Population differences in proinflammatory biology: Japanese have healthier profiles than Americans

Abstract

The pleiotropic cytokine, interleukin-6 (IL-6), has emerged as a key factor in the biology of aging and the physiology of inflammation. Yet much of what we know about the normal functioning of IL-6 has been generated primarily from research on European populations and Americans of European descent. Our analyses compared IL-6 levels in 382 middle-aged and older Japanese to the values found in 1209 Caucasian- and African–Americans from the Midlife in the United States survey (MIDUS). Across the life span from 30 to 80 years of age, mean IL-6 levels were strikingly lower in Japanese individuals. Significantly lower levels of C-reactive protein (CRP) and fibrinogen (FBG) provided confirmatory evidence for a population difference in proinflammatory activity. Because IL-6 release has been associated with obesity, differences in body mass index (BMI) were taken into consideration. Japanese had the lowest, and African–Americans had the highest overall BMIs, but significant group differences in IL-6 persisted even after BMI was included as a covariate in the analyses. Additional support for distinct variation in IL-6 biology was generated when systemic levels of the soluble receptor for IL-6 (sIL-6r) were evaluated. Serum sIL-6r was higher in Japanese than Americans, but was most notably low in African–Americans. Our cytokine data concur with national differences in the prevalence of age-related illnesses linked to inflammatory physiology, including cardiovascular disease. The findings also highlight the importance of broadening the diversity of people included in population studies of health and aging, especially given the relative paucity of information for some Asian countries and on individuals of Asian heritage living in the US.

Introduction

Many studies have now documented that old age is associated with a progressive decline in immune competence, which is often first manifested by signs in peripheral circulation of the impending dysregulation in cytokine activity (Bruunsgaard et al., 2001, Papanicolaou et al., 1998, Straub et al., 2000, Sauerwein-Teissl et al., 2000). Specifically, beginning in middle-aged adults, there may be an incremental rise in several of the proinflammatory cytokines, especially the ubiquitous IL-6 (Ershler and Keller, 2000, Harris et al., 1999, Wei et al., 1993). Increases in IL-6 have been shown to be a risk factor for a number of age-related illnesses associated with inflammation, including cardiovascular disease, diabetes, and osteoporosis (Loucks et al., 2006, Kristiansen and Mandrup-Pouslsen, 2005, Pradham et al., 2001, Ridker et al., 2000, Tamura et al., 1993, Whooley et al., 2007). An augmentation of IL-6 in middle-aged and older adults has also been associated with low socioeconomic status (SES), stressful life events, depression, and obesity (Black, 2003, Koster et al., 2006, Petersen et al., 2008, Pollitt et al., 2008). Thus, it is perhaps not surprising that several studies have reported that IL-6 levels tend to be higher in African–Americans than in Caucasian–Americans, which is then posited as one likely explanation for the racial differences in morbidity and mortality (Gruenewald et al., 2009, Ranjit et al., 2007, Slopen et al., 2010). However, our knowledge about the extent of population differences in cytokine biology is actually quite limited. The primary goal of the following analyses was to extend this comparative perspective: by assessing IL-6 levels in Japanese adults and comparing the results to the typical ranges found in Americans who had participated in a representative national survey (Midlife in the United States [MIDUS], Radler and Ryff, 2010).

The rising epidemic of obesity in industrialized countries is one of the more common reasons offered for elevated IL-6 levels in otherwise healthy middle-aged adults (Fried et al., 1998, McLaren, 2007, Xu et al., 2003). In addition to being produced by lymphoid cells, we now know that adipocytes are a major source of the IL-6 in circulation and further that activated monocytes present in fat tissue also release cytokines into the blood stream (Mohamed-Ali et al., 1997, Yamashita et al., 2007). The many different cellular sources of IL-6, which include hepatocytes, fibroblasts, and endothelial cells, help to explain why it has been linked to the risk for the metabolic syndrome. Because Japanese are traditionally less likely to be overweight than Americans, a secondary aim of our study was to investigate the contribution of body mass index (BMI) to any national differences observed in cytokine biology. In addition, we examined the correlation between IL-6 and two other hematological markers of inflammation, C-reactive protein (CRP) and fibrinogen (FBG) (Deepa et al., 2006, Friedlander et al., 2006, Gabay and Kushner, 1999, Yamaguchi et al., 1998). Prior research on MIDUS participants and on other healthy and patient populations had demonstrated that there is usually a positive association between IL-6 and CRP as well as between IL-6 and FBG (Friedman and Herd, 2010, Howren et al., 2009, Tracy et al., 1995). IL-6 is known to be a primary stimulator of the hepatic production and release of both CRP and FBG, especially during the acute phase response to infection and injury (Heinrich et al., 1990; Moshage, 1997).

Although most investigators interested in IL-6 assay only the protein levels in circulation, the functional activity of IL-6 can be strongly influenced by its receptor, which is comprised of a transmembrane ligand-binding subunit and signaling subunit on cell surfaces. The receptor is also found as a soluble form in the blood stream (Jones et al., 2001, Montero-Julian, 2001, Mulberg et al., 1999). High circulating levels of soluble receptors, which in the case of sIL6r are 1000-fold higher than IL-6 in the nanogram/mL range, may act as a buffer, attenuating acute over-reactions (May et al., 1992, Rose-John and Heinrich, 1994). It has been hypothesized that cells will shed the membrane-bound receptor when activated or over-stimulated by IL-6 for sustained periods of time (Yokoyma et al., 1997). However, the function of the soluble receptor varies across different types of tissue, and it can also act as an agonist. The receptor/ligand complex can facilitate uptake of the bound IL-6 into cells that do not endogenously express IL6r (i.e., a process described as transsignaling, Jones et al., 2001, Kallen, 2002). Of more immediate relevance to our aim to investigate population differences in cytokine activity, prior research has shown that the genetic regulation of IL-6 and its receptor is distinct (Galicia et al., 2004). Each factor is controlled somewhat independently by actions of different single nucleotide polymorphisms (SNPs). Thus, one cannot assume that an individual with low or high IL-6 levels in systemic circulation will necessarily have a similar receptor profile. The polymorphisms, as well as both IL-6 and sIL-6r, have been used as unique prognostic predictors of disease progression and poor outcomes in different patient populations (Galicia et al., 2006; Mehra et al., 2006, Nakjima et al., 1999, Velez et al., 2008, Yeh et al., 2010).

Japanese immunologists, such as Dr. Tadamitsu Kishimoto, were pioneers in the discovery of IL-6, and continue to generate seminal findings about its many functions (Kishimoto, 2005, Kishimoto, 2010), but there has not previously been a normative study comparing IL-6 levels in healthy Japanese individuals to other populations. Most articles on IL-6 from Japan instead tend to focus on clinical abnormalities in patients (e.g., Komatsu et al., 2004, Yamaguchi et al., 1998). A population-level comparison across countries is logistically challenging because it requires that samples be analyzed at the same laboratories to preclude differences in assay methods. In addition to meeting this quality control criterion for the cytokine measures, the CRP and FBG assays comparing Japanese and Americans were also conducted by only one laboratory. Thus, it was possible to directly contrast the test results across the two countries, and to use the CRP and FBG values to validate conclusions about IL-6. Beyond determining whether there were overall population differences in cytokine biology, we were interested in discovering if the age-related changes in proinflammatory activity would be similar, especially if both elderly Japanese and Americans progress toward higher cytokine levels with age.