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In advanced prostate cancer, cellular changes occur leading to a transition from androgen-dependent to androgen-independent growth. During this transition, proliferation of androgendependent prostate cancer cells becomes more and more dependent on growth factors, like the epidermal growth factor (EGF). Endocytosis of growth factor receptors, one of the mechanisms that controls growth factor signalling, was observed to be markedly changed in advanced metastatic prostate cancer. Internalisation and signalling of EGF receptors was examined in different prostate cancer cell lines, in relation to the expression level of the endocytosis-related REPS2 gene. It was observed that a high level of REPS2 correlates with reduced EGF-internalisation. To investigate this more thoroughly, prostate cancer cells with inducible REPS2 expression were generated. Using these cells, it was found that REPS2-induction indeed results in reduction of EGF-internalisation. Furthermore, when EGF receptor signalling was evaluated, by examination of mRNA expression for several EGF-responsive genes (EGF receptor, EGR-1, Fos and Jun), it was observed that induced expression of REPS2 exerts an inhibiting effect on this signalling. From these experiments, it is concluded that increased REPS2 expression negatively affects EGF receptor internalisation and subsequent signalling. Therefore, decreased REPS2 expression during prostate cancer progression, observed in earlier work, may result in enhanced EGF receptor expression and signalling, which could add to the androgen-independent state of advanced prostate cancer. Key words: REPS2; prostate cancer; endocytosis; EGF signalling Treatment of prostate cancer involves surgery of the affected region of the gland and treatment of advanced prostate cancer focuses on inhibition of growth inducing signals, mainly by androgen ablation. However, given enough time, the tumour will switch from androgens to growth factors as primary regulators of proliferation. This transition leads to therapy-resistance of the cancer cells and eventually will result in recurrence of the disease. 1 Epidermal growth factor (EGF) and transforming growth factor alpha are potent mitogens involved in autocrine regulation of proliferation, angiogenesis and metastatic spread of advanced prostate cancer. 2 The EGF receptor can bind many ligands, among which EGF and TGF-␣. 3 Upon ligand-binding, the receptor dimerizes and is activated by autophosphorylation of tyrosine residues in the intracellular tail of the receptor. Subsequent activation of well-characterised signal transduction pathways (MAPK) results in proliferation and survival of tumour cells. 4,5 During prostate cancer progression, the EGF receptor expression level is regulated by transcriptional and posttranscriptional mechanisms, including upregulation of receptor mRNA and downregulation of EGF receptor protein by EGF. Intracellular trafficking of the EGF receptor was suggested to cause the overall degradation of the EGF receptor protein in prostate cancer cells. 6 This traffi...

Prostate cancer development often includes a shift from androgen-dependent to androgenindependent growth. It is hypothesized that, during this transition, growth factors like the epidermal growth factor (EGF) gain importance as activators of tumour cell proliferation. To study this, androgen-and EGF-regulation of growth and gene-expression was analysed in the androgendependent human prostate cancer cell line LNCaP-FGC (FGC) and its androgen-independent derivative line LNCaP-LNO (LNO). It was observed that androgen-dependent FGC cells require exposure to either androgens or EGF to proliferate. This is in contrast to androgen-independent LNO cells that showed significant proliferation in medium depleted of androgens and growth factors. Gene expression data were obtained for the androgen-dependent FGC and androgen-independent LNO cells cultured in the presence or absence of androgens (synthetic R1881) or EGF for different time periods. Expression profiling showed that many cell cycle genes, including a number of androgen-and EGF-regulated genes, are constitutively activated in androgen-independent LNO cells. Furthermore, the overlap between changes in gene expression activated by androgen and EGF receptor signalling pathways was found to be very high (75%). These results partly explain why androgen-independent LNO cells can proliferate in the absence of androgenic stimulation. However, possibly other, so far unknown, signal transduction pathways that induce and maintain proliferation, have also been activated.