We investigated the role of Akt-1, one of the major downstream effectors of phosphoinositide 3-kinase (PI3K), in platelet function using mice in which the gene for Akt-1 had been inactivated. Using ex vivo techniques, we showed that Akt-1-deficient mice exhibited impaired platelet aggregation and spreading in response to various agonists. These differences were most apparent in platelets activated with low concentrations of thrombin. Although Akt-1 is not the predominant Akt isoform in mouse platelets, its absence diminished the amount of total phospho-Akt and inhibited increases in intracellular Ca 2؉ concentration in response to thrombin. Moreover, thrombin-induced platelet ␣-granule release as well as release of adenosine triphosphate from dense granules was also defective in Akt-1-null platelets. Although the absence of Akt-1 did not influence expression of the major platelet receptors for thrombin and collagen, fibrinogen binding in response to these agonists was significantly reduced. As a consequence of impaired ␣ IIb ␤ 3 activation and platelet aggregation, Akt-1 null mice showed significantly longer bleeding times than wild-type mice. IntroductionUnder normal conditions, platelets circulate freely in the blood without interacting with each other or the vessel wall. On vascular injury, subendothelial matrix proteins, including collagens, or soluble agonists trigger platelet activation. The hallmark of platelet activation is the transformation of the major platelet glycoprotein, ␣ IIb ␤ 3 , from its resting to active state, which serves as a fibrinogen receptor, thereby mediating platelet aggregation. One of the most potent platelet agonists, thrombin, acts via a dual system of G protein-coupled protease-activated receptors, PAR3 and PAR4. Both of these receptors are required for optimal thrombin-induced aggregation and secretion. 1 The majority of platelet agonists, including thrombin and collagen, activate phosphoinositide 3-kinase (PI3K) in platelets. Inhibitors of PI3K (wortmannin and LY294002) block fibrinogen binding and platelet aggregation induced by thrombin and collagen, indicating a role for PI3K in ␣ IIb ␤ 3 activation. 2 Platelets contain 2 major forms of PI3K, p85/p110 PI3K, composed of a p110 catalytic and p85 regulatory subunit and PI3K␥, composed of a p110␥ catalytic and p101 regulatory subunit. Both forms of PI3K are involved in the inside-out signaling that activates ␣ IIb ␤ 3 and induces platelet aggregation. 2 Recent studies demonstrated that a deficiency in the p85␣ regulatory subunit in mice leads to a significant reduction of collagen-induced platelet aggregation, particularly at low doses of stimulus. . 3 The absence of PI3K␥ activity, indicated by the lack of Akt phosphorylation, leads to impaired platelet aggregation in response to adenosine diphosphate (ADP) and protects against thrombosis. 4 PI3Ks generate phosphoinositide products that target the Tec family tyrosine kinases, serine/threonine protein kinases such as Akt, guanosine diphosphate/guanosine triphosphate exchange f...

The management of pain and morbidity due to the spreading and growth of cancer within bone remains to be a paramount problem in clinical care. Cancer cells actively transform bone, however, the molecular requirements and mechanisms of this process remain unclear. This study shows that functional modulation of the avb3 integrin receptor in prostate cancer cells is required for progression within bone and determines tumor-induced bone tissue transformation. Using histology and quantitative microCT analysis, we show that avb3 integrin is required not only for tumor growth within the bone but for tumor-induced bone gain, a response resembling bone lesions in prostate cancer patients. Expression of normal, fully functional avb3 enabled tumor growth in bone (incidence: 4/4), whereas avb3 (À), inactive or constitutively active mutants of avb3 did not (incidence: 0/4, 0/6 and 1/7, respectively) within a 35-day-period. This response appeared to be bone-specific in comparison to the subcutis where tumor incidence was greater than 60% for all groups. Interestingly, bone residing prostate cancer cells expressing normal or dis-regulated avb3 (either inactive of constitutively active), but not those lacking b3 promoted bone gain or afforded protection from bone loss in the presence or absence of histologically detectable tumor 35 days following implantation. As bone is replete with ligands for b3 integrin, we next demonstrated that avb3 integrin activation on tumor cells is essential for the recognition of key bone-specific matrix proteins. As a result, prostate cancer cells expressing fully functional but not dis-regulated avb3 integrin are able to control their own adherence and migration to bone matrix, functions that facilitate tumor growth and control bone lesion development.