Induction of solid tumor differentiation by the peroxisome proliferator-activated receptor-gamma ligand troglitazone in patients with liposarcoma

Current prospects for controlling cancer growth with non-cytotoxic agents--nutrients, phytochemicals, herbal extracts, and available drugs

In animal or cell culture studies, the growth and spread of cancer can be slowed by many nutrients, food factors, herbal extracts, and well-tolerated, available drugs that are still rarely used in the clinical management of cancer, in part because they seem unlikely to constitute definitive therapies in themselves. However, it is reasonable to expect that mechanistically complementary combinations of these measures could have a worthwhile impact on survival times and, when used as adjuvants, could improve the cure rates achievable with standard therapies. The therapeutic options available in this regard include measures that: down-regulate serum free IGF-I; suppress the synthesis of mevalonic acid and/or certain derivatives thereof; modulate arachidonate metabolism by inhibiting 5-lipoxygenase, 12-lipoxygenase, or COX-2; antagonize the activation of AP-1 transcription factors; promote the activation of PPAR-gamma transcription factors; and that suppress angiogenesis by additional mechanisms. Many of these measures appear suitable for use in cancer prevention.

Peroxisome proliferator-activated receptors: roles in tumorigenesis and chemoprevention in human cancer

Peroxisome proliferator-activated receptors are nuclear receptors that were isolated for their ability to modulate lipid metabolism. Similar to other members of the nuclear receptor family, peroxisome proliferator-activated receptors bind ligand as heterodimers and exert their effects via transcriptional regulation through their DNA binding domains. During the past decade, it has become clear that peroxisome proliferator-activated receptors also contribute to a variety of different biologic processes, including atherosclerosis, insulin resistance, and more recently, cancer. In this review, we discuss the evidence for the different peroxisome proliferator-activated receptors' roles in tumorigenesis and also their potential application for the treatment and prevention of neoplastic diseases.

LiSa-2, a novel human liposarcoma cell line with a high capacity for terminal adipose differentiation

LiSa-2 is a stable cell line derived from a poorly differentiated, pleomorphic liposarcoma. In serum-containing medium, LiSa-2 cells are fibroblastoid and rapidly dividing. In a serum-free, chemically defined culture medium containing physiological concentrations of insulin, triiodothyronine and cortisol, LiSa-2 cells divide slower and, extensively storing fat, acquire adipocyte morphology. In contrast to fibroblastoid LiSa-2 cells, these adipocyte-like LiSa-2 cells highly express transcripts for peroxisome proliferator-activated receptor-gamma, lipoprotein lipase, fatty acid synthetase, hormone-sensitive lipase, adipocyte most abundant gene transcript-1, glycerol-3-phosphate-dehydrogenase and the insulin-sensitive glucose transporter-4, all of which are specific for differentiated adipocytes. However, leptin mRNA expression was demonstrated only after preventing DNA methylation by incorporation of 5-aza-deoxycytidine into cellular DNA. Functionally, adipocyte-like LiSa-2 cells show increased insulin-dependent glucose uptake and lipid synthesis and are sensitive to lipolytic agents. This cell line may serve as an in vitro model for studying the regulation of human liposarcoma differentiation and for screening drugs for induction of differentiation-associated growth arrest in liposarcomas.

Molecular pathogenesis of thyroid nodules and cancer

Tumours derived from the thyroid follicular epithelium represent an informative model for understanding the molecular pathogenesis of multistage tumourigenesis, which is the prevailing theory on cancer development and progression nowadays. The early stages of thyroid tumour development appear to be the consequence of the activation or 'de novo' expression of several proto-oncogenes or growth factor receptors, such as ras, ret, NTRK, met, gsp and the thyrotropin (TSH) receptor. Alterations in the expression pattern of these genes are associated with the development of differentiated neoplasms, ranging from benign toxic adenomas (gsp and TSH receptor), to follicular (ras) and papillary (ret/PTC, NTRK, met) carcinomas. They may all be considered to be early events of thyroid cell transformation and, for some, experimental evidence derived from gene transfer studies supports this hypothesis. Alterations in tumour suppressor genes (p53, Rb) are associated instead with the most aggressive and poorly differentiated forms of thyroid cancer, indicating that, in the thyroid tumourigenic process, they represent late genetic events. Specific environmental factors (iodine deficiency, ionizing radiations) have been shown to play a crucial role in promoting the development of thyroid cancer, influencing both its genotypic and phenotypic features. Interestingly, a high percentage of genetic lesions causing thyroid cancer originate from gene rearrangements and chromosomal translocations (ret/PTC, NTRK, Pax-8/PPARgamma) a finding which, being a rare event in most epithelial tumours, makes the molecular pathogenesis of thyroid cancer unique. The uninterrupted flow of information on the molecular genetics of thyroid nodules and cancer will broaden the correlation between genotype and phenotype and will also provide important information for the development of more accurate preoperative diagnostic tools and more efficient treatment choices for the different forms of thyroid cancer.

