Nuclear receptors are involved in many cellular processes from embryonic development to cell death. Dysfunction of nuclear receptor signaling can lead to proliferative and metabolic diseases such as cancer and obesity. In the current report, we assessed the mRNA expression levels of nuclear receptors and COX-2 in 20 CRC specimens and sought a possible relationship with patient's weight status defined by BMI ranging from 18.7 to 38.7.
PPARγ constitutes the most extensively studied of the three PPAR subtypes (α, β, γ) since its function relates to lipid metabolism as well as cell differentiation, apoptosis and cancer. PPARγ can be activated by certain lipids and derivatives and by anti-diabetic agents. Activated PPARγ has been shown able to stimulate differentiation and apoptosis in cancer cells from various origins [35–38]. Nevertheless, in contrast with results generated in vitro, data concerning PPARγ expression in human cancer specimens raised questions about the anti-neoplastic activity of the receptor in vivo. For example, PPARγ was found highly expressed in ovarian carcinoma  and its overexpression in pancreatic carcinoma was associated with poor prognosis . By contrast, our data, in agreement with others , showed PPARγ expression globally unchanged in CRC compared with adjacent normal tissues, although Dubois et al.  found a marked increase of PPAR mRNA expression in four CRC samples and in different colon cancer cell lines. Discrepancies might be attributed to germline mutations in the adenomatous polyposis gene (APC). Indeed, PPARγ has been involved in increasing resistance towards carcinogens by preventing the accumulation of β-catenin, which is regulated by APC. However, PPARγ functions are lost when APC is mutated . Another report has shown that deregulated APC/β-catenin indirectly induced aberrant PPARγ overexpression , explaining previous experimental data in APC
Min/+ mice showing a promoting effect of PPARγ on carcinogenesis . This has relevance for humans because mutations in the tumor suppressor gene APC are the initiating event in about 85% of sporadic CRC. Therefore, APC status could dramatically affect expression and function of PPARγ and the steady-state levels of PPARγ reported here do not exclude loss of PPARγ transcriptional activity due to somatic mutations , alterations in intracellular distribution , post-translation modifications  or inhibition by PPARδ .
Like PPARγ, PPARδ gene expression is detected in the colon and the receptor can be activated by fatty acids and derivatives. Herein, we reported an elevated level of PPARδ (~18%) in CRC. Upregulation of PPARδ gene expression might be attributed to deregulation in the APC/β-catenin pathway since PPARδ is considered to be a downstream target gene . Increased levels of PPARδ expression have already been observed in rodent colorectal tumors and in primary human colorectal adenocarcinomas [20, 21]. Nevertheless, PPARδ function remains elusive, with data showing that PPARδ was dispensable for polyp formation . Our data and others suggested a contribution of PPARδ in the carcinogenesis process [16, 50] while Marin et al.  described that agonist-activated PPARδ protects against cancer development. As for PPARγ, the integrity of the tumor suppressor APC might be essential to guarantee PPARδ normal function.
Critical to the transcriptional activity of PPARs is the ability to form a complex with RXR and bind to DNA. Synthetic ligands of RXRα were shown to exhibit insulin-sensitizing activity [51, 52] and to act synergistically with PPARγ ligands to enhance PPARγ/RXRα-mediated transactivation . In addition, a positive correlation in healthy mucosa was found between RXRα and PPARγ supporting the idea of a tight relationship in the regulation of the expression of these receptors. While no change in RXRα expression level was previously noted in 17 patients with CRC , our data revealed a significant increase in tumor versus normal tissue. A similar upregulation was also observed in human esophageal , breast  and hepatocellular carcinomas . However, little is known about the function of RXRα in colon tumorigenesis and our results do not rule out the possibility of alterations in RXRα functions due to altered localization  or inhibitory effect of unliganded RXRα on PPARγ transactivation .
Recent data have also suggested that PPARγ anti-tumor activity required a functional RARβ . This implies that PPARγ function may be affected by alterations in the retinoid pathway. To our knowledge, very few reports have examined the expression of retinoid receptors in CRC. Therefore, we described here the first detailed analysis of nuclear receptor RARα, β and γ mRNA expression in CRC. RARβ has been extensively studied in cancer cells and human carcinomas, and several studies have suggested that it may play a role as a tumor suppressor gene [58–60]. However, our results showed a significant upregulation in the expression of all three RAR isotypes in CRC specimens compared to adjacent normal mucosa. Furthermore, we showed a complex association between the expression of mRNA for RXRα, RARs, and PPARs in cancer tissue, suggesting interactions and cross-talk between these receptors in tumorigenesis. These results demonstrated that alterations are not restricted to a single receptor. Instead, we observed a profound dysregulation of the retinoid pathway in this CRC. Downregulation of mRNA expression of RARβ often observed in cancer cells has been considered as a cellular mechanism to prevent retinoid-induced growth arrest [61, 62]. On the other hand, while elevated levels of RAR mRNA expression has also been described in breast, liver and esophageal tumors [53, 63, 64], mechanism and significance are unknown. Nevertheless, if the expression of RAR correlates with tissue sensitivity to retinoids, our results should be confirmed within a larger number of samples and both the mechanism leading to inappropriate RXR and RAR expression and the response of CRC to retinoids should be investigated.
There are strong correlations between the intake of fatty acids, the establishment of metabolic disorders and an increased risk of developing CRC [65, 66]. This suggests the involvement of PPARs and retinoid receptors, activated by fatty acids and derivatives  and modulated by metabolic disorders  in establishing a link between overweight prevalence and CRC pathogenesis. In the current report, we aimed to clarify whether aberrations in the expression of nuclear receptors may contribute to associate high BMI and CRC. However, alterations in nuclear receptor expression observed in tumors were similar in both patients with low or high BMI. We also investigated COX-2 expression which is involved in cellular responses to lipids and inflammatory processes that favour tumorigenesis by stimulating cell proliferation and angiogenesis . Interestingly, while COX-2 was greatly expressed in CRC as previously shown , we also found a significantly increased level of COX-2 expression in normal mucosa from patients with high BMI compared to low BMI patients. Recently, it has been reported that patients with a high risk of developing CRC presented an upregulation of the COX-2 gene in normal-looking colon mucosa . This supports the idea that COX-2 deregulation might be an early event in the process of carcinogenesis. Nevertheless, although previous reports showed COX-2 regulation by nuclear receptors [31, 71], very few associations were found in our study between COX-2 and nuclear receptor expression. In conclusion, our study described altered expression of nuclear receptors in CRC specimens. Further studies are warranted in order to determine the underlying mechanism leading to altered expression of PPARs and retinoid-activated receptors and the significance of such alterations. Moreover, alterations in nuclear receptor expression were independent of the weight status of patients. Nevertheless, COX-2 might be one early target influenced by excess weight and associated metabolic disorders and consequently might affect nuclear receptor expression and activation.