**1. Introduction**

Oxidative stress constitutes a disturbance characterized by an imbalance between the generation of free radicals and antioxidant defenses [1]. The reactive oxygen (ROS e.g., O2−, OH, ROO) and nitrogen species (RNS, e.g., NO, ONOO−) are generated in a variety of intracellular processes and their overproduction produces cell damage in lipids, proteins and DNA [2].

The liver is the largest organ in the vertebrate body and the site for intense metabolism [3]. It is involved in several vital functions and it has a great capacity to detoxify toxic substances and synthesize useful principles. Therefore damage to the liver inflicted by hepatotoxic substances is of grave consequences [4]. The hepatotoxin CCl4 has been used as a model component causing cellular necrosis in the liver. It induces hepatotoxicity in rats, rabbits and humans [5] and mediates its

hepatotoxicity after biotransformation by hepatic microsomal cytochrome 450 (CYT 450) to generate trichloromethyl free radicals which launch attacks on membrane proteins, thiols and lipids. This leads to the peroxidation of the lipid membrane which results in necrotic cell death [6]. To prevent such disease, both enzymatic and non-enzymatic mechanisms are present in the cell [7]. The body protects itself from oxygen free radical toxicity by enzymatic antioxidant mechanisms (e.g., glutathione peroxidase, GSH; glutathione reductase, GR; superoxide dismutase, SOD; and catalase, CAT) and by non-enzymatic antioxidants (e.g., vitamins, uric acid, albumin, bilirubin, and many others) [8].

Liver pathologies constitute a serious global health issue despite recent therapeutic advances. Nevertheless, some plants have been used for liver disorders and showed to be therapeutically useful agents [5]. Diverse plant extracts were assessed for their hepatoprotective effect against different experimentally induced liver toxicities [9–15].

From the large diversity of the plant components, special importance has been assigned to their polyphenols, which have shown their capacity to counteract oxidative stress through various mechanisms [16]. An interesting approach used to obtain such compounds is to source them from food industry residues, which are in general disposed of or used to produce animal feed [17]. The olive tree (*Olea europaea*, L.), amongst the oldest known cultivated trees in the world and mentioned in the Holy Qur'ân and Ahadith [18], has been known for its long history of medicinal and nutritional values. Historically, olive leaves were used as a folk remedy for combating fevers and other diseases, such as malaria. Numerous researchers showed the important role of this plant in improving cardiovascular risk factors [19], cancer [20] and other diseases. A recent publication reviewed the relevant role of phenolic compounds present in *Olea europaea*, L. products and by-products in human health [21], while another review focused on the potential protective effect of secoiridoids from *Olea europaea* L. in cancer, cardiovascular, neurodegenerative, aging-related, and immune-inflammatory diseases [20].

Olive leaves constitute a huge abundant residue resulting from olive tree pruning and olive oil processing. It has been estimated that olive pruning produces 25 kg of olive leaves and twigs per tree annually. In addition these by-products present, only about 10% of olives actually arrive at the mills [22]. All of this makes olive leaves a very interesting, cheap source with potentially useful bioactive components such as secoiridoids, triterpenes, lignans, and flavonoids [23]. Oleuropein and hydroxytyrosol, as major compounds of olive leaves, have been reported to exert numerous pharmacological properties, including anticancer, antidiabetic, and anti-inflammatory activities [14]. Recently, the hepatoprotective activity of olive leaves against damage induced by CCl4 [24,25], cadmium [26,27], paracetamol [28], thioacetamide [29], ethanol [5], fluoxetine [30] and deltamethrin [31] has receieved increasing interest in animal experiemts. However, available data about the hepatoprotective effect of olive leaf extracts or their major constituents are still scarce. Hence, there is not enough literature about the hepatoprotective effect of olive leaves against CCl4-induced damage. The existing published articles focused on methanolic or butanolic extracts of dried olive leaves obtained by Soxhlet apparatus [24] and ethanolic extract from dried olive leaves standardized to 16–24% of oleuropein [25]. As far we know, this is the first study on the hepatoprotective effect of Tunisian olive leaves on CCl4 -induced toxicity.

Nowadays, supercritical fluid extraction (SFE), an environmentally friendly and selective technique, has become one of the most popular green extraction techniques [32] used mainly in large-scale industrial applications. This technique has several advantages such as high efficacy, rapidity, non-toxicity, the absence of thermal degradation resulting a very high quality extracts, and the possible direct coupling to analytical instrumental technique [33] with a particular green interest due to the use of supercritical fluids such as CO2 instead organic solvents [34]. It has been increasingly used in recent years around the world for the processing of nutraceuticals as a "natural" alternative to traditional solvent-extraction processes [35]. It has acquired some relevance for the extraction of polyphenols from plant sources [36]. No previous literature reported the evaluation of olive leaf supercritical fluid extract for its hepatoprotective activity. In our present work, we used fresh and dried leaves from which extracts were obtained using a green advanced extraction technology.

Taking into account all these aspects, this study aimed to assess and compare the hepatoprotective capacity of supercritical carbon dioxide (SC-CO2) extracts of fresh and dried olive leaves in CCl4-induced toxicity in rat models. For this purpose, serum biochemical parameters alanine aminotransferase (ALT), aspartate aminotransferase (AST), alcaline phosphatase (ALP and lactate dehydrogenase (LDH)), liver tissue parameters malondialdehyde (MDA), protein carbonyls (PC), total superoxide dismutase (SOD), catalase (CAT) and glutathione peroxydase (GPX) in addition to histopathological and DNA damage assays were evaluated. As far as we know, this is the first evaluation of the hepatoprotective activity of (SC-CO2) extracts from fresh and dried olive leaves acquired via green technology. Moreover, no available literature evaluated the effect of olive leaf extracts containing phenolic compounds and triterpenoid on induced hepatotoxicity in rats.
