MicroRNAs (miRNAs) form a class of noncoding RNA genes whose products are small single-stranded RNAs that are involved in the regulation of translation and degradation of mRNAs. There is a fine balance between deregulation of normal developmental programs and tumor genesis. An increasing body of evidence suggests that altered expression of miRNAs is entailed in the pathogenesis of human cancers. Studies in mouse and human cells have identified the miR-17-92 cluster as a potential oncogene. The miR-17-92 cluster is often amplified or overexpressed in human cancers and has recently emerged as the prototypical oncogenic polycistron miRNA. The functional analysis of miR-17-92 is intricate by the existence of two paralogues: miR-106a-363 and miR-106b-25. During early evolution of vertebrates, it is likely that the three clusters commenced via a series of duplication and deletion occurrences. As miR-106a-363 and miR-106b-25 contain miRNAs that are very similar, and in some cases identical, to those encoded by miR-17-92, it is feasible that they regulate a similar set of genes and have overlapping functions. Further understanding of these three clusters and their functions will increase our knowledge about cancer progression. The present review discusses the characteristics and functions of these three miRNA clusters.

Nerve growth factor beta (NGF-beta) and its precursor proNGF are important for the differentiation and survival of neurons and dermal keratinocytes. The aim of this study was to determine the role that NGF might play in the differentiation and wound healing of oral mucosa. Cultured normal human oral mucosal keratinocytes expressed mRNA for NGF-beta/proNGF and for their receptors TrkA and p75(NTR). Lysates from cultured oral mucosal keratinocytes did not contain detectable amounts of mature 14-kDa NGF-beta but did contain several NGF proforms with molecular weights between 32 and 114 kDa. Culture medium from oral mucosal keratinocytes contained 75 kDa proNGF. The addition of NGF-beta significantly enhanced the proliferation of oral mucosal keratinocyte cultures and in vitro scratch closure. Immunostaining of biopsies from normal oral mucosa showed the presence of proNGF in all epithelial layers. NGF staining was observed in the granular and upper spinous cell layers. TrkA immunoreactivity was detected in basal and parabasal cells, with weak to moderate staining in spinous and granular cell layers. p75(NTR) staining was seen in basal cell layers. These findings indicate that NGF-beta/proNGF have mitogenic and motogenic effects on oral mucosal keratinocytes and therefore may aid in the healing of oral wounds. Differential expression of NGF and NGF receptors throughout the epithelium suggests a role in epithelial differentiation.

The six microRNAs (miRNA) encoded by the miR-17-92 cluster, also named oncomir-1, have been associated with carcinogenesis and typically exhibit-increased expression in tumors. Despite the well-established role for the miR-17-92 cluster in an oncogenic network, the physiological function of these miRNAs in normal tissues remains unresolved. In order to investigate whether there are similar patterns of miR-17-92 expression during embryogenesis and carcinogenesis, we have preformed a systematic study of the expression in cultured carcinoma cells, cultured primary human keratinocytes (KC), and during development of some murine tissues. Both levels of expression of the primary transcript (pri-miRNA) and levels of expression of the individual members of the cluster were monitored. Irrespectively of tissue examined we found that the level of expression decreased markedly during development. With cultured primary human KCs their levels of expression of some of these microRNAs decreased as the number of cell passages increased. Their levels of expression in cultured carcinoma cells, in contrasts, increased, or remained unchanged, with increasing number of cell passages. The results suggest these microRNAs are involved in the regulation of foetal development and that they may promote proliferation and inhibit differentiation during embryogenesis and carcinogenesis. Additionally, the six microRNAs exhibit variable tissue expression, suggesting selective processing of these microRNAs.

Met, the hepatocyte growth factor receptor, is important in transducing signals for tumour growth and metastasis. The aim of this study was to examine the pattern of Met expression and its value as a prognostic factor in oral squamous cell carcinomas (OSCCs). The material consisted of 53 OSCCs and five healthy controls from normal oral mucosa supplied with cell lines, 10 organotypic models supplied with oral cancer cells, and three organotypic models supplied with normal keratinocytes. Met protein expression was assessed by immunohistochemistry and western blotting. Met expression was scarce and limited to the basal layer in normal oral mucosa, but was more extensive in the tumours. Cytoplasmic expression of Met was found in the majority of the tumours, and nuclear expression was found in 72%, including a high fraction of the cells located at the invasive front. Organotypic models with normal or malignant oral cells yielded principally similar results as in the mucosa and the cancers, respectively. A smaller amount of Met immunoreactivity was detected, by western blotting, in the nuclear fraction of cultured oral cancer cells. In conclusion, Met was upregulated in OSCCs and was also found in the nucleus. However, Met was not a marker for prognosis in this study.

It has been suggested that epithelial cyclooxygenase-2 (COX-2) promotes oral carcinogenesis and carcinoma malignancy through increased prostaglandin E(2) (PGE(2)) production. Although oral squamous cell carcinomas (OSCC) often express COX-2, they may also produce PGE(2) in a COX-1-dependent manner. We used 6 isolated cell lines to investigate which COX isoforms OSCC may use for PGE(2) production. COX-1 and -2 expression patterns divided the 6 OSCC cell lines into 3 distinct groups: both COX isoforms low, only COX-1 high, or both COX isoforms high. Multicolor immunohistofluorescence staining confirmed the COX-expression profiles in organotypic 3D cultures and the COX-2 dominance in OSCC tumors. Epidermal growth factor (EGF) stimulation induced COX-2 (but not COX-1) expression and increased PGE(2) production, which was attenuated by COX-2 (but not COX-1) specific inhibition or siRNA-mediated COX-2 gene knockdown. Thus, PGE(2) production in OSCC cell lines was COX-2-dependent.