PPAR-gamma agonists: therapeutic role in diabetes, inflammation and cancer

The recent development of a novel class of insulin-sensitizing drugs, the thiazolidinediones (TZDs), represents a significant advance in antidiabetic therapy. One key mechanism by which these drugs exert their effects is by activation of the peroxisome proliferator activated receptor gamma (PPAR-gamma), a member of the nuclear receptor family. Evidence supporting this mechanism of action of the TZDs will be reviewed in this article. Recent data suggests that PPAR-gamma agonists might also have therapeutic potential in the treatment of inflammatory diseases and certain cancers.

Discrete roles for peroxisome proliferator-activated receptor gamma and retinoid X receptor in recruiting nuclear receptor coactivators

Peroxisome proliferator-activated receptor gamma (PPARgamma) plays a major role in adipogenesis. PPARgamma binds to DNA as a heterodimer with retinoid X receptor (RXR), and PPARgamma-RXR can be activated by ligands specific for either receptor; the presence of both ligands can result in a cooperative effect on the transactivation of target genes. How these ligands mediate transactivation, however, remains unclear. PPARgamma is known to interact with both the p160/SRC-1 family of coactivators and the distinct, multisubunit coactivator complex called DRIP. A single DRIP subunit, DRIP205 (TRAP220, PBP), binds directly to PPARgamma. Here we report that PPARgamma and RXR selectively interacted with DRIP205 and p160 proteins in a ligand-dependent manner. At physiological concentrations, RXR-specific ligands only induced p160 binding to RXR, and PPARgamma-specific ligands exclusively recruited DRIP205 but not p160 coactivators to PPARgamma. This selectivity was not observed in interaction assays off DNA, implying that the specificity of coactivator binding in response to ligand is strongly influenced by the allosteric effects of DNA-bound heterodimers. These coactivator-selective effects were also observed in transient-transfection assays in the presence of overexpressed p160 or DRIP coactivators. The results suggest that the cooperative effects of PPARgamma- and RXR-specific ligands may occur at the level of selective coactivator recruitment.

Expression of peroxisome proliferator-activated receptor (PPAR)gamma in gastric cancer and inhibitory effects of PPARgamma agonists

Peroxisome proliferator-activated receptor (PPAR) gamma is expressed in human colon cancer, prostate cancer and breast cancer cells, and PPARgamma activation induces growth inhibition in these cells. PPARgamma expression in human gastric cancer cells, however, has not been fully investigated. We report the PPARgamma expression in human gastric cancer, and the effect of PPARgamma ligands on proliferation of gastric carcinoma cell lines. Immunohistochemistry was used to demonstrate the presence of PPARgamma protein in surgically resected specimens from well differentiated, moderately differentiated and poorly differentiated adenocarcinoma. We used reverse transcription-polymerase chain reaction and Northern and Western blot analyses to demonstrate PPARgamma expression in four human gastric cancer cell lines. PPARgamma agonists (troglitazone and 15-deoxy-Delta(12,14)-prostaglandin J2) showed dose-dependent inhibitory effects on the proliferation of the gastric cancer cells, and their effect was augmented by the simultaneous addition of 9- cis retinoic acid, a ligand of RXRalpha. Flow cytometry demonstrated G1 cell cycle arrest and a significant increase of annexin V-positive cells after treatment with troglitazone. These results suggest that induction of apoptosis together with G1 cell cycle arrest may be one of the mechanisms of the antiproliferative effect of PPARgamma activation in human gastric cancer cells.

