AKT1 Overexpression in Endothelial Cells Leads to the Development of Cutaneous Vascular Malformations In Vivo FREE

A wide range of endothelial neoplasms occurs in humans. They can be classified into 3 categories based on their clinical behavior.¹ These lesions include hemangiomas of childhood, which grow rapidly and regress. The second most common vascular lesion is a vascular malformation, which starts in childhood and grows progressively with the patient and never regresses. The least common vascular lesion is angiosarcoma, which is a highly malignant tumor derived from endothelial cells. Angiosarcomas have a great propensity for local invasion and distant metastasis, leading to death. Angiosarcoma cells have been demonstrated to have mutations in the tumor suppressor gene p53 and/or oncogenic H-ras.^{2 - 4} Mouse models of hemangiomas and angiosarcomas exist, but, to our knowledge, we describe the first example of a transplantable vascular malformation.

The akt1 oncogene has been implicated in a variety of malignant neoplasms, usually in conjunction with complex oncogenes such as ras, which activate Akt1 as well as numerous other pathways.⁵ In previous studies of immortalized and H-ras–transformed endothelial cells, we have found that overexpression of oncogenic H-ras causes increased angiogenesis and malignant transformation of endothelial cells to angiosarcoma.⁵ Because Akt1 has been implicated as a major downstream effector of ras activity, we examined the effect of expression of active Akt1 alone in immortalized, poorly tumorigenic MS1 cells. Overexpression of Akt in immortalized endothelium led to the development of vascular malformations in vivo, which were histologically distinct from angiosarcomas derived from the same endothelial cell line. To our knowledge, this is the first murine model of transplantable vascular malformations, and provides a rationale for Akt inhibition in the treatment of vascular malformations.

MS1 cells are murine endothelial cells immortalized by infection with an ecotropic retrovirus encoding SV40 large T antigen. When injected subcutaneously into nude mice, these cells form benign hemangiomas.⁵ Subsequent efforts to transduce MS1 cells with a myristoylated Akt retroviral supernatant were accomplished with 293T cells by cotransfection with pBabepuroHA-myrAkt (or control pBabepuro) and pCLEco, which encodes gag, pol, and env proteins specifying the ecotropic receptor, using a FuGene 6 transfection reagent (Roche Applied Science, Indianapolis, Ind). Thirty-six hours after transfection, the 293T medium was collected and filtered through a 0.45-μm syringe filter onto the MS1 cells, and protamine sulfate was added to a concentration of 0.6 mg/mL. After 3 hours, the medium was replaced with Dulbecco modified Eagle medium plus 5% fetal calf serum. Thirty-six hours after infection, the cells were selected with 1-μg/mL puromycin for 3 days, at which time 100% of the noninfected cells were dead.

Cells were grown on 10-cm dishes and allowed to reach approximately 80% confluence before protein isolation. Sample aliquots normalized for protein content were size fractionated by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE), and the proteins were transferred to a polyvinylidene fluoride microporous membrane. The blots were incubated in blocking solution (phosphate-buffered saline with 2% [weight-volume ratio] bovine serum albumin) for 1 hour at room temperature. The blots were then incubated overnight in sheep polyclonal anti-Akt (Upstate Biotechnology, Lake Placid, NY), rabbit polyclonal antiphosphorylated Akt (Ser 473; Cell Signaling Technology, Beverly, Mass), monoclonal anti-ras antibody (Transduction Laboratories, Lexington, Ky), or monoclonal antiactin antibody (Chemicon International Inc, Temecula, Calif). Blots were washed 3 times with phosphate-buffered saline with 0.05% Tween 20 and were then incubated 1 hour at room temperature in secondary antibody solution consisting of a 1:16 000 dilution of horseradish peroxidase–conjugated anti–rabbit IgG (Amersham Pharmacia Biotech, Piscataway, NJ) or 1:10 000 dilution of horseradish peroxidase–conjugated anti–mouse IgG (Amersham Pharmacia Biotech) and washed 3 times with phosphate-buffered saline with 0.05% Tween 20. Finally, the blots were developed using enhanced chemiluminescence.

One million MS1 or MS1 m-Akt cells were injected subcutaneously into nude mice as described in our previous studies.⁵ MS1-derived cells were allowed to remain in nude mice up to 2 months prior to killing. Tumor tissue was fixed in formalin and stained with hematoxylin-eosin.

