Pp125FAK is a tyrosine kinase that appears to regulate the assembly of focal adhesions and thereby promotes cell spreading on the extracellular matrix. In some cells, the C terminus of pp125FAK is expressed as a separate protein, pp41/43FRNK. We have previously shown that overexpression of pp41/43FRNK inhibits tyrosine phosphorylation of pp125FAK and paxillin and, in addition, delays cell spreading and focal adhesion assembly. Thus, pp41/43FRNK functions as a negative inhibitor of adhesion signaling and provides a tool to dissect the mechanism by which pp125FAK promotes cell spreading. We report here that the inhibitory effects of pp41/43FRNK expression can be rescued by the co-overexpression of wild-type pp125FAK and partially rescued by catalytically inactive variants of pp125FAK. However, coexpression of an autophosphorylation site mutant of pp125FAK, which fails to bind the SH2 domain of pp60c-Src, or a mutant that fails to bind paxillin did not promote cell spreading. In contrast, expression of pp41/43FRNK and pp60c-Src reconstituted cell spreading and tyrosine phosphorylation of paxillin but did so without inducing tyrosine phosphorylation of pp125FAK. These data provide additional support for a model whereby pp125FAK acts as a "switchable adaptor" that recruits pp60c-Src to phosphorylate paxillin, promoting cell spreading. In addition, these data point to tyrosine phosphorylation of paxillin as being a critical step in focal adhesion assembly.

Akt (also known as PKB or RAC‐PK) is an intracellular serine/threonine kinase involved in regulating cell survival. Although this makes it a promising target for the discovery of drugs to treat human cancer, a complicating factor may be the role played by Akt in insulin signalling. Two human isoforms, Akt‐1 and Akt‐2, have been described previously and a third isoform has been identified in rats (here termed Akt‐3, but also called RAC‐PK‐γ or PKB‐γ). We describe the identification of the corresponding human isoform of Akt‐3. The gene encoding human Akt‐3 was localized to chromosome 1q43–44. The predicted protein sequence is 83% identical to human Akt‐1 and 78% identical to human Akt‐2, and contains a pleckstrin homology domain and a kinase domain. In contrast to the published rat Akt‐3 isoform, human and mouse Akt‐3 also possess a C‐terminal ‘tail’ that contains a phosphorylation site (Ser472) thought to be involved in the activation of Akt kinases. In addition to phosphorylation of Ser472, phosphorylation of Thr305 also appears to contribute to the activation of Akt‐3 because mutation of both these residues to aspartate increased the catalytic activity of Akt‐3, whereas mutation to alanine inhibited activation. Akt‐3 activity could be inhibited by the broad spectrum kinase inhibitor staurosporine and by the PKC inhibitor Ro 31‐8220, but not by other PKC or PKA inhibitors tested. Although Akt‐3 is expressed widely, it is not highly expressed in liver or skeletal muscle, suggesting that its principle function may not be in regulating insulin signalling. These observations suggest that Akt‐3 is a promising target for the discovery of novel chemotherapeutic agents which do not interfere with insulin signalling.

Purpose: The effective treatment of ovarian cancer is hampered by the development of drug resistance, which may be mediated by members of the Bcl-2 family of apoptosis regulators. ABT-737 is a recently described inhibitor of members of this family. We investigated whether this compound could sensitize ovarian cancer cells to chemotherapeutic agents.

Studies on the attachment and spreading of cells in culture have provided valuable insights into the mechanisms by which cells transmit information from the outside to the inside of the cell. This brief review considers recent information on the role of focal adhesion-associated protein tyrosine kinases in integrin-regulated cell signalling.