The role of melanogenesis in regulation of melanoma behavior: melanogenesis leads to stimulation of HIF-1α expression and HIF-dependent attendant pathways

To study the effect of melanogenesis on HIF-1α expression and attendant pathways, we used stable human and hamster melanoma cell lines in which the amelanotic vs. melanotic phenotypes are dependent upon the concentration of melanogenesis precursors in the culture media. The induction of melanin pigmentation led to significant up-regulation of HIF-1α, but not HIF-2α, protein in melanized cells for both lines. Similar upregulation of nuclear HIF-1α was observed in excisions of advanced melanotic vs. amelanotic melanomas. In cultured cells, melanogenesis also significantly stimulated expression of classical HIF-1-dependent target genes involved in angiogenesis and cellular metabolism, including glucose metabolism and stimulation of activity of key enzymes in the glycolytic pathway. Several other stress related genes containing putative HRE consensus sites were also upregulated by melanogenesis, concurrently with modulation of expression of HIF-1-independent genes encoding for steroidogenic enzymes, cytokines and growth factors. Immunohistochemical studies using a large panel of pigmented lesions revealed that higher levels of HIF-1α and GLUT-1 were detected in advanced melanomas in comparison to melanocytic nevi or thin melanomas localized to the skin. However, the effects on overall or disease free survival in melanoma patients were modest or absent for GLUT-1 or for HIF-1α, respectively. In conclusion, induction of the melanogenic pathway leads to robust upregulation of HIF-1-dependent and independent pathways in cultured melanoma cells, suggesting a key role for melanogenesis in regulation of cellular metabolism.

Abstract P1-05-15: Identification of a novel Hypoxia Inducible Factor-1 regulated gene involved in breast cancer growth and metastasis

Introduction: Hypoxia is a hallmark of most solid tumors. In response to hypoxic stress tumor cells adapt by regulating survival, metabolism and angiogenesis. The heterodimeric Hypoxia-Inducible Factor (HIF)-1 transcription factor is a master regulator of this response. HIF-1alpha protein is over-expressed in ∼30% of primary breast tumors and ∼70% of metastases, which independently correlates with poor prognosis and decreased survival in patients. Moreover, conditional deletion of Hif1a in the mammary epithelium of the MMTV-polyoma middle T (MMTV-PyMT) transgenic model of breast cancer significantly delays mammary tumor initiation and lung metastasis burden.

Methods: Our lab has established primary mammary tumor epithelial cells (MTECs) from late stage carcinomas originating in PyMT+; Hif1α floxed mice (FVB/Nj strain, F11). These MTECs were exposed to either Adenovirus-eta-βgal or -Cre to create wild-type (WT) and knock-out (KO) cells, respectively. PyMT MTECs are transplantable into syngeneic FVB/Nj female recipients. Deletion of HIF-1 activity reduced primary tumor growth by ∼60% and the formation of lung macrometastases originating from mammary fat pad tumors by >90%. Microarray profiling was conducted to identify genes differentially expressed between WT and KO cells cultured at normoxia (21% O2) or hypoxia (0.5% O2) as well as between end-stage WT and KO tumors derived from the parental cell lines. Several genes were down-regulated in both gene sets in response to deletion of Hif1a. One mRNA of particular interest, creatine kinase brain isoform (Ckb) was down-regulated in KO cells >100 fold and >2 fold in end-stage tumors. Pools of stable Ckb knockdown (KD) PyMT cells were generated via lentiviral transduction of two independent shRNA constructs (pLKO.1 shc59 or shc61, each producing >75% knockdown) to determine the effects of Ckb knockdown on cell proliferation, migration and metastasis.

Results: No differences in cell proliferation were observed between WT and Ckb KD cells when cultured in standard culture conditions. In contrast, when cells were introduced to the mammary fat pad, initiation of palpable tumors was delayed by >30 days for each shRNA KD line, suggesting that CKB function may be modulated by the tumor microenvironment. A significant decrease in invasion through Matrigel was also observed for each shRNA Ckb cell line. Likewise, when cells were introduced into circulation via tail vein injection, 20% of mice injected with either shc59 or shc61 cells developed lung metastases whereas 100% of mice injected with WT cells developed metastases. Bioinformatic analyses indicate that multiple putative HIF-1 binding sites (HREs) are present in both the mouse and human CKB gene promoters; chromatin immunoprecipiation (ChIP) experiments are in progress to validate that CKB is a direct HIF-1 target gene.

Conclusions: HIF-1alpha strongly promotes metastasis, the major cause of mortality in breast cancer patients. Further characterization of genes downstream of HIF-1alpha, such as CKB, that play a key role in driving invasion and tumor initiation may identify new pathways amenable to therapeutic intervention for patients with metastatic breast cancer.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P1-05-15.