Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

SLC3A2

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101895

Synonyms

Historical Background

The ability of cells to sense, respond, and adapt to their environment is essential for multicellular life. SLC3A2 protein provides cells with the capacity of adjusting to their surroundings by mediating two fundamental molecular functions: amino acid transport and integrin signaling.

SLC3A2 is involved in many cellular processes, such as early activation of T and B cells (Cantor et al. 2009, 2012), cell fusion (Deves et al. 2000; Takesono et al. 2012), cell survival and migration (Feral et al. 2005), cell proliferation (Cantor et al. 2009; Boulter et al. 2013; de la Ballina et al. 2016), mechanotransduction (Estrach et al. 2014), and angiogenesis ( Liao et al. 2016). Thus, SLC3A2 is crucial for responding to different cellular stresses (i.e., oxidative or nutritional stress (de la Ballina et al. 2016), lack of proper cell/ECM attachment (Feral et al. 2005), mechanical stress (Estrach et al. 2014)). Also, SLC3A2 interacts with partners other than amino acid transporters or integrins, forming part of macromolecular complexes (Liu et al. 2003; Xu et al. 2005; Yan et al. 2008) (detailed later). The genetic defect of SLC3A2 results in embryo lethality (Tsumura et al. 2003), and no SLC3A2 mutations have been described so far. Altogether these data highlight the pivotal role of SLC3A2 for a correct and efficient cell behavior.

SLC3A2 constitutes the ancillary protein of six human amino acid transporters (SLC7A5, SLC7A6, SLC7A7, SLC7A8, SLC7A10, and SLC7A11; Fotiadis et al. 2013); it also functions as a coreceptor of β-integrins, amplifying their downstream outside-in signaling (Feral et al. 2005). Interestingly, different domains of SLC3A2 regulate each function, namely, SLC3A2 cytoplasmic and transmembrane domains (105 aa) are responsible for integrin signaling, whereas SLC3A2 ectodomain (SLC3A2-ED, 425 aa) is implicated in amino acid transport (de la Ballina et al. 2016) without affecting integrin signaling (Feral et al. 2005).

Highly proliferative cells overexpress SLC3A2 both in physiological (Boulter et al. 2013) and pathological conditions (McCracken et al. 2013). Thus, whereas playing an important role in keeping homeostasis of healthy tissues, this signal integrator molecule also favors progression of disease, like in cancer, where SLC3A2 expression has been associated with poor prognosis (Cantor et al. 2012; Ip et al. 2016).

SLC3A2 constitutes an attractive therapeutic target for anticancer treatments (Bajaj et al. 2016; Ip et al. 2016). In order to develop drugs that affect SLC3A2-facilitated processes, it is crucial to understand the mechanisms by which this protein and its multiple partners function.

SLC3A2: Heavy Subunit of Heteromeric Amino Acid Transporters (HATs)

SLC3A2 is part of the heteromeric amino acid transporters (HATs), one of eleven families involved in amino acid transport across cell plasma membrane in mammals (Bröer et al. 2002). HATs constitute the only known example of solute carriers composed by two subunits: i) a heavy subunit (from solute carrier family SLC3) and ii) a light subunit (from solute carrier family SLC7), which are covalently linked together through a disulfide bridge. Human SLC3A2 heterodimerizes with one of six catalytic subunits, which are the actual transporters and confer substrate specificity to the heterodimeric complex (Reig et al. 2002). SLC3A2 traffics the whole transporter to the plasma membrane, where it mediates the corresponding amino acid transport activity. Combinations between SLC3A2 and each of the catalytic subunits render multiple amino acid transport systems, which cover a broad substrate range including all essential amino acids.

