Potential of retinoic acid derivatives for the treatment of corticotroph pituitary adenomas

Pituitary tumors are common neoplasms, reported to account for 10–15% of all intracranial tumors, being therefore the second most common neoplasm after meningeomas [1]. The prevalence is 300/1,000,000 inhabitants. Pituitary adenomas are composed of adenohypophysial cells and arise usually in the sella turcica. Their hormonal activity is usually reflective of the cytodifferentiation. The clinical picture can be very variable, many tumors are silent and not frequently diagnosed, while others may be life-threatening. Little is known about the precise environmental and genetic factors leading to their development, so preventive measures are unavailable. About two-thirds of pituitary tumors express and secrete pituitary hormones leading to various endocrine syndromes. One of the most severe is the Cushing’s syndrome, which results from chronic exposure to glucocorticoids in the blood from a variety of causes, including primary pituitary adenoma (known as Cushing’s disease), primary adrenal hyperplasia or neoplasia and ectopic ACTH production (e.g., from a small cell lung cancer) [2‐6]. It is characterized by a typical abnormal fat deposition around the neck, skin thinning, adrenal hyperplasia osteoporosis, insulin resistance, dyslipidemia, myopathy, amenorrhea and hypertension. Fatigue, irritation, anxiety and depression are also common clinical features in these patients [2, 3, 5, 6]. At the moment, there is no effective pharmacological therapy to control ACTH over-secretion by pituitary tumors [2, 3, 5‐8]. In the absence of efficient medical therapy, transsphenoidal adenomectomy is still the treatment of choice for ACTH-secreting tumors [7, 8]. However, some difficulties prevent this approach in many patients. In the first place, ACTH-secreting pituitary adenomas are usually very small, although they secrete abundant amounts of ACTH. Their small size and structure makes it difficult to localize them with the current imaging methods. This fact leads to a higher rate of recurrence in these tumors due to partial resection. In some cases, total hypophysectomy is needed to ensure the resection of the tumor. In these cases a side effect of surgery is the insufficiency of pituitary hormones. These patients require constant monitoring of the endocrine axis and hormone replacement therapy. Beside that, surgery is associated, with significant post-operative morbidities [9, 10]. In case of adrenal tumor or ectopic ACTH secretion, surgical removal of the tumor is mandatory, when possible. Future studies are needed, on one hand to provide tools for a better prediction of tumor behaviour and on the other to provide novel targets for pharmaceutical therapy.

Only very recently, molecular biology studies have provided novel potential targets for therapy. The role and the ability of nuclear receptors such as retinoic acid receptors to regulate normal and pathological pituitary tumor hormone secretion and cell growth will be discussed.

The retinoids are natural and synthetic derivatives of vitamin A that regulate diverse cellular growth and differentiation programmes, survival and death [11]. Retinoids activate the transcription of target genes through interaction with the nuclear transcription factors of the retinoic acid receptor (RARα, RARβ, RARγ) and retinoid X receptor (RXRα, RARβ, RXRγ) families [12‐14]. The α, β and γ isoforms are each expressed from the three isotypic genes (α, β and γ), expressing by differential promoter usage and splicing [15, 16]. At the molecular level, RARs and RXRs form heterodimers that respond to RAR ligands. RXRs also heterodimerize with other nuclear receptors, some of which become transcriptionally active in the sole presence of an RXR selective ligand (rexinoid) [11]. However rexinoid agonists superactivate transcription induced by RAR-RXR in the presence of RAR agonists. In the absence of ligand, retinoid receptors bind specific retinoic-acid response elements on target genes and recruit nuclear co-repressors, such as NCOR (nuclear receptor co-repressor) and SMRT (silencing mediator of RAR and thyroid hormone receptor). Retinoid binding to these receptors leads to release of co-repressors and recruitment of transcriptional co-activators to regulate a diverse range of genes involved in transcriptional and apoptosis regulation, proliferation and protein modification [17].

Retinoic acid receptors are important drug targets for cancer therapy and prevention [18, 19]. The activities mediated by the retinoid receptors encompass, for example, the induction of cell-cycle regulators (cyclin-dependent kinase inhibitor p21^WAF1/CIP1), the repression of AP1 (the Fos-Jun proto-oncogene product), and the induction of the tumor-cell-selective apoptosis ligand TRAIL [19, 20]. Strong evidence indicates that the RA-inducible RARβ, acts as a tumor suppressor [11, 20].

