Growth of the androgen-dependent tumor of the prostate: Role of androgens and of locally expressed growth modulatory factors

The crucial role played by androgens in the growth of prostatic carcinoma is now well established. However, the mechanisms of this proliferative action are still poorly understood. Experiments have been performed to clarify: (1) the metabolism of androgens in prostatic tumor cells; and (2) the role played by locally produced growth factors in the autocrine regulation of prostatic tumor cell proliferation and the possible regulation exerted by testosterone (T) on the activity of these factors. These studies have been performed by utilizing the human androgen-responsive prostatic cancer LNCaP cell line. (1) By incubating LNCaP cells with different ¹⁴C-labeled androgenic precursors, it has been shown that all the major key enzymes involved in the metabolism of androgens (5-reductase, 17β-hydroxysteroid-oxidoreductase, 3- and 3β-hydroxysteroid-oxidoreductases) are present and active in these cells. In particular, the 5-reductase, which converts T and Δ₄ to DHT and 5-A respectively, seems to be more active when Δ₄ is the substrate, suggesting a preference for this precursor. (2) The hypothesis that LNCaP cells might produce LHRH (or a LHRH-like peptide) has been verified by RT-PCR, performed in the presence of a pair of specific oligonucleotide primers. A cDNA band of the expected size (228 bp), which specifically hybridized with a ³²P-labeled LHRH oligonucleotide probe, has been obtained in LNCaP cells. To clarify the possible role played by this factor in the regulation of tumor growth, LNCaP cells, cultured in steroid-free conditions, have been treated with a LHRH antagonist; the treatment resulted in a significant increase of cell proliferation. Taken together, these data indicate that a LHRH (or LHRH-like) growth modulatory system is expressed in LNCaP cells and plays an inhibitory role in the regulation of tumor cell proliferation. This system seems to be regulated in a negative way by steroids. Growth factors endowed with stimulatory activity, such as EGF and TGF, have also been shown to be produced by LNCaP cells. The present studies show that the immunoprecipitation of the EGF receptor with a specific monoclonal antibody (Ab225) reveals a protein band of the expected size (170 kDa) which is phosphorylated even in basal conditions. Moreover, the treatment of LNCaP cells, cultured in serum-free conditions, either with a monoclonal antibody against the EGF receptor, or with immunoneutralizing antibodies against EGF and TGF, results in a significant decrease of cell proliferation. These observations clearly confirm the expression, in prostatic tumor cells, of an EGF/TGF loop which exerts a stimulatory action on cell proliferation. T seems to exert a positive regulation on this loop, at least in terms of EGF receptor concentration.