SLC3A2 is the only member of the SLC3 family exhibiting a ubiquitous distribution; however, in epithelial cells SLC3A2 is exclusively localized at the basolateral membrane. Such distinct distribution might determine the surface localization of its respective light chains allowing a vectorial amino acid transport in polarized cells. This is particularly relevant in membranes of epithelial cells of the kidney and intestine, where HATs are in charge of the reabsorption and absorption of amino acids filtered from the blood before their excretion and from digested proteins, respectively (Fig.1) (Fotiadis et al. 2013).
SLC3A2, Fig. 1

Amino acid transport mediated by HATs. (a) Interplay between HATs and other transporters allows vectorial transport of amino acids across epithelia, such as in the small intestine and kidney proximal tubule. Cystine (CSSC) and dibasic amino acids (AA+) fluxed through epithelial cells are ensured by the presence of distinct transport systems at the apical and basolateral membranes. The apical transport system b0,+ (catalyzed by SLC3A1/SLC7A9) mediates influx of AA+ and CSSC in exchange of neutral amino acids (AA 0 ), which are accumulated in the cell by the Na+-dependent transporter SLC6A19. AA+ exit the basolateral membrane through system y+L (catalyzed by SLC3A2/SLC7A7) in exchange of AA0 and Na+; this is favored by the Na+ gradient generated by the basolateral Na+, K+ ATPase. The apical small intestine transporter SLC15A1 contributes to the amino acid pool of the cell by mediating the uptake of di- and tripeptides. Transporters SLC16A10 (aka TAT1, for aromatic amino acids) and SLC43A2 (aka LAT4, for branched-chained amino acids, phenylalanine, and methionine) are candidates for mediating the basolateral efflux of intracellular AA0 (black circle with question mark). The involvement of SLC3A2/SLC7A8 in this process has also been proposed. (b) In non-epithelial cells HATs SLC3A2/SLC7A5, SLC3A2/SLC7A6, and SLC3A2/SLC7A11 provide cells with a balanced AA content, which allows them to counterbalance oxidative stress (via SLC3A2/SLC7A11 transport, import of CSSC, and its rapid conversion to cysteine (L-Cys), which constitutes a rate determining step in the glutathione (GSH) synthesis) and to fuel protein synthesis and concomitant cell proliferation

SLC3A2 and associated transporters are involved in several human pathologies. SLC3A2, SLC7A5 and SLC7A11 present an increased expression in tumors (being the amino acid transporters overexpressed in most cancer types) (McCracken et al. 2013). Moreover, SLC3A2 expression levels correlate with poor prognosis in several tumor types. SLC7A5 and SLC7A11 are both considered to be good targets for antitumor therapies, and several inhibitors against these transporters have been designed and tested for reducing tumor proliferation and progression (Ip et al. 2016). Besides their role in cancer, SLC3A2-associated light subunits are also involved in other diseases. Whereas mutations in SLC7A5 cause autism spectrum disorders (Tărlungeanu et al. 2016), SLC7A11 is essential for the Kaposi’s sarcoma-associated herpesvirus (KSHV) infection (Veettil et al. 2008). Finally, mutations in SLC7A7 cause lysinuric protein intolerance (LPI) (MIM 222700), a rare aminoaciduria (Torrents et al. 1999; Borsani et al. 1999).

SLC3A2-Associated Amino Acid Transport

Six different catalytic subunits (SLC7A5, SLC7A6, SLC7A7, SLC7A8, SLC7A10, and SLC7A11) require the ancillary protein SLC3A2 to be trafficked to the plasma membrane, where they yield four different transport activities (L, y+L, asc, and xc). SLC3A2-associated transporters function as Na+-independent mainly obligatory antiporters (SLC7A10 can also function as uniporter), meaning that they mediate the translocation of one amino acid across the membrane in exchange to another amino acid being transported in the opposite direction (1:1 stoichiometry). For a detailed review on HATs, see Fotiadis et al. (2013); here, an outline of SLC3A2-associated light subunits and the amino acid transport systems they mediate is presented (Table 1).
SLC3A2, Table 1

SLC3A2-associated SLC7 l-type amino acid transporters

Gene name

Protein name

Transport system

Substrates

Transport type

Link to disease

SLC7A5

LAT1

System L

Large neutral l-amino acids, thyroid hormones T3 and T4, l-DOPA, BCH

Antiporter. Similar intra-and extracellular selectivity, lower intracellular apparent affinity.

