Microcirculation of the aging liver: is getting old like having cirrhosis?

The decay of stem cell nourishment at the niche

One of the main features of human aging is the loss of adult stem cell homeostasis. Organs that are very dependent on adult stem cells show increased susceptibility to aging, particularly organs that present a vascular stem cell niche. Reduced regenerative capacity in tissues correlates with reduced stem cell function, which parallels a loss of microvascular density (rarefraction) and plasticity. Moreover, the age-related loss of microvascular plasticity and rarefaction has significance beyond metabolic support for tissues because stem cell niches are regulated co-ordinately with the vascular cells. In addition, microvascular rarefaction is related to increased inflammatory signals that may negatively regulate the stem cell population. Thus, the processes of microvascular rarefaction, adult stem cell dysfunction, and inflammation underlie the cycle of physiological decline that we call aging. Observations from new mouse models and humans are discussed here to support the vascular aging theory. We develop a novel theory to explain the complexity of aging in mammals and perhaps in other organisms. The connection between vascular endothelial tissue and organismal aging provides a potential evolutionary conserved mechanism that is an ideal target for the development of therapies to prevent or delay age-related processes in humans.

The liver sieve and atherosclerosis

SUMMARY

The 'liver sieve' is a term developed to describe the appearance and the role of fenestrations in the liver sinusoidal endothelial cell (LSEC). LSECs are gossamer-thin cells that line the hepatic sinusoid and they are perforated with pores called fenestrations clustered in sieve plates. There is growing evidence that fenestrations act like a permselective ultrafiltration system which is important for the hepatic uptake of many substrates, particularly chylomicron remnant lipoproteins. The liver sieve is a very efficient exchange system, however in conditions such as hepatic cirrhosis and fibrosis, diabetes mellitus and old age, there is defenestration of the liver sieve. Such defenestration has been shown to influence the hepatic uptake of various substrates including lipoproteins. In the future, pharmacological manipulation of the liver sieve may play a number of therapeutic roles including the management of dyslipidaemia; increasing the efficiency of liver-targeted gene therapy; and improving regeneration of old livers.

Relevance of b-values in evaluating liver fibrosis: a study in healthy and cirrhotic subjects using two single-shot spin-echo echo-planar diffusion-weighted sequences

PURPOSE

To investigate the relevance of increasing b-values in evaluating liver fibrosis through the agreement of two diffusion-weighted (DW) sequences.

MATERIALS AND METHODS

A total of 29 cirrhotic patients and 29 healthy volunteers were studied on a 1.5T system. Two single-shot spin-echo echo-planar sequences were acquired using sets of increasing b-values: 0, 150, 250, and 400 seconds/mm(2) (first sequence: DW1a) and 0, 150, 250, 400, 600, and 800 seconds/mm(2) (second sequence: DW2a). Apparent diffusion coefficients (ADCs) of the hepatic parenchyma were calculated on ADC maps. Noise-scaled single-point ADCs were calculated for each sequence from b = 400 seconds/mm(2).

RESULTS

ADCs resulted significantly lower in cirrhotic patients compared to controls using both DW1a (mean 1.14 +/- 0.20 x 10(-3)mm(2)/second vs. 1.54 +/- 0.12 x 10(-3)mm(2)/second; P < 0.0001) and DW2a (mean 0.91 +/- 0.18 x 10(-3)mm(2)/second vs. 1.04 +/- 0.18 x 10(-3)mm(2)/second; P = 0.0089). DW1 and DW2, respectively significantly differed in diagnostic performance at receiver operating characteristic (ROC) curve analysis (P = 0.003), showing AUCs of 0.93 (sensitivity 89.7%, specificity 100%) and 0.73 (sensitivity 62.1%, specificity 79.3%), respectively. Noise-scaled single-point ADCs showed a progressive convergence to similar values in cirrhotic and healthy livers at b = 800 seconds/mm(2) (1.12 +/- 0.27 x 10(-3)mm(2)/second vs. 1.13 +/- 0.17 x 10(-3)mm(2)/second).

CONCLUSION

A DW sequence is accurate in assessing liver fibrosis using intermediate (400 seconds/mm(2)) rather than high (800 seconds/mm(2)) maximum b-values, but after proper recalculation of ADCs the effects of perfusion rather than diffusion should be considered responsible for the higher accuracy at lower b-values.

The response of fenestrations, actin, and caveolin-1 to vascular endothelial growth factor in SK Hep1 cells

To study the regulation of fenestrations by vascular endothelial growth factor in liver sinusoidal endothelial cells, SK Hep1 cells were transfected with green fluorescence protein (GFP)-actin and GFP-caveolin-1. SK Hep1 cells had pores; some of which appeared to be fenestrations (diameter 55 +/- 28 nm, porosity 2.0 +/- 1.4%), rudimentary sieve plates, bristle-coated micropinocytotic vesicles and expressed caveolin-1, von Willebrand factor, vascular endothelial growth factor receptor-2, endothelial nitric oxide synthase and clathrin, but not CD31. There was avid uptake of formaldehyde serum albumin, consistent with endocytosis. Vascular endothelial growth factor caused an increase in porosity to 4.8 +/- 2.6% (P < 0.01) and pore diameter to 104 +/- 59 nm (P < 0.001). GFP-actin was expressed throughout the cells, whereas GFP-caveolin-1 had a punctate appearance; both responded to vascular endothelial growth factor by contraction toward the nucleus over hours in parallel with the formation of fenestrations. SK Hep1 cells resemble liver sinusoidal endothelial cells, and the vascular endothelial growth factor-induced formation of fenestration-like pores is preceded by contraction of actin cytoskeleton and attached caveolin-1 toward the nucleus.