Management of soft-tissue sarcomas: an overview and update

Soft-tissue sarcomas (STS) are relatively uncommon, especially when considered as individual histological subtypes (of which there are more than 50). Their incidence increases with age, although they are disproportionately common among children. When diagnosed and managed in a non-specialist environment, outcome is generally significantly poorer than if patients are managed by a multidisciplinary team in a tertiary centre of excellence. Prompt referral of patients with clinically suspicious masses is strongly advocated, before any type of intervention is attempted. This brief, opinion-based overview emphasises the team approach and provides a synopsis of the strategies used at our institution for pre-operative assessment and biopsy, surgical management, and the delivery of radiation therapy when appropriate (focusing on limb preservation and optimisation of function). Predictable variations in the natural history of these tumours, based on accurate histological subclassification, merit wider recognition. The role of systemic chemotherapy for soft-tissue sarcoma is still evolving, but at present the main aims are improved local control, disease-free survival, and quality of life. There are overall survival benefits for specific histological types, but this is a relatively small subgroup. Novel therapies, based on disease mechanisms at the molecular level, show promise for future advances.

Effects of ligand activation of peroxisome proliferator-activated receptor gamma in human prostate cancer

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a nuclear hormone receptor that plays a key role in the differentiation of adipocytes. Activation of this receptor in liposarcomas and breast and colon cancer cells also induces cell growth inhibition and differentiation. In the present study, we show that PPARgamma is expressed in human prostate adenocarcinomas and cell lines derived from these tumors. Activation of this receptor with specific ligands exerts an inhibitory effect on the growth of prostate cancer cell lines. Further, we show that prostate cancer and cell lines do not have intragenic mutations in the PPARgamma gene, although 40% of the informative tumors have hemizygous deletions of this gene. Based on our preclinical data, we conducted a phase II clinical study in patients with advanced prostate cancer using troglitazone, a PPARgamma ligand used for the treatment of type 2 diabetes. Forty-one men with histologically confirmed prostate cancer and no symptomatic metastatic disease were treated orally with troglitazone. An unexpectedly high incidence of prolonged stabilization of prostate-specific antigen was seen in patients treated with troglitazone. In addition, one patient had a dramatic decrease in serum prostate-specific antigen to nearly undetectable levels. These data suggest that PPARgamma may serve as a biological modifier in human prostate cancer and its therapeutic potential in this disease should be further investigated.

PAX8-PPARgamma1 fusion oncogene in human thyroid carcinoma [corrected]

Chromosomal translocations that encode fusion oncoproteins have been observed consistently in leukemias/lymphomas and sarcomas but not in carcinomas, the most common human cancers. Here, we report that t(2;3)(q13;p25), a translocation identified in a subset of human thyroid follicular carcinomas, results in fusion of the DNA binding domains of the thyroid transcription factor PAX8 to domains A to F of the peroxisome proliferator-activated receptor (PPAR) gamma1. PAX8-PPARgamma1 mRNA and protein were detected in 5 of 8 thyroid follicular carcinomas but not in 20 follicular adenomas, 10 papillary carcinomas, or 10 multinodular hyperplasias. PAX8-PPARgamma1 inhibited thiazolidinedione-induced transactivation by PPARgamma1 in a dominant negative manner. The experiments demonstrate an oncogenic role for PPARgamma and suggest that PAX8-PPARgamma1 may be useful in the diagnosis and treatment of thyroid carcinoma.

Central role of peroxisome proliferator-activated receptors in the actions of peroxisome proliferators

Peroxisome proliferators (PPs) are a large class of structurally dissimilar chemicals that have diverse effects in rodents and humans. Most, if not all, of the diverse effects of PPs are mediated by three members of the nuclear receptor superfamily called peroxisome proliferator-activated receptors (PPARs). In this review, we define the molecular mechanisms of PPs, including PPAR binding specificity, alteration of gene expression through binding to DNA response elements, and cross talk with other signaling pathways. We discuss the roles of PPARs in growth promotion in rodent hepatocarcinogenesis and potential therapeutic effects, including suppression of cancer growth and inflammation.

