Tuberin negatively affects BCL-2’s cell survival function

Cardiac rhabdomyomas in tuberous sclerosis complex show apoptosis regulation and mTOR pathway abnormalities

Cardiac rhabdomyoma (CR) is the most common heart tumor in children and is usually associated with tuberous sclerosis complex (TSC). Tuberous sclerosis complex is a genetic disorder caused by a mutation in either of 2 genes (TSC1 or TSC2) and characterized by the formation of hamartomas in multiple organs. The 2 TSC proteins, hamartin and tuberin, antagonize the mammalian target of rapamycin (mTOR) signaling pathway, thus regulating cell growth and proliferation. Recently, some trials treating TSC with the mTOR inhibitor rapamycin have been published; however, the impact of such treatment on heart tumors is not known. The aim of the present paper was to study the molecular pathobiology of CRs. Six CR samples were studied. The expression of S6K1, pErk, Erk, Akt, pAkt, 4E-BP1, hamartin, tuberin, mTOR, bcl-2, Bax, and Ki-67 was examined using immunohistochemistry and Western blot methods. Increased expression of Bax, mTOR, pS6K, pErk, and 4E-BP1 was found in all CR samples. Hamartin and tuberin expression was decreased in tumors versus normal heart tissues. This is the first study showing mTOR pathway dysregulation and an increased expression of proapoptotic Bax protein in CRs associated with TSC.

Skp2 inversely correlates with p27 and tuberin in transformed cells

The cyclin-dependent kinase inhibitor p27Kip1 (p27) is a major gatekeeper of the mammalian cell cycle progression known to be regulated by both, its subcellular localization and its degradation. To allow entrance into S phase and thereby mammalian cell cycle progression p27 must be degraded by a skp2-containing E3 ubiquitin ligase whose task is to target p27 for degradation by the proteasome. The tumor suppressor gene product tuberin directly binds to p27 and protects it from degradation via skp2. Whereas, p27 and tuberin are known to be localized to both, the cytoplasm and the nucleus, the localization of skp2 remained elusive. Here we demonstrate that skp2 is a cytoplasmic and nuclear protein. In addition we found an inverse correlation of the endogenous protein levels of skp2 with p27 and tuberin in different transformed cells and under different growth conditions. These data allow new important insights into this molecular network of cell cycle control.

Tuberin, p27 and mTOR in different cells

Mutations in the genes TSC1 or TSC2 cause the autosomal dominantly inherited tumor suppressor syndrome tuberous sclerosis, which is characterized by the development of tumors, named hamartomas, in different organs. The TSC gene products, hamartin and tuberin, form a complex, of which tuberin is assumed to be the functional component. Both, hamartin and tuberin have been implicated in the control of the cell cycle by activating the cyclin-dependent kinase inhibitor p27 and in cell size regulation by inhibiting the mammalian target of rapamycin (mTOR) a regulator of the p70 ribosomal protein S6 kinase (p70S6K) and its target the ribosomal protein S6. The tuberin/hamartin complex was shown to protect p27 from protein degradation. Within the mTOR signaling pathway tuberin harbors GTPase activating (GAP) potential toward Rheb, which is a potent regulator of mTOR. In this study, we have analyzed the protein levels of tuberin, p27, cyclin D1, mTOR and phospho mTOR Ser2448 (activated mTOR), S6 and phospho S6 Ser240/244 (activated S6) and as controls alpha-tubulin and topoisomerase IIbeta, in ten different cells, including primary normal cells, immortalized and transformed cell lines.

Tuberous sclerosis complex and epilepsy: recent developments and future challenges

Tuberous sclerosis complex (TSC) is a congenital syndrome characterized by the widespread development of benign tumors in multiple organs, caused by mutations in one of the tumor suppressor genes, TSC1 or TSC2. About 80% of affected patients have a new mutation, and the remaining 20% have inherited a TSC gene mutation from a parent. The disorder affects approximately 1 in 6000 individuals. Cortical tubers are the neuropathological hallmark of TSC. The most common neurological manifestations of TSC are epilepsy, mental retardation, and autistic behavior. Epilepsy occurs in up to 80-90% of patients and is often intractable, with a poor response to anticonvulsant medications. While the molecular basis of TSC is well established, far less is known about the mechanisms of epilepsy in this disorder. In this article, we first summarize known clinical aspects of TSC with emphasis on its neurological features. Then, based on the molecular, pathological, immunohistochemical, neurochemical, and physiological properties of tubers in patients with TSC and in animal models, we discuss possible mechanisms of seizures and epileptogenesis in TSC. Finally, we provide an updated literature review and a consensus statement from the Tuberous Sclerosis Complex Working Group for future research into the mechanisms of epilepsy in TSC.