Conditions Associated with Circulating Tumor-Associated Folate Receptor 1 Protein in Healthy Men and Women

et al. (2014) Conditions Associated with Circulating Tumor-Associated Folate Receptor 1 Protein in Healthy Men and Women. PLoS ONE 9(5): e96542. doi:10.1371/journal.pone.0096542 Conditions Associated with Circulating Tumor- Associated Folate Receptor 1 Protein in Healthy Men and Women Linda E. Kelemen 0 James D. Brenton 0 Christine Parkinson 0 Hayley C. Whitaker 0 Anna M. Piskorz 0 Ilona Csizmadi 0 Paula J. Robson 0 Arun Rishi, Wayne State University, United States of America 0 1 Department of Population Health Research, Alberta Health Services , Calgary, Alberta , Canada , 2 Department of Medical Genetics, University of Calgary , Calgary, Alberta , Canada , 3 Cancer Research United Kingdom Cambridge Institute, University of Cambridge, Li Ka Shing Centre, Cambridge, United Kingdom, 4 Department of Oncology, University of Cambridge, Hutchison/Medical Research Council Research Centre, Cambridge, United Kingdom, 5 National Institute for Health Research Cambridge Biomedical Research Centre, Cambridge, United Kingdom, 6 Cambridge Experimental Cancer Medicine Centre, Cambridge, United Kingdom, 7 Departments of Oncology and Community Health Sciences, University of Calgary , Calgary, Alberta , Canada , 8 Alberta Health Services , Edmonton, Alberta , Canada , 9 Department of Agricultural, Food and Nutritional Sciences, University of Alberta , Edmonton, Alberta , Canada Background: Serum concentrations of the tumor-associated folate receptor 1 (FOLR1) protein may be a marker for early cancer detection, yet concentrations have also been detected in cancer-free women. We investigated the conditions associated with circulating FOLR1 protein in healthy individuals and sought to clarify the range of normal serum values. Methods: Sera of cancer-free men and women (N = 60) enrolled in a population-based cohort study in Alberta, Canada were analyzed for FOLR1 protein using an electrochemical luminescence immunoassay. Dietary, lifestyle, medical and reproductive history information was collected by questionnaires. Differences in serum FOLR1 concentrations between groups were assessed by non-parametric tests, and predictors of serum FOLR1 concentrations were estimated using multivariable linear regression. Results: Median serum FOLR1 concentration was higher in women (491 pg/ml, range = 327-693 pg/ml) than in men (404 pg/ml, range = 340-682 pg/ml), P = 0.001. FOLR1 concentration was also positively associated with vitamin A intake (P = 0.02), and showed positive trends with age and with oral contraceptive hormone use among women and an inverse trend with body mass index. All variables examined explained almost half of the variation in serum FOLR1 (model R2 = 0.44, P = 0.04); however, the retention of gender (P = 0.003) and vitamin A intake (P = 0.03) together explained 20% (P = 0.001) of serum FOLR1 variation. No other predictor was significant at P,0.05. Conclusions: The positive association between serum FOLR1 concentration and female gender independent of an age effect suggests caution against statements to exploit serum FOLR1 for early cancer detection without further understanding the biological underpinnings of these observations. Serum FOLR1 concentrations may be influenced by the steroid retinoic acid (vitamin A) but do not appear to be associated with folate nutritional status. These findings require confirmation in larger independent studies. - Funding: LEK was supported by a Canadian Institutes of Health Research Investigator award MSH-87734. The Tomorrow Project is hosted by Alberta Health Services, and is supported by the Alberta Cancer Foundation, the Alberta Cancer Prevention Legacy Fund (Government of Alberta), and the Canadian Partnership Against Cancer. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: A donation from Hutchison Whampoa Limited to the University of Cambridge was used towards construction of the Cancer Research UK Cambridge Institute and there is no ongoing research or infrastructure support. This does not alter the authors adherence to PLOS ONE policies on sharing data and materials. Folate receptor 1 (FOLR1) is a membrane-bound protein with high affinity for binding nanomolar concentrations of folate and, in particular, the oxidized form of folate, folic acid [1,2]. FOLR1 exhibits restricted expression in normal epithelial tissues, including kidney tubules and intestine, placenta and choroid plexus, where it prevents against folate losses from excretion and prioritizes folate uptake by critical tissues, respectively [1]. It is also expressed in specific malignant tumors of epithelial origin [1] and is currently under investigation as a diagnostic and chemotherapeutic target [38]. FOLR1 expression has been associated with various factors. In non-population-based samples (cell cultures, animal models and anonymous tumor bank samples), negative correlates of FOLR1 expression in tissue or cell lines were 17b-estradiol [9,10] and folate concentrations [1113] and positive correlates were tamoxifen [9], glucocorticoid receptor [14] and retinoic acid [15]. In a clinical population, however, FOLR1 expression in ovarian tumors was associated positively with multivitamin use [16]. Understanding the relevance of these exposures at a population level is needed. For example, adipose tissue can produce 17bestradiol, and the increasing prevalence of obesity may influence in vivo FOLR1 expression. Exposure to folic acid is also common due to mandatory folic acid fortification of grain products in several countries [1719] and from the popularity of folic-acid containing multivitamin use [20]. A soluble form of the receptor with the folate binding site intact arises from proteolytic cleavage of the glycosylphosphatidylinositol (GPI) membrane anchor by the combined action of a membraneassociated protease and GPI-specific phospholipases C and D, which are abundant in plasma [2123]. The existence of a soluble form was first recognized 40 years ago and referred to as a serum folate binding protein [24]. At that time, investigations focused on correlates of total serum folate binding capacity. Investigators reported increased binding during pregnancy [25,26] and among users of oral contraceptive hormones [2527] but no association with folate nutritional status [25,28]. Binding capacity differences between the genders led to the speculation that the serum folate binders may be influenced by sex hormones [27,28]. These early, initial findings in small numbers of clinical populations are similar to the correlates of membrane-bound FOLR1 expression observed from contemporary in vitro and in vivo studies. More recently, FOLR1 protein was reported in the sera of women without cancer [29,30]. Before serum FOLR1 protein can be considered for investigation as a marker for early cancer detection [2931], it is important to investigate if it is associated with the same factors repor (...truncated)