Compartment-specific activation of PPARγ governs breast cancer tumor growth, via metabolic reprogramming and symbiosis

Abstract

The role of PPARγ in cancer therapy is controversial, with studies showing either pro-tumorigenic or antineoplastic effects. This debate is very clinically relevant, because PPARγ agonists are used as antidiabetic drugs. Here, we evaluated if the effects of PPARγ on tumorigenesis are determined by the cell type in which PPARγ is activated. Second, we examined if the metabolic changes induced by PPARγ, such as glycolysis and autophagy, play any role in the tumorigenic process. To this end, PPARγ was overexpressed in breast cancer cells or in stromal cells. PPARγ-overexpressing cells were examined with respect to (1) their tumorigenic potential, using xenograft models, and (2) regarding their metabolic features. In xenograft models, we show that when PPARγ is activated in cancer cells, tumor growth is inhibited by 40%. However, when PPARγ is activated in stromal cells, the growth of co-injected breast cancer cells is enhanced by 60%. Thus, the effect(s) of PPARγ on tumorigenesis are dependent on the cell compartment in which PPARγ is activated. Mechanistically, stromal cells with activated PPARγ display metabolic features of cancer-associated fibroblasts, with increased autophagy, glycolysis and senescence. Indeed, fibroblasts overexpressing PPARγ show increased expression of autophagic markers, increased numbers of acidic autophagic vacuoles, increased production of L-lactate, cell hypertrophy and mitochondrial dysfunction. In addition, PPARγ fibroblasts show increased expression of CDKs (p16/p21) and β-galactosidase, which are markers of cell cycle arrest and senescence. Finally, PPARγ induces the activation of the two major transcription factors that promote autophagy and glycolysis, i.e., HIF-1α and NFκB, in stromal cells. Thus, PPARγ activation in stromal cells results in the formation of a catabolic pro-inflammatory microenvironment that metabolically supports cancer growth. Interestingly, the tumor inhibition observed when PPARγ is expressed in epithelial cancer cells is also associated with increased autophagy, suggesting that activation of an autophagic program has both pro- or antitumorigenic effects depending on the cell compartment in which it occurs. Finally, when PPARγ is expressed in epithelial cancer cells, the suppression of tumor growth is associated with a modest inhibition of angiogenesis. In conclusion, these data support the "two-compartment tumor metabolism" model, which proposes that metabolic coupling exists between catabolic stromal cells and oxidative cancer cells. Cancer cells induce autophagy, glycolysis and senescence in stromal cells. In return, stromal cells generate onco-metabolites and mitochondrial fuels (L-lactate, ketones, glutamine/aminoacids and fatty acids) that are used by cancer cells to enhance their tumorigenic potential. Thus, as researchers design new therapies, they must be conscious that cancer is not a cell-autonomous disease, but rather a tumor is an ecosystem of many different cell types, which engage in metabolic symbiosis.

Emerging functional roles of nuclear receptors in breast cancer

“…PPARG is the best studied of the 3 PPAR subtypes and has been reported to have an anti-proliferative effect on breast cancer cells (Pon et al 2015) and is associated with better outcomes based on survival analysis of PPARG protein expression level in BC tissue microarrays (Abduljabbar et al 2015a). The downstream effect of PPARG seems to be dependent on the cell compartment in which PPARG is activated though, with activation in cancer cells resulting in growth inhibition while activation in stromal cells results in growth enhancement of co-injected breast cancer cells (Avena et al 2013). Recent studies have also suggested an involvement of PPARG and energy metabolism in breast cancer.…”

“…Secondly, overexpression of PPARG in fibroblasts increased the production of l-lactate and mitochondrial dysfunction. In addition, PPARG induces the activation of HIF1A, a transcription factor that promotes glycolysis (Avena et al 2013).…”

The update on transcriptional regulation of autophagy in normal and pathologic cells: A novel therapeutic target

“…Vara et al [91] also pointed out that PPAR gamma knockdown with siRNA caused accumulation of the autophagy markers LC3-II and p62, suggesting that PPAR gamma was necessary for the autophagy flux. From another angle, Avena et al [92] reported that overexpressing PPAR gamma showed increased expression of autophagic markers, increased numbers of acidic autophagic vacuoles, increased production of L-lactate, cell hypertrophy and mitochondrial dysfunction. More interestingly, they [92] also presented that PPAR gamma can induce the activation of the two major transcription factors that promote autophagy and glycolysis in stromal cells, such as HIF-1 alpha and NF-kappa B.…”

“…Many investigators [88][89][90][91][92][93][94][95][96][97] reported that PPAR gamma was also involved in the control of transcriptional regulation of autophagy. The target genes of PPAR gamma has been reported including ATG7, ATG12, LC3, P62, ULK1, LAMP1, BCL2, Beclin1, Pink1, and Bnip3 [88][89][90][91][92][93][94][95][96][97] (Table 1). More specifically, Yan et al [88] discovered that PPAR gamma agonist troglitazone (TZ), a synthetic ligand of PPAR gamma, could induce autophagy and apoptosis in a variety of cancer cells.…”

“…PPAR gamma is also present in other tissues, such as breast [70,71], colon [72][73][74], lung [75,76], ovary [77,78], prostate [79,80] and thyroid [81,82] wherein it mediates several specific functions, such as early development [83], cell proliferation [84], differentiation [85], apoptosis [86], and metabolic homeostasis [87]. Many investigators [88][89][90][91][92][93][94][95][96][97] reported that PPAR gamma was also involved in the control of transcriptional regulation of autophagy. The target genes of PPAR gamma has been reported including ATG7, ATG12, LC3, P62, ULK1, LAMP1, BCL2, Beclin1, Pink1, and Bnip3 [88][89][90][91][92][93][94][95][96][97] (Table 1).…”