Id proteins antagonize basic helix-loop-helix proteins , inhibit differentiation , and enhance cell proliferation. In this study we compared the expression of Id-1 , Id-2 , and Id-3 in the normal pancreas , in pancreatic cancer , and in chronic pancreatitis (CP). Northern blot analysis demonstrated that all three Id mRNA species were expressed at high levels in pancreatic cancer samples by comparison with normal or CP samples. Pancreatic cancer cell lines frequently coexpressed all three Ids , exhibiting a good correlation between Id mRNA and protein levels , as determined by immunoblotting with highly specific anti-Id antibodies. Immunohistochemistry using these antibodies demonstrated the presence of faint Id-1 and Id-2 immunostaining in pancreatic ductal cells in the normal pancreas , whereas Id-3 immunoreactivity ranged from weak to strong. In the cancer tissues, Basic helix-loop-helix (bHLH) proteins play an important role as transcription factors in cellular development, proliferation, and differentiation. 1,2 The basic domain of the bHLHs is required for binding to an E-box DNA sequence, thus promoting transcription of specific target genes. The HLH domain promotes dimer formation with various members of the bHLH protein family. 1,2 Homodimers of the class B family of bHLH proteins, including MyoD, NeuroD, and numerous other proteins, are known to activate tissue-specific genes. 3-5 These tissuespecific bHLHs typically form heterodimers with widely expressed class A bHLHs, which include proteins encoded by E2A, E2-2, HEB, and other genes (also termed E-proteins). 6 -9 These heterodimers activate transcription of genes that are associated with differentiation.Id genes encode a family of four HLH proteins that lack the basic DNA binding domain. 1,10 They act as dominantnegative HLH proteins by forming high affinity heterodimers with other bHLH proteins, thereby preventing them from binding to DNA and inhibiting transcription of differentiation-associated genes. 10 -12 Id gene expression is down-regulated on differentiation in many cell types in vitro and in vivo. [13][14][15][16][17][18] In addition, Id proteins seem to be required for cell cycle progression through G 1 /S phase in certain cell types, and interaction between Id-2 and pRB is associated with enhanced proliferation in some cell lines in vitro. 19 -23 Pancreatic cancer is the fifth leading cause of cancer death in the United States, with a mortality rate that virtually equals its incidence rate. 24 This malignancy is often associated with the overexpression of a variety of mitogenic growth factors and their receptors, and by oncogenic mutations of K-ras and inactivation of the p53 tumor suppressor gene. 25 We have recently reported that pancreatic cancers overexpress the HLH protein Id-2, and that enhanced expression of this protein is evident in the cytoplasm of the cancer cells within the pancreatic tumor mass. 26 It is not known, however, whether the expression of other Id proteins is altered in this malignancy, or whether their expressio...
