Ectopic expression of Cdk6 circumvents transforming growth factor-beta mediated growth inhibition

Discovery of Anti-TNBC Agents Targeting PTP1B: Total Synthesis, Structure-Activity Relationship, In Vitro and In Vivo Investigations of Jamunones

Twenty-three natural jamunone analogues along with a series of jamunone-based derivatives were synthesized and evaluated for their inhibitory effects against breast cancer (BC) MDA-MB-231 and MCF-7 cells. The preliminary structure-activity relationship revealed that the length of aliphatic side chain and free phenolic hydroxyl group at the scaffold played a vital role in anti-BC activities and the methyl group on chromanone affected the selectivity of molecules against MDA-MB-231 and MCF-7 cells. Among them, jamunone M (JM) was screened as the most effective anti-triple-negative breast cancer (anti-TNBC) candidate with a high selectivity against BC cells over normal human cells. Mechanistic investigations indicated that JM could induce mitochondria-mediated apoptosis and cause G0/G1 phase arrest in BC cells. Furthermore, JM significantly restrained tumor growth in MDA-MB-231 xenograft mice without apparent toxicity. Interestingly, JM could downregulate phosphatidylinositide 3-kinase (PI3K)/Akt pathway by suppressing protein-tyrosine phosphatase 1B (PTP1B) expression. These findings revealed the potential of JM as an appealing therapeutic drug candidate for TNBC.

MicroRNA-Mediated Suppression of the TGF-β Pathway Confers Transmissible and Reversible CDK4/6 Inhibitor Resistance

CDK4/6 inhibition is now part of the standard armamentarium for patients with estrogen receptorpositive (ER⁺) breast cancer, so that defining mechanisms of resistance is a pressing issue. Here, we identify increased CDK6 expression as a key determinant of acquired resistance after palbociclib treatment in ER⁺ breast cancer cells. CDK6 expression is critical for cellular survival during palbociclib exposure. The increased CDK6 expression observed in resistant cells is dependent on TGF-b pathway suppression via miR-432-5p expression. Exosomal miR-432-5p expression mediates the transfer of the resistance phenotype between neighboring cell populations. Levels of miR-432-5p are higher in primary breast cancers demonstrating CDK4/6 resistance compared to those that are sensitive. These data are Furthermore confirmed in pre-treatment and post-progression biopsies from a parotid cancer patient who had responded to ribociclib, demonstrating the clinical relevance of this mechanism. Finally, the CDK4/6 inhibitor resistance phenotype is reversible in vitro and in vivo by a prolonged drug holiday.

Cornell et al. demonstrate a mechanism of acquired CDK4/6 inhibitor resistance that is independent of inherent genetic mutations, is conferred through extracellular signaling, and is reversible in vitro and in vivo. Resistance was mediated by exosomal miRNA, causing increased expression of CDK6 to overcome G1 arrest and promote cell survival.

Downregulation of microRNA‑33a promotes cyclin‑dependent kinase 6, cyclin D1 and PIM1 expression and gastric cancer cell proliferation

Although microRNA‑33 (miR‑33) family members are known to be involved in the regulation and balancing of cholesterol metabolism, fatty acid oxidation and insulin signaling, their functions in carcinogenesis are controversial and the underlying mechanisms have remained elusive. Gastric cancer is the fourth most common malignancy in the world; however, the dysregulation and function of miR‑33 family members in gastric cancer have not been extensively studied. The present study reported that a miR‑33 family member, miR‑33a, was significantly downregulated in gastric cancer tissues and gastric cancer cell lines. Of note, the expression of miR‑33a was inversely correlated with pathological differentiation and metastasis as well as gastric cancer biomarker CA199. A cell‑counting kit‑8 assay showed that transfection of the SGC‑7901 gastric cell line with miR‑33a‑overexpression plasmid inhibited the capability of the cells to proliferate. Furthermore, overexpression of miR‑33a led to cell cycle arrest of SGC‑7901 cells in G1 phase. In addition, a luciferase reporter assay showed that miR‑33a directly targeted cyclin‑dependent kinase 6 (CDK6), cyclin D1 (CCND1) and serine/threonine kinase PIM‑1. In gastric cancer specimens, the reduced expression of miR‑33a was associated with increased expression of CDK‑6, CCND1 and PIM1. However, only PIM1 expression was significantly increased in cancer tissues compared with that in their adjacent tissues. The present study revealed that miR‑33a was downregulated in gastric cancer tissues and cell lines, while forced overexpression of miR‑33a decreased CDK‑6, CCND1 and PIM1 expression to inhibit gastric cancer cell proliferation by causing G1 phase arrest. miR‑33a overexpression may therefore resemble an efficient strategy for gastric cancer therapy.

