ANTITUMOUR ACTIVITY OF ORAL MYCOVIRUS-EXTRACT FROM LENTINUS EDODES (SHIITAKE MUSHROOM) ON MURINE LYMPHOMA

Abstract

Lentinus edodes (shiitake mushroom) have been regarded as a panacea since centuries. Scientific studies have been done on several bioactive components isolated from L. edodes. Potentiation of host immunity has been proposed to explain its medicinal effects. Most studies have been done on its polysaccharide component - lentinan. Potent antitumour activity has been observed on oral administration of lentinan. The present study evaluated antitumour activity of double-stranded (ds) RNA mycovirus, present abundantly in 2-day old buds of L. edodes. These mycovirus were easily concentrated by polyethylene glycol and their presence confirmed by negative staining electron microscopy. The hypotheses was that ds-RNA genome of mycovirus could contribute to host immunomodulation. As a therapeutic agent against cancer, such immunomodulators are effective without severe toxicity that is associated with standard cytotoxic agents. Antitumour activity with oral administration of mycovirus-extract from L edodes was evaluated on K36 cell-line induced murine lymphoma. In particular, the difference in antitumour activity by three types of intervention in relation to leukemia cell inoculation to mice was assessed. The three interventions were: pre-feeding mycovirus-cetract before K36 cell inoculation, simultaneously feeding extract with K36 cell inoculation and administering extract after tumours were induced. Tumours, obtained on day-15/16 after K31, cell inoculation, were assessed for tumour inhibition rate and microscopic differences. Pre-feeding mycovirus-extract conferred the best antitumour activity with a tumour inhibition rate of 80.7%. Simultaneous-feeding and administration of extract after tumours were induced were also effective with tumour inhibition rates of 73.8 and respectively. Electron microscopy revealed apoptotic cells in all three regimes. The number of apoptotic cells was highest in lymphoma sections obtained on pre-feeding and least on feeding mycovirus-extract after tumours were induced. These findings were confirmed by confocal microscopy with TUNEL-staining, a hallmark of apoptosis. Interestingly, electron microscopy also revealed abundant defective C-type tumour retrovirus in tumours of pre-feeding regime, and far lesser in the other two regimes. To explain the mechanism of action of mycovirus-extract from L. edodes, interferon-y and tumour necrosis factor-a levels, in serum from healthy mice were assayed after oral administration of the extract. These cytokines, which are surrogate markers of immunomodulation, were significantly elevated. In addition, mycovirus-extract was not associated with weight loss or any other toxic effects in mice. This proved the formulated hypothesis that immunomodulation by mycovirus-extract contributed to the observed antitumour activity. Induction of cytokines is probably one of the mechanisms. The final tumoricidal pathway, however, is two fold. Firstly, antitumour effect is brought about by induction of apoptosis in lymphoma cells. Secondly, production of defective retrovirus in these cells caused a decrease in tumour cell's potential to proliferate. Thus, the present study highlighted the antitumour potency of endogenous dsRNA mycovirus from L. edodes on oral administration in AKR mice-tumour model. Immunomodulation is the underlying mechanism of action.