Although epidemiological studies have pointed to a relationship between obesity and prostate cancer, these data come mainly from clinical studies and identify hyperglycemic and inflammatory factors as key factors for inducing prostate injuries. Although PPARα promotes strong anti-inflammatory properties, lipoprotein metabolism, and apoptosis in various animal models 30, PPAR-null mice treated with HFD led to greater suppression of endogenous glucose production as revealed by hyperinsulinemia, indicating less insulin resistance in the absence of PPARα. In the current study, we seek to understand the action of the gene transcription factor PPARα in the ventral prostate of mice fed a high-fat diet or standard diet, and in what ways aerobic exercise modulates the prostatic microenvironment in the absence of this nuclear receptor.
The high-fat diet (HFD, 59% kcal fat) is related to the development of obesity, glucose tolerance, and inflammation. Metabolic alterations related to obesity are strong stimulators of the development of prostate cancer 31. HFD was efficient to promote an increase in weight gain and the adipose index compared to animals fed a standard diet in WT mice. First, to relate the effect of PPARα null and diet-induced changes in fatty acid oxidation, we observed that PPARα null mice fed with HFD showed reduced weight gain and adipose index compared to the wild-type mice. Data already published by Batatinha et al., 24 the PPARα null reduced glycemia and increased level of triacylglycerol, cholesterol, and FFA in relation to WT. The expression of PPARα is maintained by the presence of endogenous ligands, such as Fas, which is important for the maintenance of basal FA metabolism 32. FFA has been reported to inhibit glucose uptake, causing insulin resistance 33. As shown in the current study, cholesterol and FFA are increased in the absence of PPARα. Nevertheless, the absence of PPARα reduces the oxidation of FA, favoring the use of glucose by the tissues, even in the HFD-fed mice with less glucose availability and, thus, the excess calorie intake is not stored in the white adipose tissue, leading to a reduction in weight gain and the adipose index of PPARα null animals.
On the other hand, aerobic exercise can regulate energy metabolism via PPARα and mitochondrial enzymes, preventing obesity and lipid disorders in rats fed with HFD 34. We know that the absence of PPARα associated with aerobic physical exercise and HFD increased energy expenditure and did not regulate the energy balance, using lipid as an energy source 24. In this way, animals of the PPARα KO group submitted to aerobic physical exercise had a lower fat index and higher levels of circulating FFA and marked glucose reduction. The action mechanism of reduced fatty acid oxidation in PPARα null mice is accompanied by twice the concentration of malonyl CoA and decreased activity of Malonyl-CoA decarboxylase 35. Silveira et al., 15 demonstrated that after 24 hours, a moderate intensity session of aerobic physical training, performed in PPARα KO mice promoted a reduction in glucose and increase in non-esterified fatty acids, cholesterol, and triglycerides, indicating that the absence of PPARα impairs β-oxidation and, consequently, NEFA and the accumulated acetyl-CoA from glycolysis can be converted to triacylglycerol. Thus, the lack of PPARα even in conditions of physical demand with aerobic training reduces the transport of fatty acids to the mitochondria and the oxidation capacity.
The role of PPARα in the prostate has been little discussed, although there is evidence of greater expression of PPARα in prostatic adenocarcinomas 36. Similarly, the impact of HFD on tumor growth is well documented 37. Our results support the idea that PPARα null has an incidence of prostatic intraepithelial neoplasia in 70% of the analyzed group and when we associated HFD with PPARα null it increased to 72% PIN, and when we analyzed the effect of HFD in WT mice we verified a higher incidence (84% PIN). The high expression of PIN in animals fed with HFD is related to increased expression of prostatic AR, however, the increase in PIN in the PPARα null group fed the standard diet is via AR independent routes. Lin et al., 38 found that AR−/− negatively alters PPARα expression in the liver, indicating that PPARα is androgen dependent. The absence of PPARα does not interfere with HFD-induced AR activation. On the other hand, aerobic physical exercise has the potential to reduce prostatic changes, as demonstrated by Teixeira et al 25,39, by reducing the expression of AR. Our results showed lower expression of AR in the groups submitted to aerobic exercise and HFD in both genotypes, with a reduction from 72% (PPARα KO HF) to 44% (PPARα KO HFT) in the incidence of PIN. The protective action of physical exercise on the prostate is independent of the presence of PPARα.
