Hormones as cancer growth factors.

It is postulated that some hormones may regulate proliferation of cancer cells in the same way as growth factors produced by cellular oncogenes. The gene coding for the hormone's specific receptor would also act as a cellular oncogene. Normal adult breast cells show few if any oestrogen receptors. In the model put forward the oestrogen receptors in breast cancer cells should not be regarded as a marker of differentiation but as a survival advantage for the tumour when oestrogens are present. Prolactin and somatomedin may also behave as growth factors. In relation to the antitumour effects of hormone antagonists such as tamoxifen, it is postulated that cancer cells are immortalised and prevented from full differentiation by the presence of growth factors and their receptors. If receptor genes are re-expressed through the process of neoplastic transformation, their presence in cancers from unresponsive normal tissues should be regarded as a common event.