Overexpression of RII beta regulatory subunit of protein kinase A in human colon carcinoma cell induces growth arrest and phenotypic changes that are abolished by site-directed mutation of RII beta

Ala99ser mutation in RI alpha regulatory subunit of protein kinase A causes reduced kinase activation by cAMP and arrest of hormone-dependent breast cancer cell growth

Expression of the RIalpha regulatory subunit of protein kinase A type I is increased in human cancer cell lines, in primary tumors, in cells after transformation, and in cells upon stimulation of growth. Ala99 (the pseudophosphorylation site) of human RIalpha was replaced with Ser (RIalpha-p) for the structure-function analysis of RIalpha. MCF-7 hormone-dependent breast cancer cells were transfected with an expression vector for the wild-type RIalpha or mutant RIalpha-p. Overexpression of RIalpha-P resulted in suppression of protein kinase A type II, the isozyme of type I kinase, production of kinase exhibiting reduced cAMP activation, and inhibition of cell growth showing an increase in G0/G1 phase of the cell cycle and apoptosis. The wild-type RIalpha overexpression had no effect on protein kinase A isozyme distribution or cell growth. Overexpression of protein kinase A type II regulatory subunit, RIIbeta, suppressed RIalpha and protein kinase A type I and inhibited cell growth. These results show that the growth of hormone-dependent breast cancer cells is dependent on the functional protein kinase A type I.

Antisense oligonucleotide inhibition of serine/threonine kinases: an innovative approach to cancer treatment

The identification of genes that confer a growth advantage on neoplastic cells and the understanding of the genetic mechanism(s) responsible for their activation have made possible a direct genetic approach to cancer treatment using nucleic acid therapeutics. Moreover, the ability to block the expression of individual genes that promote carcinogenesis provides a powerful tool to explore the molecular basis of normal growth regulation, as well as the opportunity for therapeutic intervention. One technique for turning off a single activated gene is the use of antisense oligodeoxynucleotides and their analogs for inhibition of gene expression. The serine/threonine kinases are involved in mediating intracellular responses to external signals, such as growth factors, hormones, and neurotransmitters, and are involved in cell proliferation and oncogenesis. Described herein are recent studies supporting the potential use of oligonucleotides targeting these kinases as chemotherapeutic agents for cancer treatment. The serine/threonine kinases included here are protein kinase A, protein kinase C, and c-raf-1 kinase.

Dual blockade of cyclic AMP response element- (CRE) and AP-1-directed transcription by CRE-transcription factor decoy oligonucleotide. gene-specific inhibition of tumor growth

Alteration of gene transcription by inhibition of specific transcriptional regulatory proteins has important therapeutic potential. Synthetic double-stranded phosphorothioate oligonucleotides with high affinity for a target transcription factor can be introduced into cells as decoy cis-elements to bind the factors and alter gene expression. The CRE (cyclic AMP response element)-transcription factor complex is a pleiotropic activator that participates in the induction of a wide variety of cellular and viral genes. Because the CRE cis-element, TGACGTCA, is palindromic, a synthetic single-stranded oligonucleotide composed of the CRE sequence self-hybridizes to form a duplex/hairpin. Herein we report that the CRE-palindromic oligonucleotide can penetrate into cells, compete with CRE enhancers for binding transcription factors, and specifically interfere with CRE- and AP-1-directed transcription in vivo. These oligonucleotides restrained tumor cell proliferation, without affecting the growth of noncancerous cells. This decoy oligonucleotide approach offers great promise as a tool for defining cellular regulatory processes and treating cancer and other diseases.

Cyclic adenosine 3':5'-monophosphate-dependent protein kinase on the external surface of LS-174T human colon carcinoma cells

The analysis of purified plasma membranes and the surface of intact cells revealed the presence of cyclic adenosine 3':5'-monophosphate-(cAMP) dependent protein kinase (PKA) on the external surface of LS-174T human colon carcinoma cells. Photoaffinity labeling of intact cells at confluence with 8-azido-[32P]cAMP identified the cAMP-binding proteins on the surface. Immunoprecipitation identified the photoaffinity-labeled cAMP-binding proteins as the RIIalpha regulatory subunit of PKA. During the logarithmic stage of growth, both the RIalpha and RIIalpha subunits of PKA were localized on the cell surface. Intact LS-174T cells catalyzed the phosphorylation of Kemptide in a cAMP-dependent manner; upon substitution of cAMP in the medium with 8-chloroadenosine, which did not compete with cAMP for the binding on intact cells, the ecto-PKA was no longer activated. The specific inhibitory protein for PKA, PKI, abolished the stimulation of phosphorylation by cAMP. Forskolin, which elevates intracellular levels of cAMP, activated ecto-PKA. Moreover, probenecid, which blocks the export of cAMP, inhibited the forskolin-mediated activation of ecto-PKA. These results demonstrate that LS-174T colon carcinoma cells possess an ecto-PKA on the external surface. This ecto-PKA is similar, if not identical, to the soluble intracellular PKA.

