Involvement of vimentin in the reverse transformation

An organized cytoskeleton is required for the cAMP-induced reverse transformation reaction in CHO-KI cells. In the course of the reaction a considerable fraction of the genome changes its nuclease sensitivity. The current paper presents the following evidence that cAMP-induced phospho- rylation of vimentin is an early step in this reaction complex. (i) Vimentin is only slightly phosphorylated in transformed CHO-KI cells but is heavily phosphorylated in normal fibro- blasts. (ii) cAMP addition almost triples the vimentin phos- phorylation of CHO-KI cells but does not change that of normal cells. (iii) Vimentin phosphorylation is one of the earliest phenomena to occur after addition of cAMP to CHO- KI cells, preceding the cell-stretching reaction and other manifestations of reverse transformation. (iv) Indirect immu- nofluorescence experiments demonstrate that vimentin ap- pears as a condensed mass in transformed CHO-KI cells but cAMP addition restores the filamentous structure character- istic of the normal fibroblast. (v) Other transformed cells unresponsive to reverse transformation by cAMP failed to demonstrate increased phosphorylation of vimentin on treat- ment with cAMP. These results support the proposed scheme that phosphorylation of cytoskeletal elements initiates a large- scale genetic regulatory action in which a substantial change in the spectrum of genome exposure and sequestration occurs. A function for intermediate filaments in reverse transformation is implied.