Overexpressed in cancer

SLC7A6

y+LAT2

System y+L

Na + -independent: l-cationic AA

Na + -dependent: large neutral l-AA

Antiporter. Preferentially intracellular cationic AA against extracellular neutral AA/Na+

 

SLC7A7

y+LAT1

System y+L

Na + -independent: l-cationic AA

Na + -dependent: large neutral l-AA

Antiporter. Preferentially intracellular cationic AA against extracellular neutral AA/Na+

LPI

SLC7A8

LAT2

System L

Neutral L-amino acids, thyroide hormone T3, l-DOPA, BCH

Antiporter. Similar intra-and extracellular selectivity, lower intracellular apparent affinity

 

SLC7A10

Asc1

System asc

Small neutral l-AA and d-serine

Preferentially antiporter

 

SLC7A11

xCT

System xc

l-cystine (anionic form) /l-glutamate

Antiporter. Preferentially extracellular cysteine against intracellular glutamate

Overexpressed in cancer/ KSHV infection

AA amino acid

Adapted from Fotiadis et al. (2013)

System L: SLC7A5 (LAT1) and SLC7A8 (LAT2)

System L is responsible for Na+-independent transport of large branched-chain and aromatic neutral amino acids in almost all types of cells. There are four isoforms of system L; two of them, SLC7A5 (LAT1) and SLC7A8 (LAT2), are SLC3A2-associated transporters, whereas the other two, SLC43A1 (LAT3) and SLC43A2 (LAT4), are SLC3A2-independent isoforms.

SLC3A2/SLC7A5 is an obligatory exchanger. The uptake selectivity range of this transporter is relatively broad (large neutral l-amino acids, thyroid hormones T3 and T4, l-3,4-dihydroxyphenylalanine (l-DOPA), and 2-aminobicyclo-[2,2,1]-heptane-2-carboxylic acid (BCH)). Because the apparent affinity for extracellular amino acids (in the micromolar range) is 100-fold higher than for intracellular ones, the concentration of intracellular amino acids controls the transport rate. SLC3A2/SLC7A5 does not mediate net uptake of amino acids; instead, this transporter is designed for equilibrating the relative concentrations of different amino acids across a membrane (Fotiadis et al. 2013). Its essential role in the uptake of branched-chain and aromatic amino acids has been recently demonstrated; SLC3A2/SLC7A5 function cannot be compensated for alternative transporters (de la Ballina et al. 2016). SLC3A2/SLC7A5 is also key in mTOR regulation (Nicklin et al. 2009; Cormerais et al. 2016) by providing neutral-branched amino acids to activate mTOR. Recent data show that SLC3A2/SLC7A5 is recruited to the lysosome membrane and mediates Leu uptake into the lysosome, being thus essential for mTOR activation, cell growth, and proliferation (Milkereit et al. 2015). SLC7A5 is expressed in most tested tumors and tumor cell lines.

SLC3A2/SLC7A8 exchanges all neutral amino acids, with the exception of proline, across the membrane in a Na+-independent manner and thereby equilibrates their relative concentration. This transporter presents a wider selectivity of neutral amino acids than SLC3A2/SLC7A5, transporting also small ones. Similar to SLC7A5, SLC7A8 also displays a much lower apparent affinity for intracellular amino acids than for extracellular ones (exception for glycine). SLC3A2/SLC7A5 is mainly expressed, but not only, at the basolateral membrane of the proximal kidney tubule and small intestine (Fotiadis et al. 2013).

System y+L: SLC7A7 (y+LAT1) and SLC7A6 (y+LAT2)

System y+L mediates the efflux of dibasic amino acids in exchange for, preferably, large neutral amino acids and Na+. The affinity of this system depends on the type of neutral amino acid and the cation concentration present in the medium. SLC7A7 (y+LAT1) and SLC7A6 (y+LAT2) are the two isoforms responsible for this transport system.

SLC3A2/SLC7A7 catalyzes the transport of dibasic amino acids in the absence of Na+ and the uptake of neutral amino acids together with Na+, which (as well as protons) increases the affinity for neutral amino acids without altering the maximal velocity of the transporter. Na+ is cotransported in a 1:1 stoichiometry with neutral amino acids (Fotiadis et al. 2013). SLC3A2/SLC7A7 presents a basolateral distribution in kidney and small intestine. It participates in the renal reabsorption and intestinal absorption of basic amino acids (lysine, arginine, and ornithine). This transporter is defective in the autosomal recessive disease lysinuric protein intolerance (LPI) characterized by the urinary hyperexcretion and intestinal malabsorption of cationic amino acids (arginine, lysine, and ornithine) (Torrents et al. 1999; Borsani et al. 1999).

