Different metabolic targets exist to modulate ECM expression. In general, when there is an excess of nutrients, AMP-activated protein kinase (AMPK) is decreased and mammalian target of rapamycin complex 1 (mTORC1) is increased, which subsequently increases fibronectin assembly and integrin activation (Table 1). AMPK has also been shown to regulate hyaluronan synthesis by phosphorylating and inhibiting hyaluronan synthase 2 (Xing et al., 2020). In addition, another in fibroblasts and epithelial cells from the lung, kidney, liver, and skin, it has been shown that the Sterol regulatory element-binding proteins (SREBP)/mevalonate metabolism regulates the Yes-associated protein 1 (YAP1)/transcriptional coactivator with PDZ-binding motif (TAZ) signaling (Sorrentino et al., 2014; Noguchi et al., 2018). This activation of YAP/TAZ signaling induces the synthesis of (profibrotic) ECM proteins like collagen I (Table 1). Conversely, when nutrients are in short supply, fibronectin assembly is decreased and integrin internalization and ECM degradation are subsequently increased (Dornier and Norman, 2017).

This relationship between nutrients and ECM also exists in differentiating stem cells. For example, during adipocyte differentiation, cellular metabolism changes due to the uptake and storage of fatty acids (Mariman and Wang, 2010). This change affects the ECM composition through peroxisome proliferator-activated receptor gamma (PPARγ), a key transcription factor in adipocyte differentiation, which upregulates the synthesis ECM components collagen VI and thrombospondin 1 (Okuno et al., 2002; Table 1). All in all, the energetic substrate leads to metabolic changes that can affect the expression of ECM proteins like fibronectin or collagen.

Another means by which the ECM can be modulated is through oxygen availability, which can also affect the induction of metabolic pathways, leading to mechanical modulation of the ECM. In hypoxia, which is a low level of oxygen, more anaerobic metabolic pathways are upregulated, and the expression of collagen I is reduced while the expression of collagen IV is increased (Tajima et al., 2001; McKay et al., 2017). However, it is not just the ECM synthesis and composition that are affected by metabolism; the physical properties of the ECM are also influenced. When the metabolism of chondrocytes is affected by hypoxia and there is a subsequent diminished glucose oxidation, the collagen synthesis is also reduced (Stegen et al., 2019). As the cells switch from a metabolism based on glucose and fatty acids to glutamine, the enhanced α-ketoglutarate formation from glutamine leads to increased proline and lysine hydroxylation of collagen (Stegen et al., 2019; Table 2). This hydroxylation of collagen increases ECM stiffness and prevents matrix degradation. As another example, in hepatic stellate spheroids, the addition of the glycolytic metabolite phosphoenolpyruvate has been shown to increase α-smooth muscle actin, thereby stiffening the matrix (Fujisawa et al., 2020; Table 2).

Metabolism can also affect the degradability of the ECM through, for example, matrix metalloproteinases (MMPs). Briefly, MMPs can be classified in specific groups, one of which are the collagenases. These MMPs cleave collagen at specific sites, except when the collagen is hydroxylated at a nearby proline or lysine residue (Taddese et al., 2010). An example of how metabolism affects the degradability of the ECM is the activation of PPARγ, which increases the expression of MMP-1 (François et al., 2004; Table 2). Similar relationships are seen in medicine, where obesity leads to adipocyte hypertrophy and overgrowth, leading to hypoxia and decreased energy production via the tricarboxylic acid cycle. This inhibits proline hydroxylation, which increases ECM degradation by MMPs and leads to ECM instability (Mariman and Wang, 2010). Finally, in breast cancer cells, it has been shown that pyruvate metabolism induces collagen hydroxylation by activating prolyl-4-hydroxylase, which decreases MMP8 degradation and subsequently decreases metastatic growth of the cancer cells (Elia et al., 2019; Table 2). Overall, due to low oxygen, alternative metabolic pathways are induced that affect ECM crosslinking or release enzymes that are involved in ECM remodeling affecting its mechanical properties.

It is also interesting to note that in addition to the cellular metabolism determining ECM properties, it is also the case that the ECM conversely affects cellular metabolism. However, this is outside of the scope of this mini-review and has been extensively described elsewhere (Mah et al., 2018; Fernie et al., 2020). For example, when cells detach from the ECM that leads to decreased glucose uptake, glycolysis and oxidative phosphorylation (Mason et al., 2017). In addition, a low level of hyaluronan leads to diminished internalization of the glucose transporter GLUT1 that subsequently decreases all glucose-related cellular metabolism (Sullivan et al., 2018). Decreased hyaluronan activates receptor tyrosine kinase (RTK), which subsequently induces ZFP36. ZFP36 leads to thioredoxin interacting protein (TXNIP) degradation and decreases GLUT1 internalization (Katsu-Jiménez et al., 2019). ECM detachment of ECM expression can both affect substrate uptake or increase metabolism.

In summary, cellular metabolism can affect the ECM synthesis, modification, composition, signaling, stiffness, and degradation. In addition, the ECM can also influence the cellular metabolism. These studies indicate interesting metabolic targets that can be used to modulate the ECM. Next, we will describe how (bio)materials scientists and tissue engineers can harness this information.

