Castration promotes radiosensitivity by direct regulation of DNA repair in prostate cancer

Al-Ubaidi F.1, Schultz N.1, Hamberg H.2, Granfors T.3, Helleday T.1

1Karolinska Institute, Dept. of Medical Biochemistry and Biophysics, Stockholm, 2Västmanland Hospital Västerås, Dept. of Pathology, Västerås, 3Västmanland Hospital Västerås, Dept. of Urology, Västerås

Introduction & Objectives

Neoadjuvant castration promotes radiosensitivity in prostate cancer, yet the mechanism is still not well defined. We hypothesized that neoadjuvant castration by GnRH analogue impairs DNA repair of double-strand breaks (DSBs) mediated by classical non-homologous end joining (NHEJ), and hence enhanced radiosensitivity.

Material & Methods

Forty-eight patients with clinically localized or locally advanced prostate cancer, eligible for curative radiotherapy (RT), were enrolled and divided in two arms. Patients in arm 1 received neoadjuvant GnRH analogue leupropelin followed by external beam radiotherapy (EBRT) to 78 Gy to the prostate. On the contrary, patients in arm 2 received first 2 Gy/fraction for 5 days followed by neoadjuvant GnRH analogue and then EBRT to a total of 82 Gy. Before treatment, prostatic needle core biopsy specimens were taken. In arm 1, a second round of prostatic needle core biopsy specimens were taken before EBRT, eight weeks after administration of neoadjuvant GnRH analogue and a third round of prostatic needle core biopsy specimens about three hours after the fifth dose EBRT. In arm 2, a second round of prostatic needle core biopsy specimens were taken about three hours after the fifth dose EBRT, before hormone treatment was started, and a third round of prostatic needle core biopsy specimens took place after eight weeks of neoadjuvant GnRH analogue administration. The levels of Ku70, DNA-dependent protein kinase catalytic subunit (DNA-PKcs) and androgen receptor (AR) were determined by immunofluorescence in verified cancer tissue.

Results

In arm 1, we found a significant decrease in nuclear AR (p <0.001, p <0.001), nuclear Ku70 protein (p <0.001, p <0.001) and DNA-PKcs protein (p = 0.01, p = 0.004) after castration, respectively, after combined castration and radiotherapy.
In arm 2, we saw no decrease in nuclear AR (p = 0.5) after radiotherapy alone, but an increase of nuclear Ku70 protein (p = 0.01), and DNA PKcs protein in cell nuclei (p = 0, 01). The changes in AR expression correlated significantly with changes in Ku70 and DNA-PKcs after castration, suggesting that AR activity regulates the levels of these proteins in prostate cancer tissue.

Conclusions

We show for the first time in vivo material that AR activity is directly involved in DNA repair. Ku70 and DNA-PKcs are essential proteins for NHEJ repair of DSBs. Since downregulation of these proteins impairs NHEJ mediated DNA repair machinery, proves our studies that neoadjuvant castration improve radiosensitivity in prostate cancer cells.