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Recent studies from our laboratory have indicated that the metabolism of vitamin A (retinol) to retinyl esters, carried out primarily by the enzyme lecithin:retinol acyltransferase (LRAT), is greatly reduced in human carcinoma cell lines of the oral cavity, skin, breast, and kidney as compared with their normal epithelial counterparts. These studies suggest that human carcinoma cells are retinoid-deficient relative to normal epithelial cells. In this study, we examined the metabolism of [(3)H]retinol and [(3)H]retinoic acid (RA) in human prostate cancer lines and in primary cultures of human prostate epithelial cells. Normal cells esterified all of the [(3)H]retinol added to the cultures. In contrast, all seven prostate cancer cell lines and four primary cultures derived from prostatic adenocarcinomas metabolized only trace amounts of [(3)H]retinol to [(3)H]retinyl esters. Correlated with this relative lack of esterification of [(3)H]retinol by the cancer cells was loss of expression of LRAT protein, whereas normal cells expressed abundant levels of LRAT protein by Western analysis. The metabolism of [(3)H]RA was also examined in these prostatic cells. Two of the prostate cancer tumor lines, DU 145 and PJ-1, exhibited rapid metabolism of [(3)H]RA; in contrast, the other tumor lines or primary cultures metabolized [(3)H]RA at a much slower rate. We also found that the immortalization of normal human prostatic epithelial cells by SV40 T antigen led to a reduction in LRAT protein expression and esterification of [(3)H]retinol. Further transformation to tumorigenicity with the ras oncogene resulted in loss of detectable LRAT expression. Finally, we analyzed LRAT protein expression in tissue sections from six prostatectomy specimens by immunohistochemistry. LRAT protein was predominantly expressed in the basal cells of normal prostatic epithelium, whereas its expression was lost in prostate cancer. Collectively, these data implicate aberrant retinoid metabolism in the process of prostatic carcinogenesis.

Vitamin A is required for a number of developmental processes and for the homeostatic maintenance of several adult differentiated tissues and organs. In human neuroblastoma (NB) cells as well as some other tumor types, pharmacological doses of retinoids are able to control growth and induce differentiation in vitro and in vivo. In a search for new genes that are regulated by retinoids and that contribute to the biological effects retinoids have on NB cells, we have isolated five differentially expressed transcripts. Here we report on the characterization of one of them (DD83.1) in NB cell lines. DD83.1 is identical to the human retSDR1, a short chain dehydrogenase/reductase that is thought to regenerate retinol from retinal in the visual cycle. Its expression is strongly, but differently, regulated by retinoids in NB cell lines, and it is widely expressed in human tissues, which suggests that it is involved in a more general retinol metabolic pathway. Both the retinoic acid-dependent and the exogenous expression of retSDR1 in SK-N-AS cells induce the accumulation of retinyl esters, which indicates that it is involved in generating storage forms of retinol in tissues exposed to physiological retinol concentrations. We also show that the human retSDR1 gene, which maps on chromosome 1p36.1, is contained in the DNA fragment deleted in many NB cell lines bearing MYCN amplification but is conserved in a cell line with a small 1p deletion and normal MYCN. Our observations suggest that retSDR1 is a novel regulator of vitamin A metabolism and that its frequent deletion in NB cells bearing MYCN amplification could compromise the sensitivity of those cells to retinol, thereby contributing to cancer development and progression.