The intracrine hypothesis and intracellular peptide hormone action

A proposal regarding the biology of memory: participation of intracrine peptide networks

Although progress has recently been made in defining its neurobiological underpinnings, memory remains an incompletely understood process. Here it is proposed that intracrine peptide action plays a role in memory and therefore in consciousness. Intracrine function is the intracellular action of a peptide either in its cell of synthesis or in target cells following internalization. Intracrine functionality has been associated with growth factors, transcription factors, and enzymes. Some intracrines have been shown to modify their own synthesis, the synthesis of components of their signaling cascades, and/or the synthesis of other intracrines, making it likely that these factors participate in peptinergic regulatory loops/networks. Moreover, some intracrines can affect cellular ion flows over short time periods, while stimulating protein synthesis and growth over longer periods. Intracrines have been reported in the brain (for example, angiotensin, brain derived neurotrophic factor, reelin, and fibroblast growth factor 2) and it is here suggested that intracrine expression/action occurs commonly in the nervous system and, in particular, in the cerebral cortex. This intracrine action leads to the formation of long-lived intra- and intercellular intracrine feed back loops. The intracrine networks thus formed participate in producing both short and long term synaptic potentiation; the former effect is primarily related to intracrine effects on ion flows, the latter to intracrine directed protein synthesis. In addition, intracrine networks provide a previously unappreciated dynamic substrate for the integration of information and ultimately for the formation of the associations that underlay consciousness.

Tissue renin angiotensin systems

The RAAS is a powerful regulator of vascular tone and intravascular volume and of tissue architecture and a variety of other functions. The recent appreciation of the immunoregulatory role of angiotensin II and its possible involvement in the genesis of atherosclerosis and in plaque rupture all speak to the wide-ranging physiologic and pathophysiologic activities of the peptide. So do its actions in fat cell differentiation and in neuromodulation. The system exists in the circulation, and RAASs, whole or partial, exist in many tissues. These systems are regulated at many levels ranging from the synthesis of renin to the dimerization of angiotensin receptors. Regulation occurs in multiple tissues and, as a result, tissue concentrations of angiotensin II and the concentration of other RAS components and their active metabolites can vary independently of the circulating system in these tissues. An RAS seems also to function within certain cells. Therapeutic interventions involving ACEIs and ARBs seem likely to provide benefit at least in part through the interruption of local systems. It is to be expected that with enhanced understanding of the biology of the multiple RASs, new suggestions for therapeutic interventions will be forthcoming.

Intracellular renin and the nature of intracrine enzymes

Recently, the binding of renin and prorenin to cellular receptors with the subsequent generation of second messengers and the production of physiological effects has been demonstrated. In addition, the internalization of prorenin by target cells has been associated with increased cellular synthesis of angiotensin and cardiac pathology. Also, a renin transcript lacking the sequences encoding a secretory signal has been reported, and this transcript appears to produce a renin that acts in the cell that synthesized it. Some years ago, we coined the term intracrine for a peptide hormone or factor that acts in the intracellular space either after internalization or retention in its cell of synthesis. Thus defined, a wide variety of peptides display intracrine functionality, including hormones, growth factors, transcription factors, and enzymes. For example, considerable evidence indicates that angiotensin II is an intracrine. Also, general principles of intracrine functionality have been developed. Thus, recent evidence demonstrates that the prorenin/renin molecule is an intracrine enzyme. Here, the actions of intracrine enzymes (angiogenin, phosphoglucose isomerase, phospholipase A2, granzyme A and B, thioredoxin, platelet-derived endothelial growth factor, and serine protease inhibitors) are reviewed. The relation of prorenin/renin to other intracrine enzymes, and to intracrines in general, is discussed.