A key underpinning in the pathological development of PAH is thought to be the abnormal proliferation of pulmonary arterial smooth muscle cells (PASMCs), and it is well known that oxidative stress plays a key role in this process. One oxidative stress pathway is lipid peroxidation, which has been found to contribute to abnormal PASMC growth. A major end product of lipid peroxidation processes, specifically from omega-6 peroxidation, is the compound 4-hydroxynonenal (HNE). [For those of a chemistry bent, 4-HNE is an aldehyde]. Of interest is the fact that 4-HNE has been found to contribute to PASMC growth and this compound has also been found in excess in the pulmonary arteries of PH patients.
Normally, Aldehyde dehydrogenase 2 (ALDH2) is the enzyme that is responsible for conversion of 4-HNE into harmless byproducts. However, any alterations in ALDH2 activity could result in improper removal of 4-HNE.
Furthermore, 4-HNE is typically a byproduct of oxidative stress (and thus a marker of oxidative stress), 4-HNE can also contribute to disease by binding to proteins to form protein adducts which can cause dysregulation in cellular signaling pathways.
In this paper, the role of 4-HNE and ALDH2 in development of PH was investigated further to determine if 4-HNE contributes to abnormal medial remodeling in the pulmonary vasculature (i.e., stimulates PASMC growth and migration] and if ALDH2 agonists help to improve PH.
- After injection of monocrotaline (MCT), which is a toxin used to induce PH, 4-HNE accumulates in rat PASMCs
- 4-HNE was found to stimulate growth of cultured human PASMCs.
- 4-HNE increased expression of the matrix metalloproteinases MMP-2 and MMP-9 in human PASMCs in vitro. Since MMPs are involved in degrading ECM components to facilitate cell migration, this suggests that 4-HNE mediates PASMC migration within the ECM.
- NF-kB is a signaling pathway involved in oxidative stress. 4-HNE was found to decrease the presence of the NF-kB inhibitor, IκBα, and increase the presence of NF-kB. It was also found to stimulate the MMPs via NF-kB.
- 4-HNE decreases the presence of ALDH2 in human PASMCs in vitro. When ALDH2 is increased (via treatment with an agonist), it prevented the 4-HNE induced proliferation in PASMCs as well as prevented the oxidative stress pathway triggered by NF-kB (by preventing NF-kB translocation to nucleus, which occurs because ALDH2 removes 4-HNE which is was found to activate NF-kB as indicated above).
- In MCT rat lungs, 4-HNE accumulated and ALDH2 activity was decreased.
- ALDH2 agonists improve RV hypertrophy and RV systolic pressure, but does not improve pulmonary arterial pressure. The agonist also increased presence of NF-kB inhibitor and decreased the activity of NF-kB.
The study was only performed on one animal model, the MCT mouse model: “It should be noted that we only examined the role of ALDH2 in MCT-induced PAH in rats, without evaluating using other animal models of PAH, such as hypoxia-induced PAH or Sugen-5416/hypoxia-induced PAH. Another, MCT-induced PAH is only a pro-inflammatory and oxidative stress relevant animal model, whereas PAH in humans is considered to develop by multiple pathogenic factors.” Additionally, cultured PASMCs from humans were used, and studies done on cultured cells should be interpreted with caution.
Furthermore, the action of 4-HNE is complex. While 4-HNE was shown to induce PASMC proliferation, it does so at low levels. High levels of 4-HNE, however, might do something different: “high concentrations of HNE initiated apoptosis by inducing endoplasmic reticulum stress and mitochondrial dysfunction in human colon carcinoma cells and neuroblastoma cells… These findings indicated that the function of HNE is complex; whether it promotes proliferation, differentiation or cell apoptosis depends upon its concentration and the cell type involved.”
It was convincingly shown in this article that 4-HNE can stimulate PASMC growth and stimulate oxidative stress, indicating that even if oxidative stress produces 4-HNE, the 4-HNE product can contribute back to oxidative stress creating a potential feedback loop. The reasons for 4-HNE accumulation most likely have to due with a decrease in ALDH2 enzyme activity. However, it could still also be the case that even if ALDH2 expression remains the same, an overall increase in oxidative stress can still increase 4-HNE (if 4-HNE production surpasses its degradation by ALDH2).
Why is ALDH2 activity decreased? According to the paper, a few reasons could be polymorphisms in ALDH2 gene as well as reactive oxygen species (ROS), which has been shown to decrease ALDH2 activity. However, what about metabolism? Does an altered cellular metabolism affect ALDH2 activity?