Leptin-based therapeutics

Leptin-Activity Modulators and Their Potential Pharmaceutical Applications

Leptin, a multifunctional hormone primarily, but not exclusively, secreted in adipose tissue, is implicated in a wide range of biological functions that control different processes, such as the regulation of body weight and energy expenditure, reproductive function, immune response, and bone metabolism. In addition, leptin can exert angiogenic and mitogenic actions in peripheral organs. Leptin biological activities are greatly related to its interaction with the leptin receptor. Both leptin excess and leptin deficiency, as well as leptin resistance, are correlated with different human pathologies, such as autoimmune diseases and cancers, making leptin and leptin receptor important drug targets. The development of leptin signaling modulators represents a promising strategy for the treatment of cancers and other leptin-related diseases. In the present manuscript, we provide an update review about leptin-activity modulators, comprising leptin mutants, peptide-based leptin modulators, as well as leptin and leptin receptor specific monoclonal antibodies and nanobodies.

Potential Leptin Receptor Response Modifier Peptides

Drug targets for the treatment of obesity and comorbidities represent an ever-renewable source of research opportunities worldwide. One of the earliest is the leptin–leptin receptor system that was discovered in the mid-1990s. Leptin, a satiety hormone, is overproduced in overweight patients but the protein is unable to cross the blood–brain barrier and remains inactive. Circulating high levels of leptin induces a series of conditions that would not be manifested without leptin overproduction, including various forms of cancer and inflammatory and cardiovascular diseases. Current pharmaceutical research focuses on improving the blood–brain barrier penetration of leptin receptor agonists and the development of monofunctional antagonists with broad spectrum therapeutic efficacies but without unwanted side effects. Designer peptides with their expanded chemical space as well as well controllable receptor binding and elimination properties slowly replace full-sized leptin products in the drug development pipeline.

Designer Leptin Receptor Antagonist Allo-aca Inhibits VEGF Effects in Ophthalmic Neoangiogenesis Models

Experimental and clinical data suggest that pro-angiogenic, pro-inflammatory and mitogenic cytokine leptin can be implicated in ocular neovascularization and other eye pathologies. At least in part, leptin action appears to be mediated through functional interplay with vascular endothelial growth factor (VEGF). VEGF is a potent regulator of neoangiogenesis and vascular leakage with a proven role in conditions such as proliferative diabetic retinopathy, age-related macular degeneration and diabetic macular edema. Accordingly, drugs targeting VEGF are becoming mainstream treatments for these diseases. The crosstalk between leptin and VEGF has been noted in different tissues, but its involvement in the development of eye pathologies is unclear. Leptin is coexpressed with VEGF during ocular neovascularization and can potentiate VEGF synthesis and angiogenic function. However, whether or not VEGF regulates leptin expression or signaling has never been studied. Consequently, we addressed this aspect of leptin/VEGF crosstalk in ocular models, focusing on therapeutic exploration of underlying mechanisms. Here we show, for the first time, that in retinal (RF/6A) and corneal (BCE) endothelial cells, VEGF (100 ng/mL, 24 h) stimulated leptin mRNA synthesis by 70 and 30%, respectively, and protein expression by 56 and 28%, respectively. In parallel, VEGF induced RF/6A and BCE cell growth by 33 and 20%, respectively. In addition, VEGF upregulated chemotaxis and chemokinesis in retinal cells by ~40%. VEGF-dependent proliferation and migration were significantly reduced in the presence of the leptin receptor antagonist, Allo-aca, at 100-250 nmol/L concentrations. Furthermore, Allo-aca suppressed VEGF-dependent long-term (24 h), but not acute (15 min) stimulation of the Akt and ERK1/2 signaling pathways. The efficacy of Allo-aca was validated in the rat laser-induced choroidal neovascularization model where the compound (5 μg/eye) significantly reduced pathological vascularization with the efficacy similar to that of a standard treatment (anti-VEGF antibody, 1 μg/eye). Cumulatively, our results suggest that chronic exposure to VEGF upregulates leptin expression and function. As leptin can in turn activate VEGF, the increased abundance of both cytokines could amplify pro-angiogenic and pro-inflammatory environement in the eye. Thus, combined therapies targeting ObR and VEGF should be considered in the treatment of ocular diseases.

