**9. Leptin role in disease conditions**

### **9.1 Metabolic syndrome and obesity**

Fat tissue is an energy storage tissue that functions as a negative feedback loop in energy homeostasis [73]. Homozygous mutation of leptin causes extreme obesity, diabetes and suppresses glucose metabolism in insulin-deficient diabetes [8]. The ob/ ob mice model has relatively higher food intake and observed a larger volume of lipid accumulation in the liver than the control group [74]. It has been assumed that nearly 95% of individuals have resistance against leptin [75]. The type 2 diabetes condition is noted with an increased level of leptin and suggested using leptin as a biomarker to study the effect of obesity in diabetes-related morbidities [76]. Some studies also reported that higher leptin levels are associated with the risk of heart-related problems in obese individuals [76, 77]. In younger adults, elevated leptin levels are positively correlated with HOMA-IR and BMI index [78].

Development of severe early-onset obesity and hyperphagia are common in people with homozygous *LEP* mutation [79]. Replacement of leptin from a therapeutic viewpoint has improved insulin sensitivity and thus proved the role of leptin in metabolic disorders, including T2DM. In humans, serum leptin level is positively correlated with the percentage of body fat, fat mass, size of adipocytes, and BMI [80]. Obesity connected with the enlargement of adipose cells enhances the serum leptin level, which further results in the progression of chronic hyperinsulinemia. The majority of obese patients are hyper leptinaemic which supports the development of hypertension, metabolic syndrome, and cardiovascular diseases [81]. Mutation in the leptin receptor located at the hypothalamus alters the transport of leptin across the blood–brain barrier. This incidence increases the level of serum leptin and hence diet-induced obesity. Obesity connected with the leptin receptor mutation is linked with insulin resistance and in the development of T2DM [82].

### **9.2 Cardiovascular diseases**

The level of leptin could influence the function of the heart. It could lead to the progression of many heart-related problems such as coronary artery disease, stroke, chronic kidney disease (CKD), peripheral artery disease (DAP), carotid plaque instability [83]. It was observed that elevated level of serum leptin in obese patients contributes to the low-grade systemic inflammation in favor to develop cardiovascular disease. Moreover, a high level of leptin is used as a biomarker to measure the progression of heart failure in patients with dilated cardiomyopathy [84]. On the other hand, many studies using rodent, obese and diabetic models highlighted the beneficial impact of leptin on cardiac metabolism through glucose metabolism and fatty acid oxidation. This evidence suggested that leptin compensates for cardiac insults due to ischemia and heart failure [85]. Leptin signaling in the modulation of heart function is studied extensively using animal models. These studies demonstrated that impaired cardiac leptin signaling majorly reflects in metabolic inflexibility for glucose utilization, defects in cardiac contractibility, impaired recovery of cardiac function due to coronary artery ligation [86, 87]. Clinical data cemented that plasma leptin levels are associated with LV hypertrophy and increased myocardial wall thickening [88]. Leptin also increased the blood pressure level in obese individuals with a loss-of-function mutation in leptin or leptin receptor [89]. Thus, a leptin-mediated increase in blood pressure directly increases the heartbeat rate, developing into cardiac hypertrophy through the sympathetic nervous mechanism [90].

Leptin-mediated aldosterone synthesis impairs myocardial relaxation and contributes to cardiovascular diseases through a novel mechanism associated with endothelial dysfunctions [91]. Increased plasma leptin levels positively correlate with the number of stenotic coronary arteries in patients with coronary artery disease [92]. In vitro analysis using HUVEC cells demonstrated that leptin induces chronic oxidative stress in ECs and contributes to vascular pathology development [93]. Also, the cytokine hormone leptin could stimulate vascular smooth muscle cells proliferation and migration, thereby increasing calcification and vascular lesions [94]. Altogether, it was suggested that hypertension, obesity, and endothelial dysfunctions are more frequent in T2DM patients with elevated leptin levels [95].

### **9.3 Tumor progression**

Cancer progression is a complex process that includes the interaction between ECs, fibroblast, insusceptible cells, and adipocytes [96]. Normal epithelial cells do not express leptin and leptin receptors but are overexpressed in a cancerous environment. Leptin enhances the survival rate of cancer cells through the activation of a downstream signaling molecule known as sirtuin-dependent NAD-dependent deacetylase 1 (SIRT 1) [97]. Leptin can activate many signaling pathways in cancer directly by activating TNF alpha, IL-6, ROS, VEGF, MMP2, and MMP9. It can also support tumor growth by activating JAK/STAT, Akt, FGF2, and NO molecules through receptor (Ob-R) binding mechanisms in ECs [98, 99]. The appetite hormone can potentially interact with pre-neoplastic or cancerous breast epithelium in a breast cancer environment. Leptin secreted by the breast cancer surroundings inhibits inflammatory cytokines and thus blocks macrophages' production [100, 101]. The cytokine enhances neovascularization through VEGF in many cancerous conditions [102, 103].

Increased levels of serum leptin and insulin under obese conditions cause colorectal cancer [104]. Leptin supports the proliferation and invasiveness of colonic cells. Leptin receptors are found to express in human colon cell lines and are believed to initiate cancer angiogenesis. Hyperlipidemia and insulin resistance can cause low-grade systemic inflammation that promotes proliferation and angiogenesis and inhibits apoptotic rate in colon cancer [105]. Leptin and its receptors express in papillary thyroid tumors and enhance the pathogenicity through PI3K/Akt pathway [106].

Obesity enhances the concentration of leptin around the pancreatic carcinogenic environment. The enhanced concentration of leptin favors vascularization, migration, and invasiveness of pancreatic tumor cells [105]. Leptin has a crucial role in developing the non-alcoholic fatty liver disease (NAFLD) via insulin resistance. This imbalance ultimately worsens hepatic inflammation and results in the development of liver fibrosis [107]. The receptor Ob-R identified in Kupffer cells (KC), and binding of leptin with receptor enhances the expression of TGF beta, TIMP1 in liver fibrosis scenario [108]. However, the direct role of leptin in liver cancer is controversial, with some reports suggesting its role in liver cancer. In contrast, others offer its inhibitory potential on tumor size in hepatic cancer [109, 110]. The level of leptin was found to decrease in patients with cancer cachexia compared to non-cancer cachexia [111].
