From the article:
The researchers say that while more work is needed to understand the exact mechanism by which HDL cholesterol effects aneurysms, their investigations indicate that raising HDL cholesterol influences the activity of the aortic artery’s cells, which are the building blocks of its structure and function.
They found that elevated levels of HDL had two key influences on the cells. Firstly, it altered the signals sent between cells, which, in turn, reduced the activity of a protein called ERK1/2 that is known for its cell growth properties. Secondly, it increased levels of HDL cholesterol induced programmed cell death, which is an essential part of the cell lifecycle that sees old cells replaced with new ones.