MTOR promotes basal cell carcinoma growth through atypical PKC

Assessment of Mammalian Target of Rapamycin Pathway Activation in Basal Cell Carcinoma as a New Therapeutic Approach

ABSTRACT

Targeting the mammalian target of rapamycin (mTOR) pathway represents a potentially novel approach to treat basal cell carcinoma (BCC), but activation of this pathway has not been well described in human BCCs. The purpose of this study was to assess whether mTOR pathway activation occurs in BCCs (both sporadic and syndromic) and report a case of a patient with Gorlin syndrome (GS) whose clinically suspicious BCCs responded to mTOR inhibition through topical sirolimus treatment. After Stanford Institutional Review Board Approval, archived BCCs from patients with GS (n = 25), sporadic BCCs (n = 35), and control tissues were subjected to immunohistochemical analysis for the activation of mTOR pathway, and immunohistochemical staining intensity was evaluated by a dermatopathologist. BCCs (compared with normal skin) had elevated levels of eIF4EBP1 ( Padjusted = 0.0336), which is downstream of mTOR. a serine/threonine kinase Phospho-(AKT), which interacts with mTOR, was also significantly elevated (perinuclear: Padjusted < 0.0001; cytoplasmic: Padjusted = 0.0021). When off-label topical 1% sirolimus was used on a pediatric patient with GS, we noted reduction of new BCC development and decreased size of existing neoplasms clinically suspicious for BCCs. This treatment was well tolerated after 2 years of continuous use, with no other treatments needed during this period. Topical sirolimus is a promising therapeutic candidate against both sporadic and GS-associated BCC. Multicenter, prospective studies are needed to understand the efficacy and safety of topical mTOR inhibitors in BCC treatment, and ascertain whether the immunohistochemical markers downstream of mTOR could have predictive value in identifying BCCs most likely to respond to topical mTOR inhibitors, such as sirolimus.

Protein Kinase C (PKC) Isozymes as Diagnostic and Prognostic Biomarkers and Therapeutic Targets for Cancer

Protein kinase C (PKC) is a large family of calcium- and phospholipid-dependent serine/threonine kinases that consists of at least 11 isozymes. Based on their structural characteristics and mode of activation, the PKC family is classified into three subfamilies: conventional or classic (cPKCs; α, βI, βII, and γ), novel or non-classic (nPKCs; δ, ε, η, and θ), and atypical (aPKCs; ζ, ι, and λ) (PKCλ is the mouse homolog of PKCι) PKC isozymes. PKC isozymes play important roles in proliferation, differentiation, survival, migration, invasion, apoptosis, and anticancer drug resistance in cancer cells. Several studies have shown a positive relationship between PKC isozymes and poor disease-free survival, poor survival following anticancer drug treatment, and increased recurrence. Furthermore, a higher level of PKC activation has been reported in cancer tissues compared to that in normal tissues. These data suggest that PKC isozymes represent potential diagnostic and prognostic biomarkers and therapeutic targets for cancer. This review summarizes the current knowledge and discusses the potential of PKC isozymes as biomarkers in the diagnosis, prognosis, and treatment of cancers.

Mammalian Target of Rapamycin Inhibitors for prolonged secondary prevention of non-melanoma skin cancer in solid organ transplant recipients

BACKGROUND

Immunosuppressive agents are essential for graft survival in solid-organ transplant recipients (SOTRs), but they have substantial durable side effects, including a higher incidence of aggressive non-melanoma skin cancers (NMSCs). Hitherto, only one class of immunosuppressants, mammalian target of rapamycin inhibitors (mTORi), may inhibit skin tumor formation, however their durable effectiveness is controversial.

OBJECTIVE

To evaluate the sustained effectiveness of mTORi in reducing NMSCs' incidence in SOTRs.

METHODS

A retrospective study was conducted in a specialized dermatology clinic for SOTRs of a tertiary university-affiliated medical center. SOTRs with a history of at least one histologically proven NMSC, were followed for 6 years: 3 years after transplantation, before initiation of mTORi, and 3 years under mTORi treatment.

