Signal Transduction

only induces phosphorylation to activate the CREB, but also results in the accumulation of CREB-specific mRNA.20 As with other hormones of pituitary origin, FSH is released in a pulsatile manner under the command of the hypothalamus. This observation therefore suggests that the resulting pulses of cAMP could prime the cells to become more sensitive to subsequent stimulation by the hormone.

Attenuation of the cAMP response elements by dephosphorylation

Over the 15–20 min following hormonal activation, the catalytic units of PKA diffuse into the nucleus to cause phosphorylation of CREB21 and the initiation of transcription. Then, over 4–6 h (the attenuation phase), transcription of the target genes gradually declines. This is probably due to dephosphorylation of CREB since it can be prolonged by application of phosphatase inhibitors (such as okadaic acid22). Furthermore, fibroblasts that over-express phosphatase inhibitor-1 (PP1) manifest an enhanced transcriptional response to CRE.23

In hepatic cells, manipulation of the activity of the levels of PP2A causes dephosphorylation of CREB.24 It appears that the magnitude of CRE-induced responses is regulated, at least in part, through dephosphorylation, but that different phosphatases are involved in different cells.

As mentioned above, specificity of PP1 is determined by its association with regulatory subunits, which target it to particular cellular locations. In the nucleus PP1 is attached to the chromatin by its association with NIPP, the endogenous nuclear inhibitor of PP1.25 Phosphorylation of NIPP

by PKA drastically reduces its affinity for PP1 and so cAMP may act to downregulate its own signals through the activation of the phosphatase (see Chapter 21).

As will become apparent in later chapters, the binding of a dimerized transcription factor complex to a palindromic enhancer element on DNA is a recurring feature in the regulation of specific gene expression.

Protein kinase A and the activation of ERK

Most of the actions of the 2-adrenergic receptor are transduced through Gs and mediated by phosphorylation catalysed by PKA. However, following phosphorylation of the receptor by PKA (heterologous desensitization: see Figure 4.14, page 100), its specificity alters so that it shifts its attention from Gs to Gi. The mechanism that uncouples the receptor from its normal transducer also enables it to couple with Gi. This opens up a whole new range of possibilities, not least because of the much greater quantities of Gi proteins expressed in most cells, and hence the much greater availability of

250