SLC3A2/SLC7A6 transport characteristics are very similar to those of SLC3A2/SLC7A7. It also mediates the Na+-independent transport of dibasic amino acids and the Na+-dependent uptake of neutral amino acids, but it preferentially catalyzes l-arginine efflux in exchange of l-glutamine plus Na+ (Bröer et al. 2002). In contrast to SLC7A7, SLC7A6 is widely expressed in epithelial and non-epithelial tissues.

System Asc: SLC7A10 (Asc1)

System asc (alanine-serine-cysteine) is composed by two isoforms: SLC3A2-associated SLC7A10 and SLC7A12, which is associated to a yet unknown heavy subunit. SLC7A12 is present in rodent (e.g., mouse and rat) but not in the human genome.

SLC3A2/SLC7A10 mediates the Na+-independent transport of small neutral amino acids such as l-glycine, l-alanine, l-serine, l-threonine, l-cysteine, α-aminoisobutyric acid, and β-alanine. SLC7A10 mRNA is strongly expressed in the human kidney and brain; lower expression appears in the human placenta, heart, skeletal muscle, lung, liver, and pancreas. Contrary to other members of HATs, SLC3A2/SLC7A10 functions as a preferential (but not obligatory) exchanger, being able to also mediate uniport. SLC7A10 also accepts with a high-affinity d-isomers of small neutral amino acids, in particular d-serine. d-serine plays an important role in the central nervous system where it is required for the activation of the glutamate N-methyl-d-aspartate (NMDA) receptor. Because of this high affinity for d-serine, together with the high levels of expression in brain, SLC7A10 is proposed to play a significant role in d-serine mobilization in the brain (Fotiadis et al. 2013). Moreover, SLC7A10 ablation in mice shows that this transporter controls glycine levels in the central nervous system and is required for glycinergic inhibitory transmission, making it a candidate for hyperekplexia disorders (Safory et al. 2015).

System xc: xCT (SLC7A11)

SLC3A2/SLC7A11 heterodimer is the only isoform of system xc.

This heterodimer catalyzes the Na+-independent electroneutral exchange of extracellular anionic cystine for glutamate (1:1 stoichiometry). Because of the low cystine content inside the cells (as it is rapidly reduced to cysteine), the physiological direction of this exchange consists of an exit of glutamate that favors the entrance of cystine. Cystine uptake and reduction are rate limiting for glutathione (GSH) synthesis. SLC7A11 transport directly controls intracellular GSH levels and, therefore, modulates the buffering of reactive oxygen species (ROS). SLC3A2/SLC7A11 activity invalidation leads to ferroptosis, an iron-dependent oxidative (non-apoptotic) death (Dixon et al. 2012). SLC7A11 expression is elevated in cells requiring high GSH synthesis, as native brain and activated macrophages, as well as in most cell culture lines, where oxygen tension is higher than physiological levels. In the brain, SLC3A2/SLC7A11 plays a role in the homeostasis of glutamate levels (Danbolt 2001). SLC7A11 has also been identified as a fusion-entry receptor for Kaposi’s sarcoma-associated herpesvirus (KSHV) and may mediate KSHV entry either in isolation or as part of a complex with other receptors for the virus (Veettil et al. 2008). As SLC7A5, SLC7A11 is also overexpressed in tumors (McCracken et al. 2013).

Structure of SLC3A2 and Heteromeric Amino Acid Transporters

HATs are formed by members of family SLC3 (type II N-glycoproteins with a transmembrane domain and a bulky ectodomain) and family SLC7 (non-glycosylated proteins with 12 transmembrane domains). Heterodimers are bound together by a disulfide bridge. In this section the current structural knowledge of HATs is depicted. A more detailed review on this topic has recently been published (Palacín et al. 2016).

