Age-dependent decrease in the hepatic uptake of taurocholic acid resembles that for ouabain. A possible role of surface membrane protein mobility

What really declines with age? The Hayflick Lecture for 2006 35th American Aging Association

In order to understand the basic mechanisms underlying the organismic aging process, considerable efforts have been devoted in the last half-century to biochemical (enzyme activity) alterations in specific tissues and organs of various organisms associated with aging. When a decline in enzyme activities with age has been found in a study, especially for key enzymes such as antioxidant enzymes, the results have often been interpreted as a cause for the aging of the entire body. Retrospectively, however, these changes turned out to be so variable--depending on species, strains and sexes of animals--that the interpretation of these results in general terms of aging became invalid. Further, unlike the prediction for the whole human body, many enzyme activities in a vital organ, such as the liver, remained unchanged, as long as the old subjects remained healthy. However, enzyme activities in old animals and humans are often more susceptible to morbidities and frailties, which themselves are often accompanied by infections and malnutrition. Despite the rather stable enzyme functions in the liver with age, a distinct and progressive decline in the lateral diffusion coefficient of proteins of hepatocyte plasma membranes has been demonstrated by fluorescence recovery after photobleaching (FRAP), which was implicated as the cause for the decline of hepatocyte functions such as ouabain (and taurocholate) hepatic uptake and their eventual biliary excretion. Since a similar decline in protein diffusion coefficients was observed in brain and muscle cells, it is likely that these changes are occurring in common with many cell types of the body, thus causing a delay in transmembrane transport of endogenous and exogenous substances whose transports are mediated by membrane proteins. In attempts to prolong the life spans of animals other than by calorie restriction, but instead using deprenyl or tetrahydrocurcumin, works by the author and coworkers are introduced and discussed. Despite limited success along these lines thus far, further attempts are encouraged, primarily to understand the mechanisms underlying organismic aging processes and to find a practical way to prolong the health span of the elderly.

Aging of the liver: facts and theories

Although most theories of aging assume that cellular functions decline with aging, many intracellular functions in the liver, such as enzyme activities, stay fairly stable in old age. This does not appear to be an antifact caused by in vitro experimental design, since in vivo pharmacokinetic data also demonstrate that most, if not all, biotransformation capacities of the liver remain stable during the aging process, if we take the decline in liver volume with age into account. Thus, many theories to explain the decline in cellular functions during aging appear to be based on erroneous assumptions. The stability of cellular function in old age does not necessarily mean, however, that all cellular functions are identical for young and old organisms. Once unfavorable conditions, such as malnutrition, infection, etc., are involved, the response of the liver is quite different for young and old subjects, demonstrating a more efficient and versatile response in young animal livers in comparison to old livers. Large differences in enzyme activities between young and old organisms appear during stress and especially during recovery from stress. Accordingly, any aging theory needs to explain a potential difference in liver functions (such as response capability) rather than the difference in basal functions. In contrast to rather stable intracellular functions, the uptake function of the hepatocyte surface membranes was found to be progressively decreased with age. This was shown for at least two different types of carrier systems in the surface membranes. Although the decrease of carrier unit number for these substances remains a possible causal factor, we suggest that the decline in hepatic uptake with age is at least partially the result of a gradual decrease in the mobility of surface membrane proteins, which can be shown by the fluorescence recovery after photobleaching (FRAP) technique. Theories of aging need to be elaborated on the basis of unbiased observations on the actual manifestations of cellular aging.

Aging and the liver: functional aspects

The drastic decline in the function of the hepatic microsomal cytochrome monooxygenase system, initially reported in male rat livers, was shown to be due to a feminization of male rat livers with aging. In female rat livers as well as in mouse livers, this system was found to stay unchanged with age. Phase II reactions which showed some decline with aging in male rat livers again stayed fairly stable with age in female rat and mouse livers. Glutathione S-transferase (GST) enzyme activities, which are very stable with age in female rat and mouse livers, demonstrated highly age-dependent changes when dietary conditions were manipulated, suggesting a potential age difference in the homeostatic regulation of this enzyme system. Using the fluorescence recovery after photobleaching (FRAP) technique, unique studies revealed an age-dependent decline in the lateral mobility of proteins in hepatocyte surface membranes. The protease inhibitor model of aging, initially proposed by Ivy for brain cells, has been validated in hepatocytes, demonstrating an accumulation of lipofuscin-like granules in young animals treated with i.p. infusion of leupeptin for only 2 weeks. Antioxidant enzyme activities such as superoxide dismutase (SOD) and catalase (CAT) in the liver were clearly demonstrated no to be reduced in general terms with aging. Rather, a clear increase in CAT enzyme activities with age was demonstrated in female rat livers, thus challenging the concept that intracellular enzyme activities generally decline with aging. In this paper, studies performed in Japan on aging and the liver over the past 30 years, with a focus on its functional aspects, are critically reviewed in terms of the clinical implications of these studies as well as on theories of aging in general.

