Tsc1 deficiency impairs mammary development in mice by suppression of AKT, nuclear ERα, and cell-cycle-driving proteins

Cancer invasion and metastasis: Insights from murine pubertal mammary gland morphogenesis

Cancer invasion and metastasis accounts for the majority of cancer related mortality. A better understanding of the players that drive the aberrant invasion and migration of tumors cells will provide critical targets to inhibit metastasis. Postnatal pubertal mammary gland morphogenesis is characterized by highly proliferative, invasive, and migratory normal epithelial cells. Identifying the molecular regulators of pubertal gland development is a promising strategy since tumorigenesis and metastasis is postulated to be a consequence of aberrant reactivation of developmental stages. In this review, we summarize the pubertal morphogenesis regulators that are involved in cancer metastasis and revisit pubertal mammary gland transcriptome profiling to uncover both known and unknown metastasis genes. Our updated list of pubertal morphogenesis regulators shows that most are implicated in invasion and metastasis. This review highlights molecular linkages between development and metastasis and provides a guide for exploring novel metastatic drivers.

Resistance to anti-HER2 therapy associated with the TSC2 nonsynonymous variant c.4349 C > G (p.Pro1450Arg) is reversed by CDK4/6 inhibitor in HER2-positive breast cancer

HER2-positive breast cancer patients carrying the germline TSC2 nonsynonymous variant c.4349 C > G (p.Pro1450Arg) are resistant to anti-HER2 therapy. Multi-predictor in silico analysis reveals that this variant is deleterious. We explore the potential mechanism of this TSC2 variant and investigate methods for overcoming anti-HER2 resistance. TSC2 c.4349 C > G reverses the inhibitory effect on mTOR and downstream signaling by increasing TSC2 phosphorylation at Thr1462 and confers significant lapatinib resistance in vitro and in vivo. The combination of lapatinib and the CDK4/6 inhibitor palbociclib inhibits cyclin D1/CDK4/Rb alternative pathway and TSC2 phosphorylation, thereby partially attenuating mTOR activity and inducing TSC2-mutant cell blockage at G1/G0. In in vitro and xenograft models, palbociclib+lapatinib shows higher anti-tumor activity than monotherapy and overcomes the resistance of the TSC2 c.4349 C > G-related variant to anti-HER2 therapy. We reveal a new mechanism of resistance to anti-HER2 therapy and provide a strategy to increase the efficiency of anti-HER2 therapy in HER2-positive breast cancer.

Beyond controlling cell size: functional analyses of S6K in tumorigenesis

As a substrate and major effector of the mammalian target of rapamycin complex 1 (mTORC1), the biological functions of ribosomal protein S6 kinase (S6K) have been canonically assigned for cell size control by facilitating mRNA transcription, splicing, and protein synthesis. However, accumulating evidence implies that diverse stimuli and upstream regulators modulate S6K kinase activity, leading to the activation of a plethora of downstream substrates for distinct pathobiological functions. Beyond controlling cell size, S6K simultaneously plays crucial roles in directing cell apoptosis, metabolism, and feedback regulation of its upstream signals. Thus, we comprehensively summarize the emerging upstream regulators, downstream substrates, mouse models, clinical relevance, and candidate inhibitors for S6K and shed light on S6K as a potential therapeutic target for cancers.

Emerging Link between Tsc1 and FNIP Co-Chaperones of Hsp90 and Cancer

Heat shock protein-90 (Hsp90) is an ATP-dependent molecular chaperone that is tightly regulated by a group of proteins termed co-chaperones. This chaperone system is essential for the stabilization and activation of many key signaling proteins. Recent identification of the co-chaperones FNIP1, FNIP2, and Tsc1 has broadened the spectrum of Hsp90 regulators. These new co-chaperones mediate the stability of critical tumor suppressors FLCN and Tsc2 as well as the various classes of Hsp90 kinase and non-kinase clients. Many early observations of the roles of FNIP1, FNIP2, and Tsc1 suggested functions independent of FLCN and Tsc2 but have not been fully delineated. Given the broad cellular impact of Hsp90-dependent signaling, it is possible to explain the cellular activities of these new co-chaperones by their influence on Hsp90 function. Here, we review the literature on FNIP1, FNIP2, and Tsc1 as co-chaperones and discuss the potential downstream impact of this regulation on normal cellular function and in human diseases.

Intramammary rapamycin administration to calves induces epithelial stem cell self-renewal and latent cell proliferation and milk protein expression

