Abstract Background: Lin28 and Lin28B have recently been implicated in the post-transcriptional regulation of Let-7/miR-98 biogenesis. While Lin28B over-expression has been associated with worse clinical outcome in several human cancers, the prognostic significance of Lin28B expression and the molecular mechanism(s) by which it drives tumour progression remains largely unknown. Methods: Let-7-Resist Lin28B-expression plasmid was stably transfected into the FaDu human hypopharyngeal squamous cell carcinoma line. MicoRNA expression profiling was conducted using the Nanostring nCounter Analysis System. Genome-wide micro-array and in silico analyses were utilized to identify Let-7/miR-98 targets. Cell viability and apoptosis were measured using MTS assay and flow cytometry, respectively. In vivo experiments were conducted using SCID mice. Results: qRT-PCR analysis of genes involved in the miRNA biogenesis pathway (DICER, LIN28, LIN28B, and TUT4) suggested an inverse relationship between the expression of Lin28B and mature Let-7 miRNA. Over-expression of Lin28B mediated significant increase in the migration of FaDu cells in vitro, and tumor growth in SCID mice, which in turn, was associated with marked reduction in the Let-7 family of microRNAs. Using a tri-Pronged approach by combining prediction algorithms, experimentally-derived mRNA profiling, along with gene expression data in primary HNSCC samples identified several upregulated Let-7/miR98 gene targets in Lin28B-FaDu cells, including CCND2, HMGA2, and regulators of the insulin growth factor (IGF) mitogenic pathway. Concordantly, treating FaDu-Lin28B cells with IGF (10-50 nM) led to a 26% and 50% increase in cell viability under stress conditions (serum starvation), whereas treating FaDu-GFP cells with IGF had no effect on viability, suggesting the relevance of the IGF axis in FaDu-Lin28B cells. Furthermore, analyzing publicly available data sets on primary HPV positive vs. HPV negative HNSCC samples revealed a significantly higher expression of HMGA2, and genes that regulate the IGF pathway in the latter group of tumours, suggesting a possible role for Lin28B, Let-7/miR-98 and their downstream targets in mediating the more aggressive behavior of HPV negative HNSCC. Conclusions: Our data report for the first time, a potentially important role for Lin28B in regulating the IGF mitogenic pathway, as one possible mechanism in driving the biological behaviour of HPV negative HNSCC. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1192. doi:10.1158/1538-7445.AM2011-1192