HAT Heavy Subunits

The structure of SLC3A2 ectodomain (SLC3A2-ED; at 2.1 Å resolution, PDB: 2DH2) (Fort et al. 2007) constitutes the only available atomic structure of HATs (Fig. 2).
SLC3A2, Fig. 2

Structure of SLC3A2. (a) The SLC3A2-ED structure (PDB: 2DH2; 2.1 Å resolution) can be described in terms of topology and multidomain organization of α-amylases: the so-called A-domain of α-amylases is here a (β/α)8 TIM barrel. The C-domain with eight antiparallel β-strands is also present. SLC3A2 lacks a B-domain, present in many α-amylases. (b) Structural model of the heteromeric complex SLC3A2/SLC7A8, generated by combining negative-stained transmission electron microscopy images, docking analysis, and cross-linking experiments. The long intracellular N-terminus of SLC3A2 is not shown, and the position of the transmembrane domain (shown as a transparent purple cylinder) is a supposition based on compatibility with the establishment of a disulfide bond between SLC3A2 and SLC7A8. Both in a and b, the atomic structure of SLC3A2-ED is represented as a cartoon with α-helices (in purple) and β-sheets (in gray). The structural model of SLC7A8 is a cartoon in dark gray

The SLC3A2-ED structure resembles that of bacterial α-amylases, showing a triose phosphate isomerase (TIM) barrel (α/β)8 (A-domain) and eight antiparallel β-strands (C-domain). α-Amylases present, inserted between the third β-strand and the third α-helix of domain A, an additional long loop region containing calcium-binding sites (B-domain) (Fort et al. 2007). Despite the sequence identity (25% for A-domain) and structural similarities between SLC3A2 and bacterial α-amylases, the α-glycosidase activity is lost in SLC3A2 (Fort et al. 2007). The N-terminal position of SLC3A2-ED structure corresponds to Cys109, which forms a disulfide bridge with the light subunit and is just four residues away from the putative transmembrane segment. The structure of SLC3A2-ED suggests that the location of the N-terminus imposes strong structural restraints with respect to both the docking of SLC3A2-ED onto the membrane and the interactions with the light subunit in the heterodimer (Fort et al. 2007).

HAT Catalytic Subunits

Together with their prokaryotic homologous, HAT light subunits form the l-amino acid transporter (LAT) family within the large amino acids, polyamines, and organic cations (APC) superfamily of transporters. The structure of three bacterial APC (non-LAT) transporters has been solved: AdiC (an arginine/agmatine exchanger), ApcT (an H+-dependent amino acid transporter), and GadC (a glutamate/GABA antiporter). These bacterial APC transporters, despite presenting low amino acid sequence identity with catalytic subunits of human HATs (∼14–20%), constitute their present structural models (Palacín et al. 2016). Despite not having relevant amino acid sequence identity (∼10%), all APC-solved structures share the 5 + 5 inverted symmetry motif fold first described for LeuT (a Na+-dependent and Cl-dependent neurotransmitter transporter). The 5 + 5 inverted repeat fold (or LeuT fold) consists of two-membrane topology repeats of five transmembrane (TM) domains each related by a pseudo two-fold symmetry axis located in the membrane plane. The two interior pairs of symmetry related helices (TM1/TM6 domains, which are discontinuous and consist of two short α-helixes connected by a highly conserved unstructured segment, and TM3/TM8 domains) largely define the central translocation pathway that contains the binding sites for substrate and ions in transporters sharing the 5 + 5 inverted repeat fold. Atomic models of functionally disparate yet structurally related transporter families have provided insight into the principles of an alternating access mechanism in which the transporter undergoes several conformational states required to translocate the substrate across the membrane. During this transition, the transporter keeps the substrate accessible to only one side of the membrane (substrate-binding pocket accessible to extracellular solution “open to out” or to the cytoplasm “open to in”) at a given time by opening and closing different gates. Upon substrate binding to the open-to-out apo state, the substrate-bound state evolves to an occluded state, where two gates (thick and thin) prevent the diffusion of the substrate to either side of the membrane. Transition to the inward-facing states requires a transient fully occluded symmetrical intermediate. AdiC, ApcT, and GadC have been solved in “open-to-out,” closed intermediate step and “open-to-in” states, respectively. In antiporters, the return to the outward-facing states requires the binding and translocation of a new intracellular substrate that will move the transporter back through all the states but in the opposite direction (Palacín et al. 2016).

