Infantile hypophosphatasia fibroblasts proliferate normally in culture: Evidence against a role for alkaline phosphatase (tissue nonspecific isoenzyme) in the regulation of cell growth and differentiation

To explore the hypothesis that alkaline phosphatase (ALP) functions in the regulation of cell growth and differentiation, we examined in tissue culture several parameters of cell proliferation demonstrated by dermal fibroblasts from patients with infantile hypophosphatasia-a heritable form of rickets/osteomalacia which is characterized biochemically by marked deficiency of activity of the "tissue nonspecific" (bone/liver/kidney) ALP isoenzyme. Methylumbelliferyl phosphate was used as the fluorogenic substrate to assay ALP activity in cell homogenates from patients and age-, sex-, and passage-matched control cells. The nature of the enzymatic defect in the patient fibroblasts involved their failure to increase the specific activity of ALP during growth to confluency. Patient cell monolayers contained, on average, about 2-3% of control ALP activity (several lines consistently demonstrated less than 1%). Nevertheless, patient cells grew normally both in early and late passage, and in either serum-containing or defined medium. Contact inhibition appeared to be intact in the patient fibroblasts, since the protein and DNA content of their culture dishes were similar to controls just as the cells became visually confluent, and when examined 1 week after they reached monolayer. Light and phase-contrast microscopy revealed no abnormalities in the appearance of patient cells during growth or at confluency. The mean volume of proliferating patient cells was also normal. Despite profound deficiency of constitutive ALP activity, dermal fibroblasts from infants with hypophosphatasia exhibit normal growth parameters in cell culture. Although the precise biochemical defect(s) in hypophosphatasia is unknown, these findings fail to support a role for tissue nonspecific ALP in the regulation of cell growth and differentiation.