Leptin inhibitors from fungal endophytes (LIFEs): Will be novel therapeutic drugs for obesity and its associated immune mediated diseases

Treatment of obesity and its associated immune mediated diseases is challenging due to impaired function of leptin system. Thus leptin is providing an interesting target for therapeutic intervention. Leptin, an adipose tissue-derived adipokine, displays a variety of immune functions, and regulate both innate and adaptive immune responses. The increased secretion of leptin (hyperleptinemia) and production of proinflammatory cytokines has been implicated in the pathogenesis of obesity-related immune diseases such as diabetes mellitus, hypertension, atherosclerosis, cancer, systemic lupus erythematosus, rheumatoid arthritis, crohn's disease and multiple sclerosis. These disorders are managed through antibiotics and by cytokines replacement. However, the effectiveness of cytokines coupled to the complexity of the cytokine network leads to severe side-effects, which can still occur after careful preclinical evaluation. In addition, synthetic immunotherapeutics carries a degree of risk, is time-consuming and expensive. Hence, the complexity of existing therapy and adverse effects emphasizes the need of an alternative approach for the management of immune dysfunction associated with obesity and its related diseases. For the aforementioned diseases that are related to leptin overabundance, new drugs blocking leptin signaling need to be generated. The research on the discovery of clinically important novel compounds from natural source is expanding due to their safety and no side effect. The fungal endophytes are the microbes that colonize internal tissue of plants without causing negative effects to the host. They produce plethora of substances of potential use to modern medicinal and pharmaceutical industry. The increasing body of evidence associated with application of bioactive metabolites derived from fungal endophytes in diverse disease states merits its use as therapeutic drugs. In particular, the saponins have been extensively proved to modulate the immune system, which has raised a significant interest in their potential as immunomodulators. Thus, our hypothesis is that the saponins derived from fungal endophytes can be explored as clinical applicable leptin inhibitors for treating immune mediated diseases.

Leptin and adiponectin: emerging therapeutic targets in breast cancer

Obesity is a recognized risk factor for breast cancer development and poorer response to therapy. Two major fat tissue-derived adipokines, leptin and adiponectin have been implicated in mammary carcinogenesis. Leptin appears to promote breast cancer progression through activation of mitogenic, antiapoptotic, and metastatic pathways, while adiponectin may restrict tumorigenic processes primarily by inhibiting cell metabolism. Furthermore, adiponectin is known to counteract detrimental leptin effects in breast cancer models. Thus, therapeutic inhibition of pro-neoplastic leptin pathways and reactivation of anti-neoplastic adiponectin signaling may benefit breast cancer patients, especially the obese subpopulation. This review focuses on current experimental strategies aiming at leptin and adiponectin pathways in breast cancer models. Novel leptin receptor antagonists and adiponectin receptor agonists as well as other compounds for therapeutic modulation of adipokine pathways are discussed in detail, including potential pharmacological advantages and limitations of these approaches.

Exploring leptin antagonism in ophthalmic cell models

Background

Emerging evidence suggests that angiogenic and pro-inflammatory cytokine leptin might be implicated in ocular neovascularization. However, the potential of inhibiting leptin function in ophthalmic cells has never been explored. Here we assessed mitogenic, angiogenic, and signaling leptin activities in retinal and corneal endothelial cells and examined the capability of a specific leptin receptor (ObR) antagonist, Allo-aca, to inhibit these functions.

Methods and Results

The experiments were carried out in monkey retinal (RF/6A) and bovine corneal (BCE) endothelial cells. Leptin at 50-250 ng/mL stimulated the growth of both cell lines in a dose-dependent manner. The maximal mitogenic response (35±7 and 27±3% in RF6A and BCE cells, respectively) was noted at 24 h of 250 ng/mL leptin treatments. Leptin-dependent proliferation was reduced to base levels with 10 and 100 nM Allo-aca in BCE and RF6A cells, respectively. In both cell lines, leptin promoted angiogenic responses, with the maximal increase in tube formation (163±10 and 133±8% in RF6A and BCE cultures, respectively) observed under a 250 ng/mL leptin treatment for 3 h. Furthermore, in both cell lines 250 ng/mL leptin modulated the activity or expression of several signaling molecules involved in proliferation, inflammatory activity and angiogenesis, such as STAT3, Akt, and ERK1/2, COX2, and NFκB. In both cell lines, leptin-induced angiogenic and signaling responses were significantly inhibited with 100 nM Allo-aca. We also found that leptin increased its own mRNA and protein expression in both cell lines, and this autocrine effect was abolished by 100-250 nM Allo-aca.

Conclusions

Our data provide new insights into the role of leptin in ocular endothelial cells and represent the first original report on targeting ObR in ophthalmic cell models.