As previously mentioned, several SLC3A2-associated transporters (SLC7A5, SLC7A11) are promising targets for the treatment of cancer. Solving the structure of human LATs would be a breakthrough that would allow the design of specific drugs relevant for clinics.

SLC3A2-Mediated Heterodimer Structure

SLC3A2-associated transporters work as single heterodimers (Palacín et al. 2016). Whereas SLC3A2 cytoplasmic and transmembrane domains are important for interactions with β-integrins (Feral et al. 2005), it has been recently demonstrated the importance of the SLC3A2-ED for the function of all SLC3A2-associated transporters expressed in cultured cells (SLC7A5, SLC7A6, and SLC7A11) (Fig. 1) (de la Ballina et al. 2016). This experimental evidence gives support to what was previously predicted from a modeled structure of SLC3A2-containing heterodimers. The structure of human SLC3A2/SLC7A8 was obtained by 3D reconstruction from negatively stained complexes at 21 Å. Such 3D reconstruction, along with docking analysis and cross-linking experiments, showed the relative positioning of heavy and light subunits. Thus, SLC3A2-ED would be located just on top of the catalytic subunit and interact with the extracellular loops of the corresponding associated transporter, stabilizing it (Rosell et al. 2014). In this model, the position of the transmembrane domain of SLC3A2 is not yet known (Fig. 2).

SLC3A2 and Integrins

SLC3A2 was first found to regulate integrin signaling by its ability to complement isolated β1 cytoplasmic domain dominant suppression (Fenczik et al. 1997), a phenomenon that occurs when overexpression of β1 cytoplasmic domain blocks integrin activation. Co-immunoprecipitation and subcellular co-localization were also reported, strongly suggesting a physical interaction between SLC3A2 and integrins (Fenczik et al. 1997). Altogether, multiple studies have clearly shown that SLC3A2 cytoplasmic and transmembrane domains (105 aa) are responsible for integrin signaling, whereas SLC3A2 ectodomain (SLC3A2-ED, 425 aa) is implicated in amino acid transport. Each function depends on SLC3A2 contacts with either beta-integrins or amino acid transporter catalytic subunits. One can also raise the possibility that SLC3A2 physical interactions, both with integrins and SLC7 transporter family, could modulate each other. Determining the specific role of each domain of SLC3A2 was based on the use of swapped domain chimeras between SLC3A2 and CD69 – being both type II transmembrane glycoproteins. Although results obtained using this approach were central, Cibrian and coworkers recently established a functional link between these two transmembrane proteins. In T cells, CD69 associates with the amino acid transporter complex SCL3A2/SLC7A5 regulating its surface expression and the uptake of L-tryptophan (Cibrian et al. 2016). Single-point mutants could provide an alternative for analyzing the dual function of SLC3A2. Importantly, amino acid residues 82–86 (WALLL), at the beginning of the transmembrane domain, seem key for SLC3A2 oncogenic activity via its effect downstream of integrins. On the integrin side, it is the cytoplasmic domain of β1A and β3 integrins (not β1D or β7) which interacts with SLC3A2 (Zent et al. 2000). More recently, β4 integrin was also reported to bind SLC3A2 in human keratinocytes (Lemaître et al. 2011).

Processes Tuned by SCL3A2

The dual interaction of SLC3A2, with integrins and transporters, places this protein at the crossroad of a network, providing the context for a complex cross regulation. SLC3A2 was discovered as a lymphocyte-activating antigen, namely, 4F2, and was reported widely expressed in hematopoietic and lymphoid cells, as well as in all established human tissue culture cells and in most malignant human cells (reviewed in Deves et al. 2000). The first line of evidence of SLC3A2’s role in vivo resides in the embryonic lethality of its full body knockout (E3.5-E9.5), partly due to placentation defect (Tsumura et al. 2003). SLC3A2 is also a crucial determinant of human endometrial receptivity during the implantation window of the blastocyst (Domínguez et al. 2010). In order to study SLC3A2 function in vivo, Feral et al. generated a SLC3A2fl/fl murine line (Féral et al. 2007), which was then combined with multiple Cre recombinase-expressing mouse lines. Specifically, given the SLC3A2 expression pattern, lymphocyte targeting (CD19 promoter) was performed in vivo. Ginsberg’s lab established that SLC3A2 is a key player in adaptive immunity, via its integrin-signaling portion (Cantor et al. 2009). While it is not a requisite for T- or B-cell compartmentalization, initial activation, or steady-state, SLC3A2 is crucial for clonal expansion after antigen recognition (Cantor et al. 2009, 2012) making it a compelling target for autoimmune diseases.

