Intracellular localization of the PDE4A cAMP-specific phosphodiesterase splice variant RD1 (RNPDE4A1A) in stably transfected human thyroid carcinoma FTC cell lines

Abstract

Cells of two human follicular thyroid carcinoma cell lines (FTC133, FTC236) were stably transfected with a cDNA encoding the PDE4A cAMP-specific phosphodiesterase (PDE) splice variant RD1 (RNPDE4A1A) so as to generate the cloned cell lines, FTC133A and FTC236A. This allowed the expression of a novel rolipram-inhibited cAMP-specific PDE activity in these cells. Unlike the parent cell lines in which Ca2+/calmodulin caused a profound activation (approx. 3-4-fold) of homogenate PDE activity, no such stimulation was evident in the RD1-expressing cell lines, indicating loss of PDE1 activity. Reverse transcriptase-PCR analysis indicated that this was due to the down-regulation of the PDE1C isoform. The novel PDE4 activity in transfected cells was located exclusively in the membrane fraction, as was immunoreactive RD1. Low concentrations of the detergent Triton X-100, but not high NaCl concentrations, allowed RD1 to be solubilized. Laser scanning confocal immunofluorescence analyses identified RD1 immunoreactivity in a discrete perinuclear region of these RD1-expressing transfected cell lines. A similar pattern of labelling was observed using the antiserum Tex1, which specifically identified the Golgi apparatus. Treatment of FTC133A cells with the Golgi-perturbing agents monensin and brefeldin A led to a similar redistribution of immunoreactive species detected using both the Tex1 and anti-RD1 antisera. It is suggested that the PDE4A splice variant RD1 contains a membrane-association signal which allows the targeted expression of RD1 within the Golgi complex of these human follicular thyroid carcinoma cell lines.