Activation of PPARγ in myeloid cells promotes lung cancer progression and metastasis

Tumour-associated macrophages and cancer

Our understanding of the complex roles and functions of tumour-associated myeloid cells has improved vastly over the last few years. Alternatively activated macrophages, TAMs, are an abundant part of solid and haematological malignancies and have been linked with progression, metastasis and resistance to therapy. Still, characterisation and TAM targeting is hindered by a lack of TAM specific markers, but advances in next generation technologies are rapidly increasing our understanding of the sheer diversity of myeloid differentiation and phenotypic regulation. These technologies help to shed light on the heterogeneous phenotypic states of myeloid cells within the tumour. Alternative approaches to influence the myeloid compartment within cancers surround inhibition of myeloid recruitment or 're-education' of the plastic TAM phenotype. Our knowledge continuously grows on how even 'established' therapies might influence the myeloid compartment within tumours. Now the promising results from elegant pre-clinical studies at first translate into the clinic and use combination therapies with myeloid inhibitors and standard chemotherapy.

Obesity-driven inflammation and cancer risk: role of myeloid derived suppressor cells and alternately activated macrophages

During carcinogenesis, tumors induce dysfunctional development of hematopoietic cells. Myeloid lineage cells, in the form of myeloid derived suppressor cells (MDSCs) and alternatively polarized M2 macrophages, influence almost all types of cancers by regulating diverse facets of immunosuppression, angiogenesis, cell proliferation, growth and metastasis. One-third of Americans are obese, and accumulating evidence suggests that obesity is a risk factor for various cancers. However, the relationship between these immune players and obesity are not well-described. In this review, we evaluate potential mechanisms through which different aspects of obesity, namely insulin resistance, increased estrogen, adiposity and low grade chronic inflammation from adipose tissue macrophages, may coalesce to promote MDSC induction and M2 macrophage polarization, thereby facilitating cancer development. Detailed understanding of the interplay between obesity and myeloid mediated immunosuppression may provide novel avenues for therapeutic targeting, with the goal to reduce the challenge obesity presents towards gains made in cancer outcomes.

The conditional role of inflammation in pregnancy and cancer

Cancer growth is characterized by proliferation of tumor cells in conjunction with invasion of all different immune cells that also invade healing wounds. This inflammatory response is necessary for cell proliferation but a second purpose of the inflammatory process is so that a low Th1/Th2 ratio is present with overexpression of IL-10, TGF-β and IFN-γ. Down regulation of NO activity also shifts the balance between M1 and M2 macrophages. Both aspects allow the antigenous nature of the tumor to escape anti-tumor effects of the host. Support for this view comes from observations in pregnancy in which the placenta exhibits identical immune responses and downregulation of NO production to allow trophoblast cells to invade the uterine tissues without being rejected. Cell proliferation requires a metabolic set-up in which the organism produces adequate substrate for growth. This also bears the characteristics of a systemic inflammatory response delivering a similar substrate mix required for cancer and fetal growth. This arrangement is clearly beneficial in pregnancy and therefore supports the view that cancer growth is facilitated by the organism: the cancerous tumor elicits an immunological response opposing anti-tumor effects and induces the host to produce building blocks for growth.

Anti- and Protumorigenic Effects of PPARγ in Lung Cancer Progression: A Double-Edged Sword

Peroxisome proliferator-activated receptor-γ (PPARγ) is a member of the nuclear receptor superfamily of ligand-activated transcription factors that plays an important role in the control of gene expression linked to a variety of physiological processes, including cancer. Ligands for PPARγ include naturally occurring fatty acids and the thiazolidinedione class of antidiabetic drugs. Activation of PPARγ in a variety of cancer cells leads to inhibition of growth, decreased invasiveness, reduced production of proinflammatory cytokines, and promotion of a more differentiated phenotype. However, systemic activation of PPARγ has been reported to be protumorigenic in some in vitro systems and in vivo models. Here, we review the available data that implicate PPARγ in lung carcinogenesis and highlight the challenges of targeting PPARγ in lung cancer treatments.