Altered leptin signaling on dendritic cells as a potential mechanism for cancer immunotherapy
Leptin is a pleiotropic hormone synthesized primarily by white adipocytes and its receptors are expressed in a variety of tissues and cells such as in the hypothalamus and cells of the immune system. Multiple cell types can produce a considerable amount of leptin such as skeletal muscle, placenta, and osteoblasts to name a few and its synthesis has been shown to be regulated by sex hormones and a broad range of inflammatory mediators. Although leptin has been shown to directly affect immune response, we are interested in how leptin affects dendritic cell function and their ability to induce a proper and effective anti-tumor response. The leptin receptor primarily activates the Janus kinase/Signal Transducer and Activators of Transcription (Jak/STAT) pathway with its own negative feedback loop in the production of suppressor of cytokine signaling 3 (SOCS3). Leptin resistance and leptin sensitivity have been suggested to be due to increased SOCS3 expression. Although this pathway is the target pathway, the MAPK/ERK and PI3K pathways are also known to be activated and exert different functions within the cells. Due to the presence of the long isoform of the receptor, Ob-Rb, on dendritic cells and the known effects of leptin on both branches of the immune system, we hypothesized that all three of the signaling pathways activated by leptin will be activated in both bone marrow-derived dendritic cells (BM-DC) and splenic dendritic cells (sDC) making them susceptible to leptin resistance/sensitivity. In determining leptin's signaling pathways in BM-DCs and sDCs, the cells were exposed to obese concentrations of leptin for 0-30min or 0-45mins, then fixed, permeabilized, and intracellularly stained for activation levels of pSTAT3, p-p38 MAPK, and p-AKT. Initial studies assessed by flow cytometry have shown significant increases in all 3 signaling pathways upon leptin stimulation indicating sensitivity of these cells to leptin and the potential for leptin resistance. Several studies have indicated that tumor derived factors (TDF), which hyperactivate STAT3 in both cancerous cells and cells of the immune system, reduced the ability of immune cells to differentiate and function properly allowing the cancer to evade immune surveillance and at the same time maintain an unregulated cellular proliferation. Nefedova and colleagues have suggested the use of a novel inhibitor, JSI-124 (curcubitacin I), as an immunotherapeutic agent to increase DC maturation and immunogenicity. Obese individuals have an increased risk of developing certain cancers, some of which are leptin-dependent. Leptin signaling, in this case, may be a possible target in cancer immunotherapies for these individuals. Taking the Nefedova and colleagues studies into consideration we proposed to determine if, in obese individuals, the inhibition of STAT3 activation via the leptin receptor but still allowing MAPK & PI3K signaling pathways to continue their role, may lead to enhanced immune functions. The Jak2/STAT3 signaling pathway is essential in almost all cellular functions and development and we sought to determine the effects of JSI-124 on dendritic cells in a pharmaceutical attempt to inhibit STAT3 phosphorylation and cellular function. We were able to determine that the compound reduced viability of the dendritic cells, both BM- and sDCs, by 50% at 0.5uM concentrations and it was also at this concentration where a significant reduction in phosphorylated STAT3 was observed. Because dendritic cells are the link to between both branches of the immune system with the ability to activate naïve T cells, we sought to determine the effects of acute inhibitor exposure on DCs and their ability to activate T cells with the use of T cell hybridomas (TCH). BM- and sDCs were pretreated for 30 minutes with the inhibitor prior to coculturing with TCH for a 72 hour period. The inhibitor reduced both the ability of the DCs to activate TCH through the production of IL-12 and IL-2 production by the TCH. Although these studies are in the preliminary stages, much is yet to be understood regarding leptin's role in these interactions and the ability to specifically target leptin signaling in an attempt to improve cancer immunotherapies in overweight and obese individuals.
De los Santos, Lorena, "Altered leptin signaling on dendritic cells as a potential mechanism for cancer immunotherapy" (2011). ETD Collection for University of Texas, El Paso. AAI1494342.