Use of siRNA and Antisense Knockdown to Study Regulation of PKA by PKI

The protein kinase inhibitor (PKI) family consists of three proteins (PKIα, PKIβ, and PKIγ), all of which are potent and specific inhibitors of cAMP-dependent protein kinase (PKA). In response to a stimulus that increases the intracellular cAMP level, PKA-C is released from the regulatory subunit (PKA-R), translocates into the nucleus, and regulates gene expression by phosphorylating transcription factors such as CREB. PKI also translocates into the nucleus, inactivates PKA-C by binding to it, and then translocates PKA-C out of the nucleus. This translocation out of the nucleus is due to interactions between the nuclear export signal (NES) motif found in all three members of the PKI family and the leptomycin-sensitive nuclear export machinery. However, translocation out of the nucleus is not necessary for PKI to terminate PKA-C signaling in the nucleus, as binding to PKI is sufficient. It is important to note that PKI may also have important functions in regulating PKA-C activity in the cytoplasm of cells. Short-term knockdown of PKI, or other proteins, can be achieved by either siRNA or antisense approaches, either of which can be applied to cells as oligonucleotides or by expression of gene constructs. Activation of Gs-coupled receptors also induces a transient anti-apoptotic effect in osteoblasts that depends on PKA signaling. It demonstrates that endogenous levels of PKI are sufficient to terminate physiological effects of PKA-C in the nucleus following stimulation of cAMP levels by Gs-coupled receptors.