Calcium-dependent second-messenger regulation of low-density lipoprotein oxidation by human aortic smooth muscle cells.

BACKGROUND

We postulated that the response of vascular smooth muscle cells (SMCs) to oxidized low-density lipoprotein (LDL) may be modulated through calcium and 3', 5' cyclic adenosine monophosphate (Ca+2-cAMP) second-messenger activity.

METHODS

Changes in cytosolic calcium [Ca+2]i in aortic SMCs exposed to native (N-) and oxidized (Ox-) LDL were measured with a Fura 2-AM indicator. The influence of cAMP on this response was determined by incubating the cells with either forskolin or 8-bromo-cAMP (stimulatory) or galanin (inhibitory). The cells were then activated by an initial preincubation with N- or Ox-LDL, and the subsequent cellular oxidation of N-LDL was measured. The effect of cAMP mediators alone or in conjunction with calcium antagonism was studied.

RESULTS

Exposure of SMCs to Ox-LDL resulted in a marked elevation of [Ca+2]i(306 +/- 12 nmol/L) compared with that in the control group (192 +/- 15 nmol/Liter; p < 10(-6)). This response was augmented by cAMP stimulation (406 +/- 8 nmol/L; p < 10(-6)) but reduced by cAMP inhibition (247 +/- 8 nmol/L; p < 10(-6)). The activation of intracellular signaling by initial Ox-LDL priming increased the subsequent oxidation of N-LDL (0.40 +/- 0.02 nmol malondialdehyde versus 0.24 +/- 0.02 nmol MDA control; p < 10(-4)). This response was enhanced by cAMP (0.45 +/- 0.03 nmol MDA; p < 10(-4)) and inhibited by galanin (0.26 +/- 0.02; p < 10(-4)). The cAMP effect was reversed by the blockade of calcium mobilization via membrane channels and reticular release.

CONCLUSIONS

Oxidized LDL-induced Ca+2-cAMP signaling modulates the cellular oxidation of N-LDL. This finding suggests a mechanism through which the scavenger uptake of modified LDL may potentially be regulated.