F13a1^−/− MEFs show increased adipogenesis, decreased proliferation, and a defect in pFN matrix assembly. (A) F13a1^−/− MEFs accumulate significantly more lipids in 8 days when subjected to adipogenic differentiation. Lipid accumulation was visualized and quantified by Oil Red O staining on day 8 of differentiation (n = 3). *P < .05. (B) F13a1^−/− MEFs show a significantly poorer response to the proproliferative effect of exogenous pFN given alone to cells or in combination with INS (n = 3). *P < .05; **P < .01; ***P < .001. Error bars represent SEM. (C) pFN assembly is impaired in F13a1^−/− MEF cultures compared with F13a1^+/+ cultures as assessed by incorporation of exogenous bpFN (green) into extracellular matrix on day 1 of differentiation. Nuclei are stained in blue (DAPI) (n = 2). Scale bar represents 100 µm. (D) Proposed mechanism for the role of FXIII-A in preadipocytes. FXIII-A acts on the cell surface of preadipocytes where it promotes liver-derived, circulating pFN assembly into preadipocyte extracellular matrix. pFN matrix promotes cell proliferation and potentiates the proproliferative effects of INS via the IR and activation of the Erk pathway. (E) In the absence of FXIII-A TG activity, pFN assembly is reduced, which switches INS signaling to activation of the Akt pathway, resulting in increased PPARγ expression and adipocyte differentiation. Thus, the extent of FXIII-A–mediated pFN assembly in preadipocytes and adipocytes can modulate the mitogenic and metabolic effects of INS.