Introduction of AKT1 into endothelial cells leads to a high-level expression of activated Akt (Figure 1). Akt is also known to cause phosphorylation of another transcription factor, FKHRL-1, which is accomplished in our cells by Akt introduction (Figure 1). To determine the effect of Akt activation on in vivo transformation, 1 million cells were injected subcutaneously into the right flank of 3 male nude mice. Long-term implantation (1 month) of MS1 m-Akt cells in nude mice led to the development of progressive vascular malformations (Figure 2B). Histological examination of the tumors from mice injected with MS1 m-Akt cells showed a vascular malformation with wide lumens and minimal investment by pericytes and/or smooth muscle, reminiscent of human vascular malformations (Figure 2). Note the contrast in histologic features to SVR angiosarcomas, which are highly cellular and do not possess large lumens (Figure 2).

We have previously shown that ras transformation increased angiogenic activity in immortalized endothelial cells and induces Akt as well as other signaling pathways.^{4 - 6} Akt is a well-studied downstream mediator of ras activity, and Akt has been implicated in transformation and protection from apoptosis.^{7 - 9} We studied the effect of Akt overexpression alone to determine whether Akt could confer the ability of endothelial cells to persist in vivo. Surprisingly, we found that constitutive overexpression of Akt led to the development of lesions that closely resemble vascular malformations in that they have wide lumens with small amounts of smooth muscle investment.

Recent data have shown that too much Akt can perturb angiogenesis. Targeting Akt to endothelial cells of transgenic mice leads to perinatal lethality owing to the development of vascular malformations.¹⁰ Interestingly, in the mice that have the induced akt transgene, total Akt levels remain the same, even though induction of Akt by the withdrawal of doxycycline leads to increased phosphorylation of Akt.⁹ This may be due to a feedback suppression of endogenous Akt expression by Akt induction. Our model has the advantage that it can be transplanted to nude mice and thus be used to evaluate potential therapies for vascular malformations.

Our findings provide a rationale for the use of Akt inhibitors and drugs that work downstream of Akt, such as rapamycin,¹¹ for the treatment of vascular malformations. Akt inhibitors in clinical trial include perifosine and miltefosine, drugs originally developed for the treatment of leishmaniasis.^{12 - 13} Another protein, mammalian target of rapamycin (mTOR), and downstream activators of mTOR, such as S6 kinase, can be detected in paraffin sections by immunohistochemical analysis.¹⁴ Rapamycin has already been used for the treatment of other vascular neoplasms, including Kaposi sarcoma, as well as dermatomyositis, and is well tolerated by patients.^{15 - 16} Our findings suggest that vascular malformations that demonstrate activation of the Akt pathway, including mTOR and S6 kinase, may be treated with currently available drugs such as rapamycin and novel drugs such as Akt inhibitors.

Correspondence: Jack L. Arbiser, MD, PhD, Department of Dermatology, Emory University School of Medicine, WMB 5309, 1639 Pierce Dr, Atlanta, GA 30322 (jarbise@emory.edu).

Financial Disclosure: None reported.

Accepted for Publication: August 14, 2006.

Author Contributions:Study concept and design: Cohen and Arbiser. Acquisition of data: Perry, Banyard, McLaughlin, Watnick, Sohn, Brindley, and Cohen. Analysis and interpretation of data: Perry, Banyard, Brindley, and Obata. Drafting of the manuscript: Perry, Banyard, McLaughlin, Le Good, Cohen, and Arbiser. Critical revision of the manuscript for important intellectual content: Perry, Banyard, Watnick, Sohn, Brindley, Obata, and Cantley. Obtained funding: Arbiser. Administrative, technical, and material support: Perry, McLaughlin, Watnick, Brindley, Obata, Cohen, and Arbiser. Study supervision: Arbiser.

Funding/Support: Dr Arbiser was supported by grants RO1 AR47901, P30 AR42687, and Emory Skin Disease Research Core Center Grant P30 AR42687 from the National Institutes of Health, as well as by a VA Merit Award.

Acknowledgment: We thank J. Ann Le Good, PhD, Mary Hendrix, PhD, and Elisabeth Seftor, PhD, for their critical evaluation of this manuscript and J. Dewald, BS, for performing Western blot analysis.