Abstract 2365: Defining the role of Akt activation in melanoma development and progression
Authors: Robinson James P. , VanBrocklin Matthew W. , Holmen Sheri L. 
Published: August 2012

Abstract
We have developed a somatic cell gene delivery mouse model of human melanoma that allows for rapid validation of genetic alterations identified in this disease. A major advantage of this system is the ability to model the multi-step process of carcinogenesis in immune-competent mice using retroviral-mediated gene delivery. Mutant BRaf has been demonstrated to cooperate with Pten loss to induce metastatic melanoma in mice but the role of Akt in this context has not been evaluated. This is relevant because ∼60% of human melanomas contain amplification and activation of AKT. In this study, we evaluated the role of mutant Braf in melanoma initiation in the context of Ink4a/Arf loss and Akt activation. Ink4a/Arflox/lox mice expressing the viral receptor TVA specifically in melanocytes under control of the dopachrome tautomerase (DCT) promoter were crossed to Ptenlox/lox mice and mice carrying a conditional BrafV600E allele, which can be activated by Cre expression (BrafCA). Cohorts of newborn mice were injected subcutaneously with retroviruses containing Cre recombinase. Melanoma developed in 100% (8/8) of the DCT-TVA/BrafCA mice following Cre mediated Braf activation combined with Pten and Ink4a/Arf loss; median survival was 62 ± 6.7 days in this cohort. The incidence was reduced to 43% (6/14) if Pten was retained (Pten+/+ mice); median survival of tumor bearing mice in this group was 72 ± 6.7 days. Kaplan Meier survival analysis revealed a highly significant difference between these cohorts (P = 0.01). Interestingly, Akt expression could substitute for Pten loss in this context. Melanoma developed in 88% (15/17) of the DCT-TVA/BrafCA mice following viral delivery of activated Akt and Cre, which mediated Braf activation combined with Ink4a/Arf loss; median survival of tumor bearing mice in this group was 60 ± 6.6 days. Kaplan Meier survival analysis revealed that there was no significant difference between cohorts with Pten loss or Akt expression (P = 0.658). Expression of virally delivered Akt in the absence of Cre did not lead to tumor formation in control cohorts (0/7) and no uninfected mice developed melanoma (0/14). Histological examination of the tumors generated in the BrafCA mice with delivery of Cre and Akt viruses demonstrated that they were very similar to tumors lacking Pten. Importantly, the tumors were similar to human melanoma consisting primarily of short spindle cells exhibiting high grade nuclear features and prominent nucleoli. Mitotic figures were abundant, and some tumors possessed giant cells with epitheliod features. Occasional neoplasms exhibited regions of necrosis. Extensive lung metastases were evident following Pten loss or Akt expression, but not in tumors lacking activation of Akt, confirming a role of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway in melanoma progression and metastasis. Our ongoing experiments seek to further characterize the role(s) of the PI3K/AKT pathway in melanoma progression.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2365. doi:1538-7445.AM2012-2365