Hypoxia is a common feature of solid tumors, and develops because of the rapid growth of the tumor that outstrips the oxygen supply, and impaired blood flow due to the formation of abnormal blood vessels supplying the tumor. It has been reported that tumor hypoxia can: activate angiogenesis, thereby enhancing invasiveness and risk of metastasis; increase survival of tumor, as well as suppress anti-tumor immunity and hamper the therapeutic response. Hypoxia mediates these effects by several potential mechanisms: altering gene expression, the activation of oncogenes, inactivation of suppressor genes, reducing genomic stability and clonal selection. We have reviewed the effects of hypoxia on tumor biology and the possible strategiesto manage the hypoxic tumor microenvironment (TME), highlighting the potential use of cancer stem cells in tumor treatment.

CAFs (cancer-associated fibroblasts) are highly flexible cells of the cancer microenvironment. They produce the extracellular matrix (ECM) constituents that form the structure of the tumor stroma but are also a source of metabolites, growth factors, chemokines, and exosomes that impact every aspect of the tumor, including its response to treatment. It is believed that exosomal miRNAs facilitate intercellular signaling, which is essential for the development of cancer. The role of miRNAs and CAFs in the tumor microenvironment (TME) and carcinogenesis is reviewed in this paper. The preferred reporting items for systematic reviews and meta-analyses (PRISMA) 2020 guidelines were used to perform a systematic review. Several databases, including Web of Science, Medline, Embase, Cochrane Library, and Scopus, were searched using the following keywords: CAFs, CAF, cancer-associated fibroblasts, stromal fibroblasts, miRNA, exosomal miRNAs, exosome and similar terms. We identified studies investigating exosomal miRNAs and CAFs in the TME and their role in carcinogenesis. A total of 12,572 papers were identified. After removing duplicates (n = 3803), 8774 articles were screened by title and Of these, 421 were excluded from further analysis. It has been reported that if exosomal miRNAs in CAFs are not functioning correctly, this may influence the secretory phenotype of tip cells and contribute to increased tumor invasiveness, tumor spread, decreased treatment efficacy, and a poorer prognosis. Under their influence, normal fibroblasts (NFs) are transformed into CAFs. Furthermore, they participate in metabolic reprogramming, which allows for fast proliferation of the cancer cell population, adaptation to growing energy demands, and the capacity to avoid immune system identification.

Pancreatic Ductal Adenocarcinoma (PDAC) is among the most lethal solid tumors with grim prognosis. This dismal outcome can partially be explained by the resistance to currently available chemotherapy regimens or the failure of most anticancer agents, which prompted the development of new and effective therapeutic-approaches, such as inhibitors of the epidermal growth factor receptor (EGFR). Some of these EGFR inhibitors (e.g., erlotinib) are approved for lungcancer, however available data are inconclusive for treatment of pancreatic cancer patients with EGFR-targeted-therapies. Here we describe the critical role of EGFR pathway in pancreatic-cancer, strategies to enhance the effectiveness of EGFRinhibitors as well as the preclinical/clinical studies with particular emphasis on recent findings with monoclonal antibodies and tyrosine-inhibitors. Several combinations of EGFR inhibitors with other agents illustrate inhibition of tumor-induced angiogenesis and cell growth. Moreover, combination of erlotinib with gemcitabine showed statistically significance in overall-survival, compared to gemcitabine-alone. However high cost, little survival gain and increased risk of toxicities have limited its efficacy. Considering the multiple genetic mutations and the crosstalk of signaling pathways, (1) development of multiple targeted-therapies; (2) identification of predictive-biomarkers; and (3) those patients who are most likely benefit from therapy, could provide valuable direction for the clinical development of EGFR inhibitors. Moreover further preclinical/clinical studies are warranted to identify determinants of the activity of EGFR-inhibitors and mechanisms leading to resistance to EGFR inhibitors, through the analysis of genetic and environmental alterations affecting EGFR and parallel pro-cancer pathways. These studies will be critical to improve the efficacy and selectivity of current anticancer strategies targeting EGFR in pancreatic cancer.