There are many drugs that can target metabolism, for example, by targeting a co-factor for a metabolic pathway or directly targeting a transcription factor. Examples of drugs that target an essential co-factor for a metabolic pathway are deferoxamine (DFO) and dimethyloxalylglycine (DMOG), which both affect the metabolic transcription factor HIF1α. DFO is an iron chelator, while DMOG is a synthetic analogue of α-ketoglutarate. Both iron and α-ketoglutarate are essential substrates for prolyl-4-hydroxylase, which hydroxylates HIF1α in normoxia to target HIF1α for proteosomal degradation (Iommarini et al., 2017; Donneys et al., 2019). However, during hypoxia or when adding DFO or the competitive inhibitor DMOG, prolyl-hydroxylases do not hydroxylate HIF1α. This leads to HIF1α-mediated signaling, but also decreases proline and lysine hydroxylation of collagen, which decreases matrix stiffness and leads to a higher sensitivity of the matrix for degradation (Rappu et al., 2019; Stegen et al., 2019).

An example of a metabolic drug that directly targets a metabolic transcription factor is the inhibitor AS1842856 that inhibits FOXO1 by binding to its active non-phosphorylated form. FOXO1 mediates the activation of Glucose 6-phosphatase and inhibits glucokinase. In hepatocytes, insulin represses glucose production and enhances lipogenesis by inhibiting FOXO1 activation (Langlet et al., 2017). Depending on the metabolic pathway that needs to be activated, a different metabolic drug can be chosen.

In this strategy, it is important that the characteristics of the drug of interest be taken into account. There are numerous considerations that should be given to selecting a drug for the purpose of modulating cellular metabolism and thereafter the ECM. Firstly, it is essential that the drug does not have any toxic effects on the cells at its effective concentration and exposure time (Suzman et al., 2019; Hoy, 2020). Preferably, the drug is FDA/EMA-approved like DFO, which has been approved for iron overload (Kontoghiorghes and Kontoghiorghe, 2016; Sun et al., 2020; Xu et al., 2020; Takpradit et al., 2021). After selecting the drug of interest based on characteristics, mechanism of action and cellular target, the next step will be to include it in the right material and apply the right release strategy.

The physical properties of the tissue of interest is important for the choice of the material, while the mechanism of action of the metabolism-modulating drug defines the coupling strategy. For example, depending on the stiffness of the ECM of the target tissue, a different material could be selected, ranging from solid materials to soft hydrogels. PEG, PVA, and PHEMA are frequently used for engineering the ECM, and they can be co-polymerized with different polymers to tune the physical properties (Unal and West, 2020), which can also be an advantage in 3D printing applications (Da Silva et al., 2020). For an optimal mechanical strength, dynamic viscoelastic hydrogels are of particular interest because of the tunability of their stiffness and their resemblance to the ECM (Hafeez et al., 2018; Table 3).

The drug coupling or delivery method needs to be chosen based on the drug of interest and its cellular target (Table 3). For example, if the metabolic drug needs to be released to be functional or taken up by the cell to induce its biological effect, it is essential to include a reversible linker into the material design. Biodegradable linkers can be used, including: carbonates, anhydrides, esters, urethanes, amides, or orthoesters (Elvira et al., 2005). If the drug of interest is of amphiphilic or hydrophobic nature, it can self-associate in surfactant materials (Schreier et al., 2000). In order to control drug release, linkers that depend on the environmental stimulus could be applied or designed to modulate the timing of drug release (Table 3).

For some metabolic modulators that regulate the ECM, a more sustained release is required. Many strategies are available to induce a sustained release, ranging from coupling, host-guest chemistry to encapsulating in cellulose capsules (Bakker et al., 2018a; Bakker et al., 2018b; Sun et al., 2019). Furthermore, many different intelligent biomaterials have been developed that respond to the environment, including cues such as pH, reactive oxygen species (ROS), MMPs, or even mechanotransduction. Depending on the degradation rate by MMPs, a different polymer can be chosen (Unal and West, 2020). An example of how environmental cues can be used is in the setting of an engineered tissue that lacks vasculature. This will lead to hypoxia and enhanced glycolysis and lactic acidosis in the surrounding tissue that decreases the pH. This pH decrease can be used to prevent associated changes in the surrounding ECM by releasing a drug that influences the metabolism and subsequent ECM remodeling. A hydrazine linker is a commonly used pH-inducible release linker, since it is cleaved in an acidic environment (Rao N et al., 2012). Similarly, the decreased oxidative phosphorylation present in hypoxia also leads to incomplete mitochondrial respiration and the formation of ROS. These ROS could be another stimulus that releases metabolic drugs influencing ECM in response to metabolic changes. ROS can reduce disulfide bonds inducing the release of the coupled drug molecule. Next to the disulfide bonds, there exist many different ROS or redox-responsive linkers (Liang and Liu, 2016; Tao and He, 2018).

In summary, the tissue of interest defines the material choice. Depending on the local ECM stiffness and presence of local cells, more or less metabolic modulator will be released, making synthesis and stiffness of the ECM tissue-specific. In addition, depending on the characteristics of the metabolic drug modulating ECM and the local ECM, different drug delivery strategies should be used. Because most metabolic drugs that affect the ECM require cellular uptake for their function, reversible linkers should be applied. Intelligent materials that respond to the pH, ROS, MMPs, or mechanical signals from the environment enhance the biological application of these materials.