Lateral mobility of proteins and lipids of cell surface membranes during aging: do the data support 'The Membrane Hypothesis of Aging'?

Many previous studies regarding the change with age in surface membrane fluidity of different cell types, including hepatocytes, as determined by the fluorescence anisotropy method, are in conflict, demonstrating decreased, unchanged or even increased fluidity with age. In contrast, the results of our series of works using the fluorescence recovery after photobleaching (FRAP) technique, which measures protein lateral diffusion coefficients of hepatocyte surface membranes (Dp), have demonstrated that Dp generally declines in a linear fashion with age in hepatocytes of all animal strains and species examined. The major coworker (I. Zs.-Nagy) of these studies insists that our observations support his original hypothesis, 'The Membrane Hypothesis of Aging' (MHA), the primary assumption of which is that changes in cell surface membranes with age cause a general decline in intracellular enzyme activities. However, while it seems clear that cell surface membrane changes do occur with age, a number of past observations including those from the laboratory of this author, provide strong evidence that intracellular enzyme activities do not generally decline with age. This paper presents the data in detail, along with the author's view that the results do not support the main assumption of the MHA, but are more likely related to alterations in membrane functions with age.

Age-dependence of the lateral diffusion coefficient of Con-A receptor protein in the skeletal muscle membrane of C57BL/6J mice

The lateral diffusion coefficient (Dp) of the Con-A receptor protein was measured in the sarcolemma of the quadriceps femoris (QF) muscle of male and female C57BL/6JNia mice in four age groups between 2 and 26 months. Freshly prepared, ex vivo taken muscle strips were stained with Con-A-FL conjugate for 10 min, and fluorescence recovery after photobleaching (FRAP) measurements were carried out on 20-30 cells per animal, at 37 degrees C. Using this technique, Dp, and the fractional recovery (mobile fraction = FR%) of these proteins can be measured. In the youngest male and female age groups, Dp values of 5.72E-10 and 5.43E-10 cm2/s, and FR% values of 43.3 and 36.3%, were found, respectively. Dp displayed a characteristic, significant, negative, linear correlation with age in both sexes. The slope of the linear regression line calculated per month of age was 1.06E-11 and 0.96E-11 cm2/s for males and females, respectively; both of them differ from zero highly significantly. FR% values tended to increase slightly with age, yet the estimated average Dp = D(FR), calculated for the total Con-A receptor pool, maintained its significant, negative, linear age-correlation. The physiological significance of these changes needs to be clarified in the future.

Age-dependence of the lateral mobility of lipids in hepatocyte plasma membrane of male rats and the effect of life-long dietary restriction

The lateral diffusion constant of lipids (D(1)) in hepatocyte plasma membranes was measured in liver smears by means of the fluorescence recovery after photobleaching (FRAP) method, applying the label, N-4-nitrobenzo-2-oxa-1,3-diazolyl phosphatidylethanolamine (NBD-PE). Nineteen ad libitum fed, male Fischer-344 rats in four age groups (2.1-29.8 months of age) were studied. A highly significant negative linear age-correlation of D(1) (cc = 0.958) was found. D(1) values were 1.39 x 10(-9) cm2/s in the young rats, and only 6.77 x 10(-10) cm2/s in the oldest rats. Lipid lateral mobility is changing in parallel with that of proteins, having been measured previously also with the FRAP method by the authors. Fractional recovery values (FR%) of the lipids were lower than those of proteins even in the young ages, but also decreased linearly with age, therefore, the parameter, D, x FR decreased even steeper with age than D(1) itself. D(1) was also measured in a group of six male Fischer 344 rats having been kept on dietary restriction (DR) since their age of 1 month until 30 months of age (applying the every-other-day (EOD) feeding). DR caused an increase of D(1), compared with the age-matched ad libitum fed animals: the mean was 9.24 x 10(-10) cm(2)/s. FR% and D(I), x FR again increased considerably under DR. The results are interpreted in terms of the increased protein and lipid turnover under DR.

