The role of peroxisome proliferator-activated receptor-β/δ in epithelial cell growth and differentiation

Abstract

The physiological and pharmacological roles of peroxisome proliferator-activated receptor-β (PPARβ-also referred to as PPARδ) are just beginning to emerge. It has recently become clear that PPARβ has a function in epithelial tissues, but controversy exists due to inconsistencies in the literature. There is strong evidence that ligand activation of PPARβ can induce terminal differentiation of keratinocytes, with a concomitant inhibition of cell proliferation. However, the role of PPARβ in keratinocyte-specific apoptosis is less clear. Additionally, the role of PPARβ in colonic epithelium remains unclear due to conflicting evidence suggesting that ligand activation of PPARβ can potentiate, as well as attenuate, intestinal cancer. Recent studies revealed that ligand activation of PPARβ can induce fatty acid catabolism in skeletal muscle and is associated with improved insulin sensitivity, attenuated weight gain and elevated HDL levels thus demonstrating promising potential for targeting PPARβ for treating obesity, dyslipidemias and type 2 diabetes. Therefore, it becomes critical to determine the safety of PPARβ ligands. This review focuses on recent literature describing the role of PPARβ in epithelial tissues and highlights critical discrepancies that need to be resolved for a more comprehensive understanding of how this receptor modulates epithelial homeostasis.

Introduction

In 1990, the peroxisome proliferator-activated receptor-α (PPARα) was cloned from rodent liver [1]. Soon thereafter, two additional members of the PPAR subfamily were first identified in Xenopus [2] and later in mammals [3], [4], [5], and termed PPARβ (also referred to as PPARδ) and PPARγ. The three PPARs are expressed in many species including rodents [6], [7], humans [8], [9], [10], [11] fish [12], [13], [14], [15] and amphibians [2]. PPARs belong to the type 2 class of ligand-activated nuclear receptor superfamily, historically referred to as “orphan receptors”. The PPARs are involved in a number of biological responses, including adipogenesis, lipid homeostasis, immune function, cell proliferation/apoptosis and carcinogenesis (reviewed in [16], [17], [18], [19]). There are a number of mechanisms by which PPARs modulate transcription (Fig. 1). The classic mechanism of action is initiated by ligand binding to the PPAR. Ligand binding causes a conformational change in receptor structure that allows for dissociation of co-repressors, recruitment of co-activators and heterodimerization with retinoid X receptor-α (RXRα), followed by activation of target genes having a direct repeat 1 (DR-1) element referred to as peroxisome proliferator response elements (PPREs). There is also recent evidence that PPARs can repress transcription by binding to PPREs in the presence of RXRα and co-repressors [20]. PPARs can also physically interact with other transcription factors to modulate transcription. For example, PPARα and PPARγ interfere with NF-κB- or AP-1-mediated gene transactivation through direct protein–protein interactions, respectively [21], [22]. PPARα can also physically interact with C/EBPβ and inhibit target gene expression [23]. A relatively unique mechanism of transcriptional repression has also been reported for PPARβ whereby PPARβ represses expression of PPARα and/or PPARγ-mediated target genes by binding to PPREs in association with co-repressors [24], a finding that could explain the potentiation of adipogenesis by PPARβ [25]. More recently, it was demonstrated that during adipogenesis, there is an altered activation of certain PPARγ target genes that depends upon differential displacement of co-repressors by ligand-activated PPARγ depending on the sequence context of the PPRE [20]. In some cases, the physiological significance of these interactions remains to be demonstrated in intact animal models. However, these observations illustrate the complexity of the mechanisms by which PPARs mediate signal transduction that can significantly influence numerous physiological functions.

The biological roles of PPARα and PPARγ are considerably better defined as compared to PPARβ. This is due in part to the findings that they are targets for drugs used in the treatment of dyslipidemias and type 2 diabetes, respectively. It is well accepted that PPARα, a target of the fibrate class of hypolipidemic drugs [26], has a critical role in regulating lipid homeostasis by increasing transcription of target genes encoding products that mobilize lipids from adipose, proteins that transport fatty acids in the cell and bloodstream and enzymes that facilitate fatty acid catabolism in tissues such as liver, kidney and heart. In contrast, PPARγ, a target of the thiazolidinedione class of type 2 diabetes drugs [27], is required for adipocyte differentiation and fatty acid storage. Ligands for PPARα, PPARβ and PPARγ have also been found to suppress inflammation and potentially decrease atherosclerotic lesions [28], [29]. In addition to these well-accepted functional roles for PPARα and PPARγ, there is also good evidence that these receptors are involved in processes that regulate cell proliferation, apoptosis and differentiation. However, it is of interest to point out that the roles of PPARα and PPARγ in the aforementioned pathways are, in some cases, still controversial due to conflicting reports in the literature. This is also true for PPARβ, since the functional roles of this PPAR isoform have only recently been demonstrated, due in large part to recent advances made with null mouse models coupled with highly specific ligands.