Medical management of unresectable, recurrent low-grade retroperitoneal liposarcoma: integration of cytotoxic and non-cytotoxic therapies into multimodality care

Liposarcomas of the retroperitoneum are rare tumors, and best managed by an expert multidisciplinary team consisting of a surgical oncologist with appropriate medical oncology and radiation oncology collaboration. For large tumors, surgical excision with wide margins is difficult to achieve, and even wide margins cannot ensure that microscopic remnants of residual disease will not grow back in the future. For these reasons, even following expert resection, local recurrence is common. For patients with recurrent and unresectable liposarcoma, treating the sarcoma while maintaining quality of life becomes the major therapeutic goal. Importantly, patients with advanced recurrent disease demonstrate the need for multidisciplinary team involvement, with timely consideration of palliative surgical, radiation therapy, and chemotherapy options. Such patients also represent ideal candidates for investigational approaches aimed at identifying new agents with which to treat this disease. In addition to the development of new cytotoxic agents, patients may be candidates for novel strategies such as differentiation therapies or anti-angiogenic approaches. The recent explosion of knowledge regarding the cytogenetics, molecular, and cellular biology of liposarcomas allows us to remain positive that new translational therapies will be developed to improve the clinical outcomes of patients with these diseases. Current strategies, such as the use of PPARgamma ligands to differentiate liposarcomas, will soon be tested in major national collaborative trials, and the cooperation of surgeons and medical oncologists at all levels of community and academic practice will be crucial to obtain answers in this field. This review will summarize an illustrative case in the process of describing the natural history and potential interventions which should be considered for patients with this disease.

Cytogenetics and the biologic basis of sarcomas

In this past year, a large number of reports have described cytogenetic and biologic studies of sarcomas. The cytogenetic studies provide further evidence that a growing number of sarcomas seem to be defined by consistent chromosomal abnormalities that can be detected using a variety of molecular genetic tests. However, in addition to these specific abnormalities, many sarcomas have other extremely complex genetic changes. This complexity has made it quite difficult to understand the importance of any single abnormality. Laboratory studies complementing these genetic studies have provided further understanding of sarcoma cellular and molecular biology. Importantly, both types of studies have had significant impact in the clinic in the form of more objective diagnostic tests, potential novel prognostic markers, and even new therapeutic strategies. Together, these papers highlight how genetic studies may offer tremendous insight into sarcoma biology. However, they also highlight some limitations of these approaches as well. Novel experimental approaches may be required to facilitate the continued progress in this field toward the development of better therapeutic strategies.

Degradation of the peroxisome proliferator-activated receptor gamma is linked to ligand-dependent activation

The nuclear hormone receptor peroxisome proliferator-activated receptor (PPAR) gamma is a ligand-activated transcription factor that regulates several crucial biological processes such as adipogenesis, glucose homeostasis, and cell growth. It is also the functional receptor for a new class of insulin-sensitizing drugs, the thiazolidinediones, now widely used in the treatment of type 2 diabetes mellitus. Here we report that PPARgamma protein levels are significantly reduced in adipose cells and fibroblasts in response to specific ligands such as thiazolidinediones. Studies with several doses of different ligands illustrate that degradation of PPARgamma correlates well with the ability of ligands to activate this receptor. However, analyses of PPARgamma mutants show that, although degradation does not strictly depend on the transcriptional activity of the receptor, it is dependent upon the ligand-gated activation function 2 (AF2) domain. Proteasome inhibitors inhibited the down-regulation of PPARgamma and ligand activation enhanced the ubiquitination of this receptor. These data indicate that, although ligand binding and activation of the AF2 domain increase the transcriptional function of PPARgamma, these same processes also induce ubiquitination and subsequent degradation of this receptor by the proteasome.

Induction of drug metabolising enzymes: pharmacokinetic and toxicological consequences in humans

Currently, 5 different main mechanisms of induction are distinguished for drug-metabolising enzymes. The ethanol type of induction is mediated by ligand stabilisation of the enzyme, but the others appear to be mediated by intracellular 'receptors'. These are the aryl hydrocarbon (Ah) receptor, the peroxisome proliferator activated receptor (PPAR), the constitutive androstane receptor (CAR, phenobarbital induction) and the pregnane X receptor [PXR, rifampicin (rifampin) induction]. Enzyme induction has the net effect of increasing protein levels. However, many inducers are also inhibitors of the enzymes they induce, and the inductive effects of a single drug may be mediated by more than one mechanism. Therefore, it appears that every inducer has its own pattern of induction; knowledge of the main mechanism is often not sufficient to predict the extent and time course of induction, but may serve to make the clinician aware of potential dangers. The possible pharmacokinetic consequences of enzyme induction depend on the localisation of the enzyme. They include decreased or absent bioavailability for orally administered drugs, increased hepatic clearance or accelerated formation of reactive metabolites, which is usually related to local toxicity. Although some severe drug-drug interactions are caused by enzyme induction, most of the effects of inducers are not detected in the background of nonspecific variation. For any potent inducer, however, its addition to, or withdrawal from, an existing drug regimen may cause pronounced concentration changes and should be done gradually and with appropriate monitoring of therapeutic efficacy and adverse events. The toxicological consequences of enzyme induction in humans are rare, and appear to be mainly limited to hepatoxicity in ethanol-type induction.

