Expression of p16(INK4a) in peripheral blood T-cells is a biomarker of human aging

Abstract

Expression of the p16(INK4a) tumor suppressor sharply increases with age in most mammalian tissues, and contributes to an age-induced functional decline of certain self-renewing compartments. These observations have suggested that p16(INK4a) expression could be a biomarker of mammalian aging. To translate this notion to human use, we determined p16(INK4a) expression in cellular fractions of human whole blood, and found highest expression in peripheral blood T-lymphocytes (PBTL). We then measured INK4/ARF transcript expression in PBTL from two independent cohorts of healthy humans (170 donors total), and analyzed their relationship with donor characteristics. Expression of p16(INK4a), but not other INK4/ARF transcripts, appeared to exponentially increase with donor chronologic age. Importantly, p16(INK4a) expression did not independently correlate with gender or body-mass index, but was significantly associated with tobacco use and physical inactivity. In addition, p16(INK4a) expression was associated with plasma interleukin-6 concentration, a marker of human frailty. These data suggest that p16(INK4a) expression in PBTL is an easily measured, peripheral blood biomarker of molecular age.

Figures

Figure 1. Expression of p16INK4a in PBTL is associated with chronologic age

(A) Comparison of relative p16INK4a expression in different cell types of the peripheral blood. Error bars indicate standard error of the mean (SEM). (B) Linear relationship between log2-transformed p16INK4a mRNA expression and chronological age. Aggregate data from the exploratory and validation cohorts are shown. The correlation coefficients and p-values of individual cohort are shown in Table 1. Dotted lines indicate the 95% confidence intervals (CI) of the fitted line. (C) Expression of p16INK4a protein increases with chronologic age in a representative subset of patients.

Figure 2. Expression of other INK4/ARF transcripts is associated with p16INK4a expression, but not age

Dotted lines indicate the 95% confidence intervals (CI). The correlations between the variables shown in the left panels are significant (p0.4). Data shown are from the exploratory cohort.

Figure 3. Expression of p16INK4a is associated with smoking, exercise and plasma IL-6 concentrations

(A) The comparison of p16INK4a -age linear regression among never smokers, former smokers and current smokers is shown for the combined cohort. Slope comparison: Current, 7.5 ± 0.1 × 10-2; Never, 3.9 ± 0.7 × 10-2. *p<0.05, adjusted for multiple comparisons. (B) Expression of p16INK4a is associated with tobacco exposure. Categorized pack-years: Mild (>0, ≤5); Moderate (5-10); Heavy (11-30); Severe (31-50); Extreme (51 and up). (C) Decreased p16INK4a mRNA expression is associated with increased exercise intensity. (D) Plasma IL-6 levels are associated with increased p16INK4a expression. Aggregate data from the exploratory and the validation cohorts are shown in a, b, and c; whereas d represents data only from the validation cohort. The Pearson correlations and p-values in each cohort are shown in Table 1. Dotted lines in c and d indicate 95% CI.