Modulation of HGF-induced tubulogenesis and branching by multiple phosphorylation mechanisms

MDCK cells cultured in Type I collagen gels can be induced to develop branching tubular structures with demonstrable lumens in the presence of hepatocyte growth factor (HGF). As we have now shown by immunofluorescent localization of specific marker proteins, these tubules retain apical-basolateral polarity. However, the secondary signaling events which lead to these characteristic morphogenetic changes induced by HGF remain largely unelucidated. In order to examine these signaling events, particularly the role of protein phosphorylation in the formation of branching tubular structures, Madin-Darby canine kidney (MDCK) cells in collagen gels were treated with HGF plus well-characterized agents that affect protein phosphorylation. We quantified the formation of branching processes, an early step in the development of tubular structures in this model. Protein kinase C (PKC) inhibition resulted in more complex branching processes in the presence of HGF, when compared with HGF alone. In contrast, treatment with activators of protein kinase A (PKA), as well as calmodulin antagonists, caused a marked decline in process formation. Consistent with an important role for protein phosphorylation in HGF-induced morphogenesis, protein phosphatase inhibition by okadaic acid or calyculin A was found to markedly inhibit process formation. Tyrosine kinase (TK) inhibition also decreased the percentage of processes. This is consistent with data indicating that one of the HGF receptors is identical to the c-met protooncogene product, which is known to possess TK activity. Our results suggest that the HGF-mediated induction of branching processes in MDCK cells, an early step in the development of branching tubular structures, can be modulated by multiple phosphorylation mechanisms including those mediated by PKC, PKA, and Ca2+/calmodulin-dependent kinase(s). We discuss how these phosphorylation events may play crucial roles in determining the degree of tubule formation and their length, as well as the extent of their arborization during the early development of epithelial tissues.