**1.4 Development of liver steatosis**

While triglycerides are the main lipid component that contributes to the development of fatty liver [48], and account for 60–70% of intrahepatic fat accumulation, also low density lipoprotein (LDL) cholesterol is an important contributor to the development of NAFLD, mainly in cases of high serum LDL levels, where it is known that in subjects with NAFLD, the lipid uptake by the liver is increased, due to enhanced expression on hepatocyte cell surface of the LDL receptor (LDLR), LDL receptor related protein (LRP), and the hepatic fatty acid transporter CD36. In addition, in subjects with high serum levels of lipids, hepatic uptake of lipid particles occurs through direct endocytosis by cell membrane, secondary to increased activity of hepatic lipase and lipoprotein lipase, a process that depends on the activity of HSPGs embedded in the cell wall, in response to augmented insulin resistance that complicates patients with obesity and NAFLD [49, 50].

Following lipid uptake by hepatocytes, lipid particles undergo metabolism and β-oxidation through several intra-cellular metabolic pathways, which include ACAT2, ACAT1, HMGCoA reductase, pPAR-α, pPAR- , PCSK-9 and DGAT1 [51]. In fact, also De novo lipogenesis contributes to lipid accumulation in the liver of patients with obesity and NAFLD [51, 52]. In this pathway, acetyl CoA and melonyl CoA are converted into fatty acids in the liver, a process that is accelerated in the presence of insulin resistance, which occurs in adjunction with obesity and adiposity [52].

The progression of liver steatosis occurs through several stages. Upon lipid uptake by hepatocytes, fatty acids undergo β-oxidation in mitochondria, peroxisomes, and microsomes, still with existing controversy regarding the rate of lipid oxidation (whether increased or decreased) in patients with NAFLD [51, 53], and accumulation of triglyceride-rich lipoproteins in hepatocytes is increased [51, 54]. Likewise, in patients with NAFLD increased production of small dense LDL-C was demonstrated [55].

The endoplasmic reticulum is the main intra-cellular site for lipid synthesis and protein folding and maturation, and is an important participant in the development of NAFLD, especially in the presence of impaired LDL-Triglyceride assembly, which occurs due to activation of intracellular signaling pathways, in response to higher levels of intracellular lipid particles [56]. Lipotoxicity is more prominent under higher endoplasmic reticulum oxidative stress, that occurs more frequently in obese subjects with NAFLD, apparently due to activation of the unfolded protein response (UPR). One more mechanism that contributes to the development and progression of NAFLD is the accelerated formation of extracellular vesicles, that are nano-sized particles which are over-secreted by hepatocytes in response to higher content of toxic lipid particles, and play critical role in the pathogenesis of NAFLD by acting as mediators of paracrine signaling, causing HSCs activation, angiogenesis and activation of macrophages, all leading to liver inflammation and chemotaxis [57, 58].

## **1.5 The enzyme heparanase**

Heparanase is the only enzyme in mammalians that is responsible for cleavage of HS side chains in the HSPGs. The human heparanase gene (heparanase 1) is located on chromosome 4q21.3 [59]. Also heparanase 2 was demonstrated, sharing 40% similarity with heparanase 1, but does not exert similar activity like heparanase 1 [60]. The enzyme heparanase is synthesized in the endoplasmic reticulum, then processed in the Golgi apparatus to 65 kDa proheparanase, and then released to the extra-cellular space, where it interacts with many membrane molecules, of which are membrane HSPGs like syndecans [61], resulting in endocytosis and localization into the lyzosomes, where it undergoes cleavage to its two active forms- 50 and 6 kDa. It was proven that the active enzyme has several final destinations in the cell: it could undergo anchorage on the surface of exosomes, included in autophagosomes, translocated into cell nucleus, or even be secreted to the ECM [62]. The enzyme expresses both enzymatic and non-enzymatic activities, inside the cell and in the ECM. In its enzymatic intracellular role, activity of heparanase is mainly degradation and turnover of membrane-associated HSPGs. The extracellular enzymatic activity is mainly degradation of transmembrane and ECM located HSPGs, leading both to alterations in structure and function of HSPGs in the ECM, and resulting in attenuation of HS-bound ligands and proteins which are released into the surrounding environment, causing diffusion of cytokines, growth factors, and lipoproteins, and facilitating cell motility, angiogenesis, inflammation, coagulation, and stimulating autophagic and exosome production [63–67].

Non-enzymatic activity of heparanase has been demonstrated, although the receptors that could mediate this activity have not been identified so far. Of the nonenzymatic activities of the enzyme, one can mention that the heparanase proenzyme (65 kDa) was demonstrated to induce signaling cascade towards phosphorylation of

*Role of the Enzyme Heparanase in the Development of Fatty Liver DOI: http://dx.doi.org/10.5772/intechopen.107530*

several proteins, including those involved in intracellular signaling pathways, such as Akt, ERK, p38, and Src [68]. Of the resultant effects of this activity were noted endothelial cell migration and invasion, which are enhanced by the proheparanase-Akt phosphorylation and activation of PI3K [69]. Moreover, latent heparanase was implicated in induction of tumorogenesis, such as the development of glioma, lymphoma, and T-cell adhesion, apparently due to activation of Akt, PyK2 and ERK, and Akt/PKB phosphorylation [70].
