Broccoli and lung health

“Broccoli may ‘help protect lungs’” reported BBC News. It said that research suggests that a compound found in broccoli, sulforaphane, increases the expression (activity) of a gene found in lung cells that protects the organ from damage caused by toxins. The news service said that scientists have found that the gene is less active in the lungs of smokers who have chronic obstructive pulmonary disease (COPD) and increasing expression of the gene may lead to useful treatments.

This laboratory study has shed some light on important cellular pathways involved in human lung disease. This is early research however, and it will be some time before it is clear how it can be used to treat disease. Although the focus of the news story was on sulforaphane, only a small part of the study assessed its effects, and this was on the restoration of particular chemical reactions in cells that had been genetically modified. It is too great a leap to conclude from this that eating broccoli will protect the lungs. Smoking is the main cause of COPD, and not smoking, rather than eating broccoli, is the best way to protect the lungs from damage.

Where did the story come from?

Dr Deepti Malhotra and colleagues from John Hopkins School of Public Health in Baltimore, St Paul’s Hospital in Vancouver, the University of Chicago, and the University of Colorado carried out this study. The research was funded by the National Institutes of Health, the Flight Attendant Medical Research Institute, the National Institute of Environmental Health Sciences, and the Maryland Cigarette Restitution Fund. The study was published in the peer-reviewed medical journal: American Journal of Respiratory and Critical Care Medicine.

What kind of scientific study was this?

Chronic obstructive pulmonary disease (COPD) is an umbrella term that covers several conditions including chronic emphysema and bronchitis. It is mainly caused by smoking and is an incurable, long-term condition.

In this laboratory study, the researchers used human lung tissue and mice to investigate the link between severity of COPD, oxidative stress (an excess of potentially damaging chemicals called reactive oxygen species, which include free radicals) and expression of the gene NRF2. This gene produces a protein that regulates levels of antioxidants in the lungs of patients with COPD. The chemical sulforaphane is a known stabiliser of NRF2.

In the first part of this experiment, samples of lung tissue from people with COPD of varying severity and people with normal lungs were obtained from tissue banks. The researchers assessed amounts of various proteins in the cells, such as NRF2, DJ-1 and others, and assessed the link between severity of disease and these markers. Data on the patients’ lung function and severity of disease was obtained through patient registries. The lung tissue was cultured (grown) in vitro (in test tubes).

In the second part of the experiment, the researchers grew healthy human lung cells in the laboratory. Some of the cells were then exposed to a cigarette smoke extract (CSE) and the effect of ‘oxidative stress’ on the cells was assessed by measuring the change in levels of particular proteins (namely NRF2, and DJ-1, which is a stabiliser for NRF2). The researchers then investigated if those cells that had been exposed to the CSE and been affected (i.e. had disrupted levels of DJ-1 and NRF2), could be encouraged to produce higher levels of NRF2. They did this by exposing the cells to sulforaphane, a known stabiliser of NRF2. The purpose was to investigate more fully the mechanisms behind the restoration of NRF2 activity, i.e. antioxidant activity. These cells were also exposed to an antioxidant (N-acetyl cysteine) before or after CSE to see whether this would protect the cells from oxidative stress.

In the third part of the experiment, the researchers investigated what effect disruption of the antioxidant pathway has on protein and gene expression in the lungs. They used mice for this, dividing them into four groups of five. The first group of mice had been exposed to a compound (a small interfering RNA or siRNA) that interferes with manufacturing of the DJ-1 gene (i.e. it would not be able to make DJ-1 protein). The absence of DJ-1 leads to destabilisation and breakdown of the NRF2 protein. Half of these mice were exposed to cigarette smoke and half were not. The second set of mice had been exposed to a similar compound that did not specifically target DJ-1 (i.e. control (non-targeted) siRNA). Half of these were exposed to cigarette smoke and half were not.

What were the results of the study?

The researchers found that the lung tissue of people with severe COPD had depleted levels of NRF2 protein (six smokers and three ex-smokers) compared to the lungs of people without lung disease (five smokers and one non-smoker). There were also decreases in levels of other proteins involved in the antioxidant pathway. This depletion was only present in smokers with COPD and not in smokers without the disease, suggesting that the effect is caused by COPD, and not directly caused by smoking. Through their experiments that further explored the mechanism underlying this, the researchers found that levels of DJ-1 (which has the function of stabilising NRF2) was decreased in people with severe disease compared to those with no disease.

The effect of exposure to cigarette smoke on levels of DJ-1 and NRF2 was confirmed in mice experiments. The researchers also found that exposure to an antioxidant (N-acetyl cysteine) before being exposed to cigarette smoke or sometime after, lessened the reduction in DJ-1 expression (i.e. protected against oxidative stress). Exposure to sulforaphane helped to restore some antioxidant reactions in mice whose DJ-1 expression was affected.

What interpretations did the researchers draw from these results?

The researchers conclude that NRF2 activity is a ‘susceptibility factor in development of COPD’. They say that restoring the antioxidant defences associated with NRF2 alongside smoking cessation and using anti-inflammatory agents may help to slow down progression of COPD.

What does the NHS Knowledge Service make of this study?

This laboratory study will be of interest to the scientific community as it highlights the details of important cell reactions that are involved in the development of lung disease. It will be some time before it is clear what the applications of these findings to human health are, and, in particular, to treatment for COPD.

The news article may give the impression that this study assessed the effect of eating broccoli on human lung health. This is not the case. The researchers examined the effect of sulforaphane (an antioxidant found in some vegetables) on a genetically disrupted pathway in human lung cells in test tubes. COPD is primarily caused by smoking - and not smoking, rather than eating broccoli, is the optimal way to protect the lungs from damage.

Sir Muir Gray adds...

Everyone would benefit from eating more broccoli, it’s one of the good guys.