Mammary epithelial stem cells differentiate to create the basal and luminal layers of the gland. Inducing the number of differentiating bovine mammary stem cells may provide compensating populations for the milk-producing cells that die during lactation. Inhibition of mTOR activity by rapamycin signals self-renewal of intestinal stem cells, with similar consequences in the mouse mammary gland and in bovine mammary implants maintained in mice. The implementation of these results in farm animals for better mammary development and production was studied in 3-month-old calves. mTOR activity decreased by ~50% in mammary epithelial cells subjected to 3-week rapamycin administration, with no negative consequences on mammary morphology or β-casein expression. Subsequently, stem cell self-renewal was induced, reflected by a higher propagation rate of cultures from rapamycin-treated glands compared to respective controls and higher expression of selected markers. Followed by 4-day estrogen and progesterone administration, rapamycin significantly induced proliferation rate. Higher numbers of basal and luminal PCNA⁺ cells were detected in small ducts near the elongating sites as compared to large ducts, in which only luminal cells were affected. Rapamycin administration resulted in induction of individual milk protein genes’ expression, which was negatively correlated to their endogenous levels. The inductive effect of rapamycin on luminal cell number was confirmed in organoid cultures, but milk protein expression decreased, probably due to lack of oscillation in rapamycin levels. In conclusion, intramammary rapamycin administration is an effective methodology to reduce mTOR activity in bovine mammary epithelial cells and consequently, induce stem cell self-renewal. The latent positive effect of rapamycin on epithelial cell proliferation and its potential to improve milk protein expression in calves may have beneficial implications for mature cows.

Role of TSC1 in physiology and diseases

Since its initial discovery as the gene altered in Tuberous Sclerosis Complex (TSC), an autosomal dominant disorder, the interest in TSC1 (Tuberous Sclerosis Complex 1) has steadily risen. TSC1, an essential component of the pro-survival PI3K/AKT/MTOR signaling pathway, plays an important role in processes like development, cell growth and proliferation, survival, autophagy and cilia development by co-operating with a variety of regulatory molecules. Recent studies have emphasized the tumor suppressive role of TSC1 in several human cancers including liver, lung, bladder, breast, ovarian, and pancreatic cancers. TSC1 perceives inputs from various signaling pathways, including TNF-α/IKK-β, TGF-β-Smad2/3, AKT/Foxo/Bim, Wnt/β-catenin/Notch, and MTOR/Mdm2/p53 axis, thereby regulating cancer cell proliferation, metabolism, migration, invasion, and immune regulation. This review provides a first comprehensive evaluation of TSC1 and illuminates its diverse functions apart from its involvement in TSC genetic disorder. Further, we have summarized the physiological functions of TSC1 in various cellular events and conditions whose dysregulation may lead to several pathological manifestations including cancer.

A novel mouse model of anterior segment dysgenesis (ASD): conditional deletion of Tsc1 disrupts ciliary body and iris development

Development of the cornea, lens, ciliary body and iris within the anterior segment of the eye involves coordinated interaction between cells originating from the ciliary margin of the optic cup, the overlying periocular mesenchyme and the lens epithelium. Anterior segment dysgenesis (ASD) encompasses a spectrum of developmental syndromes that affect these anterior segment tissues. ASD conditions arise as a result of dominantly inherited genetic mutations and result in both ocular-specific and systemic forms of dysgenesis that are best exemplified by aniridia and Axenfeld-Rieger syndrome, respectively. Extensive clinical overlap in disease presentation amongst ASD syndromes creates challenges for correct diagnosis and classification. The use of animal models has therefore proved to be a robust approach for unravelling this complex genotypic and phenotypic heterogeneity. However, despite these successes, it is clear that additional genes that underlie several ASD syndromes remain unidentified. Here, we report the characterisation of a novel mouse model of ASD. Conditional deletion of Tsc1 during eye development leads to a premature upregulation of mTORC1 activity within the ciliary margin, periocular mesenchyme and lens epithelium. This aberrant mTORC1 signalling within the ciliary margin in particular leads to a reduction in the number of cells that express Pax6, Bmp4 and Msx1 Sustained mTORC1 signalling also induces a decrease in ciliary margin progenitor cell proliferation and a consequent failure of ciliary body and iris development in postnatal animals. Our study therefore identifies Tsc1 as a novel candidate ASD gene. Furthermore, the Tsc1-ablated mouse model also provides a valuable resource for future studies concerning the molecular mechanisms underlying ASD and acts as a platform for evaluating therapeutic approaches for the treatment of visual disorders.

Repression of TSC1/TSC2 mediated by MeCP2 regulates human embryo lung fibroblast cell differentiation and proliferation

Pulmonary fibrosis (PF) is a severe inflammatory disease with limited effective treatments. It is known that the transdifferentiation of human embryo lung fibroblast (HELF) cells from pulmonary fibroblasts into myofibroblasts, contributes to the progression of pulmonary fibrogenesis. The tuberous sclerosis proteins TSC1 and TSC2 are two key signaling factors which can suppress cell growth and proliferation. However, the roles of TSC1 and TSC2 in lung fibroblast are unclear. Here, we developed a PF model with bleomycin (BLM) in mice and conducted several simulation experiments in HELF cells. Our study shows that the expression of TSC1 and TSC2 in fibrotic mice lung was reduced and stimulation of HELF cells with TGF-β1 resulted in a down-regulation of TSC1 and TSC2. In addition, overexpression of TSC1 or TSC2 decreased cell proliferation and differentiation. Furthermore, we found that reduced expression of TSC1 and TSC2 caused by TGF-β1 is associated with the promoter methylation status of TSC1 and TSC2. MeCP2, controls an epigenetic pathway that promotes myofibroblast transdifferentiation and fibrosis. We found that expression of TSC1 and TSC2 can be repressed by MeCP2, which regulates HELF cell differentiation and proliferation as myofibroblasts and lead to PF ultimately.