Regulation of MAP kinase activity by peptide receptor signalling pathway: paradigms of multiplicity

G protein-coupled receptors (GPCRs) can stimulate the mitogen-activated protein kinase (MAPK) cascade and thereby induce cellular proliferation like receptor tyrosine kinases (RTKs). Work over the past 5 years has established several models which reduce the links of G(i)-, G(q)-, and G(s)-coupled receptors to MAPK on few principle pathways. They include (i) Ras-dependent activation of MAPK via transactivation of RTKs such as the epidermal growth factor receptor (EGFR), (ii) Ras-independent MAPK activation via protein kinase C (PKC) that converges with the RTK signalling at the level of Raf, and (iii) activation as well as inactivation of MAPK via the cAMP/protein kinase A (PKA) pathway in dependency on the type of Raf. Most of these generalizing hypotheses are founded on experimental data obtained from expression studies and using a limited set of individual receptors. This review will compare these models with pathways to MAPK found for a great variety of peptide hormone and neuropeptide receptor subtypes in various cells. It becomes evident that under endogenous conditions, the transactivation pathway is less dominant as postulated, whereas pathways involving isoforms of PKC and, especially, phosphoinositide 3-kinase (PI-3K) appear to play a more important role as assumed so far. Highly cell-specific and unusual connections of signalling proteins towards MAPK, in particular tumour cells, might provide points of attacks for new therapeutic concepts.