EGFR signals downregulate tumor suppressors miR-143 and miR-145 in Western diet-promoted murine colon cancer: role of G1 regulators

Epidermal growth factor receptors (EGFR) contribute to colonic tumorigenesis in experimental models of colon cancer. We previously showed that EGFR was also required for colonic tumor promotion by Western diet. The goal of this study was to identify EGFR-regulated microRNAs that contribute to diet-promoted colonic tumorigenesis. Murine colonic tumors from Egfr(wt) and hypomorphic Egfr(wa2) mice were screened using micro RNA (miRNA) arrays and miR-143 and miR-145 changes confirmed by Northern, real-time PCR, and in situ analysis. Rodent and human sporadic and ulcerative colitis (UC)-associated colon cancers were examined for miR-143 and miR-145. Effects of EGFR on miR-143 and miR-145 expression were assessed in murine and human colonic cells and their putative targets examined in vitro and in vivo. miR-143 and miR-145 were readily detected in normal colonocytes and comparable in Egfr(wt) and Egfr(wa2) mice. These miRNAs were downregulated in azoxymethane and inflammation-associated colonic tumors from Egfr(wt) mice but upregulated in Egfr(wa2) tumors. They were also reduced in human sporadic and UC colon cancers. EGFR signals suppressed miR-143 and miR-145 in human and murine colonic cells. Transfected miR-143 and miR-145 inhibited HCT116 cell growth in vitro and in vivo and downregulated G(1) regulators, K-Ras, MYC, CCND2, cdk6, and E2F3, putative or established targets of these miRNAs. miRNA targets Ras and MYC were increased in colonic tumors from Egfr(wt) but not Egfr(wa2) mice fed a Western diet. EGFR suppresses miR-143 and miR-145 in murine models of colon cancer. Furthermore, Western diet unmasks the tumor suppressor roles of these EGFR-regulated miRNAs.

Cyclin-Dependent Kinase 6 Inhibits Proliferation of Human Mammary Epithelial Cells

Many defects in cancer cells are in molecules regulating G1-phase cyclin-dependent kinases (cdks), which are responsible for modulating the activities of Rb family growth-suppressing proteins. Models for understanding how such defects affect proliferation assume that cdks are responsible for sequentially phosphorylating, and hence inactivating, the growth-suppressing functions of Rb family proteins, thus promoting cell cycle progression. However, cdks also play a role in formation of growth-suppressing forms of pRb family molecules, including the “hypophosphorylated” species of pRb itself. Here, it is shown that normal human mammary epithelial cells have a high amount of cdk6 protein and activity, but all breast tumor-derived cell lines analyzed had reduced levels, with several having little or no cdk6. Immunohistochemical studies showed reduced levels of cdk6 in breast tumor cells as compared with normal breast tissue in vivo. Cdk6 levels in two breast tumor cell lines were restored to those characteristic of normal human mammary epithelial cells by DNA transfection. The cells had a reduced growth rate compared with parental tumor cells; cells that lost ectopic expression of cdk6 reverted to the faster growth rate of parental cells. Cell lines with restored cdk6 levels accumulated higher amounts of the Rb family protein p130 as well as E2F4, a suppressing member of the E2F family of transcription factors, in their nuclei. The results suggest that cdk6 restrains rather than stimulates breast epithelial cell proliferation and that its loss or down-regulation could play a role in breast tumor development.

Deregulated G1-cyclin expression induces genomic instability by preventing efficient pre-RC formation

Although genomic instability is a hallmark of human cancer cells, the mechanisms by which genomic instability is generated and selected for during oncogenesis remain obscure. In most human cancers, the pathway leading to the activation of the G1 cyclins is deregulated. Using budding yeast as a model, we show that overexpression of the G1 cyclin Cln2 inhibits the assembly of prereplicative complexes (pre-RCs) and induces gross chromosome rearrangements (GCR). Our results suggest that deregulation of G1 cyclins, selected for in oncogenesis because it confers clonal growth advantage, may also provide an important mechanism for generating genomic instability by inhibiting replication licensing.

Proteasomal activity modulates TGF-ss signaling in a gene-specific manner

Transforming growth factor-beta (TGF-beta) signaling relies on Smad-signaling pathway controlled in part by the proteasome. Here we demonstrate that inhibition of the proteasome function in mink epithelial cells accumulates both positive and negative modulators of TGF-beta signaling, phospho-Smad2 and SnoN. Inhibition of the proteasome led to abrogation of TGF-beta target gene regulation in a gene-specific manner. While regulation of p15Ink4b and myc by TGF-beta are lost, PAI-1 induction, previously shown to occur in a Smad3-dependent manner, was not affected by treatment of the cells with the proteasomal inhibitor MG132. The results suggest that proteasomal activity is required for TGF-beta signaling in a gene-specific manner.