Dr. Shujun Liu Cancer Epigenetics & Experimental Therapeutics M.S., Ph.D.
Primary interests of our research section are to understand the molecular mechanisms and the roles of aberrant epigenetics and protein kinase activities in cancer pathogenesis, and to translate the discoveries from bench to bedside by means of characterizing novel therapeutic reagents and developing innovative vehicles to efficiently and specifically deliver the drugs to the disease sites. In our laboratory, studies have included the cause of DNA hypermethylation and abnormal protein kinase activity, the mechanistic link between obesity and cancer, the identification of new therapeutic reagents, the dissection of molecular basis underlying the anti-cancer actions of bioactive compounds and the development of innovative nanoparticulates for drug delivery.
The molecular rules underlying aberrant epigenetics
DNA methylation occurs at the 5-position of cytosine in a CpG dinucleotide context and is a major epigenetic mechanism regulating chromosomal stability and gene expression. DNA methylation is under control of DNA methyltransferases (DNMTs), including DNMT1, DNMT3a and DNMT3b. In cancers, DNMTs are highly expressed and the tumor suppressor genes (TSGs) are frequently silenced by promoter hypermethylation. However, the molecular processes behind abnormal DNMT expression remain are largely unknown. We have demonstrated that Sp1/miR29 feedback loop critically regulates the expression of DNMTs. Ten-eleven translocation (TET) protein family also significantly modulates DNA methylation program through controlling DNMT gene transcription. Pharmacological modulation of Sp1/miR29 network by siRNA or/and small molecules changes DNA methylation status leading to the reexpression of TSGs and the inhibition of leukemia growth. Since Sp1 and miR29b target DNA methylation at multiple levels, we demonstrated that the combination of Sp1 inhibitors (siRNA or bortezomib) with synthetic miR29b synergistically suppresses their downstream signaling cascades. Furthermore, miRNA target prediction indicates that TET1 is a putative target of miR29b. Hence, we reasoned that miR29b could monitor DNA methylation through TET1 abrogation. In addition, we demonstrated the critical role of miR101 in controlling DNMT3a expression thereby DNA methylation program. All these observations point out the complexity of DNA methylation machinery, which may explain, at least in part, why current epigenetic therapies in cancers are unsatisfactory.
The molecular mechanisms controlling the transcription of protein kinases
It is well known that abnormal kinase activities are essential in cancer initiation and metastasis, but the main focuses currently are to pharmacologically inhibit kinase activities without the consideration of kinase overamplification. Others and we have demonstrated that kinase gene overexpression is involved in the development, progression and drug resistance of cancer and in survival in some patient subpopulations. However, how protein kinases are transcriptionally regulated remains largely undefined. We previously reported that receptor tyrosine kinases (i.e., KIT and FLT3) are regulated by Sp1/miR29 network. Given the critical roles of Sp1/miR29 in DNMT gene regulation, we proposed that aberrant DNMT function may control hyperactive kinase pathway. Indeed, our studies showed that DNMT upregulation increases KIT expression leading to KIT specific and global kinase over-function. These findings identify the regulatory and functional interactions between kinases and DNA methyltransferases, and suggest that misregulated KIT signaling and DNA hypermethylation cooperatively contribute to cancerous lesions.
Mechanistic links between obesity and leukemia
Cancer is the representatively systemic lesions taking over the first place of lethal diseases throughout the world. Obesity is a “disease” with abnormal body fat accumulation. The World Health Organization estimates that approximately one quarter of population worldwide are obese. Emerging data indicate that obesity is a major risk factor for human malignancies. It can increase the occurrence of cancerous lesions and decrease the benefit of therapy. However, the molecular mechanisms underlying these phenomena are poorly understood. We observed that higher BMI (body mass index) associates with shorter overall survival in leukemia patients. When leukemia cells were transplanted into obese or lean mice, we found that, compared to the lean counterparts, obese mice display exacerbated leukemic disease, thus experimentally demonstrating the contribution of obesity to leukemogenesis in mice. The future directions are to delineate the mechanistic pathways controlling the obesity-associated leukemogenesis.
Molecular mechanisms of anti-cancer actions of bioactive compounds
Due to the essential roles of aberrant DNA methylation in cancers, DNA hypomethylating agents (i.e., 5-Azacytidine and 5-aza-2′-deoxycytidine) have been in clinical use for decades. While some positive response has been achieved, the majority of leukemia patients relapse and eventually die of their disease, arguing for the new type of DNMT inhibitors with different structure and distinct mechanisms. Because of the anti-cancer activity and lower toxicity to normal cells, numerous plant extracts have been tested in vitro and in vivo with huge therapeutic potentials. We have demonstrated that certain types of bioactive compounds [i.e., thymoquinone (TQ), echinomycin or emetine] suppresses the expression of DNMT1, DNMT3a and DNMT3b resulting in global DNA hypomethylation and the re-expression of TSGs through promoter hypomethylation. Functionally, these compounds suppress cancer cell proliferation in vitro and metastatic growth in vivo. While the underlying molecular mechanisms remain elusive, these compounds may hold a promising in human cancer therapy.
Developing multifunctional drug and gene delivery nanoparticles for cancer therapy
The current chemotherapeutic drugs (i.e., small molecules, siRNA or miRs), although they display promising anti-cancer activity, suffer from a variety of drawbacks when administered particularly in vivo, including rapid clearance, lack of tissue selectivity, high affinity to plasma proteins and poor cellular uptake. We have developed new liposomal formulations and synthesized nanoparticles to efficiently deliver the aforementioned drugs. We have successfully delivered bortezomib, miR29 and Sp1 siRNA by nanoparticles in vitro and in vivo. As a consequence of efficient delivery, we observed that liposomal bortezomib has a decrease of clearance and thereby an increase of drug exposure to leukemia cells existing in blood, compared to those of free bortezomib in mice. Due to this change, liposomal bortezomib induced a long-term disease free remission in 80% of DKI AML mice and 100% of LGL leukemia IL-15 Tg mice. We also evidenced the synergistic effects of combined liposomal bortezomib with nano-miR29b on leukemia cell growth in mice. Recently, we synthesized HDL/AuNP nanoparticle and successfully delivered small molecule compounds into leukemia cells. These results revealed that nano-drug delivery displays huge potential to improve therapeutic efficacy while reducing its side effects, including decreased drug toxicity, altered pharmacokinetics, improved drug solubility and more specific target binding.
Overall, our discoveries offer new insights into the molecular biology of cancer, advance our understanding of nanoscience with efficient delivery vehicle for miRs and small molecule compounds, and foster the translation of nanotechnology solutions to biomedical applications thereby improving the management of cancerous lesions.
PubMed (a service of the National Library of Medicine) provides a comprehensive list of Dr. Liu’s recent publications.