Adenosine triphosphate-dependent taurocholate transport in human liver plasma membranes

Transport systems involved in uptake and biliary secretion of bile salts have been extensively studied in rat liver; however, little is known about these systems in the human liver. In this study, we investigated taurocholate (TC) transport in canalicular and basolateral plasma membrane vesicles isolated from 15 human livers (donor age 6-64 yr). ATP stimulated the uptake of TC into both canalicular and basolateral human liver plasma membrane vesicles (cLPM and blLPM, respectively). Considerable interindividual variations in the transport velocity were observed in the different membrane preparations used: 9.0 +/- 1.3 (mean +/- SEM, n = 17; range 1.6-18.0) and 9.3 +/- 2.0 (range 1.1-29.8) pmol TC.mg protein-1.min-1 at 1.0 microM TC for cLPM and blLPM, respectively. TC transport was temperature sensitive and showed saturation kinetics with a high affinity for TC (Km 4.2 +/- 0.7 microM and 3.7 +/- 0.5 microM for cLPM and blLPM, respectively). Transport was dependent on the ATP concentration and saturable (Km 0.25 +/- 0.03 mM, n = 3). Neither nitrate, which reduces membrane potential, nor the protonophore FCCP strongly inhibited ATP-dependent TC transport, indicating that membrane potential and proton gradient are not involved in this process. TC transport was significantly inhibited by the classical anion transport inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonate (250 microM) and the glutathione conjugate S-(2,4-dinitrophenyl)glutathione (100 microM). In conclusion, high affinity ATP-dependent TC transport is present in human liver at both the canalicular and the basolateral sides of the hepatocyte.

The effect of age on the biliary transport maximum (Tm) of bile salts in rats

Age-related differences in the biliary transport maxima (Tm) values for taurocholate (TC) and tauroursodeoxycholate (TUDC) were examined in female Fischer-344 rats as well as Wistar-derived rats of both sexes. The Tm values for TUDC were more than two times higher than corresponding values for TC in young (3-month-old) rats of both sexes. Tm values for both bile salts tended to decline with age, demonstrating a significant negative correlation between the Tm (micromol/min per g liver) and rat age (months) for all rat groups. The decline in Tm value was, however, dominant in the first year with little significant change after 1 year. The results in the present study coupled with our previous observations for Tm values of sulfobromophthalein (BSP) and conjugated BSP support our hypothesis that the transport capacity for bile canalicular membrane generally declines with age in rats.

Effect of leupeptin on the lateral mobility of proteins in the plasma membrane of hepatocytes of C57BL/6 mice: FRAP studies on liver smears

Four groups of male C57BL/6 mice (ages 2, 3.8, 11.8-12.5 and 24 months) were administered leupeptin at 5 mg/100 g body weight/day via intraperitoneally implanted osmotic minipumps. Both untreated and saline-treated mice served as controls. The time and dose-dependence of the effects of leupeptin on the lateral mobility of proteins in hepatocyte surface membranes were analyzed first in mice treated for 2, 5, 8, 11 and 14 days, respectively, by using the fluorescence recovery after photobleaching (FRAP) technique. The age dependence of the response to this treatment was also studied after 14 days of treatment in mice of various ages. The average lateral diffusion constant (D) and the fractional recovery (FR) were measured, and D x FR calculated. Leupeptin treatment at a daily dose of 5 mg/100g for 2 weeks increased plasma glutamic-pyruvlc transaminase levels 2-fold in all age groups. Leupeptin treatment caused a linear increase of D and a decrease of FR with respect to the duration of the treatment in adult mice. In all age groups about 30% of the membrane proteins became immobile after 14 days of treatment, whereas the still mobile fraction displayed a large increase of D. The values of D x FR in the leupeptin-treated groups were slightly higher than those in the untreated mice until the age of 15 months but decreased after this age. The results are interpreted in terms of the known inhibitory effect of leupeptin on cytoplasmic and lysosomal thiol proteases, causing a general slowing down of the protein turnover and, specifically, of proteins in the hepatocyte membrane.