Recent mechanistic studies have confirmed a role for PPARβ in epithelial cell differentiation, particularly in the skin epidermis. The epidermis is divided into four distinct cell layers, with the primary cell type consisting of keratinocytes (Fig. 2). Cell proliferation occurs primarily in the basal layer while most of the cells in the spinous layer do not retain the ability to divide [30]. Cells within the granular layer contain keratohyalin granules in their cytoplasm and are present between the spinous layer and the layer of cornified cells that are characterized as having no nucleus or cytoplasmic organelles [30]. The cornified cells, composed primarily of keratin surrounded by an insoluble protein structure (cornified envelope), represent the endproduct of keratinocyte terminal differentiation [30]. The cornified layer of cells functions to protect the organism from the extracellular environment and to prevent excessive loss of water. Cornified cells are continually lost and replaced by proliferating keratinocytes that arise from the basal layer and undergo subsequent terminal differentiation. There are a number of markers used to determine the relative state of differentiation of epidermal keratinocytes including early to late markers of differentiation (e.g. K1, K10, involucrin, etc.), assessment of cell proliferation and apoptosis and the biochemical analysis of cornified envelopes. It is well accepted that induction of terminal differentiation leads to a concomitant decrease in cell proliferation. This was classically illustrated by studies showing that increasing calcium concentration in the culture medium causes terminal differentiation of primary keratinocytes and inhibition of DNA synthesis [31]. Thus, it is not surprising that induction of keratinocyte terminal differentiation follows a pathway very similar to apoptosis [32], [33], [34], [35], since these cells are destined to differentiate into a relatively quiescent cell type, the cornified envelope, that functions to limit permeability. Therefore, the regulation of keratinocyte terminal differentiation is mediated by a number of signaling pathways that cause the cell to increase production of proteins required for formation of the cornified envelope, increase signaling that limits cell cycle progression and induction of an apoptotic-like pathway that allows for formation of the cornified layer of epithelial cells. The regulatory mechanisms that mediate signaling to induce keratinocyte differentiation are therefore critical for maintenance of a functional epithelium and have implications for skin carcinogenesis.

Similar to the epidermis, the colonic crypts consist of a number of different cell types that have varying potentials for proliferation and/or differentiation (Fig. 3). Stem cells are found at the base of the crypt and actively proliferate [36]. As the stem cells divide and give rise to daughter cells, these cells will differentiate to form enteroendocrine cells, goblet cells and colonocytes, each with a different function [36]. The terminally differentiated colonocytes will ultimately undergo apoptosis, and are replaced with other migrating cells that originate from the proliferating stem cells. This process is remarkably similar to the differentiation pattern observed in epidermal keratinocytes. However, in contrast to keratinocytes where a well-documented series of mRNA or protein markers are available to identify and characterize the sequence of events leading to the formation of a cornifed cell, the molecular markers for differentiating and terminally differentiated colonocytes are less certain. For example, keratin 20 [37], carbonic anhydrase [38], mucins [39], and fatty acid binding protein [40] have all been postulated to represent markers of differentiation of colonocytes, but their expression pattern is not always consistently limited to specific cells in the colonic crypt. Similar to epidermis, the signaling molecules that regulate cell proliferation, apoptosis and differentiation in colonic epithelium are likely good targets to inhibit or prevent colon cancer.

A growing body of literature provides the basis for the hypothesis that PPARβ can modulate cell growth in the epithelium. Specific molecular events that are mediated by PPARβ have not been clearly delineated, but significant progress has been made in past years. This review focuses on advances made in recent years demonstrating the role of PPARβ in epithelial cell growth and differentiation, with an emphasis on the significance of this receptor with respect to epithelial cancers.