Given the failure of chemo- and biotherapies to fight advanced pancreatic cancer, one major challenge is to identify critical events that initiate invasion. One priming step in epithelia carcinogenesis is the disruption of epithelial cell anchorage to the basement membrane provided by hemidesmosomes (HDs). However, the existence of HDs in pancreatic ductal epithelium and their role in carcinogenesis remain unexplored.
First, presence of HDs has been explored in normal and cancer pancreatic cells and patient samples by transmission electron and confocal microscopy. Mature type-1 HDs, composed of the integrin α6β4 and plectin, as well as of the bullous pemphigoid antigens BP180 and BP230, are known to be present in stratified epithelia, whereas simple epithelia rather contain immature type-2 HDs composed of the integrin α6β4 and plectin only. We here describe for the first time the presence of mature type-1 HDs in human pancreatic ducts, a simple epithelium. Importantly, these structures are disrupted in pancreatic cancer cells, in association with the proteolytic cleavage of BP180 into a non-functional BP120 form (no adhesive properties to the basement membrane).
To investigate the role and molecular mechanisms of HD dynamic during pancreatic carcinogenesis, unique cancer cell models where HD assembly can be pharmacologically manipulated by activation of the somatostatin/sst2 receptor signaling have been used. Using these in vitro as well as in vivo models of pancreatic cancer cells xenografted into nude mice or chick chorioallantoic membrane, we provide molecular evidences that HD breakdown relies on the activation of the master oncogenic phosphoinositide 3-kinase (PI3K) pathway in cancer cells: high PI3K activity induces the expression of the matrix-metalloproteinase MMP9, subsequently promoting the proteolytic cleavage of the hemidesmosomal BP180 protein. Using a siRNA strategy targeting BP180, we demonstrate that HD disruption is a prerequisite for integrin α6β4 delocalization from HD (at the cell basal layer) to the cell leading edges where it is phosphorylated and paradoxically promotes cell migration and invasion by inducing the expression of the pro-migratory S100A4 protein. S100A4 then stimulates MMP-9 expression, thereby inducing a positive feedback loop sustaining BP180 cleavage and HD disruption. Pharmacological inactivation (activation of the somatostatin/sst2 signaling or specific PI3K inhibitor) of this PI3K-MMP9-S100A4 signaling loop conversely blocks BP180 cleavage, induces HD reassembly and inhibits cell invasion, whereas rescuing S100A4 or MMP9 expression in a PI3K-inhibited context restores HD breakdown and cell invasion.
Our results therefore demonstrate that HDs are critical anchoring structures for the pancreatic ductal epithelium whose PI3K-dependent disruption during carcinogenesis promotes cell migration and invasion. By inhibiting PI3K and the subsequent MMP9-S100A4-MMP9 vicious cycle, activation of sst2 forces HD reassembly and inhibits pancreatic cancer cell migration and invasion. This is consistent with our data demonstrating that sst2 deficiency accelerates pancreatic carcinogenesis induced by the oncogenic KrasG12D allelic mutation in the transgenic mouse PDX-1-Cre;LSL-KRASG12D ;sst2+/- model.
Citation Format: Séverine Laval, Mounira Chalabi, Hanane Laklai, Marjorie Fanjul, Hubert Lulka, Talal Al Saati, Marie-Bernadette Delisle, Stéphane Pyronnet, Corinne Bousquet. Critical role of hemidesmosome breakdown for human pancreatic cancer cell migration and invasion. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Progress and Challenges; Jun 18-21, 2012; Lake Tahoe, NV. Philadelphia (PA): AACR; Cancer Res 2012;72(12 Suppl):Abstract nr B60.