Insulin sensitiser drugs

Insulin resistance is the predominant early pathological defect in Type 2 diabetes. As well as being a risk factor for the development of Type 2 diabetes, insulin resistance is also associated with increased cardiovascular risk and other metabolic disturbances including visceral adiposity, hyperinsulinaemia, impaired glucose tolerance, hypertension and dyslipidaemia [1-4]. The newest approach to oral antidiabetic therapy is to target improvements in insulin sensitivity at muscle, adipose tissue and hepatic level. This results in improvements in glycaemic control and other features of the insulin resistance syndrome, with potential long-term benefits in preventing/delaying the onset of diabetic complications and macrovascular disease.

Roles of PPARs in health and disease

In developed societies, chronic diseases such as diabetes, obesity, atherosclerosis and cancer are responsible for most deaths. These ailments have complex causes involving genetic, environmental and nutritional factors. There is evidence that a group of closely related nuclear receptors, called peroxisome proliferator-activated receptors (PPARs), may be involved in these diseases. This, together with the fact that PPAR activity can be modulated by drugs such as thiazolidinediones and fibrates, has instigated a huge research effort into PPARs. Here we present the latest developments in the PPAR field, with particular emphasis on the physiological function of PPARs during various nutritional states, and the possible role of PPARs in several chronic diseases.

Peroxisome proliferator-activated receptors: insight into multiple cellular functions

Peroxisome proliferator-activated receptors, PPARs, (NR1C) are nuclear hormone receptors implicated in energy homeostasis. Upon activation, these ligand-inducible transcription factors stimulate gene expression by binding to the promoter of target genes. The different structural domains of PPARs are presented in terms of activation mechanisms, namely ligand binding, phosphorylation, and cofactor interaction. The specificity of ligands, such as fatty acids, eicosanoids, fibrates and thiazolidinediones (TZD), is described for each of the three PPAR isotypes, alpha (NR1C1), beta (NR1C2) and gamma (NR1C3), so as the differential tissue distribution of these isotypes. Finally, general and specific functions of the PPAR isotypes are discussed, namely their implication in the control of inflammatory responses, cell proliferation and differentiation, the roles of PPARalpha in fatty acid catabolism and of PPARgamma in adipogenesis.

Peroxisome proliferator-activated receptors in the cardiovascular system

Peroxisome proliferator-activated receptor (PPAR)s are a family of three nuclear hormone receptors, PPARalpha, -delta, and -gamma, which are members of the steriod receptor superfamily. The first member of the family (PPARalpha) was originally discovered as the mediator by which a number of xenobiotic drugs cause peroxisome proliferation in the liver. Defined functions for all these receptors, until recently, mainly concerned their ability to regulate energy balance, with PPARalpha being involved in beta-oxidation pathways, and PPARgamma in the differentiation of adipocytes. Little is known about the functions of PPARdelta, though it is the most ubiquitously expressed. Since their discovery, PPARs have been shown to be expressed in monocytes/macrophages, the heart, vascular smooth muscle cells, endothelial cells, and in atherosclerotic lesions. Furthermore, PPARs can be activated by a vast number of compounds including synthetic drugs, of the clofibrate, and anti-diabetic thiazoldinedione classes, polyunsaturated fatty acids, and a number of eicosanoids, including prostaglandins, lipoxygenase products, and oxidized low density lipoprotein. This review will aim to introduce the field of PPAR nuclear hormone receptors, and discuss the discovery and actions of PPARs in the cardiovascular system, as well as the source of potential ligands.

Effect of the retinoid X receptor-selective ligand LGD1069 on mammary carcinoma after tamoxifen failure

BACKGROUND

We have previously shown that a retinoid X receptor (RXR)-selective ligand (a rexinoid), called LGD1069, is highly efficacious in both the chemoprevention and the chemotherapy for N-nitrosomethylurea-induced rat mammary carcinomas. To evaluate a possible role for rexinoids in breast cancer therapy further, we have designed and characterized a novel carcinogen-induced model to mimic the clinical situation in which the tumors of patients stop responding to tamoxifen therapy and develop resistance to this drug.