At pharmacological dosages, retinoids are used to treat patients with acute promyelocytic leukaemia (APL), as well as other malignant, pre-malignant and non-malignant disorders. The efficacy of retinoids in suppressing tumor development has been demonstrated in animal carcinogenesis models [21].

All-trans-retinoic acid (ATRA) and 9-cis retinoic acid (9-cis-RA) are the most biologically active retinoic acid isomers, whereas 13-cis-retinoic acid (13-cis-RA) is considered less active and may require isomerization to ATRA in order to exert biologic function [22, 23]. Differences in ATRA and 13-cis-RA metabolism and pharmacokinetics have been reported in cancer patients [24‐30]. ATRA is rapidly cleared from plasma (plasma half-life <1 h) and induces its own metabolism, which is a significant therapeutic obstacle. In contrast, 13-cis-RA is slowly cleared from plasma (plasma half-life >13 h) and does not induce its own metabolism. Different strategies to overcome self-induced ATRA metabolism and thus improve therapeutic efficacy were proposed in emphysema patients [31]. The authors concluded that intermittent therapy with high-dose ATRA produced the greatest ATRA exposure, but alternative approaches for limiting self-induced ATRA catabolism should be sought. A recent study on pharmacokinetics of ATRA in adults and children with acute promyelocytic leukaemia shows that although its bioavailability is similar in both age groups the incidence of CNS toxicity due to ATRA is higher in children than in adults [32]. Unfortunately, a key limitation of retinoid therapy is that concentrations required for anticancer actions cause several side effects, including teratogenicity, mucocutaneous toxicity, defects in liver function, conjunctivitis, mucositis and severe photosensitivity. Rexinoids are, in general, much less toxic [33].

The search for isotype-selective modulators [15, 34‐36] is pursued to first, reduce side effects associated with current retinoid therapy; second, elicit more specific biological responses, because the tissue distribution of retinoid receptors isotypes is not uniform, third, better define the physiological role of each retinoid receptor isotype in the control of certain biological phenomena such as proliferation, differentiation and apoptosis [15].

A summary on retinoids and rexinoids that are of clinical importance or being evaluated in clinical trials is provided in Tables 1 and 2 [11].

CRH increases pro-opiomelanocortin (POMC) gene expression and stimulates adrenocorticotrophin (ACTH) synthesis and secretion [37] through the interaction with the CRH receptor type 1 (CRHR1) localized in corticotrophs (Fig. 1). Activation of the CRHR1 results in Gs-mediated stimulation of adenylate cyclase, leading to increased levels of intracellular cyclic AMP (cAMP), and the activation of protein kinase A (PKA) [38], which stimulates different transcription factors [39‐41]. Two Nur DNA binding sites have been identified on the POMC promoter. The proximal binding sequence named Nur77-binding response element (NBRE) binds Nur 77 or Nurr1 monomers. The distal Nur response element (NurRE), constituted of two everted NBRE related sites, binds Nur77 homodimers or Nur77/Nurr1 heterodimers and plays a dominant role in mediating stimulation by CRH [42‐44]. Upon stimulation with CRH, the ERK pathway is activated in corticotrophs and this activation is instrumental in the regulation of Nur77/Nurr 1 [38]. The expression of these nuclear receptors in ACTH-secreting cells and also in the hypothalamus and adrenal glands might make them useful therapeutic targets for drugs which may aim to modulate the activity of the HPA axis.

CRH also induces transcriptional activity of activating protein 1 (AP-1) and cAMP response element binding protein (CREB), which have been proposed to be involved in POMC transcription at the level of the AP-1 site located in the first exon [40, 45]. Other hormones and neuropeptides [e.g. arginine vasopressin (AVP)] also modify POMC transcription by acting on the second messengers, cAMP and Ca⁺⁺ (the calcium/calmodulin pathway). Upon glucocorticoid receptor binding, glucocorticoids exert their negative effect by binding on the glucocorticoid responding element (GRE) situated on the POMC promoter and thus, inhibiting ACTH synthesis.

Retinoic acid might represent a potential therapeutic option to inhibit ACTH production, as well as tumor growth in Cushing’s disease. However, the efficiency of these treatments in patients with Cushing’s syndrome still needs to be tested in clinical trials.