Low‐level β1 protein kinase C expression in cloned rat embryo fibroblast cells enhances transformation induced by the adenovirus type 5 E1A gene

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Abstract
Expression of the E1A gene of adenovirus type 5 (Ad5) in a cloned rat embryo fibroblast (CREF) cell line results in morphological transformation. The efficiency of E1A‐mediated transformation of CREF cells is increased if a wild‐type Ad5 E1A gene is cotransfected with a rat β1 protein kinase C (β1 PKC) gene. A direct demonstration of complementation between a functional‐transforming Ad5 E1A gene and β1 in inducing transformation was demonstrated using Ad5 E1A cold‐sensitive mutant (E1ACS) genes. The E1Acs gene enhanced transformation only at the transformation‐permissive temperature of 37° C and not at the nonpermissive transforming temperature of 32° C. CREF cells constitutively expressing low levels of β1, PKC mRNA were transformed at a higher frequency than parental CREF cells after transfection with an Ad5 E1A gene or infection with wild‐type Ad5 or the Ad5 host‐range cold‐sensitive mutant H5hr1. There was no enhancement of transformation in low‐level β1 PKC‐expressing CREF cells when cultures were grown continuously in the presence of the PKC‐inhibitor 1‐(5‐isoquinolynsulfonyl)‐2‐methylpiperazine dihydrochloride. Transfected CREF cells expressing low levels of β1 PKC mRNA displayed CREF‐like morphology and did not form colonies when grown in agar. In contrast, retroviral vector‐transformed CREF cells expressing high levels of β1 PKC mRNA and β1 PKC enzyme activity were morphologically transformed and grew efficiently in agar. These findings indicate that the β1, PKC gene, when expressed at low levels, can cooperate with the Ad5 E1A gene in the initiation of viral oncogene‐mediated transformation.