Targeting the PPARγ and ER pathways via modulation of inflammation in the tumor microenvironment: a novel lung cancer prevention strategy

Abstract

Lung cancer exceeds all other diagnosed cancers in annual mortality, surpassing the top two annually diagnosed cancers breast and prostate, combined. The vast majority of diagnosed lung cancer cases are in current or former smokers, accounting for 85% of all cases. The number of diagnosed lung cancer cases continues to rise, addressing the need for novel intervention strategies. Despite current advances in chemoprevention for other less-fatal types of cancer, the only currently recognized chemopreventive strategy for lung cancer is smoking cessation. However, former smokers retain a 2.5-fold increased risk of developing lung cancer compared with never smokers, despite cessation efforts. About 40% of all newly diagnosed lung cancers occur in former smokers. As such, chemoprevention strategies for lung cancer are direly needed for the large and ever-growing high-risk population. Preclinical evaluation of existing therapies with established safety and efficacy profiles represents a fruitful opportunity to advance the field. Identification of ERβ expression has been found to be a lucrative method to identify lung cancers that confer poor survival and presents as a potential target for chemopreventive efforts. Preclinical evaluation of anti-estrogens in cell lines and mouse models of lung cancer shows great promise in advancing this class of drugs towards future clinical use in lung cancer prevention. Furthermore, anti-estrogens such as fulvestrant, a complete ER antagonist, have shown anti-tumorigenic activity in lung cancer and others such as tamoxifen have already been successfully implemented in both primary and secondary breast cancer prevention modalities. Preclinical and clinical evidence underpinning the importance of managing ER signaling to control lung cancer initiation and progression, although efficacious, alludes to the potential for increased efficacy when used in combination with other agents. Pioglitazone, a synthetic peroxisome proliferator-activated receptor gamma (PPARγ) agonist belonging to the thiazolidinediones (TZDs) drug class has also been used in preclinical studies to mitigate lung tumorigenesis, progression, and metastasis after a retrospective analysis found that diabetics using TZDs experienced a 33% reduction in lung cancer incidence. PPARγ has also been implicated as a protective pathway in lung cancer initiation and progression in early phase clinical testing. Patients with improved histology scores demonstrated a link between an increased ER gene signature and positive-response to PPARγ activation, which conferred a chemopreventive effect in dysplasias with a persistent and progressive phenotype. This link presents a unique opportunity to utilize two known mechanisms that are efficacious in protecting against carcinogen-induced lung cancer initiation and progression. Cross-talk between PPARγ signaling and estrogen receptor (ER) signaling has also been previously reported in other cancer models. NNK is a principal carcinogen in cigarette smoke, and along with its ability to induce mutations in oncogenes, NNK can act as an inflammatory mediator of the tumor microenvironment (TME) by promoting macrophage infiltration into the lungs. Furthermore, in previously reported NNK-models of lung cancer, immune cells thought to be macrophages that were positive for both aromatase and estradiol were localized to preneoplastic lesions. Taken together, macrophages are hypothesized to play a key role in regulation of the lung TME both through ER-dependent and independent mechanisms, and both ER and PPARγ pathways are also known to be functional. To develop a novel approach to prevent lung cancer, preclinical studies were developed to evaluate the therapeutic potential and chemopreventive capabilities of two FDA-approved agents, pioglitazone and fulvestrant, re-purposed in a lung cancer tumor microenvironment (TME) in vitro model and an NNK-induced adenocarcinoma chemoprevention in vivo model. To test pioglitazone and fulvestrant in a preclinical model simulating the lung TME, we selected a human NSCLC adenocarcinoma cell line with a similar KRAS mutational signature found in smoking-induced lung cancer and a human immortalized macrophage cell line. Additionally, we tested the effects of pioglitazone and fulvestrant in a murine primary cell culture model, utilizing mouse-derived adenocarcinoma cells immortalized from NNK-induced in vivo tumors and primary murine bone marrow-derived macrophages (BMDMs).