A Heated Combat

A Heated Combat
Suppressing the Growth of Tumour Cells with Hyperthermia

Traditional treatments of cancer include chemotherapy, radiotherapy, and surgery. Researchers are continuously seeking new treatment methods that are effective and, at the same time, cause minimal side effects. Hyperthermia is a relatively new method of treating cancer, which can be applied singly or in combination with other anti-tumour treatments. It involves the application of heat to suppress the growth of tumour cells through the use of devices that generate microwaves or electromagnetic waves, or by directly immersing the patient in a hot water bath.

Hyperthermia

The use of hyperthermia to treat cancer was suggested by researchers in the early seventies but has remained unpopular. This is partly due to the high costs of sophisticated facilities for the treatment and partly because its effect is obvious only on tumours located close to the body surface, such as melanoma, breast cancer, and cancer of the head and neck, where heat can be easily transmitted. Hypermotherapy can, however, be made more effective by being used in combination with radiotherapy and chemotherapy.

What is the underlying mechanism of hyperthermia on tumour cells? Prof. C.Y. Lee and Prof. K.P. Fung of the Department of Biochemistry began investigating the issue in 1991. Their project, 'The Effect of Hyperthermia on Tumour Cells', received a grant of HK$350,000 from the Research Grants Council and was rated 'excellent' by the council in late 1996.

Reaction of Tumour Cells to Hyperthermia

Human cells thrive best at 37^oC, the normal human body temperature. Results in recent studies indicate that normal cells continue to thrive under temperatures as high as 45^oC and beyond. In contrast, the growth of many tumour cells is suppressed at 43^oC. The temperature used in hyperthermia is, therefore, set at 43^oC.

It is not clear why normal cells have a greater tolerance for heat than tumour cells. But it is known that when treated with heat, normal cells produce high quantities of a class of protein known as 'heat shock proteins' which serve to protect the integrity of other proteins in the cell by maintaining their three-dimensional structures, and hence, their functions. The ability of tumour cells to produce 'heat shock proteins', however, is found to be greatly reduced under hyperthermia. Led by Profs. Lee and Fung, the research team investigated, from a biochemical angle, why tumour cells die at high temperatures, and the changes in cells produced by hyperthermia. The team studied the effect of hyperthermia at 41^o- 43^oC on the cells of various tumours, including human breast tumour, glioma, leukaemia, and hepatoma, as well as Ehrlich ascites tumour and sarcoma in mice. Three main biochemical changes were observed.

Suppressed Glucose Transport in Tumour Cells

Glucose is a major nutrient for living organisms, and tumour cells are especially dependent on glucose for growth and division. In normal cells, glucose is taken into the cell with the aid of glucose transporters on the cell membrane. Once inside the cell, it is oxidized in the cytoplasm and mitochondria through a series of reactions to produce energy. In tumour cells, however, the number of mitochondria is relatively small, and the oxidation of glucose often cannot proceed to completion. The tumour cells thus try to compensate by making more glucose transporters to boost glucose intake. The researchers found that hyperthermia significantly suppresses glucose transport in tumour cells through reduced transporter synthesis. Faced with an insufficient energy supply, the tumour cells undergo a 'programmed cell death'.

Increased Acidity in Tumour Cells

The acidity of body fluids is rigidly controlled. The metabolic functions of human cells proceed optimally at pH 7.4. Small changes in this value by even ±0.2 unit to 7.2 or 7.6 can cause great physical distress. Any changes beyond that can be life-threatening. The researchers found that hyperthermia, through an as yet unknown mechanism, increases the acidity in the tumour cells. As a result DNA and protein synthesis is affected, and the proliferation and growth of tumour cells are reduced.

Nucleophosphoprotein B23 in Tumour Cells Translocated

The function of B23, a phosphoprotein found in the nucleolar compartment of cells, is to maintain the normal assembly of ribosomal RNA in order to facilitate synthesis of cellular proteins. The study found that hyperthermia lowers the level of B23 in nucleoli and translocates it, leading to cell death. These effects are similar to the action of certain effective anti-tumour drugs such as doxorubicin. The researchers are currently studying the relationship between B23 - its reduction and translocation - and the growth of tumour cells, focusing in particular on that between B23 and heat shock proteins under hyperthermia, as well as possible synergistic effects of drugs and hyperthermia in the treatment of cancer.

The effect of hyperthermia on translocation of B23 proteins in Ehrlich ascites tumour cells, as observed in immunofluorescence photomicrographs of tumour cells after treatment for two hours.

Control cells after treatment at 37^oC show the presence of B23 proteins in the nucleoli (see A).

Tested cells after treatment at 43^oC show reduced content (see B), or absence of B23 proteins in the nucleoli (see C).

Research to Continue

The foregoing discoveries point cancer treatment in a new direction. However, as both researchers pointed out, hyper-thermotherapy is still in its research stages, far removed from clinical application.

The research team received a further grant of HK$310,000 from the Research Grants Council in 1995 to continue their study on 'The Mechanism of Combination Treatment of Tumour Necrosis Factor-a and Hyperthermia on Tumour Cells'. Tumour necrosis factor-a (TNF-a) is a special kind of protein released by the human immune system to kill cancer cells. Through genetic engineering, TNF-a can be manufactured as an anti-cancer drug. The researchers now focus their attention on the combined use of hyperthermia and TNF-a, in particular the optimal protocol for applying heat to minimize the side effects of hyperthermia on normal cells. They will also study the effectiveness of combination treatments of hyperthermia and chemotherapy or immunotherapy. The results of these studies may provide useful information on the best mode of applying hyperthermia in clinical settings.

A recent as yet unpublished observation is that hyperthermia when used with TNF-a can effectively suppress the growth of breast cancer cells, and that the order of TNF-a injection and the application of heat can affect treatment results. Another significant finding is that hyperthermia can enhance the entry of Taxol (a new drug for breast cancer) in breast cancer cells to augment the cytotoxic effect of the drug. These and other findings of the research will be published in due course, to assist medical scientists to devise the best treatment strategy for different cancers.

Investigators

Prof. Lee Cheuk-yu received his Ph.D. from the University of British Columbia in Canada. He was postdoctoral fellow and later assistant professor at the New England Institute, USA, before joining the CUHK Department of Biochemistry as lectuer in 1972. He was promoted to professorship in January 1985.

Prof. K.P. Fung received his Ph.D. in microbiology from the University of Hong Kong in 1978. He joined the Department of Biochemistry of The Chinese University of Hong Kong as assistant lecturer in 1978. He was promoted to lectureship in 1979, senior lectureship in 1987, and readership in 1995.