Purpose: Previous studies identified rod photoreceptor cyclic GMP phosphodiesterase (PDE6) transcripts in the human Y79 retinoblastoma cell line. To assess the potential to utilize this cell line for structure/function studies of PDE6, we analyzed 3',5' cyclic nucleotide phosphodiesterase activity focusing on expression of PDE6.
Methods: DEAE-chromatography was used to fractionate PDE activity from Y79 cell homogenates. PCR was performed on cDNA generated from Y79 cells and retina with PDE isoform specific primers. Western blots were performed with antibodies to PDE1, PDE4, or rod PDE6. DNA sequencing and protein truncation tests were performed with plasmids containing the entire coding region of Y79 rod PDE6 transcripts. Proteasome mediated degradation of PDE6 subunits was analyzed with a pathway specific inhibitor. Polysome isolation was performed by fractionation on sucrose gradients followed by RT-PCR for the PDE6 transcripts.
Results: Of three peaks of PDE activity, peaks 1 and 2 were activated by Ca2+/calmodulin, inhibited by dipyridamole and zaprinast, and were reactive with a PDE1 antibody. Peak 3 hydrolyzed only cAMP and was rolipram sensitive, indicative of PDE4. Transcripts for rod and cone PDE6 isoforms were detected in Y79 total RNA, however PDE6 antibodies recognized only a single 99 kDa polypeptide from immunoprecipitated 35S labeled Y79 extracts. DNA sequencing of PDE6 alpha, beta, gamma, and PDE6 associated delta-subunit cDNA revealed some polymorphism, but no apparent mutations. Each of the PDE6 transcripts could be translated into protein of the correct length. The concentration of cGMP in the cells was greatly reduced in comparison to that reported in the photoreceptor cell. Addition of cyclic nucleotide analogues, zinc, or butyrate did not enhance the expression of PDE6. Transduction into Y79 cells of adenovirus expressing PDE6 subunits failed to produce functional enzyme
Conclusions: PDE1 and PDE4 enzyme activities predominate in Y79 cells. Despite the presence of PDE6 transcripts and the ability to translate each into protein in vitro, a functional PDE6 enzyme could not be detected. Attempts to enhance expression with cell culture or with introduction of virus expressing PDE6 were not successful. The results indicate that expression of a fully active stable PDE6 enzyme requires other post-transcriptional events that do not occur or are inhibited in Y79 cells.