Caracterización funcional de las isoformas de Id1 (Id1a e Id1b) en cáncer

Resumen

Id1 is a member of the helix-loop-helix family of proteins that regulates the activity of transcription factors to suppress cellular differentiation and promote cell growth. Overexpression of Id1 in tumor cells correlates with increased malignancy and resistance to chemotherapy and radiotherapy. The Id1 gene generates two isoforms through alternative splicing: Id1a (classical isoform) and Id1b. At present, the function of Id1b is largely unknown. We have investigated the role of each isoform in cancer development. The expression of Id1b in 17 non-small cell lung cancer specimens and their matched non-malignant counterparts are significantly lower (p<0.01) in lung tumors compared to non-tumor tissues. These data indicate the low expression of Id1b. Overexpression of Id1b in human A549 lung and PC3 prostate cancer cells reduces anchorage-dependent and independent proliferation and clonogenic potential. Moreover, overexpression of Id1b increases the accumulation of cells in the G1 phase of the cell cycle, with elevation of p57 and p27 levels, whereas phospho-ERK and cyclin A levels are reduced. Similar results were obtained with overexpression of Id1b in murine CT26 and 3LL cancer cells. However, high levels of Id1a have an opposite effect and the proportion of cells in the S phase and proliferation rates increased significantly. Interestingly, Id1b overexpressing A549 cells treated with 4Gy irradiation dose are significantly less resistant to cell death than control cells. In vivo models confirmed the inhibitory role of Id1b in primary tumor growth and metastasis and its effects on radiosensitization. Recent reports have involved Id1 in maintenance of cancer stem cell features in some tumor types. Through microarray analysis we found that the cancer stem cell (CSC) markers ALDH1A1 and Notch-1 were up-regulated specifically in Id1b-overexpressing cells. By using qPCR we also found overexpression of Sca-1, Tert, Sox-2 and Oct-4 in these cells. Increased levels of Id1b promoted self-renewal and CSC-like properties, as shown by their high capacity of developing secondary tumorspheres and retaining the PKH26 dye. Our results show that Id1b decreases the malignancy of cancer cells, sensitizes them to radiotherapy-induced cell death, maintains cells in a quiescent state and promotes self-renewal and CSC-like features. On the contrary, Id1a promotes cell proliferation.