SLC3A2 is also strongly expressed in epithelia such as the skin and intestine. Spatiotemporal SLC3A2 deletion in mouse basal keratinocytes (K14 promoter) impaired skin homeostasis with major defect in hair growth and wound healing (Boulter et al. 2013). Importantly, this study strongly suggests a key role for SLC3A2 during skin aging. Collagen and fibronectin organization are impacted upon loss of epidermal SLC3A2 in vivo, resulting in a more compliant skin through a reduction in RhoA/ROCK signaling and YAP/TAZ target gene expression (Estrach et al. 2014). Concomitantly, integrin-SLC3A2 interaction is also necessary in vitro for cellular contractility to exert the necessary force on the matrix to mediate this mechanical assembly process (Féral et al. 2007). SLC3A2 binding to integrins is necessary for proper integrin outside-in signaling (Feral et al. 2005), thus influencing cellular behavior downstream of integrins (spreading, migration, anoikis). Consequently, SLC3A2 overexpression, such as in transformed or tumor cells, and SLC3A2-activating antibodies enhance integrin signaling (Deves et al. 2000). D. Merlin’s laboratory, by generating gain- and loss-of-function genetic mouse models specifically in intestinal epithelial cells (villin promoter), elegantly showed that SLC3A2 expression controls homeostatic and innate immune responses in the gut (Nguyen et al. 2011). SLC3A2 overexpression induced barrier dysfunction and stimulated cell proliferation, as well as production of pro-inflammatory mediators. The authors raise a possible mechanism in which SLC3A2 overexpression would affect integrin signaling in intestinal epithelial cells, disrupting the homeostatic regulation of cell proliferation and survival, which would result in abnormal basal intestinal phenotype and contribute to increased intestinal permeability.

In humans, SLC3A2 overexpression is a marker of bad prognosis for many cancers. Early in vivo studies consisting of injecting either overexpressing cells or SLC3A2 null cells in nude mice pointed out a role for this protein in tumor development. Mouse models, either genetic or chemical, completed our understanding by showing that both integrin signaling and amino acid transport are involved in tumorigenesis in vivo. In the aforementioned intestine model, the ultimate effect downstream of SLC3A2 overexpression is the induction of tumorigenesis. As mentioned previously, this relates to increased inflammation, which could be integrin dependent (Nguyen et al. 2011). In a Ras-driven skin cancer model, SLC3A2 increased the stiffness of the tumor microenvironment (Estrach et al. 2014). It also amplified the capacity of cells to respond to matrix rigidity, an essential factor in tumor development. Thus, by potentiating integrins role in ECM assembly in vivo, SLC3A2 supports tumor progression. A recent study also reported a role for SLC3A2/integrin in angiogenesis through VEGFR activation in endothelial cells (Liao et al. 2016), bringing a novel viewpoint. Notably, SLC3A2 capacity to regulate amino acid transport, and more specifically SLC7A5 and SLC7A11, participates in proliferation activation and tumorigenic potential (Nicklin et al. 2009; de la Ballina et al. 2016).

A humanized monoclonal antibody anti-SLC3A2 was recently generated and tested from a therapeutic point of view for acute myelogenous leukemia (AML) (Bajaj et al. 2016). SLC3A2 specifically promotes AML propagation and lethality by driving engagement of leukemia cells with their microenvironment. This antibody has several combined mechanisms of action including antibody-dependent cell death, caspase-dependent apoptosis, lysosomal membrane permeability increase, and amino acid transport decrease. Importantly, although the integrin-binding function is critical for the observed effect of SLC3A2 on AML growth, authors reported that more complete rescue of SLC3A2 null defects is only observed following restoration of the amino acid transport as well as integrin signaling functions. This work nails the previously suspected dual role of SLC3A2 in tumorigenesis via integrin and amino acid transporter binding, suggesting that targeting SCL3A2 may be more powerful than solely targeting upstream adhesive signals.

