| Abstract: | A cAMP-resistant mutant (Kin-8) isolated from Y1 mouse adrenocortical tumor cells harbors a specific lesion in the regulatory subunit of the type 1 cAMP-dependent protein kinase. This mutant also is resistant to the effects of corticotropin and cAMP on steroidogenesis, growth and morphology, suggesting an obligatory role for the protein kinase in regulation of adrenocortical functions. In this study, the cAMP-resistant phenotype of the Kin-8 mutant was reverted by transformation with DNA from cAMP-responsive Y1 cells, and the biochemical basis of the transformation was explored. Initially, Y1 mouse adrenocortical tumor cells were evaluated for their competence as recipients in DNA-mediated transformation experiments, by measuring their ability to incorporate and express a bacterial gene (neo) encoding resistance to neomycin. Y1 cells were transfected with the plasmid pSV2-neo (an SV40-neo hybrid vector designed for expression in animal cells) and screened for resistance to the neomycin analog, G418. Neomycin-resistant transformants were recovered from Y1 cells at a frequency of approximately one per 10(3) cells per 10 micrograms of DNA, and had specific neo sequences integrated into their high molecular weight (mw) DNA. The Y1 mutant, Kin-8, then was transformed with pSV2-neo DNA plus high mw DNA prepared from cAMP-responsive Y1 cells. Cells competent for transformation were recovered by selective growth in the neomycin analog G418, and these transformants were screened for recovery of morphological responses to cAMP. Several colonies capable of rounding up in the presence of cAMP were recovered after transformation with DNA from Y1 cells. These transformants also recovered the ability to round up in the presence of corticotropin, and were able to respond to both corticotropin and cAMP with increased steroidogenesis. Transformants generated from either Y1 or Kin-8 cells were unstable. Y1 cells lost resistance to neomycin when grown in the absence of G418 at a frequency of 4% per generation. Similarly, Kin-8 transformants lost their sensitivity to cAMP in subsequent culture passages. In some of the cAMP-responsive transformants, cAMP-dependent protein kinase activity was recovered and approached the activity seen in cAMP-responsive Y1 cells. The recovery of a normal protein kinase by transformation appeared to have been sufficient to reverse the cAMP-resistant phenotype of Kin-8 cells. In other cAMP-responsive transformants, protein kinase activity was not appreciably affected by cAMP.(ABSTRACT TRUNCATED AT 400 WORDS) |
