucleoplasm and the nucleolus [6]. Nucleolar TP53INP2 stimulates rDNA transcription by facilitating the assembly of POLR1 (RNA polymerase I) preinitiation complex at rDNA promoter regions [6]. In addition, exclusion of TP53INP2 from the nucleolus by deletion of the nucleolar localization signal (NoLS) dramatically represses rDNA transcription [6]. Intriguingly, TP53INP2 directly interacts with upstream binding transcription factor (UBTF) but not TATA box-binding protein-associated factor RNA polymerase I subunit A (TAF1A), another component of the RNA polymerase I preinitiation complex [6], implying that TP53INP2 may mainly act as the co-activator for UBTF in rDNA transcription. It is noteworthy that mechanistic target of rapamycin kinase complex 1 (MTORC1) inhibition is sufficient to release TP53INP2 from the nucleolus [6], suggesting that TP53INP2 seems to function as one of the downstream effectors for MTORC1 in the regulation of rDNA transcription.

TP53INP2 was first identified as a binding partner of Atg8)-family proteins by yeast two-hybrid screening [9]. Upon starvation or rapamycin treatment, nuclear TP53INP2 moves to the cytoplasm targets to autophagic structures, and brings nuclear Atg8-family proteins, including MAP1LC3/LC3 (microtubule associated protein 1 light chain 3), into the cytoplasm to participate in autophagosome formation [7,9]. In the cytoplasm, TP53INP2 still localizes to the autophagic structures and contributes to autophagosome biogenesis through enhancing the binding of LC3 to its E1-like enzyme ATG7 [12]. In addition, in the cytoplasm, TP53INP2 is able to bind to VMP1 (vacuole membrane protein 1), a component seeming to function in phagophore formation [9,13]. All these studies suggest that TP53INP2 is involved in multiple stages of autophagy, including phagophore formation and LC3 lipidation, by binding to different autophagy-related proteins.

Several studies have reported that TP53INP2 is able to interact with ubiquitin and ubiquitinated proteins [2,10,14]. Recently, two groups have identified the ubiquitin-interacting motif (UIM), which mediates the interaction with ubiquitin and ubiquitinated proteins for TP53INP2 [10,14]. During selective autophagy, a receptor protein is required for the recruitment of autophagic substrates to phagophores [15,16]. To act as an autophagic receptor, the protein must be capable of binding to autophagic substrates and targeting to phagophores [15,17]. Interestingly, overexpression of TP53INP2 with deletion of the UIM but not TP53INP2 with deletion of the UIM and the LC3-interacting region (LIR), which is responsible for the binding to LC3 and/or other Atg8-family proteins, causes accumulation of ubiquitinated proteins in cells [10]. These findings suggest that both binding to cargoes and targeting to phagophores are required for TP53INP2 to promote the degradation of ubiquitinated proteins. It is noteworthy that knockdown of SQSTM1 (sequestosome 1), one of the autophagic receptors, but not TP53INP2, leads to obvious accumulation of ubiquitinated proteins in HeLa cells [10]. Although there are several well-known autophagy receptors proteins in mammalian cells, SQSTM1 seems to be sufficient to mediate the degradation of ubiquitinated proteins. Intriguingly, knockdown of TP53INP2 promotes the degradation of SQSTM1 [10], implying that other autophagic receptor proteins, SQSTM1 at least, are able to replace the role of TP53INP2 in autophagic degradation in the absence of TP53INP2. Although TP53INP2 is capable of binding to ubiquitinated substrates and targeting to phagophores, the current evidence for TP53INP2 as a novel autophagic receptor remains vague. So far, no studies demonstrate that TP53INP2 can be localized to autolysosomes, one feature of receptors adaptors. As TP53INP2 moves from the nucleus to the cytoplasm upon autophagy induction, TP53INP2 may play a role in the autophagic degradation of some nuclear proteins other than nuclear LC3 [7], by bringing the nuclear proteins from the nucleus to the cytoplasm and targeting them to phagophores. Interestingly, TP53INP2 is recently reported to promote the sequestration of GSK3B/GSK3β (glycogen synthase kinase 3 beta) into late endosomes in an autophagy-dependent manner, which leads to an increase of CTNNBIP1 (catenin beta interacting protein 1) levels and activation of TCF/LEF transcription factors [18]. Functionally, TP53INP2 ablation increases the expression of adipogenic genes and leads to enhanced adiposity in mice [18]. Taken together, TP53INP2 appears to be able to target client proteins to CTNNBIP1 or endosomes. It would be interesting to investigate whether TP53INP2 can also target cellular proteins to other membrane-bound organelles, such as mitochondria or peroxisomes.

We have summarized the current knowledge of the role of TP53INP2 in the regulation of transcription and autophagy. Interestingly, in addition to transcription and autophagy, accumulating evidence indicates that TP53INP2 is also involved in various other biological processes, such as apoptosis and cell migration [14,19]. Considering the multifaceted role of TP53INP2, cells must integrate intracellular and extracellular signals to tightly control the distinct function of TP53INP2 in transcription, autophagy, apoptosis, cell migration, and so on. The subcellular localization of TP53INP2 seems to be a key determinant switching its function between transcription activation and autophagy initiation. So far, MTORC1 is shown to be the master upstream regulator controlling the distribution of TP53INP2 in the nucleus, including the nucleolus, and in the cytoplasm [6,9]. However, the underlying molecular mechanism by which MTORC1 regulates the subcellular distribution of TP53INP2 remains unclear. Although MTORC1 mainly regulates downstream proteins by phosphorylation [20–22], TP53INP2 seems to be an exception. TP53INP2 localizes to the nucleus depending on its C-terminal domain, but mutation of the serine/threonine residues in this region has no effect on the nuclear localization of TP53INP2 [6]. There may be a missing link between MTORC1 and TP53INP2, which is supposed to be regulated by MTORC1 and control the subcellular distribution of TP53INP2. In addition to the protein TP53INP2, the transcript of TP53INP2 also carries out biological functions [23]. A recent study reports that the transcript of TP53INP2 is the most abundant mRNA in sympathetic neuron axons, but is nearly not translated into protein [23]. Functionally, the transcript of TP53INP2 is required for axon growth by regulating NTRK1/TrkA endocytosis and nerve growth factor (NGF) signaling [23]. It would be interesting to explore the underlying mechanism that controls the translation of the transcript of TP53INP2, which is supposed to determine whether TP53INP2 functions as mRNA or as protein.

Funding Statement

This work was supported by the National Natural Science Foundation of China (31970694, and 31701213), and the Hunan Provincial Natural Science Foundation of China (2017JJ3047).

Disclosure statement

No potential conflict of interest was reported by the authors.