Thyroid hormone modulation of transformation induced by Kirsten murine sarcoma virus

We have investigated the effect of triiodothyronine (T3) on the transformation of normal rat kidney (NRK) cells by the Kirsten strain of murine sarcoma virus (Ki-MSV). When NRK cells were grown and infected with Ki-MSV in medium lacking T3, the yield of transformed foci was about one-half that observed in the cultures supplemented with T3. Individual foci appeared somewhat later in cells grown out in medium devoid of T3. The yield of Ki-MSV released from transformed NRK cells was lower when these cells were maintained in T3-depleted medium. The results cannot be attributed to cell growth modification by T3. Normal and Ki-MSV-transformed NRK cells grew equally well in mono-layer culture in medium containing or lacking T3. Selective maintenance and removal of T3 during various phases of the transformation process indicated that T3 exerted its maximum effect on transformation rates when added to the medium 24 h prior to virus infection. T3 was less effective in modulating transformation when added simultaneously with virus infection and was ineffective if added 24 h after virus infection. The results indicate that thyroid hormone is a required factor for optimal transformation by Ki-MSV and that the hormone exerts its effects during the early phase of Ki-MSV-induced transformation.