In the context of gene regulation: transactivation is the increased rate of gene expression triggered either by biological processes or by artificial means, through the expression of an intermediate transactivator protein.
In the context of receptor signaling, transactivation occurs when one or more receptors activate yet another;[1][2] receptor transactivation may result from the crosstalk of signaling cascades or the activation of G protein–coupled receptor hetero-oligomer subunits, among other mechanisms.[1]
For instance,there are indications that both D1 and D2 receptors can trans-activate the brain-derived neurotrophic factor (BDNF) receptor in neurons (Swift et al., 2011). These two dopamine receptors can also regulate calcium channels through a direct protein–protein interaction in vivo (Kisilevsky and Zamponi, 2008; Kisilevsky et al., 2008). Direct interaction of D1 and D2 receptors and Na+-K+-ATPase has also been demonstrated (Hazelwood et al., 2008; Blom et al., 2012).