The RIIbeta regulatory subunit of protein kinase A binds to cAMP response element: an alternative cAMP signaling pathway

cAMP, through the activation of cAMP-dependent protein kinase (PKA), is involved in transcriptional regulation. In eukaryotic cells, cAMP is not considered to alter the binding affinity of CREB/ATF to cAMP-responsive element (CRE) but to induce serine phosphorylation and consequent increase in transcriptional activity. In contrast, in prokaryotic cells, cAMP enhances the DNA binding of the catabolite repressor protein to regulate the transcription of several operons. The structural similarity of the cAMP binding sites in catabolite repressor protein and regulatory subunit of PKA type II (RII) suggested the possibility of a similar role for RII in eukaryotic gene regulation. Herein we report that RIIbeta subunit of PKA is a transcription factor capable of interacting physically and functionally with a CRE. In contrast to CREB/ATF, the binding of RIIbeta to a CRE was enhanced by cAMP, and in addition, RIIbeta exhibited transcriptional activity as a Gal4-RIIbeta fusion protein. These experiments identify RIIbeta as a component of an alternative pathway for regulation of CRE-directed transcription in eukaryotic cells.

Activation of cyclic AMP-dependent kinase is required but may not be sufficient to mimic cyclic AMP-dependent DNA synthesis and thyroglobulin expression in dog thyroid cells

Thyrotropin (TSH), via a cyclic AMP (cAMP)-dependent pathway, induces cytoplasmic retractions, proliferation, and differentiation expression in dog thyroid cells. The role of cAMP-dependent protein kinase (PKA) in the induction of these events was assessed by microinjection into living cells. Microinjection of the heat-stable inhibitor of PKA (PKI) inhibited the effects of TSH, demonstrating that activation of PKA was required in this process. Overexpression of the catalytic (C) subunit of PKA brought about by microinjection of the expression plasmid pC alpha ev or of purified C subunit itself was sufficient to mimic the cAMP-dependent cytoplasmic changes and thyroperoxidase mRNA expression but not to induce DNA synthesis and thyroglobulin (Tg) expression. The cAMP-dependent morphological effect was not observed when C subunit was coinjected with the regulatory subunit (RI or RII subunit) of PKA. To mimic the cAMP-induced PKA dissociation into free C and R subunits, the C subunit was coinjected with the regulation-deficient truncated RI subunit (RIdelta1-95) or with wild-type RI or native RII subunits, followed by incubation with TSH at a concentration too low to stimulate the cAMP-dependent events by itself. Although the cAMP-dependent morphology changes were still observed, neither DNA synthesis nor Tg expression was stimulated in these cells. Taken together, these data suggest that in addition to PKA activation, another cAMP-dependent mechanism could exist and play an important role in the transduction of the cAMP signal in thyroid cells.

Type II protein kinase A up-regulation is sufficient to induce growth inhibition in SK-N-SH human neuroblastoma cells

We have previously reported that overexpression of RII beta subunit of protein kinase A, which markedly reduces RI alpha protein, induces growth inhibition in SK-N-SH human neuroblastoma cells. To determine whether the reduction of RI alpha or protein kinase A isozyme type I is essential in the growth inhibition of SK-N-SH cells, we overexpressed RI alpha in sense and antisense orientation. Type I protein kinase A activity was increased in the RI alpha-overexpressing cells and was decreased in the RI alpha antisense-expressing cells. However, the changes in type I protein kinase A activities did not affect cell growth. Overexpression of RII beta or C alpha increased type II protein kinase A and inhibited cell growth in both cell lines regardless of the type I protein kinase A level. These results indicate that type II protein kinase A is the main effector in the cAMP-mediated growth regulation of SK-N-SH human neuroblastoma cells.