José Lozano

The product of par-4, a gene induced during apoptosis, interacts selectively with the atypical isoforms of protein kinase C.

The atypical PKCs are involved in a number of important cellular functions, including cell proliferation. We report here that the product of the par-4 gene specifically interacts with the regulatory domains of zeta PKC and lambda/LPKC, which dramatically inhibits their enzymatic activity. This is particularly challenging, because expression of par-4 has been shown to correlate with growth inhibition and apoptosis. Results are shown here demonstrating that the expression of par-4 in NIH-3T3 cells induces morphological changes typical of apoptosis, which are abrogated by cotransfection of either wild-type zeta PKC or lambda/LPKC, but not by their respective kinase-inactive mutants. These findings support a role for the atypical PKC subspecies in the control of cell growth and survival.

Protein kinase C ζ isoform is critical for mitogenic signal transduction

The requirement of protein kinase C zeta (zeta PKC) for maturation of X. laevis oocytes in response to insulin, p21ras, and phosphatidylcholine-hydrolyzing phospholipase C has recently been shown. Here we present experimental evidence demonstrating that activation of zeta PKC is not only necessary but also sufficient by itself to activate maturation in oocytes and to produce deregulation of growth control in mouse fibroblasts. Furthermore, by using a dominant kinase-defective mutant of zeta PKC, we confirm that this kinase is required for mitogenic activation in oocytes and fibroblasts. These results permit us to propose zeta PKC as a critical step downstream of p21ras in mitogenic signal transduction.

Evidence for a role of MEK and MAPK during signal transduction by protein kinase C zeta.

Protein kinase C zeta (zeta PKC) is critically involved in the control of a number of cell functions, including proliferation and nuclear factor kappa B (NF‐kappa B) activation. Previous studies indicate that zeta PKC is an important step downstream of Ras in the mitogenic cascade. The stimulation of Ras initiates a kinase cascade that culminates in the activation of MAP kinase (MAPK), which is required for cell growth. MAPK is activated by phosphorylation by another kinase named MAPK kinase (MEK), which is the substrate of a number of Ras‐activated serine/threonine kinases such as c‐Raf‐1 and B‐Raf. We show here that MAPK and MEK are activated in vivo by an active mutant of zeta PKC, and that a kinase‐defective dominant negative mutant of zeta PKC dramatically impairs the activation of both MEK and MAPK by serum and tumour necrosis factor (TNF alpha). The stimulation of other kinases, such as stress‐activated protein kinase (SAPK) or p70S6K, is shown here to be independent of zeta PKC. The importance of MEK/MAPK in the signalling mechanisms activated by zeta PKC was addressed by using the activation of a kappa B‐dependent promoter as a biological read‐out of zeta PKC.

zeta PKC induces phosphorylation and inactivation of I kappa B-alpha in vitro.

The zeta isotype of protein kinase C (zeta PKC), a distinct PKC unable to bind phorbol esters, is required during NF‐kappa B activation as well as in mitogenic signalling in Xenopus oocytes and mammalian cells. To investigate the mechanism(s) for control of cellular functions by zeta PKC, this enzyme was expressed in Escherichia coli as a fusion protein with maltose binding protein (MBP), to allow immobilization on amylose beads to study signalling proteins in cell extracts that might form complex(es) with zeta PKC. The following evidence for interaction with the NF‐kappa B/I kappa B pathway was obtained. MBP‐zeta PKC, but not MBP, bound and activated a potentially novel I kappa B kinase of approximately 50 kDa molecular weight able to regulate I kappa B‐alpha function. Activation of the I kappa B kinase was dependent on zeta PKC enzymatic activity and ATP, suggesting that zeta PKC controls, directly or indirectly, the activity of a functionally significant I kappa B kinase. Importantly, zeta PKC immunoprecipitates from TNF‐alpha‐stimulated NIH‐3T3 fibroblasts displayed a higher I kappa B phosphorylating activity than untreated controls, indicating the in vivo relevance of these findings. We also show here that zeta PKC associates with and activates MKK‐MAPK in vitro, suggesting that one of the mechanisms whereby overexpression of zeta PKC leads to deregulation of cell growth may be accounted for at least in part by activation of the MKK‐MAPK complex. However, neither MKK nor MAPK is responsible for the putative I kappa B phosphorylating activity. These data provide a decisive step towards understanding the functions of zeta PKC.

The Functional Interaction of 14-3-3 Proteins with the ERK1/2 Scaffold KSR1 Occurs in an Isoform-specific Manner

Identifying 14-3-3 isoform-specific substrates and functions may be of broad relevance to cell signaling research because of the key role played by this family of proteins in many vital processes. A multitude of ligands have been identified, but the extent to which they are isoform-specific is a matter of debate. Herein we demonstrate, both in vitro and in vivo, a specific, functionally relevant interaction of human 14-3-3γ with the molecular scaffold KSR1, which is mediated by the C-terminal stretch of 14-3-3γ. Specific binding to 14-3-3γ protected KSR1 from epidermal growth factor-induced dephosphorylation and impaired its ability to activate ERK2 and facilitate Ras signaling in Xenopus oocytes. Furthermore, RNA interference-mediated inhibition of 14-3-3γ resulted in the accumulation of KSR1 in the plasma membrane, all in accordance with 14-3-3γ being the cytosolic anchor that keeps KSR1 inactive. We also provide evidence that KSR1-bound 14-3-3γ heterodimerized preferentially with selected isoforms and that KSR1 bound monomeric 14-3-3γ. In sum, we have demonstrated ligand discrimination among 14-3-3 isoforms and shed light on molecular mechanisms of 14-3-3 functional specificity and KSR1 regulation.

Exploring the interactions of the RAS family in the human protein network and their potential implications in RAS-directed therapies

RAS proteins are the founding members of the RAS superfamily of GTPases. They are involved in key signaling pathways regulating essential cellular functions such as cell growth and differentiation. As a result, their deregulation by inactivating mutations often results in aberrant cell proliferation and cancer. With the exception of the relatively well-known KRAS, HRAS and NRAS proteins, little is known about how the interactions of the other RAS human paralogs affect cancer evolution and response to treatment. In this study we performed a comprehensive analysis of the relationship between the phylogeny of RAS proteins and their location in the protein interaction network. This analysis was integrated with the structural analysis of conserved positions in available 3D structures of RAS complexes. Our results show that many RAS proteins with divergent sequences are found close together in the human interactome. We found specific conserved amino acid positions in this group that map to the binding sites of RAS with many of their signaling effectors, suggesting that these pairs could share interacting partners. These results underscore the potential relevance of cross-talking in the RAS signaling network, which should be taken into account when considering the inhibitory activity of drugs targeting specific RAS oncoproteins. This study broadens our understanding of the human RAS signaling network and stresses the importance of considering its potential cross-talk in future therapies.