PPARα is best known as a critical regulator of lipid metabolism and inflammation 40 and is expressed in tissues that catabolize fatty acids, such as prostatic epithelial cells 36. Prostatic inflammation is related to the development and progression of prostate cancer 41. The HFD induces stromal infiltrates of inflammatory cells such as macrophages, T cells, monocytes, and mast cells in the prostate 42,43. Studies show that the activation of NF-kB is dependent on PPARɑ, consequently animals PPARɑ −/− present overexpression of NF-kB in the liver, stimulating the inflammatory pathway 44,45. In the prostate, there was an increase in mast cells, IL-6, and TNF-ɑ, but not NF-kB in response to low PPARɑ expression in animals fed a standard diet compared to WT. The absence of PPARɑ may induce greater expression of PPARγ in the liver 24. The increase in PPARγ induced by the absence of PPARɑ may have reduced the expression of NF-kB but did not alter the expression of inflammatory cytokines, suggesting that there is a compensatory effect. One mechanism would be the greater induction of PPARɑ−/− recruiting inflammatory macrophages (M1) in the prostate, modulating the secretion and expression of the cytokines IL-6 and TNF-α, regardless of NF-kB activation 46,47.
The increased expression of IL-6, an NF-kB inducible gene, is an autocrine growth factor for prostate cancer cells. However, the increase in the inflammatory response to an HFD occurs due to the increase in the expression of MCP-1, IL-6,and TNF-ɑ and inhibits the anti-inflammatory effect of GR 48. The absence of PPARɑ did not interfere with the expression of GR, however, the HFD up-regulated GR in the prostate. Activation of GR seems to act in synergism with the activation of PPARɑ 49. GR inhibited the activity of numerous transcription factors, including Activating Protein-1 (AP-1), NF-kB, signal transducer, and activator of transcription 1 (STAT1), many of which are regulated via the MAPK cascade 50,51. Inflammatory mediators such as TNF-a and growth signals such as insulin-like growth factor-1(IGF-1) may activate the transcription factors AP-1 and NF-kB, thus inhibiting the GR-mediated transactivation effects in the prostate 52. Our study suggested that a high-fat diet increases the expression of GR, NF-kB, and TNF-ɑ in both phenotypes. On the other hand, inflammation was reduced by the presence of aerobic exercise that mitigated the effects of HFD on the prostate regardless of the presence of PPARɑ, inducing the highest expression of IL-10.
Fas (CD95/Apo-1) is a cell membrane glycoprotein that belongs to the TNF-α family, with the function of triggering proteolytic cleavage of caspases, culminating in the process of cell apoptosis 53.The extrinsic pathway of cell apoptosis induced by Fas activation (CD95/Apo-1) initiates caspase 8-mediated cleavage of proteins such as BID, which translocates to mitochondria to activate BAX and BAK, resulting in membrane permeability and release of cytochrome C 54. The absence of PPARɑ promoted up-regulation of the BCL-2 protein and down-regulation of BAX and Fas (CD95/Apo-1). HFD induced an increase in Fas (CD95/Apo-1) in wild type mice, possibly induced by an increase in BCL-2. An increase in Fas (CD95/Apo-1) has been related to the increase in liver disease and insulin resistance and failure in mitochondrial oxidation 55. The inhibition of PPARɑ modulates proteins related to mitochondrial apoptosis in the ventral prostate with the high consumption of lipids in HFD provoking lipotoxicity, promoting an increase in Fas expression (CD95/Apo-1). The downregulation of Fas (CD95/Apo-1) is a promising result of regulation and insulin resistance and mitochondrial alteration induced by aerobic physical exercise . In the current study, aerobic training reduced Fas (CD95/Apo-1) levels, reinforcing the potential effect of exercise on prostatic lipid regulation, and increased the BAX/BCL-2 ratio.
Given the accumulated findings pointing to the importance of developing prostatic neoplasms of non-cancerous tissues, such as androgenic modifications, inflammation, and other functions mediated by stroma and infiltrating cells, our results add a new element to the emerging paradigm that the formation of neoplasms can be metabolically induced by the absence of PPARα and the great importance of energy regulation for the prostate environment. Inhibition of PPARɑ reduces the energy balance of prostatic epithelial cells and increases inflammation, inducing anti-apoptotic stimuli.
The increase in lipid intake due to the consumption of HFD in PPARɑ null animals induces a higher percentage of PIN and, consequently, an increase in NF-kB, which in turn increases the expression of GR as an attempt to restore the prostate environment by increasing cell apoptosis. The practice of aerobic exercise as a metabolic control therapy, associated with the absence of PPARɑ reduces the impact of HFD on prostatic lesions, increases the expression of anti-inflammatory proteins regulating the expression of AR and GR, and induces greater apoptotic expression via BAX and less activation via Fas (CD95/Apo-1), promoting a better prostatic mitochondrial environment.
The absence of PPARα promotes reduced adaptations to aerobic physical exercise in prostatic regulation through prostatic alterations in mice fed a standard diet and HFD, indicating that PPAR is a key factor in the effects of physical training on the prostate (Fig. 5). Therefore, the action of genes involved in prostate cell metabolism must be seen in a broader context. Several pro-inflammatory factors stimulate the tumor growth. Herein, we report that PPARα null induces an inflammation process and prostatic changes, however, these inflammatory processes induce the increase in apoptosis and may inhibit cell growth even more when associated with physical exercise, thus expanding the spectrum for anticancer therapies that aim to interfere with prostatic stromal processes.