Acidic pH and atherosclerosis

1
Acidic pH and atherosclerosis

• David S. Leake
• Cardiovascular Research Group
• Cell and Molecular Biology Research Division
• School of Animal and Microbial Sciences
• The University of Reading
• United Kingdom

2
Do atherosclerotic lesions have an extracellular
acidic pH?

• It has been proposed that atherosclerotic lesions
  may have a localised acidic pH1
• They may be acidic by analogy to
• Inflammatory sites
• Ischaemic sites
• Tumours
• Macrophages release lactic acid and protons and
  acidify their immediate environment to as low as
  pH 3.62.

3
Atherosclerotic lesions have recently been shown
to have a reduced pH

• Maghavi et al. (2002) have shown using a glass
  microelectrode and pH-sensitive fluorescent
  probes that human and rabbit atherosclerotic
  lesions (but not the normal arterial wall) have a
  reduced extracellular pH3. The mean pH in the
  lipid-rich areas of human lesions was 7.15 (some
  areas were 7.0 or below). Due to technical
  difficulties, there may possibly have been very
  localised areas with lower pH values that could
  not be detected.
• The mean pH in the calcified areas was 7.73

4
Oxidised LDL and atherosclerosis

• The local oxidation of LDL in atherosclerotic
  lesions may be important in this disease4.
• Oxidised LDL is taken up faster by macrophages
  and may contribute to foam cell formation
• Oxidised LDL activates inflammatory genes,
  inhibits the activity of nitric oxide and induces
  apoptosis in cells

5
Macrophages oxidise LDL faster at acidic pH

• Morse peritoneal macrophages were incubated with
  Hanks balanced salt solution containing 6
  micromolar FeSO45.
• The oxidation of LDL did not occur at pH 7.4 but
  did at pH 6.25 or below.

6
LDL oxidation by iron is faster at acidic pH

• LDL oxidation by FeSO4 and cysteine in the
  absence of cells in a simple buffer or Hams F10
  culture medium was much faster at pH 6.5 or
  below6.
• The increased rate of oxidation was observed in
  terms of lipid hydroperoxides, TBARS or
  macrophage uptake.

7
LDL oxidation by copper at acidic pH

• The effects of pH on LDL oxidation by copper are
  more complex than those by iron6.
• In the absence of antioxidants, the oxidation
  measured in terms of conjugated dienes, lipid
  hydroperoxides or TBARS is slower at acidic pH,
  but the uptake by macrophages increases more
  rapidly.

8
Antioxidants inhibit LDL oxidation by copper less
at acidic pH

• Human serum, cysteine and histidine inhibit LDL
  oxidation by copper less at acidic pH than at pH
  7.47.
• In the absence of these antioxidants, the
  oxidation is slower at acidic pH
• In the presence of these antioxidants, the
  oxidation is faster at acidic pH

9
Serum inhibits LDL oxidation by copper less at
acidic pH
10
Transferrin oxidises LDL at acidic pH

• Iron circulates bound to transferrin
• Transferrin cannot oxidise LDL at pH 7.4
• At acidic pH (beginning at pH 6.5) iron
  dissociates from transferrin and can oxidise LDL
  in the presence of cysteine8.

11
Acidic pH increase LDL oxidation by macrophages
catalysed by caeruloplasmin

• Copper circulates bound to caeruloplasmin.
• Pre-incubating caeruloplasmin at acidic pH (or
  even pH 7.0) increases its ability to catalyse
  LDL oxidation by macrophages9.

12
Metmyoglobin oxidises LDL faster at acidic pH

• The haem protein metmyoglobin oxidises LDL faster
  at acidic pH10.

13
Lysosomal proteases modify LDL at acidic pH to
increase its uptake by macrophages

• Incubating LDL with a macrophage sonicate at
  acidic pH results in the proteolysis of the LDL
  due to the lysosomal cathepsins B and D and to
  its aggregation and increased uptake by
  macrophages11.
• If these cathepsins are released by exocytosis by
  macrophages or by lysis of dead cells, this may
  help to explain why macrophages are converted
  into foam cells in atherosclerosis.

14
Conclusions

• A lowered pH in localised regions of
  atherosclerotic lesions may help to explain why
  LDL becomes oxidised at these sites.

15
References (1)

• 1 Leake, D. S. (1997) Atherosclerosis 129,
  149-157. Does an acidic pH explain why low
  density lipoprotein is oxidised in
  atherosclerotic lesions?
• 2 Silver, I. A., Murrills, R. J. Etherington,
  D. J. (1988) Experimental Cell Research 175,
  266-276. Microelectrode studies on the acid
  microenvironment beneath adherent macrophages and
  osteoclasts
• 3 Naghavi, M., John , R., Naguib, S., Siadaty, M.
  S., Grasu, R., Kurian, K. C., van Winkle, W. B.,
  Soller, B., Litovsky, S., Madjid, M., Willerson,
  J. T. Cassells, W. (2002) Atherosclerosis 164,
  27-35. pH Heterogeneity of human and rabbit
  atherosclerotic plaques a new insight into
  detection of vulnerable plaque
• 4 Steinberg, D. (1997) J. Biol. Chem. 272,
  20963-20966. Low density lipoprotein oxidation
  and its pathobiological significance
• 5 Morgan, J. Leake, D. S. (1993) FEBS Lett.
  333, 275-279. Acidic pH increases the oxidation
  of LDL by macrophages

16
References (2)

• 6 Morgan, J. Leake, D. S. (1995) J. Lipid Res.
  36, 2504-2512. Oxidation of low density
  lipoprotein by iron or copper at acidic pH
• 7 Patterson, R. A. Leake, D. S. (1998) FEBS
  Lett. 434, 317-321. Human serum, cysteine and
  histidine inhibit the oxidation of low density
  lipoprotein less at acidic pH
• 8 Lamb, D. J. Leake, D. S. (1994) FEBS Lett.
  352, 15-18. Iron released from transferrin at
  acidic pH can catalyse the oxidation of low
  density lipoprotein
• 9 Lamb, D. J. Leake, D. S. (1994) FEBS Lett.
  338, 122-126. Acidic pH enables caeruloplasmin to
  catalyse the modification of low-density
  lipoprotein
• 10 Rodriguez-Malaver, A. J., Leake, D. S.
  Rice-Evans, C. A. (1997) FEBS Lett. 406, 37-41.
  The effects of pH on the oxidation of low-density
  lipoprotein by copper and metmyoglobin are
  different
• 11 Leake, D. S., Rankin, S. M. Collard, J.
  (1990) FEBS Lett. 269, 209-212. Macrophage
  proteases can modify low density lipoproteins to
  increase their uptake by macrophages