METHODS

Rats with experimentally induced mammary tumors were treated with tamoxifen to select a population with primary tumors that failed to respond completely to the drug. Once the failure of tamoxifen therapy had been established, LGD1069 was added to the treatment regimen, and the tumors in these animals were compared with tumors in a group of animals that remained on tamoxifen alone.

RESULTS

LGD1069 in combination with tamoxifen for up to 20 weeks yielded an overall objective response rate of 94% (95% confidence interval [CI] = 86%-100%) (includes complete and partial responses) in primary tumors compared with a rate of 33% (95% CI = 11%-56%) in primary tumors treated with tamoxifen alone, a statistically significant difference (two-sided P<.0001). In addition, the LGD1069 and tamoxifen combination was associated with a statistically significant decrease in total tumor burden (two-sided P =.03). In a second study, tumors that failed to respond to tamoxifen therapy exhibited a 51% (95% CI = 34%-71%) objective response rate when treated with LGD1069 alone for 6 weeks after tamoxifen therapy was withdrawn.

CONCLUSION

We have demonstrated that the RXR-selective ligand LGD1069 in combination with tamoxifen is a highly efficacious therapeutic agent for tumors that fail to respond completely to tamoxifen. This finding suggests that rexinoid therapy offers a novel approach to the treatment of breast tumors that may have developed resistance to antihormonal therapies such as tamoxifen.

Pharmacological peroxisome proliferator-activated receptorgamma ligands: emerging clinical indications beyond diabetes

The discovery of peroxisome proliferator-activated receptor gamma (PPARgamma) as the molecular target for antidiabetic thiazolidinediones has heralded a new era in the approach to understanding the pathophysiology of insulin resistance and its relationship to cardiovascular disease. However, the subsequent discovery of PPARgamma-dependent modulation of immune function and the cell cycle has led to a new paradigm in the approach to treating proliferative, inflammatory diseases. Moreover, PPARgamma agonists can promote apoptosis, block angiogenesis and inhibit pathological remodelling in a variety of malignant and non-malignant pathological states. These findings imply that the pharmacological modulation of this key nuclear transcription factor and its co-factors could be important tools in understanding the relationships between multigenic diseases, and pave the way to a focused interventional approach in their treatment. With the availability of the PPARgamma protein crystal structure, the ligand binding domain co-ordinates and a better knowledge of the interaction of PPARgamma with co-factor assemblies, libraries of simple synthetic organic PPARgamma ligands can be constructed. High throughput screening can identify the best candidates for targeting cellular phenotypic transition, cell cycle control, inflammation and apoptosis. Instead of single agents for single pathologies, one can envisage the development of multifunctional therapeutic agents that target the multiple cellular processes that contribute to multifactorial diseases such as diabetes, hypertension, atherosclerosis, psoriasis and other inflammatory diseases, and carcinogenesis. The considerable potential of PPARgamma ligands in the treatment of diseases other than diabetes is the subject of this review.

The biologic basis for the use of retinoids in cancer prevention and treatment

Retinoids (vitamin A and related molecules) are biologic agents that have demonstrated, in preclinical and clinical models, potent activity in the prevention and treatment of a variety of malignancies. Presented in this article is a review of recent clinical studies and correlative laboratory findings that advance our understanding of the biologic basis for the use of retinoids in cancer prevention and treatment.

Loss-of-function mutations in PPAR gamma associated with human colon cancer

The gamma isoform of the peroxisome proliferator-activated receptor, PPAR gamma, regulates adipocyte differentiation and has recently been shown to be expressed in neoplasia of the colon and other tissues. We have found four somatic PPAR gamma mutations among 55 sporadic colon cancers: one nonsense, one frameshift, and two missense mutations. Each greatly impaired the function of the protein. c.472delA results in deletion of the entire ligand binding domain. Q286P and K319X retain a total or partial ligand binding domain but lose the ability to activate transcription through a failure to bind to ligands. R288H showed a normal response to synthetic ligands but greatly decreased transcription and binding when exposed to natural ligands. These data indicate that colon cancer in humans is associated with loss-of-function mutations in PPAR gamma.