RESULTS

The cohort consisted of 44 SOTRs. Treatment with mTORi was initiated on average 6.27 (3.34-6.34) years following transplantation. In the 3 years before mTORi treatment initiation, the mean number of new NMSCs per patient was 2.11 (1-14). This value decreased to 1.2 (0-19) in the 3 years under mTORi treatment (P=0.0007). Analysis by NMSC type yielded a significant decrease in both SCCs and BCCs.

CONCLUSION

This study found that mTORi are effective for prolonged secondary prevention of NMSCs in SOTRs. This article is protected by copyright. All rights reserved.

Single-cell analysis of human basal cell carcinoma reveals novel regulators of tumor growth and the tumor microenvironment

How basal cell carcinoma (BCC) interacts with its tumor microenvironment to promote growth is unclear. We use singe-cell RNA sequencing to define the human BCC ecosystem and discriminate between normal and malignant epithelial cells. We identify spatial biomarkers of tumors and their surrounding stroma that reinforce the heterogeneity of each tissue type. Combining pseudotime, RNA velocity-PAGA, cellular entropy, and regulon analysis in stromal cells reveals a cancer-specific rewiring of fibroblasts, where STAT1, TGF-β, and inflammatory signals induce a noncanonical WNT5A program that maintains the stromal inflammatory state. Cell-cell communication modeling suggests that tumors respond to the sudden burst of fibroblast-specific inflammatory signaling pathways by producing heat shock proteins, whose expression we validated in situ. Last, dose-dependent treatment with an HSP70 inhibitor suppresses in vitro vismodegib-resistant BCC cell growth, Hedgehog signaling, and in vivo tumor growth in a BCC mouse model, validating HSP70's essential role in tumor growth and reinforcing the critical nature of tumor microenvironment cross-talk in BCC progression.

Basal cell carcinomas acquire secondary mutations to overcome dormancy and progress from microscopic to macroscopic disease

Basal cell carcinomas (BCCs) frequently possess immense mutational burdens; however, the functional significance of most of these mutations remains unclear. Here, we report that loss of Ptch1, the most common mutation that activates upstream Hedgehog (Hh) signaling, initiates the formation of nascent BCC-like tumors that eventually enter into a dormant state. However, rare tumors that overcome dormancy acquire the ability to hyperactivate downstream Hh signaling through a variety of mechanisms, including amplification of Gli1/2 and upregulation of Mycn. Furthermore, we demonstrate that MYCN overexpression promotes the progression of tumors induced by loss of Ptch1. These findings suggest that canonical mutations that activate upstream Hh signaling are necessary, but not sufficient, for BCC to fully progress. Rather, tumors likely acquire secondary mutations that further hyperactivate downstream Hh signaling in order to escape dormancy and enter a trajectory of uncontrolled expansion.

Trieu et al. generate BCC mouse models in which rare macroscopic tumors form alongside numerous failed microscopic lesions. Successful macroscopic tumors acquire secondary changes that elevate Gli1, Gli2, and/or Mycn levels, causing hyperactivation of downstream Hedgehog (Hh) signaling. Loss of p53 and Notch1 also contributes to tumor progression.

PI3K Promotes Basal Cell Carcinoma Growth Through Kinase-Induced p21 Degradation

Basal cell carcinoma (BCC) is a locally invasive epithelial cancer that is primarily driven by the Hedgehog (HH) pathway. Advanced BCCs are a critical subset of BCCs that frequently acquire resistance to Smoothened (SMO) inhibitors and identifying pathways that bypass SMO could provide alternative treatments for patients with advanced or metastatic BCC. Here, we use a combination of RNA-sequencing analysis of advanced human BCC tumor-normal pairs and immunostaining of human and mouse BCC samples to identify a PI3K pathway expression signature in BCC. Pharmacological inhibition of PI3K activity in BCC cells significantly reduces cell proliferation and HH signaling. However, treatment of Ptch1^fl/fl; Gli1-Cre^ERT2 mouse BCCs with the PI3K inhibitor BKM120 results in a reduction of tumor cell growth with no significant effect on HH signaling. Downstream PI3K components aPKC and Akt1 showed a reduction in active protein, whereas their substrate, cyclin-dependent kinase inhibitor p21, showed a concomitant increase in protein stability. Our results suggest that PI3K promotes BCC tumor growth by kinase-induced p21 degradation without altering HH signaling.