Attenuated ALK5 receptor expression in human pancreatic cancer: Correlation with resistance to growth inhibition

Transforming growth factor-β (TGF-β) receptors constitute a family of transmembrane proteins that bind TGF-β ligands. In this study we assessed the growth responsiveness to TGF-βI in pancreatic cancer cell lines and characterized the levels of expression of TGF-β receptors in these cell lines and in human pancreatic cancer tissues. COLO 357 cells were most sensitive to the growth inhibitory actions of TGF-βI, PANC-1 cells exhibited moderate sensitivity, Hs766T cells exhibited slight sensitivity and MIA PaCa-2 and T3M4 cells were resistant to TGF-βI. Only COLO 357 cells expressed high levels of ALK5, the major type I TGF-β receptor (TβRI). Hs766T and PANC-1 cells expressed high levels of SKRI, another TβRI subtype. Only MIA PaCa-2 cells did not exhibit the type II TGF-β receptor (Tβ-RII) transcript, whereas type III TGF-β receptor (Tβ-RIII) mRNA levels were elevated in this cell line and in HS766T cells. All the cell lines expressed TGF-βI, but TGF-β2 and TGF-β3 mRNA levels were variable. ALK5 and SKRI mRNA levels were 6.8- and 9-fold greater in the pancreatic tumors in comparison with the corresponding levels in the normal pancreas. However, in the cancer cells, ALK5 immunoreactivity was faint, whereas TβRII immunoreactivity was focal and intense. Conversely, in ductal cells adjacent to cancer cells ALK5 immunoreactivity was strong, whereas TβRII immunoreactivity was weak. Since ALK5 heterodimerization with TβRII is crucial for TGF-β-mediated signaling, our findings suggest that low levels of ALK5 in pancreatic cancer cells within a tumor may protect against growth inhibition.