25-Hydroxyvitamin D-1α-hydroxylase activity is diminished in human prostate cancer cells and is enhanced by gene transfer

The hormone 1α,25-dihydroxyvitamin D (1α,25(OH)₂D) inhibits growth and induces differentiation of prostate cells. The enzyme responsible for 1α,25(OH)₂D synthesis, 25-hydroxyvitamin D (25(OH)D)-1α-hydroxylase (1α-OHase), has been demonstrated in human prostate cells. We compared the levels of 1α-OHase activity in prostate cancer cell lines, LNCaP, DU145 and PC-3 and in primary cultures of normal, cancerous and benign prostatic hyperplasia (BPH) prostate cells. We observed a marked decrease in 1α-OHase activity in prostate cancer cells, including an undetectable level of activity in LNCaP cells. Transient or stable transfection of 1α-OHase cDNA into LNCaP cells increased 1α-OHase activity from undetectable to 4.95pmole/mg±0.69pmole/mg and 5.8pmole/mg±0.7pmole/mg protein per hour, respectively. In response to 25(OH)D, the prohormone of 1α,25(OH)₂D, the transfected LNCaP cells showed a significant inhibition of ³H-thymidine incorporation (37%±6% and 56%±4% at 10^-8M for transiently and stably transfected cells, respectively). These findings support an important autocrine role for 1α,25(OH)₂D in the prostate and suggest that the re-introduction of the 1α-OHase gene to prostate cancer cells, in conjunction with the systemic administration of 25(OH)D, constitutes an endocrine form of gene therapy that may be less toxic than the systemic administration of 1α,25(OH)₂D.