Breaking through cancer’s acid shell
By Peter Canning
Weatherall Institute of Molecular Medicine, Oxford, UK
This summary was awarded 3rd place for Access to Understanding 2015
Cells typically generate energy by aerobic respiration – breaking down sugar in the presence of oxygen. When cells work hard, they use more oxygen. If the circulatory system can’t supply enough oxygen then cells have to resort to anaerobic respiration for energy. This means they consume sugar without using any oxygen, creating lactic acid as a by-product.
When cells become cancerous, they grow far faster than healthy cells and eventually form tumours. The demand for oxygen in these cells is far higher than normal and the cancerous cells often have to use anaerobic respiration to supply their energy. The lactic acid produced makes the area around the tumour become slightly acidic. Cancer cells have to adapt themselves to their new acidic environment. To do this they cannibalize and recycle parts of their own cellular machinery, a process called ‘autophagy’. However, once adapted to their new conditions, their acidic surroundings help protect them from the immune system and certain anti-cancer drugs. There are clinical trials underway to treat cancer by neutralising the acid to remove this protection. Other trials are looking at the possibility of blocking the process of autophagy. There is evidence that blocking autophagy makes cancer cells more vulnerable to chemotherapy and radiotherapy; in some tests it has killed them outright.
Two drugs are in trials to treat cancer by halting autophagy: Chloroquine and its derivative, Hydroxychloroquine. Chloroquine is commonly used as an anti-malaria drug but is also sometimes used as an anti-inflammatory. The process of autophagy takes place in two specialised cellular compartments: autophagosomes and lysosomes. Autophagosomes swallow up cellular contents that need to be eliminated (damaged cell components, invading bacteria, etc.) and then pass them on to lysosomes. Lysosomes are sacks full of enzymes that break down the offending material. Chloroquine prevents cells undergoing autophagy by getting inside lysosomes and stopping them from working. Researchers from the Karolinska Institute in Stockholm wanted to know whether this would still be true in a tumour protected by a layer of acid.
The researchers tested colon cancer, melanoma (skin cancer) and osteosarcoma (bone cancer) cells. They grew the cells in either neutral or slightly acidic conditions and added Chloroquine to them. They found that in neutral conditions Chloroquine was absorbed by the cells and stopped autophagy by blocking the function of the lysosomes inside. They also tested cells that had already adapted themselves to an acidic environment and found that they died when treated with Chloroquine in neutral conditions. However, cells in acidic conditions no longer absorbed the Chloroquine. As a result, the rate of autophagy was unaffected and the acid-adapted cancer cells survived.
Cells in a dish can behave very differently to a solid tumour, so the team conducted an experiment in which they used a mouse to grow a tumour. They dissected the tumour and analysed it using two stains. One stain identified areas of the tumour that were under stress due to low oxygen, meaning they will be producing acid. They used a second stain and image analysis software to measure the rate of autophagy all over the tumour. Chloroquine treatment made almost no difference to the rate of autophagy in the acidic areas of the tumour, but the rate of autophagy decreased drastically everywhere else. These results matched the findings from the cell experiments – cells in an acid environment will not absorb Chloroquine. The most likely explanation is that Chloroquine becomes positively charged on contact with the acid and this prevents it from being absorbed by the cells.
This is a serious problem for the use of Chloroquine as an anti-cancer drug. The acid around many tumours will prevent the drug from having any beneficial effect.
The researchers also tested another drug, Lys-01. This drug is similar to Chloroquine but was specifically designed to have a stronger effect on autophagy. They found that, unlike Chloroquine, Lys-01 still worked in acidic conditions. It was still absorbed by the cells and was therefore able to block autophagy by halting the activity of the lysosomes inside. They tested Lys-01 on the acid-adapted cells and found that, even in acidic conditions, the cells died. Better still, in neutral conditions the drug was even more effective. This suggests that an autophagy-blocking drug like Lys-01 could be a powerful cancer treatment, especially if the acid around the tumour could be neutralised.
Unfortunately the whole tumour experiment could not be repeated with Lys-01 as the drug is toxic to mice. Hence, we do not know whether Lys-01 treatment is as effective in solid tumours. It is also relevant because if drugs are toxic to mice they are often toxic to humans as well. It is unlikely that Lys-01 will be a new cancer drug, but these results suggest it could be an excellent starting point for one.
This article describes the research published in:
Acidic extracellular pH neutralizes the autophagy-inhibiting activity of chloroquine: implications for cancer therapies
Pellegrini, A. Strambi, C. Zipoli, M. Hägg-Olofsson, M. Buoncervello, S. Linder, A. De Milito Autophagy (2014) 10(4), 562–571
This article was selected for inclusion in the competition by Worldwide Cancer Research.