Designer peptide antagonist of the leptin receptor with peripheral antineoplastic activity

The obesity hormone leptin has been implicated in the development and progression of different cancer types, and preclinical studies suggest that targeting leptin signaling could be a new therapeutic option for the treatment of cancer, especially in obese patients. To inhibit pro-neoplastic leptin activity, we developed leptin receptor (ObR) peptide antagonists capable of blocking leptin effects in vitro and in vivo. Our lead compound (Allo-aca), however, crosses the blood-brain-barrier (BBB), inducing undesirable orexigenic effects and consequent weight gain. Thus, redesigning Allo-aca to uncouple its central and peripheral activities should produce a superior compound for cancer treatment. The aim of this study was to generate novel Allo-aca analogs and test their biodistribution in vivo and anti-neoplastic activity in vitro in breast and colorectal cancer cells. Examination of several Allo-aca analogs resulted in the identification of the peptidomimetic, d-Ser, that distributed only in the periphery of experimental animals. d-Ser inhibited leptin-dependent-proliferation of ObR-positive breast and colorectal cancer cells in vitro at 1nM concentration without exhibiting any partial agonistic activity. d-Ser efficacy was demonstrated in monolayer and three-dimensional cultures, and its antiproliferative action was associated with the inhibition of several leptin-induced pathways, including JAK/STAT3, MAPK/ERK1/2 and PI3K/AKT, cyclin D1, and E-cadherin. In conclusion, d-Ser is the first leptin-based peptidomimetic featuring peripheral ObR antagonistic activity. The novel peptide may serve as a prototype to develop new therapeutics, particularly for the management of obesity-related cancers.

Metformin inhibits leptin-induced growth and migration of glioblastoma cells

Metformin, a derivative of biguanide, is a first-line therapy for type 2 diabetes mellitus. Since the drug has been shown to significantly reduce the risk of various cancers and cancer mortality in diabetic patients, it is being considered as a potential anticancer therapeutic or preventive agent. In cellular models, metformin inhibits the growth of many types of cancer cells; however, its effects on glioblastoma multi-forme (GBM) are not well characterized. Here, we analyzed the effects of metformin on the growth and migration of LN18 and LN229 GBM cells cultured under basal conditions or exposed to leptin, a cytokine that has recently been implicated in GBM development. We found that 2-16 mM metformin reduced basal and leptin-stimulated growth of GBM cells in a dose-dependent manner. Furthermore, the drug significantly inhibited the migration of GBM cells. The action of metformin was mediated through the upregulation of its main signaling molecule, the adenosine monophosphate-activated protein kinase (AMPK), as well as through the downregulation of the signal transducer and activator of transcription 3 (STAT3) and the Akt/PKB serine/threonine protein kinase. In leptin-treated cells, the drug reversed the effects of the cytokine on the AMPK and STAT3 pathways, but modulated Akt activity in a cell-dependent manner. Our results suggest that metformin or similar AMPK-targeting agents with optimized blood-brain-barrier penetrability could be developed as potential treatments of GBM and could be used in conjunction with other target drugs such as leptin receptor antagonists.

Glioblastoma-derived leptin induces tube formation and growth of endothelial cells: comparison with VEGF effects

BACKGROUND

Leptin is a pleiotropic hormone whose mitogenic and angiogenic activity has been implicated in the development and progression of several malignancies, including brain tumors. In human brain cancer, especially in glioblastoma multiforme (GBM), leptin and its receptor (ObR) are overexpressed relative to normal tissue. Until present, the potential of intratumoral leptin to exert proangiogenic effects on endothelial cells has not been addressed. Using in vitro models, we investigated if GBM can express leptin, if leptin can affect angiogenic and mitogenic potential of endothelial cells, and if its action can be inhibited with specific ObR antagonists. Leptin effects were compared with that induced by the best-characterized angiogenic regulator, VEGF.

RESULTS

We found that GBM cell lines LN18 and LN229 express leptin mRNA and LN18 cells secrete detectable amounts of leptin protein. Both lines also expressed and secreted VEGF. The conditioned medium (CM) of LN18 and LN 229 cultures as well as 200 ng/mL pure leptin or 50 ng/mL pure VEGF stimulated proliferation of human umbilical vein endothelial cells (HUVEC) at 24 h of treatment. Mitogenic effects of CM were ~2-fold greater than that of pure growth factors. Furthermore, CM treatment of HUVEC for 24 h increased tube formation by ~5.5-fold, while leptin increased tube formation by ~ 80% and VEGF by ~60% at 8 h. The mitogenic and angiogenic effects of both CM were blocked by Aca 1, a peptide ObR antagonist, and by SU1498, which inhibits the VEGF receptor. The best anti-angiogenic and cytostatic effects of Aca1 were obtained with 10 nM and 25 nM, respectively, while for SU1498, the best growth and angiogenic inhibition was observed at 5 μM. The combination of 5 μM SU1498 and Aca1 at 25 nM (growth inhibition) or at 10 nM (reduction of tube formation) produced superior effects compared with single agent treatments.

CONCLUSIONS

Our data provide the first evidence that LN18 and LN 229 human GBM cells express leptin mRNA and might produce biologically active leptin, which can stimulate tube formation and enhance proliferation of endothelial cells. Furthermore, we demonstrate for the first time that a peptide ObR antagonist inhibits proangiogenic and growth effects of leptin on endothelial cells, and that the pharmacological potential of this compound might be combined with drugs targeting the VEGF pathway.