Effects of old age on vascular complexity and dispersion of the hepatic sinusoidal network

OBJECTIVES

In old age, there are marked changes in both the structure of the liver sinusoidal endothelial cell and liver perfusion. The objective of this study was to determine whether there are also aging changes in the microvascular architecture and vascular dispersion of the liver that might influence liver function.

METHODS

Vascular corrosion casts and light micrographs of young (4 months) and old (24 months) rat livers were compared. Fractal and Fourier analyses and micro-computed tomography were used. Vascular dispersion was determined from the dispersion number for sucrose and 100-nm microspheres in impulse response experiments.

RESULTS

Age did not affect sinusoidal dimensions, sinusoidal density, or dispersion number. There were changes in the geometry and complexity of the sinusoidal network as determined by fractal dimension and degree of anisotropy.

CONCLUSIONS

There are small, age-related changes in the architecture of the liver sinusoidal network, which may influence hepatic function and reflect broader aging changes in the microcirculation. However, sinusoidal dimensions and hepatic vascular dispersion are not markedly influenced by old age.

Age-related changes in the liver sinusoidal endothelium: a mechanism for dyslipidemia

The liver sinusoidal endothelial cell (LSEC) influences the transfer of substrates between the sinusoidal blood and hepatocytes and has a major role in endocytosis; therefore, changes in the LSEC have significant implications for hepatic function. There are major morphological changes in the LSEC in old age called pseudocapillarization. These changes include increased LSEC thickness and reduced numbers of pores in the LSEC, which are called fenestrations. Pseudocapillarization has been found in old humans, rats, mice, and nonhuman primates. In addition, old age is associated with impaired LSEC endocytosis and increased leukocyte adhesion, which contributes to reduced hepatic perfusion. Given that fenestrations in the endothelium allow passage of some lipoproteins, including chylomicron remnants, age-related reduction in fenestrations impairs hepatic lipoprotein metabolism. In old rats, caloric restriction was associated with complete preservation of LSEC morphology and fenestrations. In conclusion, pseudocapillarization of the LSEC is a newly discovered aging change that, through its effects on lipoproteins, contributes to the association between old age, dyslipidemia, and vascular disease.

Autoimmune hepatitis (AIH) in the elderly: a systematic retrospective analysis of a large group of consecutive patients with definite AIH followed at a tertiary referral centre

BACKGROUND/AIMS

A few reports have suggested that AIH may be less severe in the elderly and may be underdiagnosed, but there is a paucity of data.

METHODS

We have undertaken a systematic analysis of 164 consecutive patients (36 males, 128 females) with definite AIH (median score 23, range 18-28) attending our clinics, comparing those presenting at age >60 years (Group 1, n=43) with those presenting at <60 years (Group 2, n=121).

RESULTS

Median (range) duration of follow-up was 9 years (1-28) in Group 1 and 14 years (1-33) in Group 2. Median ages (ranges) at presentation were: Group 1=65 (60-79) and Group 2=41 (6-59). Group 1 patients had a significantly increased incidence of ascites at presentation (p<0.001) and a lower incidence of relapse (42% vs. 70%, p=0.002), but there were no significant differences between the groups with respect to mode of onset (acute, insidious, asymptomatic), other clinical signs at presentation, biochemical parameters, types or titres of autoantibodies, incidence of histological cirrhosis, response to therapy or related side effects. There were also no significant differences in liver-related deaths or transplantation, or the frequencies of HLA DR3 or DR4 - although there was an increased frequency of the A1-B8-DR3/4 haplotype in Group 2 (40% vs. 22%, p=0.138).

CONCLUSIONS

These findings suggest that AIH often presents in older patients, who frequently have severe disease. Active management in these patients can lead to a normal life expectancy.

Impaired Response to Interferon-α2B plus Ribavirin in Cirrhotic Patients with Genotype 3A Hepatitis C Virus Infection

Patients with chronic infection with the 3a genotype of hepatitis C virus (HCV) are considered as ‘easy-to-treat’ with interferon/ribavirin (IFN/RBV), independent of liver disease severity. However, patients with extensive fibrosis or cirrhosis were under-represented in all the registration Phase III trials performed so far. To assess the influence of liver fibrosis on the outcome of anti-HCV therapy, all patients with genotype 3a hepatitis C who were naive to IFN-based therapies, and received RBV combined with standard IFN or pegylated IFN-α2b (peg-IFN-α2b) as standard of care for their disease, were investigated at our centre. A sustained virological response (SVR) was achieved in 68 of 91 patients (75%) independent of IFN type, pretreatment viraemia, clearance of HCV RNA at week 4 and relevant co-morbidities. A SVR was less common in cirrhotics (6 of 17) than in non-cirrhotics (62 of 74; 35% vs 84%; P<0.0005). Compared to non-cirrhotics, the age and sex adjusted odds ratio (OR) of treatment failure for cirrhotics was 10.1 (95% confidence interval: 2.4–41.7). By multivariate analysis, cirrhosis was the only predictor of non-SVR. In conclusion, cirrhosis is an independent predictor of IFN/RBV treatment failure in patients chronically infected with HCV 3a and is associated with an increased risk of post-treatment hepatitis relapse. Evaluation of liver fibrosis is important in the management of patients with genotype 3a hepatitis C, since it helps to predict response to IFN/RBV therapy.