You are studying protein kinase A (PKA)-mediated cell signaling in a mammalian cell line, in which PKA levels are sensitive to two different hormones, XGF and YGF. Examine the results shown below and suggest a mechanism for how XGF and YGF affect PKA.

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Key: A: Controls with no XGFor YGF treatment B: Treatment of cells with XGF only C: Treatment of cells with YGF only D: Treatment with both XGF and YGF PKA Activity A B CD