SLC3A2 and Other Partners

In addition to amino acid transporters and β-integrins, several other proteins have been found to interact directly or indirectly with SLC3A2. Specific functions and mechanisms of these interactions remain somewhat elusive. Nonetheless, they could participate in the regulation of SLC3A2-dependent functions, potentially placing SLC3A2 in a complex protein network that requires further functional investigations (Fig. 3).
SLC3A2, Fig. 3

SLC3A2 partners. All SLC3A2 known partners (limited to SLC7A5 for amino acid transporters) are depicted. Suggested multiprotein complexes are shaded in blue; nevertheless, data to support one SLC3A2 molecule gathering several partners simultaneously is still missing. Described cellular processes downstream of SLC3A2/partners’ interaction are also indicated

As such, one of the first partners described was secreted galectin-3, which binds SLC3A2 via its ED glycosylation. Even though this interaction seems implicated in functions as distinct as placental cell fusion and kidney tubulogenesis, its role and mechanisms have not yet been fully revealed. Xu et al. also described the formation of a complex including SLC3A2 and the cell surface protein CD147 (basigin), which promotes production of matrix metalloproteinases and hyaluronan and associates with monocarboxylate transporters (MCTs) and integrins. The formation of this supercomplex was thought to play a critical role in energy metabolism (Xu et al. 2005). More recently, CD147 was implicated in SLC3A2 redistribution, thus regulating integrin/SLC3A2 downstream behaviors (Yan et al. 2008). Similarly, intercellular adhesion molecule-1 (ICAM-1) co-immunoprecipitates with SLC3A2, and antibody-mediated cross-linking of both proteins conversely affects leucine transport by SLC7A8 (Liu et al. 2003). More specifically, in primary endometrial epithelial cells, SLC3A2 interacts with tetraspanin CD9, within the tetraspanin-enriched microdomains at the plasma membrane, which greatly enhanced mouse blastocyst adhesion (Domínguez et al. 2010). ICAM-1 can also, in turn, associate with CD9, suggesting the idea of macroprotein complex. Finally, thanks to its C-terminal class II PDZ-binding domain (GLLLRFPYAA, amino acids 520–529) protruding into the basolateral extracellular space of the intestine, SLC3A2 was proposed to interact with membrane-associated guanylate kinase hCASK (Yan et al. 2008); the implications of such interaction are yet to be fully elucidated. As mentioned earlier, a macromolecular complex could represent a functional entity capable of controlling SLC3A2-dependent processes, such as amino acid transport activity and/or cell/extracellular matrix adhesion via β1 integrin.

The interesting and extended SLC3A2 interaction network places this protein at the crossroad of many cellular processes. Thus, although no enzymatic or ligand-binding capacity has been reported for SLC3A2, one can imagine that it is involved in tuning of a considerable number of cellular processes that may become interdependent.

Summary

SLC3A2 dual function as amino acid transporter and integrin signaling enhancer, make this protein a pivotal signal integrator. Besides, its multiple interactions with partners involved in central cellular processes only reinforce the potential impact of SLC3A2 on the regulation of numerous cell behaviors. For this reason, SLC3A2 constitutes an attractive therapeutic target for the development of new treatments for diseases such as cancer. However, in order to design specific drugs, information regarding the molecular mechanisms by which SLC3A2 regulates each cellular function and how/whether the different processes mediated by SLC3A2 are regulated and modulate each other is required. Affecting SLC3A2-mediated functions would have a greater effect than separately targeting each of the functions in which it is involved.

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Institute for Research in Biomedicine (IRB Barcelona)The Barcelona Institute of Science and TechnologyBarcelonaSpain
  2. 2.Department of Biochemistry and Molecular BiologyUniversity of BarcelonaBarcelonaSpain
  3. 3.Department of Molecular Medicine, Institute of Basic Medical ScienceUniversity of OsloOsloNorway
  4. 4.Université Côte d’Azur, INSERM, CNRS, IRCANNiceFrance
  5. 5.Spanish Biomedical Research Network in Rare Diseases (CIBERER CB06/07/0100)BarcelonaSpain